Research Items (243)
- Mar 2019
Background Optimal timing of the CT scan relative to the contrast media bolus remains a challenging task given the shorter scan durations of modern CT scanners, as well as interpatient variability. Purpose To compare contrast opacification in CT angiography of the aorta between a cohort with fixed trigger delay and a cohort with patient-specific individualized trigger delay for contrast media timing with bolus tracking. Materials and Methods In this prospective study (January-August 2018), CT angiography of the thoracoabdominal aorta with bolus tracking was performed in two different study cohorts: one with a fixed trigger delay of 4 seconds (fixed cohort) and one with a patient-specific trigger delay (individualized cohort). All CT and contrast media protocol parameters were kept identical among cohorts. Objective image quality was evaluated by one reader; two readers assessed subjective image quality. Student t test was used to test for differences in mean attenuation; the Wilcoxon-Mann-Whitney test was used to test for differences in noise, contrast-to-noise ratio, and subjective image quality. Results The fixed cohort had 108 study participants (16 women; mean age ± standard deviation, 72 years ± 10); the individualized cohort had 108 participants (16 women; mean age, 72 years ± 12). The trigger delay in the individualized cohort ranged from 6.4-11.3 seconds (mean, 9.2 seconds). There was higher overall attenuation in the individualized cohort than in the fixed cohort (486 HU ± 92 for individualized vs 438 HU ± 99 for fixed; P < .001), with increasing differences from the aortic arch (8 HU) to the iliac arteries (95 HU). The regression model indicated uniform attenuation in the individualized cohort and decreasing attenuation in the fixed cohort (decrease of 87 HU by the iliac arteries; P < .001). There was no difference between cohorts for image noise (20 vs 19; P = .41), but contrast-to-noise ratio (21 vs 19; P = .04) and subjective image quality were higher in the individualized cohort than in the fixed cohort (excellent or good image quality, 100% vs 67%; P < .001). Conclusion Compared with a fixed delay time after bolus tracking, a patient-specific individualized trigger delay improves image quality and provides uniform contrast attenuation for CT angiography of the aorta. ©RSNA, 2019.
- Feb 2019
Objectives To investigate the diagnostic accuracy of a modified three-material decomposition calcium subtraction (CS) algorithm for the detection of arterial stenosis in dual-energy CT angiography (DE-CTA) of the lower extremity runoff compared to standard image reconstruction, using digital subtraction angiography (DSA) as the reference standard. Methods Eighty-eight patients (53 males; mean age, 65.9 ± 11 years) with suspected peripheral arterial disease (PAD) who had undergone a DE-CTA examination of the lower extremity runoff between May 2014 and May 2015 were included in this IRB-approved, HIPAA-compliant retrospective study. Standard linearly blended and CS images were reconstructed and vascular contrast-to-noise ratios (CNR) were calculated. Two independent observers assessed subjective image quality using a 5-point Likert scale. Diagnostic accuracy for ≥ 50% stenosis detection was analyzed in a subgroup of 45 patients who had undergone additional DSA. Diagnostic accuracy parameters were estimated with a random-effects logistic regression analysis and compared using generalized estimating equations. Results CS datasets showed higher CNR (15.3 ± 7.3) compared to standard reconstructions (13.5 ± 6.5, p < 0.001). Both reconstructions showed comparable qualitative image quality scores (CS, 4.64; standard, 4.57; p = 0.220). Diagnostic accuracy (sensitivity, specificity, positive and negative predictive values) for CS reconstructions was 96.5% (97.5%, 95.6%, 90.9%, 98.1) and 93.1% (98.8%, 90.4%, 82.3%, 99.1%) for standard images. Conclusions A modified three-material decomposition CS algorithm provides increased vascular CNR, equivalent qualitative image quality, and greater diagnostic accuracy for the detection of significant arterial stenosis of the lower extremity runoff on DE-CTA compared with standard image reconstruction. Key Points • Calcified plaques may lead to overestimation of stenosis severity and false positive results, requiring additional invasive digital subtraction angiography (DSA). • A modified three-material decomposition algorithm for calcium subtraction provides greater diagnostic accuracy for the detection of significant arterial stenosis of the lower extremity runoff compared with standard image reconstruction. • The application of this algorithm in patients with heavily calcified vessels may be helpful to potentially reduce inconclusive CT angiography examinations and the need for subsequent invasive DSA.
Shorter scan times and the desire for higher resolution were the driving forces behind the development of spiral scanning in 1990 and the first multi-detector row computed tomography (CT) systems in 1998. With the introduction of ECG-gated scanning on four-slice CT scanners in 1999, the first step toward cardiac imaging with multi-detector row computed tomography (MDCT) had been made.
Adverse effects of intravenous contrast media (CM) in patients with renal risk factors and acute kidney injury are still controversially discussed. The aim of this study was to investigate whether dual-energy (DE) pulmonary CT angiography (CTPA) in combination with a noise optimized virtual monoenergetic imaging algorithm allows for a reduction of CM. This IRB-approved study comprised 150 patients with suspected pulmonary embolism (78 male; mean age 65 ± 17years). 50 patients with acute/chronic renal failure were examined on a 3rd generation dual-source CT with an optimized DE CTPA protocol and a low CM injection protocol (5.4 g iodine). 100 further patients were either examined with a standard CTPA protocol or a standard DE CTPA (32 g iodine). For the DE CTPA virtual monoenergetic spectral datasets (40-100 keV) were reconstructed. Main pulmonary arteries at 50 keV and peripheral pulmonary arteries at 40 keV datasets provided the highest contrast-to-noise-ratio (CNR) for both the standard DE CTPA and the optimized protocol, with significantly higher CNR values for the standard DE CTPA protocol (p < 0.05). No pulmonary embolism was missed on the optimized CM protocol. DE CTPA utilizing image reconstruction at 40/50 keV allowed for a reduction of 84% in iodine load while maintaining CNR, which is especially important in patients with acute/chronic renal failure.
- Jul 2018
Aim: The aim of the study was to assess possible correlation of fluorogeoxyglucose (FDG) uptake and iodine-related attenuation values derived from positron-emission tomography/computed tomography (PET/CT) using single-source dual-energy CT scan (DE-CT) in non-small cell lung cancer (NSCLC). Materials and Methods: Forty-eight patients with histologically-proven NSCLC underwent 18 F-FDG-PET/CT within their staging process. PET/CT included single-source DE-CT in late post-contrast phase. Direct comparison of PET and DE-related values was performed. A sub-study regarding different histological types and various thresholds for quantification of volume metabolic values was also performed. Results: A strong correlation was found of metabolic tumor volume and total lesion glycolysis with total iodine content using Pearson correlation analysis (r=0.965-0.983; p<0.0001) with various thresholds for FDG lesion segmentation. The strongest correlations with iodine content were reached using 10% threshold for segmentation. Only a weak correlation was found between iodine content and the maximal standard uptake value. A significant difference between adenocarcinomas and other histological subtypes was found for selected parameters of metabolic PET and DE-CT data. Conclusion: Our study demonstrated a strong correlation of the iodine content calculated from single-source DE-CT with volumetric FDG parameters in NSCLC. without a significant effect of the threshold value for FDG lesion segmentation.
Objectives: The aim of this study was to evaluate the accuracy of a 3-dimensional (3D) camera algorithm for automatic and individualized patient positioning based on body surface detection and to compare the results of the 3D camera with manual positioning performed by technologists in routinely obtained chest and abdomen computed tomography (CT) examinations. Materials and methods: This study included data of 120 patients undergoing clinically indicated chest (n = 68) and abdomen (n = 52) CT. Fifty-two of the patients were scanned with CT using a table height manually selected by technologists; 68 patients were automatically positioned with the 3D camera, which is based on patient-specific body surface and contour detection. The ground truth table height (TGT) was defined as the table height that aligns the axial center of the patient's body region in the CT scanner isocenter. Off-centering was defined as the difference between the ground truth table height (TGT) and the actual table position used in all CT examinations. The t test was performed to determine significant differences in the vertical offset between automatic and manual positioning. The χ test was used to check whether there was a relationship between patient size and the magnitude of off-centering. Results: We found a significant improvement in patient centering (offset 5 ± 3 mm) when using the automatic positioning algorithm with the 3D camera compared with manual positioning (offset 19 ± 10 mm) performed by technologists (P < 0.005). Automatic patient positioning based on the 3D camera reduced the average offset in vertical table position from 19 mm to 7 mm for chest and from 18 mm to 4 mm for abdomen CT. The absolute maximal offset was 39 mm and 43 mm for chest and abdomen CT, respectively, when patients were positioned manually, whereas with automatic positioning using the 3D camera the offset never exceeded 15 mm. In chest CT performed with manual patient positioning, we found a significant correlation between vertical offset greater than 20 mm and patient size (body mass index, >26 kg/m, P < 0.001). In contrast, no such relationship was found for abdomen CT (P = 0.38). Conclusions: Automatic individualized patient positioning using a 3D camera allows for accurate patient centering as compared with manual positioning, which improves radiation dose utilization.
Objectives: The aims of this study were to assess the value of a dedicated sharp convolution kernel for photon counting detector (PCD) computed tomography (CT) for coronary stent imaging and to evaluate to which extent iterative reconstructions can compensate for potential increases in image noise. Materials and methods: For this in vitro study, a phantom simulating coronary artery stenting was prepared. Eighteen different coronary stents were expanded in plastic tubes of 3 mm diameter. Tubes were filled with diluted contrast agent, sealed, and immersed in oil calibrated to an attenuation of -100 HU simulating epicardial fat. The phantom was scanned in a modified second generation 128-slice dual-source CT scanner (SOMATOM Definition Flash, Siemens Healthcare, Erlangen, Germany) equipped with both a conventional energy integrating detector and PCD. Image data were acquired using the PCD part of the scanner with 48 × 0.25 mm slices, a tube voltage of 100 kVp, and tube current-time product of 100 mAs. Images were reconstructed using a conventional convolution kernel for stent imaging with filtered back-projection (B46) and with sinogram-affirmed iterative reconstruction (SAFIRE) at level 3 (I463). For comparison, a dedicated sharp convolution kernel with filtered back-projection (D70) and SAFIRE level 3 (Q703) and level 5 (Q705) was used. The D70 and Q70 kernels were specifically designed for coronary stent imaging with PCD CT by optimizing the image modulation transfer function and the separation of contrast edges. Two independent, blinded readers evaluated subjective image quality (Likert scale 0-3, where 3 = excellent), in-stent diameter difference, in-stent attenuation difference, mathematically defined image sharpness, and noise of each reconstruction. Interreader reliability was calculated using Goodman and Kruskal's γ and intraclass correlation coefficients (ICCs). Differences in image quality were evaluated using a Wilcoxon signed-rank test. Differences in in-stent diameter difference, in-stent attenuation difference, image sharpness, and image noise were tested using a paired-sample t test corrected for multiple comparisons. Results: Interreader and intrareader reliability were excellent (γ = 0.953, ICCs = 0.891-0.999, and γ = 0.996, ICCs = 0.918-0.999, respectively). Reconstructions using the dedicated sharp convolution kernel yielded significantly better results regarding image quality (B46: 0.4 ± 0.5 vs D70: 2.9 ± 0.3; P < 0.001), in-stent diameter difference (1.5 ± 0.3 vs 1.0 ± 0.3 mm; P < 0.001), and image sharpness (728 ± 246 vs 2069 ± 411 CT numbers/voxel; P < 0.001). Regarding in-stent attenuation difference, no significant difference was observed between the 2 kernels (151 ± 76 vs 158 ± 92 CT numbers; P = 0.627). Noise was significantly higher in all sharp convolution kernel images but was reduced by 41% and 59% by applying SAFIRE levels 3 and 5, respectively (B46: 16 ± 1, D70: 111 ± 3, Q703: 65 ± 2, Q705: 46 ± 2 CT numbers; P < 0.001 for all comparisons). Conclusions: A dedicated sharp convolution kernel for PCD CT imaging of coronary stents yields superior qualitative and quantitative image characteristics compared with conventional reconstruction kernels. Resulting higher noise levels in sharp kernel PCD imaging can be partially compensated with iterative image reconstruction techniques.
Background Current techniques for evaluation of bone mineral density (BMD) commonly require phantom calibration. The purpose of this study was to evaluate a novel algorithm for phantomless in vivo dual-energy computed tomography (DECT)-based assessment of BMD of the lumbar spine in comparison with dual-energy X-ray absorptiometry (DEXA). Methods Data from clinically indicated DECT and DEXA examinations within two months comprising the lumbar spine of 47 patients were retrospectively evaluated. By using a novel automated dedicated post-processing algorithm for DECT, the trabecular bone of lumbar vertebrae L1–L4 was selected and analysed. Linear correlation was analysed using Pearson’s product-moment correlation coefficient for the comparison of the results from DECT and DEXA. Results A total of 186 lumbar vertebrae in 47 patients (mean age, 58 years; age range, 24–85 years) were analysed, 24 men (mean age, 55 years; age range, 24–85 years) and 23 women (mean age, 59 years; age range, 31–80 years). Mean BMD of L1–L4 determined with DEXA was 0.985 g/cm² and 20/47 patients (42.6%) showed an osteoporotic BMD (T score lower than – 2.5) of at least two vertebrae. Average DECT-based BMD of L1–L4 was 86.8 mg/cm³. Regression analysis demonstrated a lack of correlation between DECT- and DEXA-based BMD values with a Pearson’s product-moment correlation coefficient r = 0.4205. Conclusions Dedicated post-processing of DECT data using a novel algorithm for retrospective phantomless BMD assessment of the trabecular bone of lumbar vertebrae from clinically indicated DECT examinations is feasible.
- Sep 2017
Purpose: The aim of this study was to investigate computed tomography (CT) imaging characteristics of coronary stents using a novel photon-counting detector (PCD) in comparison with a conventional energy-integrating detector (EID). Materials and methods: In this in vitro study, 18 different coronary stents were expanded in plastic tubes of 3 mm diameter, were filled with contrast agent (diluted to an attenuation of 250 Hounsfield units [HU] at 120 kVp), and were sealed. Stents were placed in an oil-filled custom phantom calibrated to an attenuation of -100 HU at 120 kVp for resembling pericardial fat. The phantom was positioned in the gantry at 2 different angles at 0 degree and 90 degrees relative to the z axis, and was imaged in a research dual-source PCD-CT scanner. Detector subsystem "A" used a standard 64-row EID, while detector subsystem "B" used a PCD, allowing high-resolution scanning (detector pixel-size 0.250 × 0.250 mm in the isocenter). Images were obtained from both detector systems at identical tube voltage (100 kVp) and tube current-time product (100 mA), and were both reconstructed using a typical convolution kernel for stent imaging (B46f) and using the same reconstruction parameters. Two independent, blinded readers evaluated in-stent visibility and measured noise, intraluminal stent diameter, and in-stent attenuation for each detector subsystem. Differences in noise, intraluminal stent diameter, and in-stent attenuation where tested using a paired t test; differences in subjective in-stent visibility were evaluated using a Wilcoxon signed-rank test. Results: Best results for in-stent visibility, noise, intraluminal stent diameter, and in-stent attenuation in EID and PCD were observed at 0-degree phantom position along the z axis, suggesting higher in-plane compared with through-plane resolution. Subjective in-stent visibility was superior in coronary stent images obtained from PCD compared with EID (P < 0.001). Mean in-stent diameter was 28.8% and 8.4% greater in PCD (0.85 ± 0.24 mm; 0.83 ± 0.14 mm) as compared with EID acquisitions (0.66 ± 0.21 mm; 0.76 ± 0.13 mm) for both 0-degree and 90-degree phantom positions, respectively. Average noise was significantly lower (P < 0.001) for PCD (5 ± 0.2 HU) compared with EID (8.3 ± 0.2 HU). The increase in in-stent attenuation (0 degree: Δ 245 ± 163 HU vs Δ 156.5 ± 126 HU; P = 0.006; 90 degrees: Δ 194 ± 141 HU vs Δ 126 ± 78 HU; P = 0.001) was significantly lower for PCD compared with EID acquisitions. Conclusions: At matched CT scan protocol settings and identical image reconstruction parameters, the PCD yields superior in-stent lumen delineation of coronary artery stents as compared with conventional EID arrays.
- Jul 2017
Photon-counting computed tomography (PCCT) uses a photon counting detector to count individual photons and allocate them to specific energy bins by comparing photon energy to preset thresholds. This enables simultaneous multi-energy CT with a single source and detector. Phantom studies were performed to assess the spectral performance of a research PCCT scanner by assessing the accuracy of derived images sets. Specifically, we assessed the accuracy of iodine quantification in iodine map images and of CT number accuracy in virtual monoenergetic images (VMI). Vials containing iodine with 5 known concentrations were scanned on the PCCT scanner after being placed in phantoms representing the attenuation of different size patients. For comparison, the same vials and phantoms were also scanned on 2nd and 3rd generation dual-source, dual-energy (DSDE) scanners. After material decomposition, iodine maps were generated, from which iodine concentration was measured for each vial and phantom size and compared with the known concentration. Additionally, VMIs were generated and CT number accuracy was compared to the reference standard, which was calculated based on known iodine concentration and attenuation coefficients at each keV obtained from the U.S. National Institute of Standards and Technology. Results showed accurate iodine quantification (root mean square error of 0.5 mgI/cc) and accurate CT number of VMIs (percentage error of 8.9%) using the PCCT scanner. The overall performance of the PCCT scanner, in terms of iodine quantification and VMI CT number accuracy, was comparable to that of EID-based dual-source, dual-energy scanners.
- May 2017
Objectives: Computed tomography angiography (CTA) is a valuable tool for the assessment of carotid artery stenosis. However, blooming artifacts from calcified plaques might result in an overestimation of the stenosis grade. The aim of this study was to investigate a new dual-energy computed tomography (DECT) technique with a modified 3-material decomposition algorithm for calcium removal in extracranial carotid artery stenosis. Materials and methods: In this retrospective, institutional review board-approved study, 30 calcified carotid plaques in 22 patients (15 men; mean age, 73 ± 10 years) with clinical suspicion of stroke were included. Dual-energy computed tomography image data were obtained using second-generation dual-source CT with tube voltages at 80 and 140Sn kVp. Conventional CTA and virtual noncalcium (VNCa) images using the modified DECT algorithm were reconstructed. By assessing spectral characteristics, the modified DECT algorithm allows for a selective removal of calcium independent of blooming. Two independent and blinded readers evaluated subjective image quality, blooming artifacts, amount of (residual) calcification, and performed stenosis measurements according to the North American Symptomatic Carotid Endarterectomy Trial (NASCET) criteria. Differences were tested using a pairwise sign test. Paired sample t tests with Bonferroni correction (P < 0.017) and Bland-Altman analyses were used to test for differences in carotid stenosis measurements between VNCa and conventional CTA using digital subtraction angiography (DSA) as the standard of reference. Results: Subjective image quality was similar among conventional CTA and VNCa image data sets (P = 0.82), whereas blooming artifacts were significantly reduced in VNCa images compared with conventional CTA (P < 0.001). Residual calcifications in VNCa images were absent in 11 (37%), minor in 12 (40%), medium sized in 2 (7%), and large in 5 (17%) arteries. Stenosis measurements differed significantly between VNCa (mean NASCET stenosis: 27% ± 20%) and conventional CTA images (mean NASCET stenosis: 39% ± 16%; P < 0.001) and between conventional CTA and DSA (23% ± 16%, P < 0.001). No significant differences in stenosis measurements were observed between VNCa and DSA (P = 0.189), with narrow limits of agreement (mean difference ±1.96 standard deviations: -4.7%, -35.1%, and 25.7%). Conclusions: A modified 3-material decomposition DECT algorithm for calcium removal was introduced, which allows for an accurate removal of calcified carotid plaques in extracranial carotid artery disease. The algorithm might overcome the problem of overestimation of calcified stenosis due to blooming artifacts in conventional CTA.
