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Abstract
To objectively characterize the performance of the gemstone spectral imaging (GSI) mode of GE CT750 HD scanner from a user's perspective.
A regular scan protocol that approximates the adult abdomen scan protocol frequently used in the authors' institute was selected as the baseline, and a GSI protocol (preset 11) that is similar to the regular protocol and has a moderate dose level (CTDI(vol) = 26.27 mGy) was compared to the baseline protocol. The resolving power of both protocols was characterized in terms of modulation transfer functions and high contrast resolution bar readings. Their noise characteristics were studied through noise power spectra, and their low contrast detectability was compared via contrast-to-noise ratio. Material decomposition capability of GSI was evaluated by scanning iodine solutions of 9-24 mg/ml iodine concentration in a Gammex CT phantom and by examining the estimated iodine concentration. In addition, a formula describing the dependency of HU in iodine enhanced area on GSI monochromatic energies and iodine concentrations was provided and the theoretical values were compared with the measured results.
The resolutions levels of 50%, 10%, and 5% MTF of GSI monochromatic images at 65 keV agree with those of the regular protocol within 0.1 1p/cm. GSI monochromatic images at 65 keV demonstrated the lowest noise level among GSI images of different monochromatic energies and showed very similar noise magnitude and noise power distribution as compared to the regular protocol images. The CNR of 60 and 65 keV GSI monoimages are approximately 100% of those of the regular protocol images. Estimated iodine concentration levels agreed with the actual values within 2% when the iodine solutions were placed at 3, 9, 12 o'clock positions of the phantom; when iodine solutions were placed at the phantom center and at 6 o'clock position, higher discrepancies of 2%-10% were observed. The observed dependency of HU on keV and iodine concentration levels agreed with the expectation from x-ray attenuations.
Equivalent performances were observed in the comparison between GSI 65 keV monochromatic images and images from a regular abdomen scan protocol. This suggests the possibility of GSI to be employed in routine abdominal scans, which would potentially offer more information through its capabilities of material decomposition.
... Recently, spectral imaging in the X-ray field has moved towards dual-energy CT (DECT) [1][2][3][4][5][6][7][8][9][10][11][12] and multi-energy CT [13,14], both based on the principle of discriminating the attenuation difference of various materials as a function of their energies [8,12,15]. While multi-energy CT remains at the research and development stage, with the potential to one day provide a greater discrimination of the spectrum, DECT is increasingly used in clinical practice [1][2][3][4][5][6][7][8][9][10][11][12]. ...
... Recently, spectral imaging in the X-ray field has moved towards dual-energy CT (DECT) [1][2][3][4][5][6][7][8][9][10][11][12] and multi-energy CT [13,14], both based on the principle of discriminating the attenuation difference of various materials as a function of their energies [8,12,15]. While multi-energy CT remains at the research and development stage, with the potential to one day provide a greater discrimination of the spectrum, DECT is increasingly used in clinical practice [1][2][3][4][5][6][7][8][9][10][11][12]. DECT platforms use both low-and high-energy photon spectra to calculate the photoelectric and Compton scattering effects separately, where the results may vary depending on the platform used [12,15]. ...
... Several studies on phantoms and patients have evaluated the effect of the technical differences of the DECT platforms on spectral separation and performance [4,[20][21][22][23][24][25][26][27]. The accuracy of the mean attenuation or of the iodine concentration has so far only been compared on phantoms [11,21,[24][25][26]. However, these studies did not assess the influence of noise magnitude and noise texture, on the spatial resolution for the different DECT platforms. ...
Objectives
To compare the spectral performance of dual-energy CT (DECT) platforms using task-based image quality assessment based on phantom data.Materials and methodsTwo CT phantoms were scanned on four DECT platforms: fast kV-switching CT (KVSCT), split filter CT (SFCT), dual-source CT (DSCT), and dual-layer CT (DLCT). Acquisitions on each phantom were performed using classical parameters of abdomen-pelvic examination and a CTDIvol at 10 mGy. Noise power spectrum (NPS) and task-based transfer function (TTF) were evaluated from 40 to 140 keV of virtual monoenergetic images. A detectability index (d′) was computed to model the detection task of two contrast-enhanced lesions as function of keV.ResultsThe noise magnitude decreased from 40 to 70 keV for all DECT platforms, and the highest noise magnitude values were found for KVSCT and SFCT and the lowest for DSCT and DLCT. The average NPS spatial frequency shifted towards lower frequencies as the energy level increased for all DECT platforms, smoothing the image texture. TTF values decreased with the increase of keV deteriorating the spatial resolution. For both simulated lesions, higher detectability (d′ value) was obtained at 40 keV for DLCT, DSCT, and SFCT but at 70 keV for KVSCT. The detectability of both simulated lesions was highest for DLCT and DSCT.Conclusion
Highest detectability was found for DLCT for the lowest energy levels. The task-based image quality assessment used for the first time for DECT acquisitions showed the benefit of using low keV for the detection of contrast-enhanced lesions.Key Points
• Detectability of both simulated contrast-enhanced lesions was higher for dual-layer CT for the lowest energy levels.
• The image noise increased and the image texture changed for the lowest energy levels.
• The detectability of both simulated contrast-enhanced lesions was highest at 40 keV for all dual-energy CT platforms except for fast kV-switching platform.
... [12][13][14][15][16] However, the image quality and quantitative accuracy of low-energy VMI typically suffer from significant amplification of noise and beam-hardening artifacts that occur during material decomposition. 17,18 Various denoising methods dedicated for VMI have been reported in the literature. We briefly describe a few examples as follows. ...
... The set of lower-density testing inserts included commercial calcium insert (50 mgCa∕cc), iodine and blood mixture inserts (2 and 4 mgI∕cc), and in-house iodine inserts (3, 6, and 12 mgI∕cc), and measured between 35 and 325 HU across 40 and 50 keV. c The set of higher-density testing inserts included commercial calcium inserts (100 and 300 mgI∕cc) and inhouse iodine inserts (18,22, and 30 mgI∕cc), and measured between 206 and 868 HU across 40 and 50 keV. insert materials and anatomical background. ...