- Apr 2017
Purpose: Dynamic CT perfusion (CTP) consists in repeated acquisitions of the same volume in different time steps, slightly before, during and slightly afterwards the injection of contrast media. Important functional information can be derived for each voxel, which reflect the local hemodynamic properties and hence the metabolism of the tissue. Different approaches are being investigated to exploit data redundancy and prior knowledge for noise reduction of such datasets, ranging from iterative reconstruction schemes to high dimensional filters. Methods: We propose a new spatial bilateral filter which makes use of the k-means clustering algorithm and of an optimal calculated guiding image. We named the proposed filter as k-means clustering guided bilateral filter (KMGB). In this study, the KMGB filter is compared with the partial temporal non-local means filter (PATEN), with the time-intensity profile similarity (TIPS) filter, and with a new version derived from it, by introducing the guiding image (GB-TIPS). All the filters were tested on a digital in-house developed brain CTP phantom, were noise was added to simulate 80 kV and 200 mAs (default scanning parameters), 100 mAs and 30 mAs. Moreover, the filters performances were tested on 7 noisy clinical datasets with different pathologies in different body regions. The original contribution of our work is two-fold: first we propose an efficient algorithm to calculate a guiding image to improve the results of the TIPS filter, secondly we propose the introduction of the k-means clustering step and demonstrate how this can potentially replace the TIPS part of the filter obtaining better results at lower computational efforts. Results: As expected, in the GB-TIPS, the introduction of the guiding image limits the oversmoothing of the TIPS filter, improving spatial resolution by more than 50%. Furthermore, replacing the time-intensity profile similarity calculation with a fuzzy k-means clustering strategy (KMGB) allows to control the edge preserving features of the filter, resulting in improved spatial resolution 35 and CNR both for CT images and for functional maps. In the phantom study, the PATEN filter showed overall the poorest results, while the other filters showed comparable performances in terms of perfusion values preservation, with the KMGB filter having overall the best image quality. Conclusion: In conclusion, the KMGB filter leads to superior results for CT images and functional maps quality improvement, in significantly shorter computational times compared to the other filters. Our results suggest that the KMGB filter might be a more robust solution for halved-dose CTP datasets. For all the filters investigated, some artifacts start to appear on the BF maps if one sixth of the dose is simulated, suggesting that no one of the filters investigated in this study might be optimal for such a drastic dose reduction scenario. This article is protected by copyright. All rights reserved.
This study evaluates the capabilities of a whole-body photon counting CT system to differentiate between four common kidney stone materials, namely uric acid (UA), calcium oxalate monohydrate (COM), cystine (CYS),and apatite (APA) ex vivo. Two different x-ray spectra (120 kV and 140 kV) were applied and two acquisition modes were investigated; The macro-mode generates two energy threshold based image-volumes and two energy bin based image-volumes. In the chesspattern-mode, however, four energy thresholds are applied. A virtual low energy image, as well as a virtual high energy image are derived from initial threshold-based images, while considering their statistically correlated nature. The energy bin based images of the macro-mode, as well as the virtual low and high energy image of the chesspattern-mode serve as input for our dual energy evaluation. The dual energy ratio of the individually segmented kidney stones were utilized to quantify the discriminability of the different materials. The dual energy ratios of the two acquisition modes showed high correlation for both applied spectra. Wilcoxon-rank sum tests and the evaluation of the area under the receiver operating characteristics curves suggest that the UA kidney stones are best differentiable from all other materials (AUC = 1.0), followed by CYS (AUC ≈ 0.9 compared against COM and APA). COM and APA, however, are hardly distinguishable (AUC between 0.63 and 0.76). The results hold true for the measurements of both spectra and both acquisition modes.
Objective To determine the accuracy of iodine quantification with dual energy computed tomography (DECT) in two high-end CT systems with different spectral imaging techniques. Methods Five tubes with different iodine concentrations (0, 5, 10, 15, 20 mg/ml) were analysed in an anthropomorphic thoracic phantom. Adding two phantom rings simulated increased patient size. For third-generation dual source CT (DSCT), tube voltage combinations of 150Sn and 70, 80, 90, 100 kVp were analysed. For dual layer CT (DLCT), 120 and 140 kVp were used. Scans were repeated three times. Median normalized values and interquartile ranges (IQRs) were calculated for all kVp settings and phantom sizes. ResultsCorrelation between measured and known iodine concentrations was excellent for both systems (R = 0.999–1.000, p < 0.0001). For DSCT, median measurement errors ranged from −0.5% (IQR −2.0, 2.0%) at 150Sn/70 kVp and −2.3% (IQR −4.0, −0.1%) at 150Sn/80 kVp to −4.0% (IQR −6.0, −2.8%) at 150Sn/90 kVp. For DLCT, median measurement errors ranged from −3.3% (IQR −4.9, −1.5%) at 140 kVp to −4.6% (IQR −6.0, −3.6%) at 120 kVp. Larger phantom sizes increased variability of iodine measurements (p < 0.05). Conclusion Iodine concentration can be accurately quantified with state-of-the-art DECT systems from two vendors. The lowest absolute errors were found for DSCT using the 150Sn/70 kVp or 150Sn/80 kVp combinations, which was slightly more accurate than 140 kVp in DLCT. Key Points• High-end CT scanners allow accurate iodine quantification using different DECT techniques.• Lowest measurement error was found in scans with largest photon energy separation.• Dual-source CT quantified iodine slightly more accurately than dual layer CT.
Introduction: Recent studies have shown a substantial reduction of radiation dose from computed tomography (CT) scans down to 0.1 mSv for lung cancer screening and cardiac examinations, when applying optimization techniques. Hence, CT localizer radiographs (LRs) might now be considered a significant contributor to the total dose of the CT examination. We investigated in our study the potential for reducing dose of the LRs by adapting the patient-specific acquisition parameters of the LR. Materials and methods: Localizer radiographs covering the lungs were acquired on 2 clinical scanners (64 slices, conventional detector [CD]; 96 slices, fully integrated detector [ID]) for 3 semianthropomorphic phantoms, representing a slim, a normal, and an obese adult. Starting at 120-kV tube voltage and 250-mA current were reduced until the image quality of the LR, and thereby the accuracy of the automatic exposure control was compromised; this was defined as a deviation of measured attenuation values in the center of the LR of more than 5% from the reference values measured at the highest tube voltage and current. Subsequent Monte Carlo calculations on anthropomorphic phantoms were performed to calculate organ and effective dose values for the respective optimal settings. In addition, effective dose values normalized to CTDIvol for tube voltages ranging from 60 to 160 kV were determined for the different combinations of phantom sizes, sexes, and LR views to evaluate dose efficiency. Results: For the CD scanner, the optimal LR settings depended strongly on phantom size. Higher tube voltage and current were necessary for the larger phantoms. The ID scanner showed uncompromised LR quality for all phantoms using the lowest possible tube voltage-tube current combination of 80 kV and 20 mA. Depending on patient size and LR direction, effective dose values for the optimal settings ranged from 6 to 53 μSv and 3 to 11 μSv for the CD and ID scanner, respectively. For the example of an anterior-posterior LR on a normal patient, using the optimal settings instead of the standard settings on the ID scanner reduced LR dose from 53 μSv (120 kV, 30 mA) to 10 μSv (80 kV, 20 mA). The simulations for the different tube voltages show that effective dose and CTDIvol behave similarly for different views and patient sizes. However, the tube voltage level itself impacts the relationship between CTDIvol and effective dose, by up to a factor of 2. Discussion: Dose from LRs may contribute significantly to the total effective dose of low-dose CT examinations such as lung cancer screening. Optimal LR settings can reduce LR dose substantially, but adaptations have to consider scanner characteristics, detector technology, and patient size. Thus, for low-dose CT examinations, such as cardiac examinations and lung cancer screening, LR optimization may result in a significant dose reduction and thereby in a substantial reduction of total dose.
Objectives: The aims of this study were to introduce the measure noise texture deviation as quantitative parameter for evaluating iterative reconstruction (IR)-specific artifacts in computed tomography (CT) images and to test whether IR-specific artifacts, quantified through this measure, are reduced in advanced modeled IR (ADMIRE) as compared with sinogram-affirmed IR (SAFIRE) images of the liver ex vivo and in patients with hypodense liver lesions. Materials and methods: In the ex vivo study part, an abdominal phantom was used. In the institutional review board-approved in vivo study part, 40 consecutive patients (mean age, 63 years) with hypodense liver lesions undergoing abdominal CT in the portal-venous phase were included. Images were reconstructed with filtered back projection, with the second-generation IR algorithm SAFIRE and with the third-generation IR algorithm ADMIRE. Noise power spectra and noise texture deviation were calculated in the phantom; image noise was measured in the phantom and in patients. Two blinded readers evaluated all image data regarding IR-specific artifacts (plastic-like, blotchy appearance); patient data were evaluated regarding conspicuity and confidence for detecting hypodense liver lesions. Results: Image noise was significantly reduced at increasing IR levels (P < 0.001) with both algorithms, with no significant differences between corresponding strength levels of SAFIRE and ADMIRE (all, P > 0.05). Noise power spectra were similar at corresponding strength levels of SAFIRE and ADMIRE (all, P > 0.05). Noise texture deviation in ADMIRE was reduced compared with corresponding strength levels of SAFIRE (all, P < 0.001) and strongly correlated with subjective IR-specific artifacts (r = 0.88, P < 0.001). Iterative reconstruction-specific artifacts were significantly reduced in ADMIRE compared with that in SAFIRE images at strength levels 3 or greater, both ex vivo and in vivo (all, P < 0.001). There were no significant differences in the readers' ratings of lesion conspicuity and lesion confidence in detecting hypodense liver lesions between SAFIRE and ADMIRE (P > 0.05). Only lesion conspicuity was superior with SAFIRE and ADMIRE compared with filtered back projection (all, P < 0.001). Conclusions: Noise texture deviation is a quantitative measure reflecting IR-specific artifacts and is reduced in CT images with ADMIRE compared with SAFIRE.
- Mar 2016
The medical treatment of a kidney stone strongly depends on its chemical composition, size and its precise location inside the urinary tract of the human body. Yet, kidney stones are highly prevalent and show increased recurrence-rates, therefore an early detection and robust characterization of the crystalline accumulations is desired. Whereas medical imaging technologies became an important tool for kidney stone diagnosis, low dose non-contrast CT is considered as today’s gold standard. Several CT-based approaches of kidney stone characterization and differentiation were investigated in previous work. The purpose of this study, however, is to detect and differentiate two particularly prominent kidney stone-materials using image information acquired by a research Photon Counting Detector CT-scanner (PCD-CT). PCD-CT provides scans with a full Field of View, fully registered image data, stability against motion artifacts and no cross scatter from a second X-ray source. Compared to conventional energy integrating CT-detectors, PCD-CT technology promises to provide an increased spatial resolution, due to the absence of optical septa, a negligible level of electronic noise and an increased dose efficiency especially for materials providing comparably higher absorptions in the lower energy ranges of the energy spectrum. Furthermore, the polychromatic spectrum as emitted by the X-ray tube can be resolved in energy bins by the introduction of energy thresholds; this allows applications of dual/multi energy algorithms as used in this study.
- Mar 2016
- SPIE Medical Imaging
The energy resolving capabilities of Photon Counting Detectors in Computed Tomography facilitate energy-sensitive measurements. In this study two algorithms (Material Decomposition and Virtual Non-Contrast) are applied on a data set obtained from a PCD-CT prototype system. Three living rabbits were measured. Two contrast agents are applied: A gadolinium based contrast agent used to enhance contrasts for vascular imaging, and a mixture of xenon and air, used to evaluate local ventilation impairments of the animal’s lung. All images are generated from two images based on energy bin information. A modified version of a commercially available software framework is capable of providing images with quantitative and full diagnostic value.
- Jan 2016
Background and purpose: Calculated monoenergetic ultra-low keV datasets did not lead to improved contrast-to-noise ratio (CNR) due to the dramatic increase in image noise. The aim of the present study was to evaluate the objective image quality of ultra-low keV monoenergetic images (MEIs) calculated from carotid DECT angiography data with a new monoenergetic imaging algorithm using a frequency-split technique. Materials and methods: 20 patients (12 male; mean age 53±17 years) were retrospectively analyzed. MEIs from 40 to 120 keV were reconstructed using the monoenergetic split frequency approach (MFSA). Additionally MEIs were reconstructed for 40 and 50 keV using a conventional monoenergetic (CM) software application. Signal intensity, noise, signal-to-noise ratio (SNR) and CNR were assessed in the basilar, common, internal carotid arteries. Results: Ultra-low keV MEIs at 40 keV and 50 keV demonstrated highest vessel attenuation, significantly greater than those of the polyenergetic images (PEI) (all p-values <0.05). The highest SNR level and CNR level was found at 40 keV and 50 keV (all p-values <0.05). MEIs with MFSA showed significantly lower noise levels than those processed with CM (all p-values <0.05) and no significant differences in vessel attenuation (p>0.05). Thus MEIs with MFSA showed significantly higher SNR and CNR compared to MEIs with CM. Conclusion: Combining the lower spatial frequency stack for contrast at low keV levels with the high spatial frequency stack for noise at high keV levels (frequency-split technique) leads to improved image quality of ultra-low keV monoenergetic DECT datasets when compared to previous monoenergetic reconstruction techniques without the frequency-split technique.
- Nov 2015
Purpose: To evaluate the effect of iterative reconstruction on the depiction of systemic sclerosis-related interstitial lung disease (ILD) when the radiation dose is reduced by 60%. Materials and methods: This study was based on retrospective interpretation of prospectively acquired data over a 12-month period and approved by the institutional review board. The requirement to obtain informed consent was waived. Fifty-five chest computed tomographic (CT) examinations were performed in 38 women and 17 men (mean age, 55.8 years; range, 23-82 years) by using a dual-source CT unit with (a) both tubes set at similar energy (120 kVp) and (b) the total reference milliampere seconds (ie, 110 mAs) split up in a way that 40% was applied to tube A and 60% to tube B. Two series of images were generated simultaneously from the same dataset: (a) standard-dose images (generated from both tubes) reconstructed with filtered back projection (group 1, the reference standard) and (b) reduced-dose images (generated from tube A; 60% dose reduction) reconstructed with sinogram-affirmed iterative reconstruction (SAFIRE) (group 2). In both groups, the analyzed parameters comprised the image noise and the visualization and conspicuity of CT features of ILD. Two readers independently analyzed images from both groups. Results were compared by using the Wilcoxon test for paired samples; the 95% confidence interval was calculated when appropriate. Results: The mean level of objective noise in group 2 was significantly lower than that in group 1 (22.02 HU vs 26.23 HU, respectively; P < .0001). The CT features of ILD in group 1 were always depicted in group 2, with subjective conspicuity scores (a) improved in group 2 for ground-glass opacity, reticulation, and bronchiectasis and/or bronchiolectasis and (b) identical in both groups for honeycombing. The interobserver agreement for their depiction was excellent in both groups (κ, 0.84-0.98). Conclusion: Despite a 60% dose reduction, images reconstructed with SAFIRE allowed similar detection of systematic sclerosis-related ILD compared with the reference standard.
Objectives: To investigate the relationship of dual-phase dual-energy CT (DE-CT) and tumour size in the evaluation of the response to anti-EGFR therapy in patients with advanced non-small cell lung cancer (NSCLC). Methods: Dual-phase DE-CT was performed in 31 patients with NSCLC before the onset of anti-EGFR (erlotinib) therapy and as follow-up (mean 8 weeks). Iodine uptake (IU; mg/mL) was quantified using prototype software in arterial and venous phases; arterial enhancement fraction (AEF) was calculated. The change of IU before and after therapy onset was compared with anatomical evaluation in maximal transverse diameter and volume (responders vs. non-responders). Results: A significant decrease of IU in venous phase was proved in responders according to all anatomical parameters (p=0.002-0.016). In groups of non-responders, a significant change of IU was not proved with variable trends of development. The most significant change was observed using the anatomical parameter of volume (cut-off 73 %). A significant difference of percentage change in AEF was proved between responding and non-responders (p=0.019-0.043). Conclusion: Dual-phase DE-CT with iodine uptake quantification is a feasible method with potential benefit in advanced assessment of anti-EGFR therapy response. We demonstrated a decrease in vascularization in the responding primary tumours and non-significant variable development of vascularization in non-responding tumours. Key points: • Dual-phase DE-CT is feasible for vascularization assessment of NSCLC with anti-EGFR therapy. • There was a significant decrease of iodine uptake in responding tumours. • There was a non-significant and variable development in non-responding tumours. • There was significant difference of AEF percentage change between responders and non-responders.
The aim of this study was to introduce a new theoretical framework describing the relationship between the blood velocity, computed tomography (CT) acquisition velocity, and iodine contrast enhancement in CT images, and give a proof of principle of contrast gradient-based blood velocimetry with CT. The time-averaged blood velocity (vblood) inside an artery along the axis of rotation (z axis) is described as the mathematical division of a temporal (Hounsfield unit/second) and spatial (Hounsfield unit/centimeter) iodine contrast gradient. From this new theoretical framework, multiple strategies for calculating the time-averaged blood velocity from existing clinical CT scan protocols are derived, and contrast gradient-based blood velocimetry was introduced as a new method that can calculate vblood directly from contrast agent gradients and the changes therein. Exemplarily, the behavior of this new method was simulated for image acquisition with an adaptive 4-dimensional spiral mode consisting of repeated spiral acquisitions with alternating scan direction. In a dynamic flow phantom with flow velocities between 5.1 and 21.2 cm/s, the same acquisition mode was used to validate the simulations and give a proof of principle of contrast gradient-based blood velocimetry in a straight cylinder of 2.5 cm diameter, representing the aorta. In general, scanning with the direction of blood flow results in decreased and scanning against the flow in increased temporal contrast agent gradients. Velocity quantification becomes better for low blood and high acquisition speeds because the deviation of the measured contrast agent gradient from the temporal gradient will increase. In the dynamic flow phantom, a modulation of the enhancement curve, and thus alternation of the contrast agent gradients, can be observed for the adaptive 4-dimensional spiral mode and is in agreement with the simulations. The measured flow velocities in the downslopes of the enhancement curves were in good agreement with the expected values, although the accuracy and precision worsened with increasing flow velocities. The new theoretical framework increases the understanding of the relationship between the blood velocity, CT acquisition velocity, and iodine contrast enhancement in CT images, and it interconnects existing blood velocimetry methods with research on transluminary attenuation gradients. With these new insights, novel strategies for CT blood velocimetry, such as the contrast gradient-based method presented in this article, may be developed.