Purpose: We developed a deep learning method to reduce noise and beam-hardening artifact in virtual monoenergetic image (VMI) at low x-ray energy levels. Approach: An encoder-decoder type convolutional neural network was implemented with customized inception modules and in-house-designed training loss (denoted as Incept-net), to directly estimate VMI from multi-energy CT images. Images of an abdomen-sized water phantom with varying insert materials were acquired from a research photon-counting-detector CT. The Incept-net was trained with image patches (
64
×
64
pixels
) extracted from the phantom data, as well as synthesized, random-shaped numerical insert materials. The whole CT images (
512
×
512
pixels
) with the remaining real insert materials that were unseen in network training were used for testing. Seven contrast-enhanced abdominal CT exams were used for preliminary evaluation of Incept-net generalizability over anatomical background. Mean absolute percentage error (MAPE) was used to evaluate CT number accuracy. Results: Compared to commercial VMI software, Incept-net largely suppressed beam-hardening artifact and reduced noise (53%) in phantom study. Incept-net presented comparable CT number accuracy at higher-density (
P
-value [0.0625, 0.999]) and improved it at lower-density inserts (
P
-
value
=
0.0313
) with overall MAPE: Incept-net [2.9%, 4.6%]; commercial-VMI [6.7%, 10.9%]. In patient images, Incept-net suppressed beam-hardening artifact and reduced noise (up to 50%,
P
-
value
=
0.0156
). Conclusion: In this preliminary study, Incept-net presented the potential to improve low-energy VMI quality.
... Существует предположение, что ДЭКТ сопровождается облучением в высокой дозе, однако ее величина варьирует в зависимости от применения различных технологий (сканер с переключением киловольтажа и детекторы с двойным слоем различаются по данной характеристике, при этом первые дают более низкую дозу облучения), протоколов, а также времени сканирования. В ходе нескольких исследований было показано, что двухисточниковая ДЭКТ подвергает пациентов такой же лучевой нагрузке, что и рутинная МЭКТ [34,35], а более поздние исследования показали, что лучевая нагрузка при проведении одноисточниковой ДЭКТ с переключением тока на трубке также не превышает дозу при МЭКТ [36]. В целом поглощенная доза рентгеновского излучения при проведении ДЭКТ не превышает значений, рекомендованных Американской коллегией радиологов, и примерно на 20% выше дозы, получаемой при рутинной КТ для данного региона тела [35]. ...
An increase in left ventricular (LV) extracellular volume (ECV) is an important parameter of LV morphology and is considered synonymous with myocardial fibrosis, as well as a reliable marker of myocardial injury and impaired cardiac function. Accurate methods for detecting and assessing myocardial fibrosis are very important for clinical practice. The current standard for myocardial fibrosis imaging is delayed gadolinium enhanced cardiac magnetic resonance imaging (MRI) or T1 mapping, but these techniques have limitations. They can be avoided by using dual-energy computed tomography (DECT), which makes it possible to identify myocardial fibrosis, including small-focal fibrosis, in two different ways (subtraction technique and iodine density measurement technique). The literature analysis carried out by the authors showed good comparability of MRI and DECT results in determining ECV in patients with various heart diseases of both ischemic and non-ischemic nature, including cardiomyopathies, aortic stenosis, pulmonary hypertension, sarcoidosis, and amyloidosis. In addition, the use of DECT to identify myocardial fibrosis is also possible if cardiac inflammation is suspected. In addition to evaluating the effectiveness of DECT compared with MRI, different scanning protocols were analyzed, since there is currently no consensus on the optimal contrast administration regimen. The issue of radiation exposure in modern DECT scanners is also separately considered. The authors showed that DECT is an important tool for determining ECV, which is of interest for clinical practice.
... The iodine concentration on non-enhanced CT was ideally regarded as almost 0 µg/cm 3 because there was no measurable iodine. A monochromatic energy projection can be synthesized by the weighted sum of material density projections using their corresponding mass attenuation coefficients at a given energy as the weighting factors, resulting in providing the estimated material densities [23,24]. The actual iodine value in non-enhanced CT is not always zero due to the point where the iodine-water material decomposition is based on the binarization concept of iodine or water, and mainly due to the effect of image noise. ...
Background: Dual-energy CT has been reported to be useful for differentiating thymic epithelial tumors. The purpose is to evaluate thymic epithelial tumors by using three-dimensional (3D) iodine density histogram texture analysis on dual-energy CT and to investigate the association of extracellular volume fraction (ECV) with the fibrosis of thymic carcinoma. Methods: 42 patients with low-risk thymoma (n = 20), high-risk thymoma (n = 16), and thymic carcinoma (n = 6) were scanned by dual-energy CT. 3D iodine density histogram texture analysis was performed for each nodule on iodine density mapping: Seven texture features (max, min, median, average, standard deviation [SD], skewness, and kurtosis) were obtained. The iodine effect (average on DECT180s—average on unenhanced DECT) and ECV on DECT180s were measured. Tissue fibrosis was subjectively rated by one pathologist on a three-point grade. These quantitative data obtained by examining associations with thymic carcinoma and high-risk thymoma were analyzed with univariate and multivariate logistic regression models (LRMs). The area under the curve (AUC) was calculated by the receiver operating characteristic curves. p values < 0.05 were significant. Results: The multivariate LRM showed that ECV > 21.47% in DECT180s could predict thymic carcinoma (odds ratio [OR], 11.4; 95% confidence interval [CI], 1.18–109; p = 0.035). Diagnostic performance was as follows: Sensitivity, 83.3%; specificity, 69.4%; AUC, 0.76. In high-risk thymoma vs. low-risk thymoma, the multivariate LRM showed that the iodine effect ≤1.31 mg/cc could predict high-risk thymoma (OR, 7; 95% CI, 1.02–39.1; p = 0.027). Diagnostic performance was as follows: Sensitivity, 87.5%; specificity, 50%; AUC, 0.69. Tissue fibrosis significantly correlated with thymic carcinoma (p = 0.026). Conclusions: ECV on DECT180s related to fibrosis may predict thymic carcinoma from thymic epithelial tumors, and the iodine effect on DECT180s may predict high-risk thymoma from thymoma.