Metal-related artifacts from spine instrumentation can obscure relevant anatomy and pathology. We evaluated the ability of CT images reconstructed with and without iterative metal artifact reduction to visualize critical anatomic structures in postoperative spines and assessed the potential for implementation into clinical practice. We archived CT projection data in patients with instrumented spinal fusion. CT images were reconstructed by using weighted filtered back-projection and iterative metal artifact reduction. Two neuroradiologists evaluated images in the region of spinal hardware and assigned a score for the visualization of critical anatomic structures by using soft-tissue and bone windows (critical structures totally obscured, n = 0; anatomic recognition with high diagnostic confidence, n = 5). Using bone windows, we measured the length of the most pronounced linear artifacts. For each patient, neuroradiologists made recommendations regarding the optimal use of iterative metal artifact reduction and its impact on diagnostic confidence. Sixty-eight patients met the inclusion criteria. Visualization of critical soft-tissue anatomic structures was significantly improved by using iterative metal artifact reduction compared with weighted filtered back-projection (median, 1 ± 1.5 versus 3 ± 1.3, P < .001), with improvement in the worst visualized anatomic structure in 88% (60/68) of patients. There was not significant improvement in visualization of critical osseous structures. Linear metal artifacts were reduced from 29 to 11 mm (P < .001). In 87% of patients, neuroradiologists recommended reconstructing iterative metal artifact reduction images instead of weighted filtered back-projection images, with definite improvement in diagnostic confidence in 32% (22/68). Iterative metal artifact reduction improves visualization of critical soft-tissue structures in patients with spinal hardware. Routine generation of these images in addition to routine weighted filtered back-projection is recommended. © 2015 American Society of Neuroradiology.
The prospective study included 54 asymptomatic high-risk patients who underwent coronary CT angiography (CTA) and regadenoson-induced stress CT perfusion (rsCTP). Diagnostic accuracy of significant stenosis (≥50%) determination was evaluated for CTA alone and CTA + rsCTP in 27 patients referred to ICA due to the positive rsCTP findings. Combined evaluation of CTA + rsCTP had higher diagnostic accuracy over CTA alone (per-segment: specificity 96 versus 68%, p = 0.002; per-vessel: specificity 95 versus 75%, p = 0.012) and high overruling rate of rsCTP was proved in intermediate stenosis (40-70%). Results demonstrate a significant additional value of rsCTP in the assessment of intermediate coronary artery stenosis found with CTA.
- Jun 2015
Purpose To evaluate the potential of advanced modeled iterative reconstruction (ADMIRE) for optimizing radiation dose of high-pitch coronary CT angiography (CCTA). Methods High-pitch 192-slice dual-source CCTA was performed in 25 patients (group 1) according to standard settings (ref. 100 kVp, ref. 270 mAs/rot). Images were reconstructed with filtered back projection (FBP) and ADMIRE (strength levels 1–5). In another 25 patients (group 2), high-pitch CCTA protocol parameters were adapted according to results from group 1 (ref. 160 mAs/rot), and images were reconstructed with ADMIRE level 4. In ten patients of group 1, vessel sharpness using full width at half maximum (FWHM) analysis was determined. Image quality was assessed by two independent, blinded readers. Results Interobserver agreements for attenuation and noise were excellent (r = 0.88/0.85, p p p Conclusions In a selected population, ADMIRE can be used for optimizing high-pitch CCTA to an effective dose of 0.3 mSv. Key points • Advanced modeled IR (ADMIRE) reduces image noise up to 50 % as compared to FBP. • Coronary artery vessel borders show an increasing sharpness at higher ADMIRE levels. • High-pitch CCTA with ADMIRE is possible at a radiation dose of 0.3 mSv.
The aim of this study was to evaluate the systematic and random errors of a new bolus tracking-based algorithm that predicts a patient-specific time of peak arterial enhancement and compare its performance with a best-case scenario for the current bolus tracking technique. All local institutional review boards approved this retrospective study, in which the test bolus signals of cardiac computed tomography angiographies of 72 patients (46 men; median age, 62 years [range, 31-81 years]) were used to simulate contrast enhancement curves for a multitude of injection protocols with iodine delivery rates (IDRs) varying between 0.5 and 2.5 gI/s, injection durations between 4 and 30 seconds, and tube voltages of 100 and 120 kV. From these simulated curves, bolus tracking signals with statistical errors of 4 Hounsfield units (HU) (standard deviation) were derived with trigger values of 100 and 150 HU at 100 and 120 kV, respectively. The new algorithm then matched the actual bolus tracking signal with a database of expected enhancement curves for that particular injection protocol, taking into account population-averaged blood circulation characteristics with variations in patient weight and cardiac output. Posttrigger delays (PTDs) were calculated as the time difference between the last bolus tracking point and the time of peak enhancement. The systematic and random errors between the predicted and true PTDs were assessed and compared with a best-case scenario for the current bolus tracking method. With the current bolus tracking technique, interpatient variations decrease with higher IDRs and earlier triggering (lower tube voltage and/or lower trigger value), and the true PTDs increase linearly with injection duration. Compared with the current bolus tracking method, the systematic and random errors of the algorithm-predicted PTDs are smaller, do not depend on the IDR, and are predictable over a large range of total iodine doses. The median difference between the true and algorithm-predicted PTD is less than ±1 second for all IDRs and injection durations, and the algorithm was able to predict patient-specific PTDs within ±2 seconds from the true PTD in more than 90% of patients for almost all injection protocols. The new algorithm can robustly predict a patient-specific time of arterial peak enhancement and is better than a best-case scenario for the current bolus tracking technique because interpatient variations are taken into account. It offers a new framework for scan timing optimization and can potentially be used for personalized scan timing in real time.
Purpose: To prospectively assess the feasibility of using virtual iron content (VIC) imaging at dual-energy computed tomography (CT) to evaluate the liver iron content (LIC) in patients suspected of having liver iron overload and to compare the LIC grading performance of VIC imaging and magnetic resonance (MR) imaging. Materials and Methods: This study was approved by the institutional review board, and informed consent was obtained from all patients. Fifty-six patients suspected of having liver iron overload (serum ferritin concentrations .500 mg/L) underwent unenhanced dual-energy CT and MR imaging of the liver. MR imaging-measured LICs were obtained in 34 of the 56 patients. VIC images were generated with dual-energy analysis. R2∗and MR-measured LIC were obtained with gradient-echo and spin-echo sequences, respectively. Correlations between CT and MR measurements were analyzed. The diagnostic performance of VIC and R2∗in the differentiation of different LIC thresholds were evaluated with receiver operating characteristic (ROC) analysis. Results: Hepatic VIC showed significant correlation with R2∗and MR-measured LIC (r = 0.885 and 0.871, respectively; P , .0001). To differentiate among different LIC thresholds of 1.8, 3.2, 7.0, and 15.0 mg of iron per gram of dry tissue, the corresponding optimal cutoff values for VIC were 2.50, 5.13, 8.93, and 17.97 HU, respectively. At a LIC threshold of 7.0 mg of iron per gram of dry tissue or higher, 100% sensitivity (15 of 15 patients) and 100% specificity (19 of 19 patients) were obtained for VIC. There was no significant difference between VIC and R2∗(area under the ROC curve, 0.964 vs 0.993, respectively; P = .299) in grading LIC levels at a LIC threshold of 3.2 mg of iron per gram of dry tissue or higher. Conclusion: Hepatic VIC is a potential index for accurately evaluating and grading clinically significant liver iron accumulation, with a diagnostic performance similar to that of MR imaging.
The purpose of this study was to determine whether ultralow-radiation-dose chest CT can be used for quantification of lung density and for emphysema detection in participants undergoing lung cancer screening. Fifty-two patients were prospectively enrolled and underwent scanning twice with low-dose CT (reference parameters, 120 kV, 50 effective mAs) and ultralow-dose CT (reference parameters, 80 kV, 4-5 effective mAs). Images were reconstructed by filtered back projection (FBP) for low-dose CT and FBP and iterative reconstruction (IR) for ultralow-dose CT. Radiation dose was recorded. Image noise, mean lung attenuation, 15th percentile of lung attenuation, and emphysema index were measured in each image series and compared. Test characteristics of ultralow-dose CT in detecting more than subtle emphysema (emphysema index ≥ 3%) were calculated. The effective dose of low-dose CT was 2.1 ± 0.5 mSv, and that of ultralow-dose CT was 0.13 ± 0.04 mSv. Compared with the findings for low-dose CT, absolute overestimation of emphysema index was 7% on ultralow-dose CT images reconstructed with FBP and 2% on those processed with IR. The 15th percentile of lung attenuation was underestimated by 21.3 HU on ultralow-dose FBP images and by 5.8 HU on IR images. No relevant bias was observed for mean lung attenuation. Four patients (8%) had more than subtle emphysema. The emphysema index measured at ultralow-dose CT with FBP and IR had 100% and 100% sensitivity and 92% and 96% specificity in identifying patients with more than subtle emphysema at a cutoff of greater than 12.1% for FBP and greater than 6.7% for IR. Ultralow-dose chest CT performed for lung cancer screening can be used for quantification of lung density and for emphysema detection. IR improves the accuracy of ultralow-dose CT in this setting.
To assess the accuracy of liver iron content (LIC) quantification and grading ability associated with clinical LIC stratification using virtual iron concentration (VIC) imaging on dual-energy CT (DECT) in an iron overload rabbit model. Fifty-one rabbits were prepared as iron-loaded models by intravenous injection of iron dextran. DECT was performed at 80 and 140 kVp. VIC images were derived from an iron-specific algorithm. Postmortem LIC assessments were conducted on an inductively coupled plasma (ICP) spectrometer. Correlation between VIC and LIC was analyzed. VIC were stratified according to the corresponding clinical LIC thresholds of 1.8, 3.2, 7.0, and 15.0 mg Fe/g. Diagnostic performance of stratification was evaluated by receiver operating characteristic analysis. VIC linearly correlated with LIC (r = 0.977, P < 0.01). No significant difference was observed between VIC-derived LICs and ICP (P > 0.05). For the four clinical LIC thresholds, the corresponding cutoff values of VIC were 19.6, 25.3, 36.9, and 61.5 HU, respectively. The highest sensitivity (100 %) and specificity (100 %) were achieved at the threshold of 15.0 mg Fe/g. Virtual iron concentration imaging on DECT showed potential ability to accurately quantify and stratify hepatic iron accumulation in the iron overload rabbit model. • Virtual iron concentration (VIC) linearly correlates with liver iron content (LIC). • VIC accurately quantifies LIC. • VIC accurately grades LIC based on clinical LIC stratification.
- Jan 2015
Objective: The objective of our study was to show the feasibility of distinguishing between uric acid (UA) and non-UA renal stones using two consecutive spatially registered low- and high-energy scans acquired on a conventional CT system. Subjects and methods: A total of 33 patients undergoing clinically indicated dual-source dual-energy CT examinations to differentiate UA from non-UA renal stones were enrolled in this study. Immediately after patients underwent clinically indicated dual-source dual-energy CT, two consecutive scans (one at 80 kV and one at 140 kV) were obtained on a conventional CT scanner over the region limited to the stones identified on the dual-source scans. After 3D deformable registration of the 80- and 140-kV images, UA and non-UA stones were identified using commercial software. The sensitivity, specificity, and accuracy of stone classification were calculated using the dual-source results as the reference standard. Results: A total of 469 stones were identified in the dual-source examinations (26 UA and 443 non-UA stones). The average in-plane stone diameter was 4.4 ± 2.5 (SD) mm (range, 2.0-18.9 mm). The overall sensitivity, specificity, and accuracy for identifying UA stones were 73.1%, 90.1%, and 89.1%, respectively. The sensitivity, specificity, and accuracy were 94.7%, 96.9%, and 96.8% for stones 3 mm or larger (n = 341 [19 UA and 322 non-UA]). Conclusion: Accurate differentiation of UA from non-UA renal stones is feasible using two consecutively acquired and spatially registered conventional CT scans.
- Jan 2015
- Dual Energy CT in Oncology
Computed tomography (CT), since its introduction in the 1970s, has not only revolutionized radiology, but made all diagnostic algorithms faster and more accurate: for example, the presence of a subdural hematoma in a trauma patient before the invention of CT could be just suspected after an accurate neurological examination and by the presence of a fracture of the skull on a conventional x-ray. Nowadays, a CT scan performed in few seconds clearly shows the presence and the characteristics of the lesion.
- Jan 2015
- Dual-Energy CT in Cardiovascular Imaging
This article describes technical principles and clinical applications of dual energy (DE) scanning with dual source CT (DSCT) systems, with a focus on vascular and cardiac applications. DSCT systems acquire DE data by simultaneously operating both x-ray tubes at different x-ray tube voltages (different kV). The quality of dual energy images relies on the effective separation of the energy spectra. In DSCT, the energy separation can be significantly improved by tin pre-filtration of the high-energy spectrum. This is a pre-requisite for DE acquisitions at similar radiation dose compared with single-energy CT exams. DSCT systems provide dedicated algorithms to restore the temporal resolution of a quarter of the rotation time (66–83 ms, depending on the scanner generation) in DE CT angiographic examinations of the heart. In addition, iterative beamhardening correction is available to significantly reduce iodine-related beamhardening artifacts, e.g. in the myocardium, which could otherwise degrade the quality of DE material decomposition. DSCT systems have to cope with certain challenges, such as cross-scattered radiation, which requires model-based or measurement-based correction, or a limited scan field of view (SFOV) of the second detector (35.5 cm with third generation DSCT). Pertinent vascular and cardiac applications are the computation of pseudo mono-energetic images to increase the iodine contrast-to-noise ratio (CNR) at low energies (keV) or to reduce metal artifacts and Ca-blooming at high keV, automated subtraction of bone and calcifications from CT angiographic scans, or the computation of iodine maps and virtual non-enhanced CT images. DE iodine maps of the myocardium acquired at rest and during stress have been used to evaluate the myocardial blood supply and to identify hemodynamically relevant stenosis. DE scanning of the heart is therefore a promising step toward comprehensive evaluation of coronary artery disease with a single modality.
A method for reconstructing picture data of a cyclically-moving object from measurement data is disclosed, with the measurement data being detected beforehand for a relative rotational movement between a radiation source of a computed tomography system and the object under examination during a plurality of movement cycles of the object under examination. In at least one embodiment, a first picture and a second picture are determined from the measurement data, with measurement data of different movement cycles being combined for reconstruction of the second picture into a measurement dataset to be used as the basis for the picture reconstruction. Difference information is computed by comparing the first picture with the second picture. Using the difference information, a result picture is computed from the first picture and the second picture.
- Nov 2014
An image processing device and method are disclosed for determining a proportion of necrotic tissue in a defined tissue area of an object under examination based on a high-energy image dataset and a low-energy image dataset, each recorded by way of x-ray measurements with different x-ray energies after a contrast medium has been applied to the object under examination. In at least one embodiment of the method, a virtual contrast medium image is determined from the high-energy image dataset and the low-energy image dataset and a segmentation image dataset is created, by the area of tissue being segmented. The segmentation result is transferred into the virtual contrast medium image for segmenting the tissue area in the virtual contrast medium image. Finally an analysis of values of the pixels lying in the segmented area is undertaken for identifying pixels which are to be assigned to necrotic tissue.
- Nov 2014
Introduction: One method to acquire dual-energy (DE) computed tomography (CT) data is to perform CT scans at 2 different x-ray tube voltages, typically 80 and 140 kV, either as 2 separate scans, by means of rapid kV switching, or with the use of 2 x-ray sources as in dual-source CT (DSCT) systems. In DSCT, it is possible to improve spectral separation with tin prefiltration (Sn) of the high-kV beam. Recently, x-ray tube voltages beyond the established range of 80 to 140 kV were commercially introduced, which enable additional voltage combinations for DE acquisitions, such as 80/150 Sn or 90/150 Sn kV. Here, we investigate the DE performance of several x-ray tube voltages and prefilter combinations on 2 DSCT scanners and the impact of the spectra on quantitative analysis and dose efficiency. Materials and methods: Circular phantoms of different sizes (10-40 cm in diameter) equipped with cylindrical inserts containing water and diluted iodine contrast agent (14.5 mg/cm) were scanned using 2 different DSCT systems (SOMATOM Definition Flash and SOMATOM Force; Siemens AG, Forchheim, Germany). Five x-ray tube voltage combinations (80/140 Sn, 100/140 Sn, 80/150 Sn, 90/150 Sn, and 100/150 Sn kV) were investigated, and the results were compared with the previous standard acquisition technique (80/140 kV). As an example, 80/140 Sn kV means that 1 x-ray tube of the DSCT system was operated at 80 kV, whereas the other was operated at 140 kV with additional tin prefiltration (Sn). Dose values in terms of computed tomography dose index (CTDIvol) were kept constant between the different voltage combinations but adjusted with regard to object size according to automatic exposure control recommendations. Reconstructed images were processed using linear blending of the low- and high-kV CT images to combined images, as well as 3-material decomposition techniques to generate virtual noncontrast (VNC) images and iodine images. Contrast and pixel noise were evaluated, as well as DE ratios, which are defined as the CT value at low kV divided by the CT value at high kV. Results: For the 10-, 20-, 30-, and 40-cm phantom, dose values in terms of CTDIvol were 1.2, 2.6, 7.3, and 21.6 mGy, respectively. In the combined images, those obtained with tin filtration showed lower noise values at similar iodine enhancement levels than did images obtained without tin filtration. The largest differences in noise were observed for the larger phantoms, in particular the 40-cm phantom. Dual-energy ratios for iodine increased with decreasing voltages of the low-kV beam and with increasing voltages of the high-kV beam, and they increased when tin prefiltration was added. In case of the 20-cm phantom, DE ratios ranged from 2.0 at 80/140 kV to 3.4 at 80/150 Sn kV. The noise level of the VNC images was strongly correlated with the inverse of the DE ratio. Irrespective of the phantom size, the lowest noise values were measured for 80/150 Sn kV. Discussion: Dual-source CT systems enable DE data to be acquired using a variety of voltage combinations. Combined (or mixed) DE images provide an image impression similar to standard 120 kV images, yet the noise level depends on the DE voltage combination that is selected. Noise in decomposed VNC images is strongly influenced by the DE ratio, and it improves substantially with tin filtration of the high-voltage beam.