... Cui et al. showed that 70 keV images yielded greater CNR compared to the single energy polychromatic images while Martin et al. showed that among the VMI series 70 keV images had the highest CNR [11,14]. It has also been noted previously that the image noise on monochromatic images was lowest at 60 keV and 70 keV within the range 40-140 keV and lower than that of conventional 120 kVp CT [21,22]. He et al. showed that distal branches of the superior mesenteric artery are better demonstrated with better image quality at 50 keV VM images [23]. ...
Purpose
To evaluate the utility of dual energy CT angiography (DECTA) in acute non-variceal gastrointestinal hemorrhage (ANVGIH) compared to digital subtraction angiography (DSA) as gold standard.
Materials and methods
111 Patients (mean age: 39.2 years; 94 males) of ANVGIH who underwent both DECTA and DSA between January 2016 and September 2021 were included. Virtual monochromatic (VM) images at 10 keV increments from 40 to 70 keV and blended (120kVp equivalent) images of arterial phase of DECTA were evaluated independently by two readers blinded to DSA information. Quantitative analysis included measurement of attenuation in the major arteries (abdominal aorta, celiac artery, superior mesenteric artery), suspected vascular lesion, and lesion feeding artery to calculate contrast-to-noise ratios (CNRs) and signal-to-noise ratios (SNRs). Qualitative analysis assessed the image quality of each data set using a 3-point Likert scale. Findings on DSA were evaluated by a third reader and both DECTA and DSA were compared.
Results
On linear blended images, vascular lesion was identified by reader 1 in 88 (79.3%) and by reader 2 in 87 (78.4%) patients and DSA showed lesion in 92 (82.9%) patients. The sensitivity and specificity of blended images and VM images of DECTA for lesion detection were not significantly different from each other. The CNR and SNR of arteries, vascular lesion and feeding artery were significantly higher at 70 keV (p < 0.005) compared to blended and other VM images. Although subjective scores for image quality were higher for 60 keV images by both readers, the difference was not statistically significant (p = 0.3). The interobserver agreement was mostly good.
Conclusion
In the assessment of ANVGIH, the 60 keV and 70 keV VM images improved the image quality and contrast, respectively, but there was no increase in diagnostic accuracy of VM image datasets compared to linearly blended images. Hence, the diagnostic utility of DECTA in ANVGIH is still uncertain.
Graphical abstract
... One can scan the same object with two distinct x-ray spectra in this category. In practice, this can be accomplished in a variety of ways, such as the dual-source/dualdetector setups, 6-8 the fast-kV-switching technique, [9][10][11] in addition to Hounsfield's original proposal to scan the image object twice at different tube potentials. Siemens also released a split-beam technique 12,13 that applies two different beam filters along the longitudinal direction to split the beam into two halves with different mean energies. ...
Background:
Single-kV CT imaging is one of the primary imaging methods in radiology practices. However, it does not provide material basis images for some subtle lesion characterization tasks in clinical diagnosis.
Purpose:
To develop a quality-checked and physics-constrained deep learning (DL) method to estimate material basis images from single-kV CT data without resorting to dual-energy CT acquisition schemes.
Methods:
Single-kV CT images are decomposed into two material basis images using a deep neural network. The role of this network is to generate a feature space with 64 template features with the same matrix dimensions of the input single-kV CT image. These 64 template image features are then combined to generate the desired material basis images with different sets of combination coefficients, one for each material basis image. Dual-energy CT image acquisitions with two separate kVs were curated to generate paired training data between a single-kV CT image and the corresponding two material basis images. To ensure the obtained two material basis images are consistent with the encoded spectral information in the actual projection data, two physics constraints, i.e., (1) effective energy of each measured projection datum that characterizes the beam hardening in data acquisitions and (2) physical factors of scanners such as detector and tube characteristics, are incorporated into the end-to-end training. The entire architecture is referred to as Deep-En-Chroma in this paper. In the application stage, the generated material basis images are sent to a deep quality check (Deep-QC) network to assess the quality of estimated images and to report the pixel-wise estimation errors for users. The models were developed using 5,592 training and validation pairs generated from 48 clinical cases. Additional 1,526 CT images from another 13 patients were used to evaluate the quantitative accuracy of water and iodine basis images estimated by Deep-En-Chroma.
Results:
For the iodine basis images estimated by Deep-En-Chroma, the mean difference with respect to dual-energy CT is -0.25 mg/mL, and the agreement limits are [-0.75 mg/mL, +0.24 mg/mL]. For the water basis images estimated by Deep-En-Chroma, the mean difference with respect to dual-energy CT is 0.0 g/mL, and the agreement limits are [-0.01 g/mL, 0.01 g/mL]. Across the test cohort, the median [25th, 75th percentiles] root mean square errors between the Deep-En-Chroma and dual-energy material images are 14 [12, 16] mg/ml for the water images and 0.73 [0.64, 0.80] mg/ml for the iodine images. When significant errors are present in the estimated material basis images, Deep-QC can capture these errors and provide pixel-wise error maps to inform users whether the DL results are trustworthy.
Conclusions:
The Deep-En-Chroma network provides a new pathway to estimating the clinically relevant material basis images from single-kV CT data and the Deep-QC module to inform end-users of the accuracy of the DL material basis images in practice. This article is protected by copyright. All rights reserved.
... [93][94][95][96] Virtual monoenergetic images are used to increase iodine contrast-to-noise ratio (CNR) using low photon energy levels or reduce metal artifacts and calcium blooming with high photon energy levels. 95,[97][98][99] In VMIs, the CT number of a material can be computed exactly for a specified photon energy level (keV) (Fig. 5), ideally making the CT number of a material independent of scanner type, tube potential, and beam filtration FIGURE 6. CT number dependence on tube potential for iodine (I, 2 and 10 mg/mL) and hydroxyapatite (HA, 50 mg/mL). Dimmer bars are from the low-energy threshold image, and brighter bars are from virtual monoenergetic images (VMIs). ...
Computed tomography (CT) images display anatomic structures across 3 dimensions and are highly quantitative; they are the reference standard for 3-dimensional geometric measurements and are used for 3-dimensional printing of anatomic models and custom implants, as well as for radiation therapy treatment planning. The pixel intensity in CT images represents the linear x-ray attenuation coefficient of the imaged materials after linearly scaling the coefficients into a quantity known as CT numbers that is conveyed in Hounsfield units. When measured with the same scanner model, acquisition, and reconstruction parameters, the mean CT number of a material is highly reproducible, and quantitative applications of CT scanning that rely on the measured CT number, such as for assessing bone mineral density or coronary artery calcification, are well established. However, the strong dependence of CT numbers on x-ray beam spectra limits quantitative applications and standardization from achieving robust widespread success. This article reviews several quantitative applications of CT and the challenges they face, and describes the benefits brought by photon-counting detector (PCD) CT technology. The discussed benefits of PCD-CT include that it is inherently multienergy, expands material decomposition capabilities, and improves spatial resolution and geometric quantification. Further, the utility of virtual monoenergetic images to standardize CT numbers is discussed, as virtual monoenergetic images can be the default image type in PCD-CT due to the full-time spectral nature of the technology.
... Recent advances in CT technology and contrast agents (CAs) in clinical and preclinical cardiac imaging have facilitated the development of functional imaging (7). Functional CT can now be achieved through the combination of electrocardiogram gating, allowing a comprehensive assessment of global and regional myocardial function, perfusion, and coronary angiography (8). CT can not only observe the cardiac structure but also accurately report on the vascular condition. ...
Background:
This study investigated the correlation between cardiac function parameters by cardiac computed tomography (CT) and the clinical outcomes of heart failure patients with preserved ejection fraction (HFpEF) to provide experimental data for the diagnosis of HFpEF.
Methods:
A total of 157 HFpEF patients admitted to our hospital from January 2017 to January 2019 were retrospectively analyzed. The patients were divided into event and non-event groups according to the occurrence or absence of adverse events. Cardiac function parameters, such as the left ventricular (LV) end-diastolic volume (LVEDV) and LV end-diastolic volume index (LVEDVI), were obtained via CT scan. Also, the N-terminal-pro hormone b-type natriuretic peptide (NT-proBNP) levels in patients' serum were measured using an enzyme linked immunosorbent assay (ELISA) kit, and echocardiographic parameters such as LV posterior wall thickness (LVPWT) were also recorded. Further, Cox regression was employed to analyze factors associated with the clinical outcomes.
Results:
Compared with patients in the non-event group, the left ventricular end-diastolic mass (LVM), LVEDVI, left ventricular end-systolic volume index (LVESVI), left atrial end-diastolic volume index (LAEDVI), and left atrial end-systolic volume index (LAESVI) were significantly increased, and the left ventricular total emptying fraction (LVTEF) and left atrial total emptying fraction (LATEF) were markedly decreased in the event group patients. Also, the E/e' and LAEDVI were related factors affecting the clinical outcomes of HFpEF patients. The above indicators displayed a significant predictive for the clinical outcomes of HFpEF patients.
Conclusions:
Several cardiac function measures, including LAEDVI, are factors associated with the clinical outcomes of HFpEF patients.
... More recently, other materials with better performance have been introduced 43 . For example, the garnet crystal (Gemstone Detector, GE Healthcare, Milwaukee, Wisconsin), has a primary decay time of 30 ns (100 times faster than Gd oxysulfide) and 25% of the afterglow of Gd oxysulfide; this material is fundamental for spectral imaging with fast kV switching (Figure 2 A-B) [44][45][46] . Other materials are available for spectral imaging 47 . ...
Many technological advances have entered the clinical routine of Computed Tomography (CT) imaging. The new CT scanners have specific solutions in gantry design to bear the mechanical solicitations. The X-ray tubes have been improved for faster acquisitions at low radiation exposure, while the innovations in CT detectors provide a better image quality. The optimization of image quality and contrast, and the reduction of radiation dose, cannot be achieved without the implementation of adequate reconstruction software, such as Iterative Reconstructions (IR) and Artificial Intelligence (AI). In recent years, dual-energy (DECT) technology has expanded the indications of CT.
In this narrative review, a panoramic overview of the technological novelties in CT imaging will be provided for optimal utilization of CT technology.
... These can evaluate the difference between the HU value measured in a given insert and its theoretical value. These two metrics are also used to assess the accuracy of the iodine concentration on iodine maps [61,[66][67][68][69][70][71]74,75]. The higher the accuracy of the HU and iodine, the better the spectral performance and the more relevant the clinical results. ...
Spectral computed tomography (CT) imaging encompasses a unique generation of CT systems based on a simple principle that makes use of the energy-dependent information present in CT images. Over the past two decades this principle has been expanded with the introduction of dual-energy CT systems. The first generation of spectral CT systems, represented either by dual-source or dual-layer technology, opened up a new imaging approach in the radiology community with their ability to overcome the limitations of tissue characterization encountered with conventional CT. Its expansion worldwide can also be considered as an important leverage for the recent groundbreaking technology based on a new chain of detection available on photon counting CT systems, which holds great promise for extending CT towards multi-energy CT imaging. The purpose of this article was to detail the basic principles and techniques of spectral CT with a particular emphasis on the newest technical developments of dual-energy and multi-energy CT systems.
... Therefore, NIC may be used as a surrogate imaging marker for assessing angiogenesis in breast cancer (21) . Studies have shown that there is no statistical significance between the measured iodine content and the true value (22) . It has also been reported that the two-material decomposition technique can obtain accurate iodine content measurements (23) . ...
... For e.g., Low energy virtual monochromatic image are used to maximize the visualization of iodinated contrast and improve the contrast to noise ratio. VMC images created at 50 keV have been reported to demonstrate peak contrast to noise ratio (10). On the other hand, high-energy > 90 keV reconstruction images minimizes artifact from metallic hardware (11,12). ...