Objectives: To compare image quality and low-contrast detectability of an integrated circuit (IC) detector in abdominal CT of obese patients with conventional detector technology at low tube voltages. Methods: A liver phantom with 45 lesions was placed in a water container to mimic an obese patient and examined on two different CT systems at 80, 100 and 120 kVp. The systems were equipped with either the IC or conventional detector. Image noise was measured, and the contrast-to-noise-ratio (CNR) was calculated. Low-contrast detectability was assessed independently by three radiologists. Radiation dose was estimated by the volume CT dose index (CTDIvol). Results: The image noise was significantly lower, and the CNR was significantly higher with the IC detector at 80, 100 and 120 kVp, respectively (P = 0.023). The IC detector resulted in an increased lesion detection rate at 80 kVp (38.1 % vs. 17.2 %) and 100 kVp (57.0 % vs. 41.0 %). There was no difference in the detection rate between the IC detector at 100 kVp and the conventional detector at 120 kVp (57.0 % vs. 62.2 %). The CTDIvol at 80, 100 and 120 kVp measured 4.5-5.2, 7.3-7.9 and 9.8-10.2 mGy, respectively. Conclusions: The IC detector at 100 kVp resulted in similar low-contrast detectability compared to the conventional detector with a 120-kVp protocol at a radiation dose reduction of 37 %.
- Oct 2014
The estimation of patient dose using Monte Carlo (MC) simulations based on the available patient CT images is limited to the length of the scan. Software tools for dose estimation based on standard computational phantoms overcome this problem; however, they are limited with respect to taking individual patient anatomy into account. The purpose of this study was to generate whole-body patient models in order to take scattered radiation and over-scanning effects into account. Thorax examinations were performed on three physical anthropomorphic phantoms at tube voltages of 80 kV and 120 kV; absorbed dose was measured using thermoluminescence dosimeters (TLD). Whole-body voxel models were built as a combination of the acquired CT images appended by data taken from widely used anthropomorphic voxel phantoms. MC simulations were performed both for the CT image volumes alone and for the whole-body models. Measured and calculated dose distributions were compared for each TLD chip position; additionally, organ doses were determined. MC simulations based only on CT data underestimated dose by 8%-15% on average depending on patient size with highest underestimation values of 37% for the adult phantom at the caudal border of the image volume. The use of whole-body models substantially reduced these errors; measured and simulated results consistently agreed to better than 10%. This study demonstrates that combined whole-body models can provide three-dimensional dose distributions with improved accuracy. Using the presented concept should be of high interest for research studies which demand high accuracy, e.g. for dose optimization efforts.
Objectives To prospectively evaluate radiation dose and image quality of a third generation dual-source CT (DSCT) without z-axis filter behind the patient for temporal bone CT. Methods Forty-five patients were either examined on a first, second, or third generation DSCT in an ultra-high-resolution (UHR) temporal bone-imaging mode. On the third generation DSCT system, the tighter focal spot of 0.2 mm2 removesthe necessity for an additional z-axis-filter, leading to an improved z-axis radiation dose efficiency. Images of 0.4 mm were reconstructed using standard filtered-back-projection or iterative reconstruction (IR) technique for previous generations of DSCT and a novel IR algorithm for the third generation DSCT. Radiation dose and image quality were compared between the three DSCT systems. Results The statistically significantly highest subjective and objective image quality was evaluated for the third generation DSCT when compared to the first or second generation DSCT systems (all p
Objectives: The objective of this study was to assess the robustness of a novel test bolus (TB)-based computed tomographic angiography (CTA) contrast-enhancement-prediction (CEP) algorithm by retrospectively quantifying the systematic and random errors between the predicted and true enhancements. Materials and methods: All local institutional review boards approved this retrospective study, in which a total of 72 (3 × 24) anonymized cardiac CTA examinations were collected from 3 hospitals. All patients (46 men; median age, 62 years [range, 31-81 years]) underwent a TB scan and a cardiac CTA according to local scan and injection protocols. For each patient, a shorter TB signal and TB signals with lower temporal resolution were derived from the original TB signal. The CEP algorithm predicted the enhancement in the descending aorta (DAo) on the basis of the TB signals in the DAo, the injection protocols and kilovolt settings, as well as population-averaged blood circulation characteristics. The true enhancement was extracted with a region of interest along the DAo centerline. For each patient, the errors in timing and amplitude were calculated; differences between the hospitals were assessed using the 1-way analysis of variance (P < 0.05) and variations between the TB signals were assessed using the within-subject standard deviation. Results: No significant differences were found between the 3 hospitals for any of the TB signals. With errors in the amplitude and timing of 0.3% ± 15.6% and -0.2 ± 2.0 seconds, respectively, no clinically relevant systematic errors existed. Shorter- and coarser-time-sampled TB signals introduced a within-subject standard deviation of 4.0% and 0.5 seconds, respectively. Conclusions: This TB-based CEP algorithm has no systematic errors in the timing and amplitude of predicted enhancements and is robust against coarser-time-sampled and incomplete TB scans.
Objective: The purpose of this study was to evaluate the simultaneous use of automatic tube current modulation (ATCM) and automatic tube voltage selection (ATVS) for abdominal and chest CT examinations regarding radiation dose reduction and image quality. Materials and methods: We enrolled 617 patients who all underwent contrast-enhanced chest or abdominal CT and divided them into two groups. In group A, 317 patients who underwent CT with only ATCM and a fixed body mass index-adjusted tube voltage (120 kV or 100 kV) were enrolled. In group B, both ATCM and ATVS were used. Image attenuation and noise were measured in different anatomic regions. Results: The mean contrast-to-noise ratio and the signal-to-noise ratio of abdomen and chest CT was higher in group B compared with group A (p < 0.0001). In total, the effective radiation doses for abdomen and chest CT examinations were significantly reduced in group B by 18% compared with group A (p < 0.0001). When only examining those who benefited from the ATVS tool, a dose reduction of 35% for chest CT and 42% for abdomen CT could be achieved (p < 0.0001 for each). Conclusion: The simultaneous use of ATVS and ATCM enables significant radiation dose reduction in abdominal and thoracic contrast-enhanced CT examinations compared with the use of ATCM alone.
- Jul 2014
Objectives To assess the influence of tube potential on radiation dose and image quality of third-generation dual-source coronary CT angiography (CTA) in a phantom simulating an obese patient. Methods A thoracic phantom was equipped with tubular inserts containing iodine solution and water. A soft-tissue-equivalent ring around the phantom simulated an obese patient. Images were acquired at tube potentials of 80, 100, 120 and 140 kV with second-generation dual-source CT (DSCT) and 70–150 kV (in 10-kV increments) with third-generation DSCT. Contrast-to-noise ratio (CNR) was calculated and CT dose index was recorded. Results With second-generation DSCT, CNR was highest for 120 kV (19.0) and decreased with lower tube potential (12.0 at 80 kV) owing to disproportionately increased image noise. With third-generation DSCT, 70- and 80-kV acquisitions showed a smaller increase in noise. CNRs for third-generation DSCT were highest for 70 and 80 kV (21.1 and 21.2, respectively). Compared to 120 kV, radiation dose was 68 % and 49 % lower at 70 kV and 80 kV, respectively. Conclusion Third-generation DSCT enables one to perform coronary CTA at 70–80 kV in obese patients without compromising CNR and thus reduces radiation dose by 49–68 %. Key points • Low tube potential CT angiography is currently not suitable for obese patients. • Third-generation DSCT offers substantially increased tube power at low tube potential. • This enables one to perform coronary CT angiography at 70–80 kV in obese patients. • Signal-to-noise ratio is maintained owing to increased tube current. • This approach can be expected to reduce radiation dose by 49–68 %.
Introduction: Low tube voltage allows for computed tomography (CT) imaging with increased iodine contrast at reduced radiation dose. We sought to evaluate the image quality and potential dose reduction using a combination of attenuation based tube current modulation (TCM) and automated tube voltage adaptation (TVA) between 100 and 120 kV in CT of the head and neck. Methods: One hundred thirty consecutive patients with indication for head and neck CT were examined with a 128-slice system capable of TCM and TVA. Reference protocol was set at 120 kV. Tube voltage was reduced to 100 kV whenever proposed by automated analysis of the localizer. An additional small scan aligned to the jaw was performed at a fixed 120 kV setting. Image quality was assessed by two radiologists on a standardized Likert-scale and measurements of signal- (SNR) and contrast-to-noise ratio (CNR). Radiation dose was assessed as CTDIvol. Results: Diagnostic image quality was excellent in both groups and did not differ significantly (p = 0.34). Image noise in the 100 kV data was increased and SNR decreased (17.8/9.6) in the jugular veins and the sternocleidomastoid muscle when compared to 120 kV (SNR 24.4/10.3), but not in fatty tissue and air. However, CNR did not differ statistically significant between 100 (23.5/14.4/9.4) and 120 kV data (24.2/15.3/8.6) while radiation dose was decreased by 7-8%. Conclusions: TVA between 100 and 120 kV in combination with TCM led to a radiation dose reduction compared to TCM alone, while keeping CNR constant though maintaining diagnostic image quality.
A method for generating a tomographic temperature map in a patient by use of a CT device, a computing unit and CT system with computing unit are disclosed. In at least one embodiment of the method, the local distribution of density and mean atomic number is determined on the basis of tomographic image data from different X-ray energy regions, and a local temperature distribution in the tissue of the patient is ascertained from previously experimentally determined or theoretically calculated relations between Z values, density and temperature.
Objectives To investigate the potential contribution of iodine uptake calculation from dual-phase dual-energy CT (DE-CT) for lymph node staging and therapy response monitoring in lung cancer patients. Methods Retrospective analysis of 27 patients with non-small cell lung carcinoma (NSCLC), who underwent dual-phase DE-CT before and after chemotherapy, was performed. Iodine uptake (mg/mL) and total iodine uptake (mg) were calculated using prototype software in the early (arterial) and late (venous) post-contrast circulatory phase in 110 mediastinal lymph nodes. The arterial enhancement fraction (AEF) was calculated and compared with lymph node size and response to chemotherapy. Results A significant difference of AEF was observed between enlarged (90.4 %; 32.3–238.5 %) and non-enlarged (72.7 %; −37.5-237.5 %) lymph nodes (p = 0.044) before treatment onset. A significantly different change of AEF in responding (decrease of 26.3 %; p = 0.022) and non-responding (increase of 43.0 %; p = 0.031) lymph nodes was demonstrated. A higher value of AEF before treatment was observed in lymph nodes with subsequent favourable response (88.6 % vs. 77.7 %; p = 0.122), but this difference did not reach statistical significance. Conclusions The dual-phase DE-CT examination with quantification of ratio of early and late post-contrast iodine uptake is a feasible and promising method for the functional evaluation of mediastinal lymph nodes including therapy response assessment. Key Points • Dual-phase DE-CT is beneficial for mediastinal lymph node assessment in NSCLC. • Arterial to venous iodine uptake ratio was higher in enlarged lymph nodes. • Change of arterial enhancement fraction correlated to therapy response.
Purpose To prospectively compare image quality of cranial computed tomography (CCT) examinations with varying slice widths using traditional filtered back projection (FBP) versus sinogram-affirmed iterative image reconstruction (SAFIRE). Materials and Methods: 29 consecutive patients (14 men, mean age: 72 ± 17 years) referred for a total of 40 CCT studies were prospectively included. Each CCT raw data set was reconstructed with FBP and SAFIRE at 5 slice widths (1-5 mm; 1 mm increments). Objective image quality was assessed in three predefined regions of the brain (white matter, thalamus, cerebellum) using identical regions of interest (ROIs). Subjective image quality was assessed by 2 experienced radiologists. Objective and subjective image quality parameters were statistically compared between FBP and SAFIRE reconstructions. Results SAFIRE reconstructions resulted in mean noise reductions of 43.8% in the white matter, 45.6% in the thalamus and 42.0% in the cerebellum (p < 0.01) compared to FBP on non contrast-enhanced 1 mm slice width images. Corresponding mean noise reductions on 1 mm contrast-enhanced studies were 45.7%, 47.3%, and 45.0% in the white matter, thalamus, and cerebellum, respectively (p < 0.01). There was no significant difference in mean attenuation of any region or slice width between the two reconstruction methods (all p > 0.05). Subjective image quality of IR images was mostly rated higher than that of the FBP images. Conclusion Compared to FBP, SAFIRE provides significant reductions in image noise while increasing subjective image in CCT, particularly when thinner slices are used. Therefore, SAFIRE may allow utilization of thinner slices in CCT, potentially reducing partial volume effects and improving diagnostic accuracy.
- May 2014
OBJECTIVES: To evaluate image quality, maximal heart rate allowing for diagnostic imaging, and radiation dose of turbo high-pitch dual-source coronary computed tomographic angiography (CCTA). METHODS: First, a cardiac motion phantom simulating heart rates (HRs) from 60-90 bpm in 5-bpm steps was examined on a third-generation dual-source 192-slice CT (prospective ECG-triggering, pitch 3.2; rotation time, 250 ms). Subjective image quality regarding the presence of motion artefacts was interpreted by two readers on a four-point scale (1, excellent; 4, non-diagnostic). Objective image quality was assessed by calculating distortion vectors. Thereafter, 20 consecutive patients (median, 50 years) undergoing clinically indicated CCTA were included. RESULTS: In the phantom study, image quality was rated diagnostic up to the HR75 bpm, with object distortion being 1 mm or less. Distortion increased above 1 mm at HR of 80-90 bpm. Patients had a mean HR of 66 bpm (47-78 bpm). Coronary segments were of diagnostic image quality for all patients with HR up to 73 bpm. Average effective radiation dose in patients was 0.6 ± 0.3 mSv. CONCLUSIONS: Our combined phantom and patient study indicates that CCTA with turbo high-pitch third-generation dual-source 192-slice CT can be performed at HR up to 75 bpm while maintaining diagnostic image quality, being associated with an average radiation dose of 0.6 mSv. KEY POINTS: • CCTA is feasible with the turbo high-pitch mode. • Turbo high-pitch CCTA provides diagnostic image quality up to 73 bpm. • The radiation dose of high-pitch CCTA is 0.6 mSv on average.
In a computed tomography apparatus and operating method, a radiation source and radiation detector are rotated around a system axis, and a patient support plate and diaphragm elements of a diaphragm associated with the x-ray source are also movable in the direction of the system axis. Movement of the patient support plate and the diaphragm plates between respective end positions is coordinated during a dynamic computed tomography examination of a subject so as to reduce and homogenize the dose of x-ray radiation to which the subject is exposed during the examination.
- Apr 2014
Following the trend of low-radiation dose computed tomographic (CT) imaging, concerns regarding the detectability of low-contrast lesions have been growing. The goal of this research was to evaluate whether a new image-based algorithm (Mono+) for virtual monoenergetic imaging with a dual-energy CT scanner can improve the contrast-to-noise ratio (CNR) and conspicuity of these low-contrast objects when using iodinated contrast media. Four circular phantoms of different diameter (10-40 cm) with an iodine insert at the center were scanned at a fixed radiation dose with different single- (80, 100, 120 kV) and dual-energy protocols (80/140 kV, 80/140 Sn kV, 100/140 Sn kV) using a dual-source CT system. In addition, an anthropomorphic abdominal phantom with different low-contrast lesions was scanned with the settings previously mentioned but also at only a half and a quarter of the initial dose. Dual-energy data were processed, and virtual monoenergetic images (range, 40-190 keV) were generated. Beside the established technique, a newly developed prototype algorithm to calculate monoenergetic images (Mono+) was used. To avoid noise increase at lower calculated energies, which is a known drawback of virtual monoenergetic images at low kilo electron-volt, a regional spatial frequency-based recombination of the high signal at lower energies and the superior noise properties at medium energies is performed to optimize CNR in case of Mono+ images. The CNR and low-contrast detectability were evaluated. For all phantom sizes, the Mono+ technique provided increasing iodine CNR with decreasing kilo electron-volt, with the optimum CNR obtained at the lowest energy level of 40 keV. For all investigated phantom sizes, CNR of Mono+ images at low kilo electron-volt was superior to the CNR in single-energy images at an equivalent radiation dose and even higher than the CNR obtained with 80-kV protocols. In case of the anthropomorphic phantom, low-contrast detectability in monoenergetic images was, for all settings, similar to the circular phantoms, best for the voltage combination 80/140 Sn kV, irrespective of the dose level. For all dual-energy voltage combinations, the Mono+ algorithm led to superior results compared with single-energy imaging. With regard to optimized iodine CNR, it is more efficient to perform dual-energy scans and compute virtual monoenergetic images at 40 keV using the Mono+ technique than to perform low kilovolt scans. Given the improved CNR, the Mono+ algorithm could be very useful in improving both detection and differential diagnosis of abdominal lesions, specifically low-contrast lesions, as well as in other anatomical regions where improved iodine CNR is beneficial.
- Mar 2014
The purpose of this study was to evaluate the image quality and sensitivity of ultralow radiation dose single-energy computed tomography (CT) with tin filtration for spectral shaping and iterative reconstructions for the detection of pulmonary nodules in a phantom setting. Single-energy CT was performed using third-generation dual-source CT (SOMATOM Force; 2 × 192 slices) at 70 kVp, 100 kVp with tin filtration (100Sn kVp), and 150Sn kV with tube current-time product adjustments resulting in standard dose (CT volume dose index, 3.1 mGy/effective dose, 1.3 mSv at a scan length of 30 cm), 1/10th dose level (0.3 mGy/0.13 mSv), and 1/20th dose level (0.15 mGy/0.06 mSv). An anthropomorphic chest phantom simulating an intermediate-sized adult with randomly distributed solid pulmonary nodules of various sizes (2-10 mm; attenuation, 75 HU at 120 kVp) was used. Images were reconstructed with advanced model-based iterative reconstruction (ADMIRE; strength levels 3 and 5) and were compared with those acquired with second-generation dual-source CT at 120 kVp (reconstructed with filtered back projection) and sinogram-affirmed iterative reconstruction (strength level 3) at the lowest possible dose at 120 kVp (CT volume dose index, 0.28 mGy). One blinded reader measured image noise, and 2 blinded, independent readers determined overall image quality on a 5-grade scale (1 = nondiagnostic to 5 = excellent) and marked nodule localization with confidence rates on a 5-grade scale (1 = unsure to 5 = high confidence). The constructional drawing of the phantom served as reference standard for calculation of sensitivity. Two patients were included, for proof of concept, who were scanned with the 100Sn kVp protocol at the 1/10th and 1/20th dose level. Image noise was highest in the images acquired with second-generation dual-source CT and reconstructed with filtered back projection. At both the 1/10th and 1/20th dose levels, image noise at a tube voltage of 100Sn kVp was significantly lower than in the 70 kVp and 150Sn kV data sets (ADMIRE 3, P < 0.01; ADMIRE 5, P < 0.05). Sensitivity of nodule detection was lowest in images acquired with second-generation dual-source CT at 120 kVp and the lowest possible dose. Protocols at 100Sn kVp and ADMIRE 5 showed highest sensitivity at the 1/10th and 1/20th dose levels. Highest numbers of false-positives occurred in second-generation dual-source CT images (range, 12-15), whereas lowest numbers occurred in the 1/10th and 1/20th dose data sets acquired with third-generation dual-source CT at 100Sn kVp and reconstructed with ADMIRE strength levels 3 and 5 (total of 1 and 0 false-positives, respectively). Diagnostic confidence at 100Sn kVp was significantly higher than at 70 kVp or 150Sn kV (ADMIRE 3, P < 0.05; ADMIRE 5, P < 0.01) at both the 1/10th and 1/20th dose levels. Images of the 2 patients scanned with 100Sn kVp at the 1/10th and 1/20th dose levels were of diagnostic quality. Our study suggests that chest CT for the detection of pulmonary nodules can be performed with third-generation dual-source CT producing high image quality, sensitivity, and diagnostic confidence at a very low effective radiation dose of 0.06 mSv when using a single-energy protocol at 100 kVp with spectral shaping and when using advanced iterative reconstruction techniques.