Dual energy CT (DECT) refers to the acquisition of CT images at two energy spectra and can provide information about tissue composition beyond that obtainable by conventional CT. The attenuation of a photon beam varies depends on the atomic number and density of the attenuating material and the energy of the incoming photon beam. This differential attenuation of the beam at varying energy levels forms the basis of DECT imaging and enables separation of materials with different atomic numbers but similar CT attenuation. DECT can be used to detect and quantify materials like iodine, calcium, or uric acid. Several post-processing techniques are available to generate virtual non-contrast images, iodine maps, virtual mono-chromatic images, Mixed or weighted images and material specific images. Although initially the concept of dual energy CT was introduced in 1970, it is only over the past two decades that it has been extensively used in clinical practice owing to advances in CT hardware and post-processing capabilities. There are numerous applications of DECT in Emergency radiology including stroke imaging to differentiate intracranial hemorrhage and contrast staining, diagnosis of pulmonary embolism, characterization of incidentally detected renal and adrenal lesions, to reduce beam and metal hardening artifacts, in identification of uric acid renal stones and in the diagnosis of gout. This review article aims to provide the emergency radiologist with an overview of the physics and basic principles of dual energy CT. In addition, we discuss the types of DECT acquisition and post processing techniques including newer advances such as photon-counting CT followed by a brief discussion on the applications of DECT in Emergency radiology.
... The material decomposition technique in dual-energy spectral CT is based on the fact that any substance can be expressed by the combination of other two basic materials in a corresponding proportion [19] . The measured value of the basis material pair can re ect the relative composition of materials in the tissue to make a relatively quantitative description of the relevant indicators [20] . Dual-energy CT has been proposed as an alternative for 3D volumetric assessment of bone mineral density [21] . ...
Background.
To explore the feasibility of using hydroxyapatite (HAP) measurement based on the material decomposition in dual-energy spectral CT imaging to evaluate bone mineral density (BMD).
Methods.
247 patients (aged 16 to 97 years, 156 males and 91 females) who underwent both unenhanced and contrast-enhanced (arterial-, venous- and delayed-) phase abdominal dual-energy spectral CT were retrospectively collected. Patients were divided into four groups according to their ages: group A (<30years, n=40), group B (30-49years, n=90), group C (50-69years, n=73) and group D (≥70years, n=44). HAP densities of trabecular bone of the third lumbar vertebrae in four phases were measured on the HAP material decomposition images using HAP-Iodine as the basis material pair and recorded. HAP as function of age was established and measurements in four phases were statistically compared using that in unenhanced phase as the reference standard.
Results.
No statistical difference was found in HAP value among different imaging phases (P>0.05). The HAP value was positively correlated with age in group A (r=0.393, P<0.05) and negatively correlated with age in groups B, C and D (r=-0.298, -0.361, -0.361, P<0.05), and overall high negative correlation with age for patients ≥30years (r=-0.775, P<0.05).
Conclusions.
The HAP measurement is not affected by contrast agent and stable in different imaging phases in dual-energy spectral CT, enabling a phase-independent HAP measurement. HAP has correlation with age which may be used to reflect the change of vertebral BMD, providing a reference for evaluating BMD according to the linear regression equation.
... These spectra are used by material decomposition algorithms to characterize tissues according to two or three basic materials including those with a low atomic number (e.g., water) and those with a high atomic number (e.g., iodine) [12]. Several DECT platforms have been developed and are now commercially available [13][14][15][16]. These platforms have different spectral performances as reported in recent studies [17][18][19]. ...
Purpose
The purpose of this study was to assess the impact of advanced modeled iterative reconstruction (ADMIRE) algorithm and dose levels on the accuracy of Hounsfield unit (HU) measurement, image noise and contrast-to-noise ratio (CNR) in virtual monochromatic images (VMIs) with low iodine concentrations, and the accuracy of iodine quantification.
Materials and methods
A CT phantom was scanned with dual-source CT using abdomen-pelvis examination parameters at four dose levels: 5, 8, 11 and 20 mGy. Images were reconstructed using filtered-back projection (FBP) and ADMIRE levels 3 and 5 (A3-A5). HU accuracy was assessed calculating the root-mean-square deviation (RMSDHU). Image noise and CNR were computed on VMIs at 40/50/60/70 keV for 4 iodine inserts with 0.5, 1, 2 and 5 mg/mL concentrations. Accuracy of iodine quantification was assessed by the RMSDiodine and iodine bias (IB).
Results
The RMSDHU decreased significantly as the dose levels increased compared to 5 mGy, except for 8 mGy with A3 (P = 0.380) and with A5 level (P = 0.945). Noise increased by 63.0 ± 3.0 (standard deviation [SD])% from 20 mGy to 5 mGy. Noise decreased significantly by -53.8 ± 0.9 (SD) % with A5 compared to FBP. The CNR decreased by -43.1 ± 6.5 (SD)% from 20 mGy to 5 mGy. It increased using ADMIRE, and as the ADMIRE levels increased. The RMSDiodine and IB decreased as the dose level increased, and this was similar for all reconstruction types.
Conclusion
ADMIRE strongly improves image quality in VMIs and slightly improves HU accuracy but does not affect the accuracy of iodine quantification.
... A novel kVp-switching dual-energy spectral CT (DESCT) imaging mode functioning in a single rotation by completing instantaneous switching between low (80 kVp) and high (140 kVp) voltages to collect data was introduced in 2008 [4]. DESCT can produce virtual monochromatic spectral images at different energy levels, spectral curves, effective atomic numbers, and material decomposition images, thereby improving its ability to differentiate the material compositions [5][6][7][8][9][10][11][12]. ...
Background:
Necrotic pulmonary lesions manifest as relatively low-density internally on contrast-enhanced computed tomography (CT). However, using CT to differentiate malignant and benign necrotic pulmonary lesions is challenging, as these lesions have similar peripheral enhancement. With the introduction of dual-energy spectral CT (DESCT), more quantitative parameters can be obtained and the ability to differentiate material compositions has been highly promoted. This study investigated the use of kVp-switching DESCT in differentiating malignant from benign necrotic lung lesions.
Methods:
From October 2016 to February 2019, 40 patients with necrotic lung cancer (NLC) and 31 with necrotic pulmonary mass-like inflammatory lesion (NPMIL) were enrolled and underwent DESCT. The clinical characteristics of patients, CT morphological features, and DESCT quantitative parameters of lesions were compared between the two groups. Binary logistic regression analysis was performed to identify the independent prognostic factors differentiating NPMIL from NLC. Receiver operating characteristic (ROC) curves were used to assess the diagnostic performance of single-parameter and multiparametric analyses.
Results:
Significant differences in age, C-reactive protein concentration, the slope of the spectral curve from 40 to 65 keV (K40-65 keV) of necrosis in non-contrast-enhanced scanning (NCS), arterial phase (AP) and venous phase (VP), effective atomic number of necrosis in NCS, and iodine concentration (IC) of the solid component in VP were observed between groups (all p < 0.05). The aforementioned parameters had area under the ROC curve (AUC) values of 0.747, 0.691, 0.841, 0.641, 0.660, 0.828, and 0.754, respectively, for distinguishing between NLC and NPMIL. Multiparametric analysis showed that age, K40-65 keV of necrosis in NCS, and IC of the solid component in VP were the most effective factors for differentiating NLC from NPMIL, with an AUC of 0.966 and percentage of correct class of 88.7%.
Conclusions:
DESCT can differentiate malignant from benign necrotic lung lesions with a relatively high accuracy.
... In 2014, Hansen found that after chemotherapy for GC, the tumor volume and surface permeability value (an indicator of tumor local vascular permeability) in CT (computed tomography) perfusion parameters both decreased significantly (96). Moreover, energy spectral CT can accurately reflect the true iodine concentration (97), and iodine concentration can accurately reflect the blood supply and vascular conditions of lesions (98), so the energy spectrum CT is also used to evaluate the vascularization after NACT for GC. In 2005, Tang's study showed that there was a significant difference in iodine concentration in the arteriovenous phase before and after NACT, and the change of iodine concentration was significantly correlated with tumor regression grade (99). ...
The overall efficacy of neoadjuvant chemoradiotherapy (NACT) for locally advanced gastric cancer (LAGC) has been recognized. However, the response rate of NACT is limited due to tumor heterogeneity. For patients who are resistant to NACT, not only the operation timing will be postponed, patients will also suffer from the side effects of it. Thus, it is important to develop a comprehensive strategy and screen out patients who may be sensitive to NACT. This article summarizes the related research progress on the sensitivity prediction of NACT for GC in the following aspects: microRNAs, metabolic enzymes, exosomes, other biomarkers; inflammatory indicators, and imageological assessments. The results showed that there were many studies on biomarkers, but no unified conclusion has been drawn. The inflammatory indicators are related to the survival and prognosis of patients under NACT. For imageological assessments such as CT, MRI, and PET, with careful integration and optimization, they will have unique advantages in early screening for patients who are sensitive to NACT.
... Although perfusion CT had previously been used to measure the decrease in tumor blood ow, dual-energy CT has recently allowed easy measurement of tumor blood ow. Studies have shown that gemstone spectral image analysis using dual-energy CT can measure material density [11,12]. Moreover, dual-energy CT images generated from contrast-enhanced CT using an iodine contrast medium and measurement of tumor iodine density enable the evaluation of tumor blood volume. ...
Lung cancer with low average iodine density measured via contrast-enhanced computed tomography (CT) using dual-energy CT technology has shown a reduced local control rate after stereotactic body radiotherapy (SBRT). The current study therefore investigated the relationship between low iodine density tumor area and its ratio and local recurrence after SBRT. Dual-energy CT was performed on the day before SBRT initiation, with a low iodine density tumor area being defined as that with an iodine density of <1.81 mg cm-3. The low iodine density tumor area, the ratio between the low iodine density tumor area and the entire tumor, and the local recurrence rate were then determined. No correlation was observed between the low iodine density tumor area and the local recurrence rate. However, tumors with a large low iodine density tumor area ratio showed an increased local recurrence rate, with the prognostic accuracy almost similar to that in previous studies using average iodine densities. Our results therefore suggest that the low iodine density tumor area ratio was a useful prognostic index after SBRT, with an accuracy comparable with that of the average iodine density.
... The details of the acquisition technique of the CE-4D-DECT was presented in our previous study [10], and all CT images were acquired using a DECT scanner (Revolution HD, GE Medical Systems, Waukesha, WI) [11]. Briefly, the images of center of PVTT were acquired after 40-80 s from the injection, and the scan delay was changed depending on the location of the tumor, patient's breathing cycle. ...
To assess the objective and subjective image quality, and respiratory motion of hepatocellular carcinoma with portal vein tumor thrombosis (PVTT) using the contrast-enhanced four-dimensional dual-energy computed tomography (CE-4D-DECT). For twelve patients, the virtual monochromatic image (VMI) derived from the CE-4D-DECT with the highest contrast to noise ratio (CNR) was determined as the optimal VMI (O-VMI). To assess the objective and subjective image quality, the CNR and five-point score of the O-VMI were compared to those of the standard VMI at 77 keV (S-VMI). The respiratory motion of the PVTT and diaphragm was measured based on the exhale and inhale phase images. The VMI at 60 keV yielded the highest CNR (4.8 ± 1.4) which was significantly higher ( p = 0.02) than that in the S-VMI (3.8 ± 1.2). The overall image quality (4.0 ± 0.6 vs 3.1 ± 0.5) and tumor conspicuity (3.8 ± 0.8 vs 2.8 ± 0.6) of the O-VMI determined by three radiation oncologists was significantly higher ( p < 0.01) than that of the S-VMI. The diaphragm motion in the L-R (3.3 ± 2.5 vs 1.2 ± 1.1 mm), A-P (6.7 ± 4.0 vs 1.6 ± 1.3mm) and 3D (8.8 ± 3.5 vs 13.1 ± 4.9 mm) directions were significantly larger ( p < 0.05) compared to the tumor motion. The improvement of objective and subjective image quality was achieved in the O-VMI. Because the respiratory motion of the diaphragm was larger than that of the PVTT, we need to be pay attention for localizing target in radiotherapy.