- Feb 2014
Objective: The purpose of this article was to assess the effect of an integrated circuit (IC) detector for abdominal CT on image quality. Materials and methods: In the first study part, an abdominal phantom was scanned with various extension rings using a CT scanner equipped with a conventional discrete circuit (DC) detector and on the same scanner with an IC detector (120 kVp, 150 effective mAs, and 75 effective mAs). In the second study part, 20 patients were included who underwent abdominal CT both with the IC detector and previously at similar protocol parameters (120 kVp tube current-time product and 150 reference mAs using automated tube current modulation) with the DC detector. Images were reconstructed with filtered back projection. Results: Image quality in the phantom was higher for images acquired with the IC compared with the DC detector. There was a gradually increasing noise reduction with increasing phantom sizes, with the highest (37% in the largest phantom) at 75 effective mAs (p < 0.001). In patients, noise was overall significantly (p = 0.025) reduced by 6.4% using the IC detector. Similar to the phantom, there was a gradual increase in noise reduction to 7.9% in patients with a body mass index of 25 kg/m(2) or lower (p = 0.008). Significant correlation was found in patients between noise and abdominal diameter in DC detector images (r = 0.604, p = 0.005), whereas no such correlation was found for the IC detector (r = 0.427, p = 0.060). Conclusion: Use of an IC detector in abdominal CT improves image quality and reduces image noise, particularly in overweight and obese patients. This noise reduction has the potential for dose reduction in abdominal CT.
- Jan 2014
A method is disclosed for the reduction of image artifacts, in particular metal artifacts, during the generation of computed tomography image data of an object. In at least one embodiment of the method, two CT image data sets are generated with different medium x-ray energies. By way of a weighted combination of the two CT image data sets, a new image data set is calculated. The weighting factor employed in the weighted combination is here selected in such a way that the image artifacts in the new CT image data set are significantly reduced compared with the image artifacts in the two original CT image data sets. In this way it is possible in a simple manner significantly to reduce in particular metal artifacts in CT images.
PURPOSE 80 kV are not routinely used for abdominal CT perfusion imaging with conventional detectors because non-linear effects occur when the electronic noise of the detector dominates the measured signal. We evaluated a new detector design (Stellar, Siemens Healthcare) with minimal electronic noise for this special application. METHOD AND MATERIALS Inserts with iodine concentrations of 0.5, 1.0 and 15 mg/ml were placed in a 30 and a 40 cm water phantom. Both phantoms were examined with 100 kV and 80 kV on two otherwise identical scanners equipped with a conventional and a Stellar detector (SOMATOM Definition Flash). Scans were performed using dynamic modes (scan duration 30 s). We varied mAs per time point from 15 to 300 (CTDIvol 0.3 to 12 mGy) and measured image noise and iodine contrast. As noise*sqrt(CTDI) is constant for Poisson distributed photon noise at the same kV, we determined when this product started to increase as an indicator of the onset of relevant electronic noise. We also compared iodine contrast-to-noise ratios (CNR) beween 100 kV and 80 kV in order to determine mAs settings of equal performance. RESULTS Noise was at least 14% lower for the Stellar detector at all kV and mAs settings. For the conventional detector at 80 kV electronic noise became dominant below 1.2 mGy for the 30 cm phantom and below 6 mGy for the 40 cm phantom with increasingly prominent visual artifacts if dose was further reduced. For the Stellar detector at 80 kV dose could be reduced to 0.3 mGy for 30 cm and to 2 mGy for 40 cm without electronic noise contamination and without any visual artifacts. Using 80 kV the same CNR and image quality of the clinical default setting of 100 kV were obtained at 60% (30 cm phantom) and at 65% (40 cm phantom) of the dose. CONCLUSION The Stellar detector allows the routine use of 80 kV for abdominal perfusion imaging. Depending on patient size this reduces the dose by 35% to 40% compared to the current standard of 100 kV at identical CNR and image quality. CLINICAL RELEVANCE/APPLICATION With the Stellar detector 80 kV can be safely used for routine abdoninal perfusion imaging without any reduction in CNR or image quality at up to 40% lower dose values.
PURPOSE To evaluate the organ and effective doses of dual-energy CT (DECT) in pediatric-sized phantoms in comparison to low tube potential single-energy CT (SECT) with the same radiation output. METHOD AND MATERIALS Two anthropomorphic phantoms simulating a 1 year-old and a 5 year-old that had inserted thermoluminescent dosimeters (TLDs) were scanned using a dual-source 128-slice CT system operated with conventional SECT at low tube potential and also DECT at 80/140 kVp with tin filtration. The scan range included both abdomen and pelvis. For the SECT scans, the tube potential and corresponding tube current were selected by using an automated tube potential selection tool (CARE kV), using 120 kVp and 150 mAs as reference, with optimization for CT angiography. The scanner output, as measured by the volume CT dose index (CTDIvol), was recorded and used to adjust the mAs in the DECT scans such that CTDIvol was the same as the SECT scan. Organ doses in mGy were measured and the effective dose in mSv was calculated by summing the absorbed doses (mGy) of individual organs considering ICRP103 weighting factors. RESULTS The resulting CTDIvol values were 0.67 mGy and 2.73 mGy for the 1 year-old and 5 year-old phantoms, respectively. The calculated effective doses were 1 and 1 mSv (1 year-old), and 3 and 3 mSv (5 year-old) for the 80 kVp and 80/140 kVp scans, respectively. In the 1 year-old phantom, organ doses were statistically the same with average difference of 0.11 mGy (P=0.07) between 80 kVp and 80/140 kVp. In the 5 year-old phantom, organ doses were also statistically the same with average difference of 0.35 mGy (P=0.15) between 80 kVp and 80/140 kVp. CONCLUSION At matched radiation scanner output, organ and effective doses of DECT scans are comparable to those from conventional SECT at a low tube potential of 80 kVp. CLINICAL RELEVANCE/APPLICATION The ability of DECT to achieve comparable organ and effective doses relative to optimized low-tube potential CT angiography in pediatrics, is a pre-requisite for consideration of its use clinically.
PURPOSE To quantitatively evaluate the performance of a novel algorithm for metal artifact reduction in computed tomography (CT). METHOD AND MATERIALS The proposed iterative metal artifact reduction algorithm starts with standard CT reconstruction. Metal pixels are then identified through HU thresholding and a metal sinogram is generated by forward projection. A prior image is calculated from the initial image by assigning soft tissue pixels (identified by upper and lower HU thresholds) to 0 HU. Then normalized sinogram interpolation is performed: The prior image is forward projected and the original rawdata is normalized pixelwise with the prior sinogram. In the normalized sinogram, pixels within the metal trace are replaced by linear interpolation from the edges of the metal trace. The interpolated sinogram is de-normalized and standard reconstruction of the corrected sinogram is performed. The procedure is repeated iteratively with the output of the previous iteration used as input for prior image calculation. Finally, a frequency split is performed to preserve valid edge information of the non-corrected images: The high frequency part of non-corrected images is merged with the low frequency part of MAR-corrected images. The algorithm was applied to 10 hip replacement cases. RESULTS Streak artifacts from hip prostheses as well as the typical dark band between bilateral hip prostheses were almost completely eliminated. Pelvic soft tissue and organ structure was restored. Typical drawbacks of previous MAR algorithms such as introduction of new artifacts or compromised bone structure close to the prostheses were minimal due to iterative normalized interpolation and frequency split, respectively. Between bilateral hip prostheses, mean HU values within regions of interest located inside the bladder (expected to be water, i.e. 0 HU) were improved from -440 HU to -7 HU on average; the maximum improvement was from (-732 ± 252) HU to (-7 ± 38) HU. CONCLUSION The proposed algorithm substantially reduces artifacts from hip prostheses. CLINICAL RELEVANCE/APPLICATION The proposed algorithm strongly facilitates the visualization of pelvic anatomy. Due to the recovery of HU values, improvements for radiotherapy treatment planning are expected.
PURPOSE To quantify the accuracy and precision of a novel test bolus-based CT angiography (CTA) contrast-enhancement prediction (CEP) algorithm by comparing the amplitude, timing and curve shape of the predicted and true enhancement in the descending aorta (DAo). METHOD AND MATERIALS After routine clinical scanning according to local scan and injection protocols, from three hospitals a total of 72 (3x24) anonymized cardiac CTA exams were collected for retrospective analysis. Patients (30f/42m) had a median age and body weight of respectively 74y (range 31-81) and 79kg (range 61-125). Since existing data were retrospectively analyzed, injection protocols, image acquisitions and reconstructions differed substantially between hospitals. Test bolus (TB) scans were performed at the level of the pulmonary artery, after which the TB signal in the DAo was processed by the CEP-algorithm. This novel algorithm takes the injection protocols and kV settings of the TB and CTA scan into account, and uses population-averaged information to predict the CTA enhancement. The true enhancement was extracted from the CTA scan with a 6mm ROI along the DAo-centerline. For each patient, the relative errors in the accuracy and precision were calculated. Deviations in the amplitude were quantified with Bland-Altman analysis and shape differences with the mean absolute error (MAE) of the normalized curves. The predicted curve was shifted along the true enhancement to find the timing error, which is the time shift for which the MAE is minimal. RESULTS Although differences in injection and acquisition protocols existed, no significant differences in the precision and accuracy were found between the hospitals. For the entire patient group, the predicted enhancement has an average deviation of 1.0±15.4% in the amplitude, 0.1±1.8s in the timing, and 5.5±2.4% in the curve shape. CONCLUSION No clinically relevant offsets in the timing and amplitude of the predicted enhancement exist, and the curve shape corresponds well with the true enhancement. With its excellent accuracy and good precision, this algorithm has high potential for CTA scan timing and injection protocol optimization. CLINICAL RELEVANCE/APPLICATION Most efficient usage of contrast agent, and thus maximum CNR in CTA images, can potentially be achieved by using this algorithm for scan timing and injection protocol optimization.
PURPOSE Following the trend of low dose imaging, concerns regarding the detectability of low contrast lesions have been growing. The goal of this research is to evaluate if a new image-based algorithm (mono+) for monoenergetic imaging can improve the contrast-to-noise ratio and conspicuity of these low contrast objects. METHOD AND MATERIALS Three different anthropomorphic dual energy phantoms of different size representing a small medium and large phantoms containing 3 different iodine inserts (known values of 20, 50 and 100HU @120kV) were scanned at 3 different dose levels (full, half and quarter dose). Images were reconstructed at both 40keV and 70keV using both a standard image-based monoenergetic algorithm and mono+ at all three dose levels, resulting in 12 different images sets per phantom size. Hounsfield units and standard deviation (noise) measurements were recorded from ROIs placed within the three inserts and one background for each image set. To calculate monoenergetic images, similar to raw data approaches a two material decomposition into base materials is performed. Based on tabulated data, from the two material images, monoenergetic (keV) images can be calculated. Since by theory, any decomposition leads to an increase in noise, keV images of very low or high energy (e.g. 40 keV or 190 keV) show a substantial noise increase. Our newly developed method to calculate keV images suppresses this increase by applying a regional analysis-dependent frequency-based recombination of the high signal at lower energies and the superior noise properties at medium energies. RESULTS The mono+ algorithm resulted in a greatly improved image quality for both the 40 keV (Fig 1.) and 70 keV. Both keV level displayed lower image artifacts and a significant reduction in image noise. CNR improved for all inserts using mono+ compared to the standard algorithm. For example for the small phantom CNR could be improved for the 40 keV by about 50%. CONCLUSION Mono+ improves CNR and low contrast lesion conspicuity in particular for low dose imaging, independent of phantom size. CLINICAL RELEVANCE/APPLICATION Mono+ provides significantly increased CNR, resulting in increased lesion conspicuity. These improvements should allow for added diagnostic confidence, higher throughput and reduced reader fatigue.
PURPOSE Tube current modulation (TCM) and automatic exposure control (AEC) are widely used in modern CT. The aim of this work was to include the effects of TCM and AEC in a software package for fast and easy organ and effective dose estimates. METHOD AND MATERIALS Measurements were carried out for a SOMATOM Definition Flash scanner (Siemens AG, Forchheim, Germany); the manufacturer provided all necessary information on their CARE Dose4D TCM/AEC product. TCM and AEC curves were derived for anthropomorphic phantoms by generating complete CT projection data sets by means of ray-tracing and predicting the flux at the detector. For all phantoms and parameter combinations studied, Monte Carlo (MC) calculations w&w/o CARE Dose4D were performed to provide tabulated dose values. These tables were included in the software package ImpactDose (CT Imaging GmbH, Erlangen, Germany) which estimates organ and effective dose depending on patient size, scan region and scan protocol. It is based on pre-tabulated dose values calculated by means of MC calculations for the ORNL family of anthropomorphic phantoms. Validation measurements were performed using thermoluminescence dosimeters (TLDs) for each of three different anthropomorphic phantoms (Rando adult, 5-y.o. and 1-y.o. CIRS) w&w/o CARE Dose4D. RESULTS Measured dose values were compared to MC results on a chip-by-chip basis. The mean differences for all TLD chips were 5%, 7%, and 6% for the adult, the 5-year old, and the 1-year old phantom, respectively. This deviation is in the range of the uncertainty associated with TLD measurements and indicates that TCM/AEC were correctly implemented. The derived dose values w&w/o TCM/AEC allowed for assessment of their effect on dose for different patients without the need for measurements or repeated MC calculations. CONCLUSION Dose estimates based on tabulated MC-derived dose distributions can provide accurate information on the effect of TCM and AEC in clinical CT if information about their implementation is provided by the manufacturer. CLINICAL RELEVANCE/APPLICATION The software package allows to obtain fast and accurate dose estimates when TCM/AEC is used and furthermore may serve as a learning tool.
To assess the accuracy of dual-energy CT (DECT) for the quantification of iodine concentrations in a thoracic phantom across various cardiac DECT protocols and simulated patient sizes. Experiments were performed on first- and second-generation dual-source CT (DSCT) systems in DECT mode using various cardiac DECT protocols. An anthropomorphic thoracic phantom was equipped with tubular inserts containing known iodine concentrations (0-20 mg/mL) in the cardiac chamber and up to two fat-equivalent rings to simulate different patient sizes. DECT-derived iodine concentrations were measured using dedicated software and compared to true concentrations. General linear regression models were used to identify predictors of measurement accuracy RESULTS: Correlation between measured and true iodine concentrations (n = 72) across CT systems and protocols was excellent (R = 0.994-0.997, P < 0.0001). Mean measurement errors were 3.0 ± 7.0 % and -2.9 ± 3.8 % for first- and second-generation DSCT, respectively. This error increased with simulated patient size. The second-generation DSCT showed the most stable measurements across a wide range of iodine concentrations and simulated patient sizes. Overall, DECT provides accurate measurements of iodine concentrations across cardiac CT protocols, strengthening the case for DECT-derived blood volume estimates as a surrogate of myocardial blood supply. • Dual-energy CT provides new opportunities for quantitative assessment in cardiac imaging. • DECT can quantify myocardial iodine as a surrogate for myocardial perfusion. • DECT measurements of iodine concentrations are overall very accurate. • The accuracy of such measurements decreases as patient size increases.
- Oct 2013
Objective: The objective of our study was to evaluate in phantoms the differences in CT image noise and artifact level between two types of commercial CT detectors: one with distributed electronics (conventional) and one with integrated electronics intended to decrease system electronic noise. Materials and methods: Cylindric water phantoms of 20, 30, and 40 cm in diameter were scanned using two CT scanners, one equipped with integrated detector electronics and one with distributed detector electronics. All other scanning parameters were identical. Scans were acquired at four tube potentials and 10 tube currents. Semianthropomorphic phantoms were scanned to mimic the shoulder and abdominal regions. Images of two patients were also selected to show the clinical values of the integrated detector. Results: Reduction of image noise with the integrated detector depended on phantom size, tube potential, and tube current. Scans that had low detected signal had the greatest reductions in noise, up to 40% for a 30-cm phantom scanned using 80 kV. This noise reduction translated into up to 50% in dose reduction to achieve equivalent image noise. Streak artifacts through regions of high attenuation were reduced by up to 45% on scans obtained using the integrated detector. Patient images also showed superior image quality for the integrated detector. Conclusion: For the same applied radiation level, the use of integrated electronics in a CT detector showed a substantially reduced level of electronic noise, resulting in reductions in image noise and artifacts, compared with detectors having distributed electronics.
In a method and x-ray device to adapt the width and the position of a central value of a greyscale windowing for imaging with the x-ray device based on CT values determined with said x-ray device, the adaptation takes place within the scope of a determination and adjustment of an acquisition tube voltage of an x-ray tube of the x-ray device for an examination of a defined tissue of a patient, assuming a reference tube voltage for the examination of the defined tissue of the patient, and in which a width and position of a central value of the greyscale windowing that are associated with the reference tube voltage are automatically adapted to the acquisition tube voltage.
OBJECTIVE. The purpose of this study is to compare sinogram-affirmed iterative reconstruction (SAFIRE) and filtered back projection (FBP) reconstruction of chest CT acquired with 65% radiation dose reduction. MATERIALS AND METHODS. In this prospective study involving 24 patients (11 women and 13 men; mean [± SD] age, 66 ± 10 years), two scan series were acquired using 100 and 40 Quality Reference mAs over a 10-cm scan length in the chest with a 128-MDCT scanner. The 40 Quality Reference mAs CT projection data were reconstructed with FBP and four settings of SAFIRE (S1, S2, S3, and S4). Six image datasets (FBP with 100 and 40 Quality Reference mAs, and S1, S2, S3, S4 with 40 Quality Reference mAs) were displayed on a DICOM-compliant 55-inch 2-megapixel monitor for blinded evaluation by two thoracic radiologists for number and location of lesions, lesion size, lesion margins, visibility of small structures and fissures, and diagnostic confidence. Objective noise and CT values were measured in thoracic aorta for each image series, and the noise power spectrum was assessed. Data were analyzed with analysis of variance and Wilcoxon signed rank tests. RESULTS. All 186 lesions were seen on 40 Quality Reference mAs SAFIRE images. Diagnostic confidence on SAFIRE images was higher than that for FBP images. Except for the minor blotchy appearance on SAFIRE settings S3 and S4, no significant artifacts were noted. Objective noise with 40 Quality Reference mAs S1 images (21.1 ± 6.1 SD of HU) was significantly lower than that for 40 Quality Reference mAs FBP images (28.5 ± 8.1 SD of HU) (p < 0.001). Noise power spectra were identical for SAFIRE and FBP with progressive noise reduction with higher iteration SAFIRE settings. CONCLUSION. Iterative reconstruction (SAFIRE) allows reducing the radiation exposure by approximately 65% without losing diagnostic information in chest CT.
Objective: The purpose of this study is to assess the ability of a novel automatic tube potential selection tool to reduce radiation dose while maintaining diagnostic quality in CT angiography (CTA) and contrast-enhanced abdominopelvic CT. Materials and methods: One hundred one CTA examinations and 90 contrastenhanced abdominopelvic examinations were performed using an automatic tube potential selection tool on a 128-MDCT scanner. Two vascular radiologists and two abdominal radiologists evaluated the image quality for sharpness, noise, artifact, and diagnostic confidence. In a subset of patients who had undergone prior studies (CTA, 28 patients; abdominopelvic CT, 25 patients), a side-by-side comparison was performed by a separate radiologist. Dose reduction and iodine contrast-to-noise ratio resulting from use of the tool were calculated. Results: For CTA, 80 or 100 kV was selected for 73% of the scans, with a mean dose reduction of 36% relative to the reference 120-kV protocol. For abdominopelvic CT examinations, 80 or 100 kV was used for 55% of the scans, with a mean dose reduction of 25%. Overall dose reduction relative to the reference 120-kV protocol was 25% and 13% for CTA and abdominopelvic CT scans, respectively. Over 98% of scans had acceptable sharpness, noise texture, artifact, and diagnostic confidence for both readers and diagnostic tasks; 94-100% of scans had acceptable noise. Iodine contrast-to-noise ratio was significantly higher than (p < 0.001) or similar to (p = 0.11) that of prior scans, and equivalent quality was achieved despite the dose reduction. Conclusion: Automatic tube potential selection provides an efficient and quantitativeway to guide the selection of the optimal tube potential for CTA and abdominopelvic CT examinations.
Purpose: With recently introduced technical innovations for CT systems, the dose of CT scan acquisitions has been substantially reduced; even effective dose values below 1 mSv have been reported. Due to this development, dose of the localizer radiograph may contribute substantially to dose of the whole CT examination. Since there are only limited data in the literature regarding patient dose for the different types of localizer radiographs, patient dose values were estimated in our study by measurements and Monte Carlo simulations and compared to dose values of typical CT examinations. Methods: First, dose distributions were measured in anthropomorphic phantoms for three different body regions (head, thorax, abdomen-pelvic) and three positions of the x-ray tube (AP, PA, and lateral views); measured values were compared to simulated data using Monte Carlo techniques for validation purposes. Second, organ and effective dose values for the various investigated localizer radiograph scenarios were calculated and compared with published dose values for standard CT and low-dose CT examinations. Results: For the anthropomorphic phantom, deviations of the dose values between measured and calculated results were in the range of 15%. Organ and effective dose values showed a strong dependence on the tube position. The largest differences were observed for chest localizer radiographs in the female phantom for the dose to the breast (AP: 1.01 mGy vs PA: 0.24 mGy). Overall effective dose values were in the range of 0.04-0.42 mSv per localizer radiograph acquisition. Conclusions: In view of the technical dose-reducing innovations in CT, localizer radiographs may substantially contribute to the total dose of the whole CT examination, particularly in the case of dedicated low-dose scans used, e.g., for young patients or screening purposes. Optimization of dose in localizer radiographs should be pursued further in the same way as it was done in CT.
- May 2013
A method and a computed tomography scanner are disclosed for carrying out an angiographic examination of a patient, wherein the utilized computed tomography scanner includes at least one recording system mounted on a gantry such that it can rotate about a z-axis. Projection data is acquired from at least one prescribed angular position of the gantry for at least two different energies of X-ray radiation. The projection data is subsequently combined to form a resulting projection image by evaluating the projection data corresponding to the respective angular position, in which projection image at least one substance, which should be displayed selectively, is imaged with a high image contrast compared to the respective individual projection data. This procedure extends the field of application of the computed tomography scanner to projection-based angiography examinations, which were previously restricted to C-arm systems. 3D image reconstruction methods and projection methods can be carried out on opposite sides and with great flexibility during an examination, without the need for an additional modality. By using a multispectral technique, it is possible to contrast agent. The projection data at dispense with recording a native projection data record without the different energies are moreover acquired with no or little time offset, and so a computationally expensive and error-prone registration of the data records can be dispensed with.
- May 2013
Purpose: To determine the value of a metal artefact reduction (MAR) algorithm with iterative reconstructions for dental hardware in carotid CT angiography. Methods: Twenty-four patients (six of which were women; mean age 70 ± 12 years) with dental hardware undergoing carotid CT angiography were included. Datasets were reconstructed with filtered back projection (FBP) and using a MAR algorithm employing normalisation and an iterative frequency-split (IFS) approach. Three blinded, independent readers measured CT attenuation values and evaluated image quality and degrees of artefacts using axial images, multi-planar reformations (MPRs) and maximal intensity projections (MIP) of the carotid arteries. Results: CT attenuation values of the internal carotid artery on images with metal artefacts were significantly higher in FBP (324 ± 104HU) datasets compared with those reconstructed with IFS (278 ± 114HU; P < 0.001) and with FBP on images without metal artefacts (293 ± 106HU; P = 0.006). Quality of IFS images was rated significantly higher on axial, MPR and MIP images (P < 0.05, each), and readers found significantly less artefacts impairing the diagnostic confidence of the internal carotid artery (P < 0.05, each). Conclusion: The MAR algorithm with the IFS approach allowed for a significant reduction of artefacts from dental hardware in carotid CT angiography, hereby increasing image quality and improving the accuracy of CT attenuation measurements. Key points: • CT angiography of the neck has proven value for evaluating carotid disease • Neck CT angiography images are often degraded by artefacts from dental implants • A metal artefact reduction algorithm with iterative reconstruction reduces artefacts significantly • Visualisation of the internal carotid artery is improved.
- Apr 2013
Purpose: The purpose of this study was to compare the effects of combined automated tube voltage selection and automated tube current modulation on radiation dose and image quality in small-sized phantoms undergoing computed tomography angiography (CTA) with the use of automated current modulation alone. Materials and methods: Three semianthropomorphic phantoms, corresponding to a neonate, a small child, and a small adult, underwent simulated abdominal CTA using an automated tube voltage selection technology, which simultaneously optimizes kilovoltage (in kilovolt [peak]) and tube-current/milliamperage (in milliampere-second) on the basis of the patient topogram and clinical task. The phantoms were scanned with 2 protocols: protocol A, using the combination of automated kilovoltage and milliamperage, and protocol B, using only automated milliamperage with the standard 120 kV(p). Radiation doses were measured for each phantom, and the measurements were then used to estimate volume computed tomography dose index. Image noise and iodine contrast, contrast-to-noise ratio, and the relative dose factor were assessed. Differences were tested using paired t tests, and percentage differences for various technical factors and the phantom sizes were calculated. Results: The computed tomography dose index was significantly lower in protocol A (mean, 3.3 mGy) compared with that in protocol B (mean, 7.7 mGy), representing a 56.0% dose reduction (P = 0.01). In protocol A, tube potential dropped from 120 to 70 kV(p) in the small and medium phantoms and to 80 kV(p) in the large phantom. For each phantom size, image noise and iodine contrast increased significantly in protocol A relative to those in protocol B (P = 0.03 and P < 0.01, respectively). Corresponding contrast-to-noise ratio values increased by 9.1% in protocol A relative to those in protocol B (P = 0.04). The relative dose factor values for protocol A relative to those for protocol B were 31%, 36%, and 44% for the small, medium, and large phantoms, respectively. Conclusions: Combined use of automated kilovoltage selection and automated tube current modulation is more effective for reducing radiation dose and maintaining image quality during simulated pediatric CTA than is automated tube current modulation in isolation.
Purpose: To assess the value of iterative frequency split-normalized (IFS) metal artifact reduction (MAR) for computed tomography (CT) of hip prostheses. Materials and methods: This study had institutional review board and local ethics committee approval. First, a hip phantom with steel and titanium prostheses that had inlays of water, fat, and contrast media in the pelvis was used to optimize the IFS algorithm. Second, 41 consecutive patients with hip prostheses who were undergoing CT were included. Data sets were reconstructed with filtered back projection, the IFS algorithm, and a linear interpolation MAR algorithm. Two blinded, independent readers evaluated axial, coronal, and sagittal CT reformations for overall image quality, image quality of pelvic organs, and assessment of pelvic abnormalities. CT attenuation and image noise were measured. Statistical analysis included the Friedman test, Wilcoxon signed-rank test, and Levene test. Results: Ex vivo experiments demonstrated an optimized IFS algorithm by using a threshold of 2200 HU with four iterations for both steel and titanium prostheses. Measurements of CT attenuation of the inlays were significantly (P < .001) more accurate for IFS when compared with filtered back projection. In patients, best overall and pelvic organ image quality was found in all reformations with IFS (P < .001). Pelvic abnormalities in 11 of 41 patients (27%) were diagnosed with significantly (P = .002) higher confidence on the basis of IFS images. CT attenuation of bladder (P < .001) and muscle (P = .043) was significantly less variable with IFS compared with filtered back projection and linear interpolation MAR. In comparison with that of FBP and linear interpolation MAR, noise with IFS was similar close to and far from the prosthesis (P = .295). Conclusion: The IFS algorithm for CT image reconstruction significantly reduces metal artifacts from hip prostheses, improves the reliability of CT number measurements, and improves the confidence for depicting pelvic abnormalities.
A method for enhancing a virtual non-contrast image, includes receiving a pair of dual scan CT images and calculating a virtual non-contrast image from the pair of CT images using known tissue attenuation coefficients. A conditional probability distribution is estimated for tissue at first and second points in each of the pair of CT images and the virtual non-contrast image as being the same type. A conditional probability distribution for tissue is estimated at the first and second points in each of the pair of CT images and the virtual non-contrast image as being of different types. An a posteriori probability of the tissue at the first and second points as being the same type is calculated from the conditional probability distributions, and an enhanced virtual non-contrast image is calculated using the a posteriori probability of the tissue at the first and second points as being the same type.
- Jan 2013
Purpose : To verify the technical feasibility of low contrast volume (40 ml) run-off CT angiography (run-off CTA) with the individual scan time optimization based on double-level test bolus technique. materials and methods : A prospective study of 92 consecutive patients who underwent run-off CTA performed with 40 ml of contrast medium (injection rate of 6 ml/s) and optimized scan times on a second generation of dual-source CT. Individual optimized scan times were calculated from aortopopliteal transit times obtained on the basis of double-level test bolus technique - the single injection of 10 ml test bolus and dynamic acquisitions in two levels (abdominal aorta and popliteal arteries). Intraluminal attenuation (HU) was measured in 6 levels (aorta, iliac, femoral and popliteal arteries, middle and distal lower-legs) and subjective quality (3-point score) was assessed. Relations of image quality, test bolus parameters and arterial circulation involvement were analysed. Results : High mean attenuation (HU) values (468; 437; 442; 440; 342; 274) and quality score in all monitored levels was achieved. In 91 patients (0.99) the sufficient diagnostic quality (score 1-2) in aorta, iliac and femoral arteries was determined. A total of 6 patients (0.07) were not evaluable in distal lower-legs. Only the weak indirect correlation of image quality and test-bolus parameters was proved in iliac, femoral and popliteal levels (r values: - 0.263, -0.298 and -0.254). The statistically significant difference of the test-bolus parameters and image quality was proved in patients with occlusive and aneurysmal disease. Conclusion : We proved the technical feasibility and sufficient quality of run-off CTA with low volume of contrast medium and optimized scan time according to aortopopliteal transit time calculated from double-level test bolus.
- Dec 2012
Objectives: To investigate the volumetric iodine-uptake (VIU) changes by dual-energy CT (DECT) in assessing the response to sorafenib treated hepatocellular carcinoma (HCC) patients, compared with AASLD (American Association for the Study of Liver Diseases) and Choi criteria. Materials and methods: Fifteen patients with HCC receiving sorafenib, monitored with contrast-enhanced DECT scans at baseline and a minimum of one follow-up (8-12 weeks) were retrospectively evaluated. 30 target lesions in total were analyzed for tumor response according to VIU and adapted Choi criteria and compared with the standard AASLD. Results: According to AASLD criteria, 67% target lesions showed disease control: partial response (PR) in 3% and stable disease (SD) in 63%. 33% lesions progressed (PD). Disease control rate presented by VIU (60%) was similar to AASLD (67%) and Choi (63%) (P>0.05). For disease control group, change in mean VIU was from 149.5±338.3mg to 108.5±284.1mg (decreased 19.1±42.9%); and for progressive disease group, change in mean VIU was from 163.7±346.7mg to 263.9±537.2mg (increased 230.5±253.1%). Compared to AASLD (PR, 3%), VIU and Choi presented more PR (33% and 30%, respectively) in disease control group (P<0.05). VIU has moderate consistency with both AASLD (kappa=0.714; P<0.005) and Choi (kappa=0.648; P<0.005), while VIU showed a better consistency and correlation with AASLD (kappa=0.714; P<0.005; r=0.666, P<0.005) than Choi with AASLD (kappa=0.634, P<0.005; r=0.102, P=0.296). Conclusion: VIU measurements by DECT can evaluate the disease control consistent with the current standard AASLD. Measurements are semi-automatic and therefore easy and robust to apply. As VIU reflects vital tumor burden in HCC, it is likely to be an optimal tumor response biomarker in HCC.
- Dec 2012
Objectives: To investigate the volumetric iodine-uptake (VIU) changes by dual-energy CT (DECT) in assessing the response to sorafenib treated hepatocellular carcinoma (HCC) patients, compared with AASLD (American Association for the Study of Liver Diseases) and Choi criteria. Materials and methods: Fifteen patients with HCC receiving sorafenib, monitored with contrast-enhanced DECT scans at baseline and a minimum of one follow-up (8-12 weeks) were retrospectively evaluated. 30 target lesions in total were analyzed for tumor response according to VIU and adapted Choi criteria and compared with the standard AASLD. Results: According to AASLD criteria, 67% target lesions showed disease control: partial response (PR) in 3% and stable disease (SD) in 63%. 33% lesions progressed (PD). Disease control rate presented by VIU (60%) was similar to AASLD (67%) and Choi (63%) (P>0.05). For disease control group, change in mean VIU was from 149.5 ± 338.3mg to 108.5 ± 284.1mg (decreased 19.1 ± 42.9%); and for progressive disease group, change in mean VIU was from 163.7 ± 346.7 mg to 263.9 ± 537.2 mg (increased 230.5 ± 253.1%). Compared to AASLD (PR, 3%), VIU and Choi presented more PR (33% and 30%, respectively) in disease control group (P<0.05). VIU has moderate consistency with both AASLD (kappa=0.714; P<0.005) and Choi (kappa=0.648; P<0.005), while VIU showed a better consistency and correlation with AASLD (kappa=0.714; P<0.005; r=0.666, P<0.005) than Choi with AASLD (kappa=0.634, P<0.005; r=0.102, P=0.296). Conclusion: VIU measurements by DECT can evaluate the disease control consistent with the current standard AASLD. Measurements are semi-automatic and therefore easy and robust to apply. As VIU reflects vital tumor burden in HCC, it is likely to be an optimal tumor response biomarker in HCC.
PURPOSE Implanted hardware (high z materials such as metal and ceramics) affect the accuracy of CT numbers used for radiation therapy planning. Here, we assess the performance of a novel raw data based algorithm (FSNMAR: Meyer et al. SPIE 2012) in improving the accuracy of CT numbers in the presence of metal implants, compared to standard weighted filtered back projection (FBP) and existing techniques for reducing metal artifacts (MAR). METHOD AND MATERIALS Several different high z material components including joint and dental prosthesis were arranged in acrylic water phantoms mimicking the head and torso. Materials tested included titanium alloys, surgical grade stainless steel, gold, ceramic and an amalgam. All 6 phantoms containing hardware were scanned at routine dose and maximum dose (high mAs) using an appropriate base protocol (routine head for dental, routine pelvis for hips, etc.) utilizing both single and dual energy protocols. Each data set was reconstructed with FBP, MAR and FSNMAR. RESULTS In phantoms, the FSNMAR algorithm consistently reduced the standard deviation of the Hounsfield units (HU – CT numbers) surrounding the hardware and also provided qualitative improvements in image quality. Not only did FNSMAR provide improvements in image quality compared to standard MAR methods, but more impressively, when compared to standard FBP, FSNMAR reduced the SD of CT numbers by: 84% for a stainless steel spine screw base plate; 72% for titanium spine screws; 85% for a titanium femoral revision; and 78% in the real tooth with Amalgam. CONCLUSION FSNMAR consistently reduces artifacts that arise from high atomic number (z) materials without increasing image noise. Compared to MAR and FBP, FSNMAR provides a significant reduction in CT number variations in the presence of metal hardware, improving CT number, and thus, radiation therapy planning accuracy. FSNMAR also has the potential to improve image quality and diagnostic accuracy in CT images when metal hardware is present. CLINICAL RELEVANCE/APPLICATION Metal artifacts in CT images lead to an unwanted shift in CT numbers, directly impacting radiation therapy planning. FSNMAR substantially improves the accuracy of CT values and thus, therapy planning.
PURPOSE The main objective was to compare image quality of low-dose images reconstructed with a raw-data-based iterative reconstruction algorithm (Sinogram Affirmed Iterative Reconstruction; SAFIRE) with standard-dose filtered back projection (FBP) CT. The secondary objective was to evaluate the impact of SAFIRE on the detection of CT features of lung infiltration. METHOD AND MATERIALS 50 consecutive dual-source chest CT datasets, acquired in the conditions of routine clinical practice (120 kVp; 110 mAs) with (a) both tubes set at similar energy, and (b) the total reference mAs split up in a way that 40% of the reference mAs was applied to tube A (i.e., 44 eff mAs) while 60% of the reference mAs was applied to tube B (i.e., 66 eff mAs) with a 4D dose modulation. Two series of images were generated: (a) full-dose images (generated from both tubes) reconstructed with FBP (Group 1); and (b) low-dose images (generated from tube A; 60% dose reduction) reconstructed with SAFIRE (Group 2). The CT parameters analyzed on both groups of images included: (a) subjective and objective image noise on lung and mediastinal images; (b) the presence and conspicuity of elementary lesions of lung infiltration. RESULTS In Group 2 images, there was: (a) a significant reduction in the objective image noise measured at the level of the trachea on mediastinal (16.04 ±5.66 vs 17.66 ±5.84) (p=0.0284) and lung images (29.77±6.79 vs 37.96 ±9.03) (p<0.0001); (b) a similar visual perception of noise on mediastinal (p=1) and lung images (p=1), mainly rated as minimal; and (d) a similar overall image quality, rated as excellent in 66% (33/50) of examinations, without loss of diagnostic information as assessed by the comparative analysis of individual CT features of lung infiltration (98.4 %; 95% CI=[96.9%-99.9%]). CONCLUSION Despite a 60% dose reduction, low-dose images reconstructed with SAFIRE had a similar subjective and a better objective image quality compared to full-dose FBP images. CLINICAL RELEVANCE/APPLICATION Iterative reconstruction is a useful tool to implement marked dose reduction in clinical practice without impairing the diagnostic value of chest CT examinations.