... An alternative to the dual-source implementation of incidence-based dual-energy CT is to rapidly switch between exposure settings [62,63]. This technology was introduced commercially in 2008 by General Electric (GE) and is denoted fast kV switching [23,64]. In the GE system, the x-ray tube voltage is switched, typically between 80 and 140 kV, several thousand times per second in order to acquire images at high-and low-energy spectra through virtually identical ray paths (c.f. Figure 2 b)). ...
Spectral imaging is an umbrella term for energy-resolved x-ray imaging in medicine. The technique makes use of the energy dependence of x-ray attenuation to either increase the contrast-to-noise ratio, or to provide quantitative image data and reduce image artefacts by so-called material decomposition. Spectral imaging is not new, but has gained interest in recent years because of rapidly increasing availability of spectral and dual-energy CT and the dawn of energy-resolved photon-counting detectors. This review examines the current technological status of spectral and dual-energy imaging and a number of practical applications of the technology.
... Reviews on clinical applications of dual-energy CT may be found in ( [9,10,11,12,13,14,15]). Dual-energy CT data can currently be acquired with dual source CT systems ( [5,16]), CT systems with fast kV switching [17] or dual-layer detector CT systems [18]. However, each of these solutions has inherent limitations. ...
Photon-counting detectors are a promising new technology for computed tomography (CT) systems. They provide energy-resolved CT data at very high spatial resolution without electronic noise and with improved tissue contrasts. This review article gives an overview of the principles of photon-counting detector CT, of potential clinical benefits and limitations, and of the experience gained so far in pre-clinical installations.
... Dual-energy (DECT) has been used in children for the diagnosis of PE, which can provide functional information for PE and identify pulmonary perfusion defects [13]. In addition, the dual-energy spectral CT imaging can re ect the true iodine concentration accurately and objectively [14], which was introduced for the diagnosis of PE in adult by quantitative depicting pulmonary blood ow and perfusion defects on Iodine-based material decomposition images [15], but there is not report on the application of dual-energy spectral in children for diagnosis of pulmonary embolism. In our study, clots were found intra consolidations and extra consolidations. ...
Background: Pulmonary embolism (PE) associated with Mycoplasma pneumoniae pneumonia (MPP) in children has already attracted more attention. We aimed to evaluate the application of dual-energy spectral CT in diagnosing PE in children with MPP.
Methods: Eight-three children with MPP and highly suspected PE, underwent CT pulmonary angiography (CTPA) with spectral imaging mode. Noise, clot-to-artery contrast-to-noise ratio, image quality and diagnosis confidence were calculated and assessed on nine monochromatic image sets (40 to 80keV). CTPA images were observed for the presence, localization, and embolic degrees of PE. Emboli were divided between intra and extra-consolidation. For extra-consolidation clots, iodine concentration (IC) of perfusion defects and normal lung, perfusion defects of 4 children before and after the treatment were measured and compared. For intra-consolidation clots, IC of consolidation areas with clots and consolidation areas without clot were measured and compared.
Results: The optimal energy level for detecting PE in children was 55 keV. 116 clots (29 extra consolidations) were found in 25 children, IC of defect regions associated with PE was 0.69±0.28mg/mL (extra-consolidations) and 0.90±0.23mg/mL (intra-consolidations), both significantly lower than the 2.76±0.45mg/mL in normal lungs and 10.25±1.76mg/mL in consolidations without clots (P<0.001). Significant difference was found in the presence or absence of perfusion defects between occlusive clots and nonocclusive clots(P<0.001). IC of the perfusion defects significantly increased after treatment (P<0.001).
Conclusions: In spectral CTPA 55 keV images optimize PE detection for children. and MD images quantify pulmonary blood flow of PE, and may help to detect small clots and quantify embolic degrees.
... In our study, VMI at a high-energy level like 110 keV did not make a significant difference for the assessment of bowel walls of CD patients. It might be attributed to the progressively increased noise at a rate that is inversely proportional to the energy separation between the low-and high-energy spectra [18,20,27]. This is also observed at VMI at energy levels lower than 60 keV; however, VMI at lower energy levels might contribute more effectively to the diagnosis of mural enhancement compared to VMI at higher energy levels. ...
Purpose:
To assess the use of virtual monochromatic images (VMI) for discrimination of affected and non-affected bowel walls in patients with Crohn's disease (CD) as well as to compare mural enhancement between patients with and without CD.
Materials and methods:
This retrospective study included 61 patients (47 with CD, 14 without CD). Attenuation value (AV), signal-to noise ratio (SNR), and contrast-to-noise ratio (CNR) were obtained at VMI energy levels from 40 to 110 keV in 10 keV increment. Analyses were performed among affected and non-affected bowel walls in CD patients, as well as from bowel walls in patients without CD. Image quality and mural enhancement were evaluated at VMI energy levels at 40, 70, and 110 keV.
Results:
At all energy levels of VMI, each quantitative data for AV, SNR, and CNR showed statistically significant difference between diseased and non-diseased bowel walls in CD patients. In the quantitative assessment of patients with and without CD, the optimal AV and SNR were obtained at 40 keV, and the optimal CNR was obtained at 70 keV. For the qualitative assessment, the best image quality and mural enhancement were obtained at 70 keV and 40 keV, respectively.
Conclusion:
VMI are helpful for the differentiation of affected bowel walls in CD patients, providing high diagnostic accuracy.
... As one of dual-energy spectral CT imaging techniques [5], GSI was based on rapid switching between high and low voltage (80kVp and 140kVp) during a single rotation of the tube. It can acquire multiple parameters, such as monochromatic spectral images at energy levels ranging from 40 to 140 keV, material decomposition images, effective atomic number and spectral HU curve [6,7]. Specially, iodine concentration measured on iodine-enhanced images, which is considered to be comparable to the real value of enhancement, could re ect the blood supply of pulmonary lesions, and is essential for differential diagnosis [8,9]. ...