PURPOSE Evaluation of a novel temporal domain extended 4D iterative cardiac reconstruction algorithm applied to clinical coronary CT angiography (CCTA) data . The reconstruction is compared to an established iterative reconstruction algorithm (SAFIRE, Siemens Healthcare, Germany) and weighted filtered back projection. METHOD AND MATERIALS The main limitation of cardiac image reconstruction especially in obese patients is the underlying goal of maximizing the temporal resolution, which subsequently does not allow noise reduction by data accumulation, e.g. by a slow rotation time or similar techniques. An iterative 4D reconstruction algorithm enables data accumulation in the temporal direction by analyzing possible cardiac motion. The 4D algorithm is an extension of the established SAFIRE iterative reconstruction algorithm. The 4D algorithm is based on an initial 4D volume reconstructed at 5 adjacent cardiac phases with a tempoiral spacing of half a reconstruction angle. In an iterative optimization a raw data supported image noise model is utilized to analyze local image contrast. In comparison to the 3D approach where each voxel is evaluated relative to its 26 neighboring voxels, the 4D volume extends this to 80 neighboring voxels. This increase in statistics yields a more robust noise modeling and therefore in the subsequent subtraction process a improvement in the contrast to noise ration in each iteration. We used 15 clinical CCTA data sets acquired with a second-generation dual-source CT (Siemens Healthcare, Germany) to evaluate the 4D algorithm. The data sets were reconstructed with a filtered back projection algorithm, the SAFIRE algorithm and the extended 4D algorithm. We evaluated image noise and contrast to-noise ratio as well as image quality with respect to spatial resolution and the presence of motion artifacts on a subjective rating five point scale. RESULTS On average the noise was reduces by 45 % in the standard SAFIRE case and by 65 % in the extended 4D cases compared to filtered backprojection. Image quality and artifact level was comparable in all three cases. CONCLUSION Inital experience with an 4D iterative algorithm indicates an greatly improved contrast to noise ratio compared to standard 3D iterative techniques. CLINICAL RELEVANCE/APPLICATION An improved contrast to noise ratio in cardiac images could be useful for CCTA diagnosis especially in obese patients where image noise is often the limiting factor.
Objectives: The objective of this study was to assess the value of an integrated circuit (IC) detector, potentially improving spatial resolution by means of reduced cross talk between detector channels, in coronary computed tomographic (CT) angiography regarding image quality and stenosis quantification compared with conventional detector technology. Materials and methods: In the ex vivo part of the study, a coronary phantom including 63 defined stenoses and 7 plaque densities (degree of stenosis, 10%-90%; plaque densities, -100 to 1000 Hounsfield unit [HU]) was loaded with contrast agent diluted to 300 HU and placed in an anthropomorphic chest phantom. The phantom was scanned in 0-, 45-, and 90-degree orientations to the z-axis of the CT scanner table. Images were acquired using 128-section dual-source CT equipped with IC and with conventional detector technology. Data were reconstructed with filtered back projection (FBP) and with sinogram-affirmed iterative reconstruction (IR) at a slice thickness of 0.6 mm (increment, 0.4 mm). Data acquired with the IC detector were additionally reconstructed with a slice thickness of 0.5 mm (increment, 0.3 mm) combined with IR. Two readers rated image quality; image noise and degree of stenosis were assessed. In the in vivo part of the study, phantom observations were validated in 30 consecutive patients (11 women; mean [SD] age, 62  years; mean [SD] heart rate, 81  beats per minute) undergoing coronary CT angiography with IC for clinical indications. Images of the patients were reconstructed with FBP (slice thickness, 0.6 mm) and with IR (slice thickness, 0.5 mm) and were assessed for image quality and degree of stenosis. Interreader agreement for image quality was evaluated using intraclass correlation coefficients. The image quality was compared with the Wilcoxon signed rank test. The image noise and the degree of stenosis were compared with the Student t test for paired samples. Results: The interreader agreement for the assessment of image quality was substantial (intraclass correlation coefficients, 0.79). The image quality was significantly (P < 0.001) higher for the images acquired with the IC detector as compared with the conventional detector. The image noise with IR was significantly (P = 0.020) reduced for the IC detector as compared with the conventional detector. The IC detector yielded significantly more accurate results regarding stenosis grading when compared with the images acquired with the conventional detector regarding both FBP (mean [SD] error FBP, 12.1% [7.6%] vs 13.7% [7.6%]; P = 0.043) and IR (mean [SD] error IR, 10.5% [6.6%] vs 13.0% [6.9%]; P < 0.001). The images with a slice thickness of 0.5 mm reconstructed with IR (mean [SD] error, 8.8% [5.9%]) obtained by the IC detector significantly (P < 0.001) improved measurement accuracy in the phantom as compared with FBP with a slice thickness of 0.6 mm (mean [SD] error, 12.1% [7.6%]). In the patients, we found a significantly (P < 0.001) higher image quality, and stenoses were quantified significantly (P = 0.009) smaller with FBP as compared with IR (mean stenosis, 47.6% vs 42.1%; mean difference, 5.5%). Conclusions: Our ex vivo and patient study indicates significantly reduced image noise and more accurate stenosis quantification in coronary CT angiography when acquiring data using an IC detector and combining IR with high-resolution images as compared with conventional detector technology and FBP reconstructions.
Objectives: The objective of this study was to evaluate the effect of, and optimal parameters for, nonlinear image blending compared with linear image blending in the late-phase dual energy computed tomography (DECT) for the visualization of delayed myocardial contrast enhancement in acute myocardial infarction (MI). Materials and methods: Acute reperfused MI was induced in 7 pigs by temporary occlusion of the left anterior descending or the left circumflex artery. Two hours after the reperfusion, a contrast-enhanced, late-phase DECT (80 kV/140 kV) scanning was performed. The DECT data were postprocessed with linear and nonlinear image blending techniques. Contrast and percentage signal differences between healthy and infarcted myocardium as well as the blood pool of the left ventricle were computed for the linear and nonlinear techniques and the low- and high-kilovolt images. Data were compared using repeated-measures analysis of variance and post hoc t tests. Results: The nonlinear blending showed the highest signal differences for all contrasts and analyses. Repeated-measures ANOVA results confirmed that the differences were statistically significant for the different postprocessing techniques (P value ranging from <0.001-0.003). Paired-samples post hoc t tests proved the significance of these results (P value ranging from <0.001-0.037). The ideal settings for the nonlinear image blending can thus be deduced from the computed tomographic values of the regions of interest in the linearly blended images with the weighting factor 0.3. Conclusions: Nonlinear image blending improves the visualization of acute MI in the late-phase DECT. It is superior to linearly blended images and source images obtained at 80 or 140 kV.
PURPOSE Dual Energy data can be acquired with different voltage combinations, beam pre-filtrations and dose splitting between high and low kV beam. Depending on the selected acquisitions settings, noise and dual energy signal might vary. We investigated in a phantom experiment the impact of the different settings on the feasibility to detect certain materials and on the uncertainty of quantification. METHOD AND MATERIALS For our measurements we used a SOMATOM Definition Flash (Siemens Healthcare, Germany). Data were acquired for three different voltage combinations (80kV/140kV, 80kV/140SN kV and 100kV/140SN). In addition, the mAs values were for one set of measurements adjusted to achieve same dose in the high and low kV beam; for an other set of measurements same mAs were used for both beams. To assess the ability for material quantification, we diluted contrast agents based on iodine (I), gadolinium (Ga), iron (Fe) and ytterbium (Yb) in water to obtain the following concentrations: 0.1, 0.5, 1, 2, 4, 6, 8, 10, 15 and 20 mg/ml. For the dual energy processing a modified version of the three material decomposition technique was used. Dual energy ratio (R = HU at low kV/HU at high kV), material concentration (C) and noise (SD) was determined for all assessed combinations. RESULTS Dual energy ratio R was not impacted by the distribution of dose between both beams (same dose vs. same mAs). However, scans with tin filtration (SN) leaded to significantly higher values for R: I 2.0, 3.1 and 2.3; Ga 1.6, 2.1 and 1.8; Fe 2.0, 2.8 and 2.1; Yb 1.3, 1.4 and 1.4 for 80kV/140kV, 80kV/140SN kV and 100kV/140SN respectively. The higher the dual energy ratio R, the lower was SD in the post-processed images. Measurements with same mAs for both beams leaded to substantially higher SD values and reduced the ability for material detectability compared to the scans with same dose. CONCLUSION Same dose values in the high and low kV beam in combination with an additional tin filtration increased substantially the ability to detect materials and it improved the precision of quantitative values. CLINICAL RELEVANCE/APPLICATION To optimize the results of post-processed dual data scans, optimal distribution of dose and a high spectral separation is recommended.
PURPOSE To evaluate in phantoms the differences in CT image noise and artifact level between commercial CT detectors with distributed electronics and those with direct coupling of the photodiode and ADC, which is intended to decrease system electronic noise. METHOD AND MATERIALS Cylindrical water phantoms of 20, 30 and 40-cm diameter were scanned using two CT scanners, one equipped with the integrated detector (Stellar detector on Siemens Definition FLASH) and the other with conventional detector (Siemens Definition AS+). All other scanning parameters were identical. Scans were acquired at four tube potentials (80, 100, 120 and 140 kV), 10 tube currents (60 to 600 mA) and a 0.5 s rotation time. Images were reconstructed with 1 mm thickness and a medium smooth kernel (B30). Two semi-anthropomorphic phantoms were also scanned to mimic the shoulder and abdomen regions, with tube potentials of 80 and 120 kV, and a quality reference mAs of 240. Image noise was quantified as the standard deviation of CT number (STD) in uniform regions of interest (ROI). Artifact level was quantified as the difference in STD between regions containing streak artifacts and adjacent artifact-free ROI. Noise power spectra were also computed. RESULTS For cylindrical phantoms, the reduction of image noise depended on phantom size and tube current. Low signal scans, e.g. low tube currents or large phantoms, had larger noise reductions, up to 40% for a 30 cm phantom using 80 kV. This translated to up to 50% in dose reduction for the equivalent image noise. The integrated detector behaved much more closely to an ideal detector, where noise should be inversely proportional to the square root of dose. Streak artifacts through regions of high attenuation (e.g. through the shoulders) were greatly reduced for the integrated detector, especially in lower dose scans, with artifact reductions up to 45%. There was no visible change in the shape of the noise power spectra between the two types of detectors. CONCLUSION The integrated detector resulted in substantially reduced levels of electronic noise, resulting in reductions in image noise and artifacts compared with conventional detectors. CLINICAL RELEVANCE/APPLICATION The integrated CT detector will enable dose reduction and improved image quality in situations where low photon counts are measured, such as in low-dose, dual-energy or large patient scans.
The purpose of this study was to determine whether the use of an automated CT kilovoltage (kV) selection tool (Auto kV) can result in lower radiation dose without sacrificing image quality in contrast-enhanced abdominopelvic CT. Tube potential, radiation dose, and iodine contrast-to-noise ratio (CNR) were retrospectively evaluated in 36 patients who underwent abdominopelvic CT with Auto kV, and compared with results from size-matched control patients using identical protocols. Two radiologists evaluated image quality (sharpness, noise, and diagnostic confidence) blinded to kV. Volume CT dose index (CTDI(vol)) was also compared with what each patient would have received from scanning at 120 kV. Mean (SD) CTDI(vol) was 16.0 (4.4) mGy after Auto kV versus 19.5 (4.0) mGy using standard 120-kV prescription and was 19.3 (6.0) mGy in control subjects (yielding dose reductions of 18.0% and 17.2%, respectively; p < 0.001 for both). Thirty of 36 patients were scanned at 100 kV (median dose reduction, 25%). Auto kV images were rated as very sharp in 33 (92%) and 36 (100%) cases versus 36 (100%) and 35 (97%) of the control cases, with all cases scored as having optimal noise. Readers had full diagnostic confidence in 34 (94%) and 36 (100%) of Auto kV cases; one reader scored "probably confident" in two cases (6%). Iodine CNRs for the aorta, liver, and portal vein were similar between Auto kV cases and control cases (p > 0.50, all comparisons). The use of an automated kV selection tool results in significant dose savings while maintaining diagnostic image quality and iodine CNR.
- Oct 2012
Rationale and objectives: Computed tomographic angiography is the standard in routine follow-up after endovascular aneurysm repair, causing radiation exposure; thus, dose-saving strategies should be applied. The aim of this study was to evaluate the novel sinogram-affirmed iterative reconstruction (SAFIRE) algorithm in terms of clinical usability and potential reduction of radiation exposure. Materials and methods: Forty-six patients underwent computed tomographic angiographic follow-up after endovascular aneurysm repair. Data were acquired using a dual-source computed tomographic scanner running both x-ray tubes at the same voltage (120 kV). Raw data were reconstructed using projections of both tubes with filtered back projection (FBP) and of only one tube with FBP and SAFIRE, corresponding to synthetic acquisition with half the radiation dose of the clinical routine radiation dose. Image sets were objectively compared regarding signal-to-noise ratio and edge sharpness. Two radiologists independently assessed a set of subjective criteria, including diagnostic usability, depiction of contrasted vessels, and image noise. Results: Half-dose (HD) SAFIRE images showed significantly higher signal-to-noise ratios compared to full-dose FBP images (P < .001), while having equal edge sharpness (P = .56). Most of the subjectively assessed parameters, such as diagnostic usability and depiction of contrasted vessels, were rated similar for HD SAFIRE and full-dose FBP images. Full-dose FBP images depicted fine anatomic structures more clearly (P < .05), while HD SAFIRE data sets showed less noise (P < .01). HD FBP images performed worse on all criteria (P < .001). Interrater agreement was good (κ = 0.74-0.80). Conclusions: Using the SAFIRE algorithm, the radiation dose of high-contrast abdominal computed tomographic angiography is reducible from routine clinical levels by up to 50% while maintaining good image quality and diagnostic accuracy.
- Sep 2012
To evaluate a method for obtaining half-dose CT images for observer studies evaluating lower-dose CT. Phantoms of varying sizes were scanned at multiple tube potentials using dose-matched dual-source (DS) and single-source (SS) protocols. Images from single-tube reconstruction of DS data were compared with SS images acquired at half-original CTDIvol. Thirty patients underwent supine SS and dose-matched prone DS CT colonography (CTC). Half-dose prone images were reconstructed with sinogram-affirmed iterative reconstruction (SAFIRE). Two radiologists scored image quality on 2-dimensional (2D) and 3D images. Image noise was similar between half-dose SS images and DS images reconstructed from one tube only with tube potential of 120 kV or more for phantoms 40 cm or smaller (P < 0.05). For both readers, the patients' CTC image quality scores were more than 84% concordant between SS or DS CTC images, and half-dose-prone CTC images with SAFIRE had 84% or more concordance with routine-dose CTC except for 3D image noise. In appropriately sized patients, DS acquisition with single-tube reconstruction can create half-dose images, permitting comparison to full-dose images. For CTC, there is comparable image quality for colonic evaluation between full-dose and half-dose images reconstructed with SAFIRE.
Purpose. To estimate effective dose and organ equivalent doses of prospective ECG-triggered high-pitch CTCA. Materials and Methods. For dose measurements, an Alderson-Rando phantom equipped with thermoluminescent dosimeters was used. The effective dose was calculated according to ICRP 103. Exposure was performed on a second-generation dual-source scanner (SOMATOM Definition Flash, Siemens Medical Solutions, Germany). The following scan parameters were used: 320 mAs per rotation, 100 and 120 kV, pitch 3.4 for prospectively ECG-triggered high-pitch CTCA, scan range of 13.5 cm, collimation 64 × 2 × 0.6 mm with z-flying focal spot, gantry rotation time 280 ms, and simulated heart rate of 60 beats per minute. Results. Depending on the applied tube potential, the effective whole-body dose of the cardiac scan ranged from 1.1 mSv to 1.6 mSv and from 1.2 to 1.8 mSv for males and females, respectively. The radiosensitive breast tissue in the range of the primary beam caused an increased female-specific effective dose of 8.6%±0.3% compared to males. Decreasing the tube potential, a significant reduction of the effective dose of 35.8% and 36.0% can be achieved for males and females, respectively (P < 0.001). Conclusion. The radiologist and the CT technician should be aware of this new dose-saving strategy to keep the radiation exposure as low as reasonablly achievable.
- May 2012
The objective of this study was to assess the effect of Sinogram Affirmed Iterative Reconstruction (SAFIRE) and filtered back-projection (FBP) techniques on abdominal computed tomography (CT) performed with 50% and 75% radiation dose reductions. Twenty-four patients (mean age, 64 ± 14 years; male-female ratio, 10:14) gave informed consent for an institutional review board-approved prospective study involving acquisition of additional research images through the abdomen on 128-slice multi-detector-row CT (SOMATOM Definition Flash) at quality reference mAs of 100 (50% lower dose) and 50 (75% lower dose) over a scan length of 10 cm using combined modulation (CARE Dose 4D). Standard-of-care abdominal CT was performed at 200 quality reference mAs, with remaining parameters held constant. The 50- and 100-mAs data sets were reconstructed with FBP and at 4 SAFIRE settings (S1, S2, S3, S4). Higher number of SAFIRE settings denotes increased strength of the algorithm resulting in lower image noise. Two abdominal radiologists independently compared the FBP and SAFIRE images for lesion number, location, size and conspicuity, and visibility of small structures, image noise, and diagnostic confidence. Objective noise and Hounsfield units (HU) were measured in the liver and the descending aorta. All 43 lesions were detected on both FBP and SAFIRE images. Minor blocky, pixelated appearance of 50% and 75% reduced dose images was noted at S3 and S4 SAFIRE but not at S1 and S2 settings. Subjective noise was suboptimal in both 50% and 75% lower-dose FBP images but was deemed acceptable on all SAFIRE settings. Sinogram Affirmed Iterative Reconstruction images were deemed acceptable in all patients at 50% lower dose and in 22 of 24 patients at 75% lower dose. As compared with 75% reduced dose FBP, objective noise was lower by 22.8% (22.9/29.7), 35% (19.3/29.7), 44.3% (16.7/29.3), and 54.8% (13.4/29.7) on S1 to S4 settings, respectively (P < 0.001). Sinogram Affirmed Iterative Reconstruction-enabled reconstruction provides abdominal CT images without loss in diagnostic value at 50% reduced dose and in some patients also at 75% reduced dose.
- Apr 2012
The problem of metal artifact reduction (MAR) is almost as old as the clinical use of computed tomography itself. When metal implants are present in the field of measurement, severe artifacts degrade the image quality and the diagnostic value of CT images. Up to now, no generally accepted solution to this issue has been found. In this work, a method based on a new MAR concept is presented: frequency split metal artifact reduction (FSMAR). It ensures efficient reduction of metal artifacts at high image quality with enhanced preservation of details close to metal implants. FSMAR combines a raw data inpainting-based MAR method with an image-based frequency split approach. Many typical methods for metal artifact reduction are inpainting-based MAR methods and simply replace unreliable parts of the projection data, for example, by linear interpolation. Frequency split approaches were used in CT, for example, by combining two reconstruction methods in order to reduce cone-beam artifacts. FSMAR combines the high frequencies of an uncorrected image, where all available data were used for the reconstruction with the more reliable low frequencies of an image which was corrected with an inpainting-based MAR method. The algorithm is tested in combination with normalized metal artifact reduction (NMAR) and with a standard inpainting-based MAR approach. NMAR is a more sophisticated inpainting-based MAR method, which introduces less new artifacts which may result from interpolation errors. A quantitative evaluation was performed using the examples of a simulation of the XCAT phantom and a scan of a spine phantom. Further evaluation includes patients with different types of metal implants: hip prostheses, dental fillings, neurocoil, and spine fixation, which were scanned with a modern clinical dual source CT scanner. FSMAR ensures sharp edges and a preservation of anatomical details which is in many cases better than after applying an inpainting-based MAR method only. In contrast to other MAR methods, FSMAR yields images without the usual blurring close to implants. FSMAR should be used together with NMAR, a combination which ensures an accurate correction of both high and low frequencies. The algorithm is computationally inexpensive compared to iterative methods and methods with complex inpainting schemes. No parameters were chosen manually; it is ready for an application in clinical routine.
CT protocols should aim for radiation doses being as low as reasonably achievable. The purpose of our study was to assess the image quality and radiation dose of neck CT at a tube potential of 70 kVp. Twenty patients (7 female, mean age 51.4 years, age range 19-81 years) underwent contrast-enhanced 64-section CT of the neck at 70 kVp (ATCM, effective tube current-time product 614 eff.mAs, range 467-713 eff.mAs). All 20 patients had a previous neck CT at 120 kVp on the same scanner. Two radiologists assessed image quality and artifacts in the upper, middle, and lower neck. Image noise and attenuation were measured, and the CNR was calculated. Effective radiation dose was calculated. Interobserver agreement regarding image quality of soft tissue for 70-kVp and 120-kVp scans was good to excellent. At 70 kVp, soft tissues were of diagnostic image quality in all scans, whereas the lower cervical spine was not of diagnostic quality in 3 and 4 scans per both readers. No difference was found among 70-kVp and 120-kVp scans for soft tissue image quality in the upper neck, while image quality was significantly better in the middle at 70 kVp (P < .05) and better in the lower third at 120 kVp (P < .05). CNR was significantly higher at 70 kVp in all levels for both readers (P < .001). Effective radiation dose at 70 kVp was significantly lower (0.88 ± 0.2 mSv) than at 120 kVp (1.33 ± 0.2 mSv, P < .001). CT of the cervical soft tissues at 70 kVp is feasible, provides diagnostic image quality with improved CNR, and reduces radiation dose by approximately 34% compared with a standard protocol at 120 kVp. In contrast, low kVp CT of the lower cervical spine suffers from compromised image quality.
- Feb 2012
Metal implants in the field of measurement lead to strong artifacts in CT images and reduce the image quality and the diagnostic value severely. We introduce frequency split metal artifact reduction (FSMAR), a conceptually new MAR method which is designed to reduce metal artifacts and preserve details and edges of structures even close to metal implants. There are many MAR methods which simply replace unreliable parts of the projection data by inpainting. FSMAR is a combination of an inpainting-based MAR method with a frequency split approach. Normalized metal artifact reduction (NMAR) is chosen as the inpainting-based MAR method in this work. The high frequencies of the original image, where all rawdata were used for the reconstruction, are combined with an NMAR-corrected image. NMAR uses a normalization step to reduce metal artifacts without introducing severe new artifacts. Algorithms using a frequency split were already used in CT for example to reduce cone-beam artifacts. FSMAR is tested for patient datasets with different metal implants. The study includes patients with hip prostheses, a neuro coil, and a spine fixation. All datasets were scanned with modern clinical dual source CT scanners. In contrast to other MAR methods, FSMAR yields images without the usual blurring close to metal implants.
- Jan 2012
- Radiation Dose from Multidetector CT
With CT being the imaging modality of choice in many situations, dose reduction is of concern for both manufacturers and users. To reduce the dose for an individual exam, the first and foremost principle to adhere to is ALARA (As Low As Reasonably Achievable), i.e., to use the lowest possible dose to obtain the required diagnostic quality images. Besides adjusting the techniques to the diagnostic question, it is vital to know and understand dose reduction techniques available on the CT system. Whereas the availability of individual techniques typically depends on the model type as well as the installed scanner software, this chapter gives an overview of technologies and algorithms available on Siemens systems to reduce the absorbed dose to a minimum.
PURPOSE Various powerful nonlinear noise reduction techniques have been proposed for Computed Tomography, claiming substantial dose reduction potentials (e.g. Thibault et al., Med. Phys. 34, pp. 4526 (2007), Bruder et al., Adaptive Iterative Reconstruction, SPIE 2011, to be published). To prove that these dose reductions are really achievable in practice remains a difficult task, however. Meanwhile the community has realized that simply measuring the noise and sharpness of images is not sufficient. Unlike in nuclear medicine where model observers are a well-established technique used to assess e.g. various collimator designs, there is no such standard for CT. Moreover, there are serious doubts if the standard techniques used in nuclear medicine, e.g. SKE-BKV with lumpy background and circularly symmetric channels, can be applied in CT where the noise is much more correlated and directional (A. Wunderlich, F. Noo, Evaluation of the Impact of Tube Current Modulation on Lesion Detectability using Model Observers, 30th IEEE EMBS Conference, 2008). METHOD AND MATERIALS We compare simulated images processed with and without a nonlinear adaptive raw data filter, applying standard (Channelized Hotelling) model observers to different tasks, performing ROC and LROC analysis. RESULTS We find that the choice of the observers and the task significantly influences the outcome of comparisons. We also find that the task must be carefully selected dependent on the question to be answered. CONCLUSION Task based image quality assessments represent a powerful tool to evaluate and optimize new non-linear image reconstruction techniques. This tool, however, comes with many degrees of freedom that will require careful analysis before they can be used as standardized black-box methods. CLINICAL RELEVANCE/APPLICATION Nonlinear data or image processing techniques can be evaluated with model observers which have to be carefully selected to provide meaningful results.
PURPOSE To prospectively demonstrate the feasibility of differentiating uric acid (UA) and non-uric acid (NUA) renal stones using two consecutive scans on a single source CT scanner and a non-rigid 3D registration algorithm. METHOD AND MATERIALS A total of 10 patients undergoing clinically-indicated dual source (DS), dual energy CT scanning for the purpose of differentiating UA and NUA kidney stones were recruited in this IRB approved prospective study. On the same day as the clinically-indicated DS exam, the patient was scanned on a single source CT scanner (Definition AS+, Siemens Healthcare, Germany). An 80 kV CT scan was performed in a cranial-caudal range limited to the identified stones, immediately followed by a 140 kV scan at the same region. The combined radiation dose for the 80 and 140 kV scans was set to be equivalent to that applied with the DS CT scan. Images from the 80 and 140 kV scans were registered using a 3D non-rigid registration program, followed by dual-energy material differentiation analysis to differentiate UA (in red) and NUA (in blue) stones. Accuracy of stone classification was calculated using the results from dual source scanner as gold standard. A GU radiologist recorded the number, size and composition of stones in each patient, and performed a side by side comparison for the image quality of the single source and dual source scans. RESULTS A total of 52 NUA stones were identified from the DS CT scans (no UA stones). Among these stones, 41 were correctly identified as pure NUA stones from the single source CT scan; 6 were majority blue but with a little dot or linear region of red along the periphery, which will be classified as NUA stones clinically; 2 (size=2mm) were incorrectly identified as UA stones and 3 (size=1mm) unclassified. The accuracy of stone classification was 90.4%. Image quality of single source CT scan was considered similar to that of the dual source scan for all patients. CONCLUSION Using a 3D non-rigid registration algorithm on two consecutive single energy scans at different beam energies, UA and NUA stones can be accurately differentiated using a single source CT scanner with stone size larger than 2mm. CLINICAL RELEVANCE/APPLICATION This technique may enable stone composition differentiation to be performed on widely available standard CT scanner with the use of a 3D non-rigid registration algorithm.
PURPOSE To introduce a novel algorithm of automated attenuation-based tube potential selection and to assess its impact on image quality and radiation dose of body CT angiography (CTA). METHOD AND MATERIALS Forty patients (mean age 71±11.8 years, body mass index (BMI) 25.7±3.8 kg/m2, range 18.8-33.8 kg/m2) underwent 64-slice thoraco-abdominal CTA (contrast material 80 ml, 5 ml/sec) using an automated tube potential selection algorithm (CAREkV) which optimizes tube-potential (70-140 kV) and tube-current (138.8±18.6 effective mAs, range 106-216) based on the attenuation profile of the topogram and on the diagnostic task. Image quality was semi-quantitatively assessed (scores 1: excellent to 5: non-diagnostic) and attenuation, noise, and contrast-to-noise ratio (CNR) were measured by two independent readers. The CT dose index (CTDIvol) was recorded and compared to the estimated CTDIvol of a standard 120kV protocol without using the algorithm in each patient. Selected tube potentials were correlated with BMI and attenuation of the topogram. RESULTS Diagnostic image quality was obtained in all patients (excellent: 14; good: 21; moderate: 5; interreader agreement: κ=0.78). Mean attenuation, noise, and CNR were 260.8±63.5HU, 15.5±3.3HU, and 14±4.2, respectively, with good to excellent agreement between readers (r=0.50-0.99, p<0.01 each). Automated attenuation-based tube potential selection resulted in a kV-reduction from 120kV to 100kV in 23 patients and to 80kV in one patient, whereas tube potential increased to 140kV in one patient. Automatically selected tube potential showed a significant correlation with both BMI (r=0.427, p<0.05) and attenuation of the topogram (r=0.831, p<0.001). CTDIvol (7.95±2.6mGy) was significantly lower when using the algorithm compared to the standard 120kV protocol (10.59±1.8mGy, p<0.001), corresponding to an overall dose reduction of 25.1%. CONCLUSION Automated attenuation-based tube potential selection based on the attenuation profile of the topogram is feasible, provides a diagnostic image quality of body CTA, and reduces overall radiation dose by around 25% as compared to a standard protocol with fixed 120kV. CLINICAL RELEVANCE/APPLICATION Automated attenuation-based tube potential selection based on the attenuation profile of the topogram provides diagnostic image quality of body CTA and reduces radiation dose.
PURPOSE To prospectively evaluate image quality of thoraco-abdominal CT angiography (CTA) in half-dose (HD) datasets reconstructed with raw-data based sinogram-affirmed iterative reconstruction (SAFIRE) compared to HD and full-dose (FD) datasets reconstructed with standard filtered back projection (FBP). METHOD AND MATERIALS Twenty-five patients (mean age 70.7±11.7 years) underwent thoraco-abdominal CTA (contrast material 80ml, 5 ml/sec) performed with a 128-section dual-source CT system, both tubes operating at 120 kV and 210 ref. mAs (mean DLP 1479.1±392.5mSv). FD images with FBP were reconstructed (serving as reference) and were compared to HD images with FBP and HD images with SAFIRE, both reconstructed using data from only one tube-detector-system. Image quality was semi-quantitatively assessed (score 1: excellent to 5: non-diagnostic), vessel attenuation and noise were measured, and contrast-to-noise ratio (CNR) calculated. Artifacts were semi-quantitatively assessed (score 1: absent to 4: affecting diagnostic quality). RESULTS Diagnostic image quality was obtained in all patients (range 1-3; excellent-moderate, κ=0.701). Image quality in HD-FBP was rated inferior compared to FD-FBP (p<0.001), and HD-SAFIRE (p<0.001), respectively. Aortoiliac attenuation was similar among all image series (FD, 220.4±38.6 HU; HD-FBP, 221.3±37.8 HU; HD-SAFIRE, 220.9±37.9 HU, p>0.05). Image noise in FD-FBP (7.98 HU) was significantly lower than noise in HD-FBP (10.44 HU, p<0.001) but significantly higher than in HD-SAFIRE (7.23 HU, p<0.05, noise reduction of 9.4% as compared to FD-FBP). No significant differences were found for artifacts between the three image series (p>0.05). CONCLUSION Our intra-individual comparisons indicate that sinogram-affirmed iterative reconstructions allow for a dose reduction of larger than 50% for body CTA while maintaining high image quality. CLINICAL RELEVANCE/APPLICATION Raw-data based sinogram-affirmed iterative reconstructions can be used to reduce radiation exposure of body CTA studies more than 50% without a compromise in image quality.
PURPOSE CT identification of abnormally thickened peripheral airway walls is a non-specific finding that typically involves imprecise visual assessment or complex computations including isolated measurements of wall dimensions. We investigate the potential of quantitatively assessing airway wall enhancement following administration of IV contrast media by using dual-energy CT (DECT) as a potential method for identifying airway wall inflammation in patients with Cystic Fibrosis (CF). METHOD AND MATERIALS A Dual-energy protocol was calibrated using 3 sets of airway phantoms with varying wall thicknesses (2, 4 and 6mm), each set containing 4 concentrations of iodine (0, 25, 50 and 100HU at 120 kVp). Protocol parameters for tissue, fat and smoothness were defined such that contrast uptake shown on the generated Iodine Uptake (IU) maps would approach the given concentrations for each set of phantoms. Following determination of optimal imaging parameters and IRB approval, DECT was used to evaluate a total of 118 bronchi in images from 5 patients with documented Cystic Fibrosis. Acquisitions were immediately done post-contrast administration timed to peak aortic enhancement. All patients had 0.5 to 0.8mm collimation CT scans obtained at 1mm intervals. For each bronchus, the inner and outer walls were automatically computed from a cross-section perpendicular to the airway orientation using a modified Full Width Half Maximum algorithm. RESULTS Despite partial volume effects, 2mm walls in phantom clearly showed increasing levels of enhancement (4.5, 15.8, 38.3 and 61HU on average) which corresponded to the increasing levels of iodine, demonstrating a possibility of iodine quantification within walls of this size. In the CF patients, the 118 identified bronchi had an average thickness of 2.38mm, and the average contrast enhancement measured 14.28 HU [range of 0 to 50], indicating probable contrast uptake. CONCLUSION In this preliminary investigation, we defined a protocol for IU image reconstruction that, associated with dedicated airway analysis tools, allows for the quantification of airway wall enhancement using DECT images. Such quantification could therefore be used to assess the degree of inflammation in the airway walls CLINICAL RELEVANCE/APPLICATION DECT technique could define a subset of patients with non-specific bronchial wall thickening in whom airway wall enhancement may serve as a potential biomarker for identifying airway wall inflammation
- Nov 2011
- Radiological Society of North America 2011 Scientific Assembly and Annual Meeting
PURPOSE Software is available for dose estimation in general CT while there is a lack of such tools for pediatric CT covering the age and gender range. We developed a software application with a graphical user interface (GUI) which provides organ and effective dose values in pediatric CT for arbitrary scan parameters based on established voxel phantoms. METHOD AND MATERIALS The proposed application is based on Monte Carlo (MC) dose calculations performed on the ORNL family of 11 mathematically defined anthropomorphic phantoms representing both genders and different ages (0, 1, 5, 10, 15 years old and adults) with all organs of interest segmented [Cristy M. Rep. ORNL/NUREG/TM-367 1980; Deak et al, Radiology 2010]. Dose contributions from primary and scattered radiation to each voxel was calculated per exposure of each single 5-mm section of the phantoms for a variety of scan parameters, e.g. for 60, 70, 80, 100, 120 and 140 kV. The dose values were tabulated for various scanner types. For any choice of gender and age and arbitrary scan ranges and parameters, the software tool extracts the data from the respective tables and provides organ doses immediately. For validation, results were compared to direct MC calculations using a validated MC tool (ImpactMC, CT Imaging GmbH, Erlangen, Germany). RESULTS The GUI allows interactive input of scan parameters and scan ranges for any of the phantoms and provides graphical displays for control. All organ doses and effective doses for ICRP 26, 60, or 103 are displayed without time delay. Values are also compared to dose reference levels which can be updated or created on line by the institution. Evaluations of the effects of variations of scan parameters are easy. E.g., a thorax examination of a 5 year-old child with 120 kV and 40 mAs results in an effective dose of 2.1 mSv and 1.4 mSv for a girl and a boy, respectively, while these are reduced to 1.3 and 0.8 mSv, respectively, for 70 kV and 110 mAs. CONCLUSION The software tool allows fast estimation of dose parameters and relates them to dose reference levels. It offers graphical output and is particularly convenient for teaching purposes. CLINICAL RELEVANCE/APPLICATION The GUI-based software covering both genders and the representative ages may be useful in practice and for teaching purposes in pediatric CT.
- Nov 2011
- Radiological Society of North America 2011 Scientific Assembly and Annual Meeting
PURPOSE To quantify the inherent properties, the potential, and the limitations of X-ray phase contrast imaging (PCI). METHOD AND MATERIALS The refractive index of materials can be characterized by its real part and imaginary part which cause a phase shift of the electro-magnetic waves and absorption, respectively. PCI visualizes the real part of the refractive index by exploiting the differential phase shift. It is believed to provide additional information on tissue properties compared with conventional absorption based imaging (AI) techniques. We have developed a mathematical formalism for the signal and noise transmission in order to assess the contrast-to-noise ratio per radiation dose. This allows for a direct performance comparison of PCI versus AI. The mathematical model was validated by numerical simulations and compared to experimental data, particularly, with regard to computed tomography (CT). RESULTS 1. PCI shows a fundamentally different noise power spectrum (NPS). AI has a bounded NPS whereas the NPS for PCI diverges at low spatial frequencies. Absolute phase projection data and CT images show increasing statistical fluctuations on growing length scales. The predicted characteristics are confirmed by simulations as well as experimental data. Consequently, the performance of PCI deteriorates with decreasing spatial resolution. Break-even is achieved at a spatial resolution of about 40 LP/cm for a typical system geometry with 20% visibility and fat/water contrast at 60 keV photon energy. 2. Due to the measurement of periodic quantities (phase), the distribution of expectation values degenerates into an equipartition at low dose where information propagation collapses. Simulations and experimental findings are in agreement with this result. This implies a minimally required radiation dose which is particularly relevant for CT due to the need for acquiring multiple projections. In contrast, AI does not collapse at low dose. CONCLUSION To achieve a benefit of PCI versus AI a minimum spatial resolution is necessary. This implies increased radiation dose if the resolution to break even is higher than that of a specific diagnostic application today. On the other hand, PCI cannot be performed at arbitrarily low dose as a matter of principle. CLINICAL RELEVANCE/APPLICATION The presented work allows for evaluating the potential of PCI in terms of radiation dose reduction compared with AI, but also demonstrates its limitations.