Background: There were still nonnegligible overlaps between lung inflammatory and malignant lesions in morphologic appearance and enhancement pattern using conventional computed tomography (CT) especially when the lesion size was smaller than 3cm, which made misdiagnosis often happen in clinical practice. The study was to verify whether spectral CT parameters can differentiate solitary lung cancer from solitary inflammatory lesions or not, and to find the correlation between GSI parameters and lesion size.
Methods: 102 patients with solitary inflammatory or lung cancer lesions underwent enhanced CT scans with GSI mode. While total of 78 ROIs (region of interest) were finally collected and divided into inflammatory group (group I: ≤3cm, IA; >3cm, IB) and cancer group (group II:≤3cm, IIA; >3cm, IIB) according to their maximum diameter (Dmax). CT values measured on 40keV and 70 keV monochromatic images (HU40kev , HU70kev), effective atomic number (Zeff) , iodine concentration (IC), normalized IC (NIC) and slopes of spectral curve (λ30, λ40) were calculated. Intergroup and intragroup comparisons were made using Mann-Whitney U test, and receiver operating characteristic curve (ROC) analysis were done. Correlation analysis was used to find the relationship between Dmax and GSI parameters.
Results: NIC and λ30 under venous phase could distinguish general solitary inflammatory from cancer group. No significant correlation was found between GSI parameters and Dmax in inflammatory group, while negative correlations were found between them in cancer group. The GSI parameters (except HU70kev) of group IIA were significantly higher than those of group IIB. When Dmax was larger than 3cm, there were significant differences between solitary inflammatory group and cancer group in HU40kev, IC, NIC, λ30 and λ40, under both AP and VP, while the AUC for λ30 under VP was largest, and the corresponding sensitivity and specificity were 96.32% and 85.71% respectively. However, only the HU40kev and HU70kev under arterial phase of IIA were significantly higher than those of IA.
Conclusions: Most of quantitative parameters of GSI can contribute to the differentiation between solitary lung cancer from solitary inflammatory lesions when Dmax is larger than 3cm, while only few parameters are meaningful when Dmax is smaller than 3cm .
In 1970, the use of a linear system of equations in the form of algebraic reconstruction technique (ART) was an initial step to be used as an iterative reconstruction technique in the domain of computer tomography (CT). The method used by Gordon and Herman was popularly known as the Kaczmarz method in numerical linear algebra. There are many advantages of such an iterative reconstruction technique, however ART never took off for clinical trials due to a lack of computational power. Hence, the filtered back projection (FBP) method enjoyed as THE most popular and commercially used CT image reconstruction algorithm for decades. In 2009, the first iterative algorithm popularly known as the iterative algorithm in image space (IRIS) got FDA approval for commercial use as a CT image reconstruction algorithm. Such an advancement truly boosted researchers and commercial vendors to look for advanced iterative algorithms. As a result, the number of publications in the domain of iterative CT image reconstruction techniques is countless. Recent advancements in artificial intelligence have attracted researchers to explore the possibilities for the development of CT image reconstruction algorithms using the techniques of neural networks or deep learning. In this work, we intend our discussion to understand and explore the developments of major iterative reconstruction algorithms, especially as inverse problems, and optimization-based approaches for CT image reconstruction. We aim to understand what these algorithms have achieved in terms of radiation dose reduction while maintaining the image quality.KeywordsAlgebraic reconstruction technique (ART)Computer tomography (CT)Filtered back projection (FBP)Compressive sensingOrthogonal matching pursuit (OMP)Restricted isometry property (RIP)Deep learningMathematics Subject Classification (2010)68T0168T07
Background:
Computed tomography (CT) number accuracy is important for quality assurance in CT imaging. However, in dual-energy CT imaging, there are no widely used action limits for CT number accuracy in spectral images, information that is urgently needed.
Purpose:
To establish action limits for spectral CT images using longitudinal spectral data and an American College of Radiology (ACR) phantom.
Methods:
An ACR accreditation phantom was scanned routinely as part of a quality control program in our institution. We selected and analyzed 57 continuous weekly scans. The CT numbers or the density values of conventional and spectral images, including virtual monoenergetic images (40, 50, 70, 120, and 200 keV), iodine maps, calcium suppressed, and virtual non-contrast images, were measured in the four inserts (solid water, bone, polyethylene, and acrylic) of the phantom. Longitudinal data were analyzed for correlation using Pearson's correlation coefficient (r) and standard deviation (SD). The SD ratios between spectral images and conventional images were calculated and the action limits for spectral images were established based on the action limits from the ACR.
Results:
Strong to very strong correlations (r > 0.70 or r < -0.70) were found among most spectral image types except the 200 keV images using solid water, polyethylene, and acrylic inserts (r = [-0.45, 0.64]). The SD ratio was highest for the 40 keV images, ranging from 2.8 to 6.5. The action limits of the bone insert were baseline ± 5.3 mg/mL for the iodine map and ranged from baseline ± 23.0 HU to baseline ± 391.9 HU for the other image types. The action limits for solid water ranged from baseline ± 4.1 HU to baseline ± 25.3 HU. The results for the polyethylene and the acrylic insert were close to those for solid water. Baselines can be established using the average of the initial 5∼10 measurements.
Conclusions:
Using longitudinal data, we estimated the action limits for CT number accuracy in the spectral images. This paves the way for establishing a comprehensive quality control program for spectral CT imaging.
Spectral photon-counting CT (SPCCT) offers quantitative, high-resolution molecular imaging using intrinsic biomarkers and introduced non-radioactive agents. Thus, SPCCT may open the door to a broader range of medical investigations and increase patient access to advanced diagnostics.Molecular imaging with SPCCT typically uses non-radioactive pharmaceuticals, which provide several advantages. Firstly, unlike PET, the pharmaceutical can be prepared in advance without the need for extensive infrastructure and an on-site radiochemist. Radioisotopes used in PET imaging have half-lives measured in hours. Therefore, PET often requires a nearby cyclotron, making PET prohibitively expensive. Secondly, clinics will be able to access a range of pharmaceuticals 24 hours a day. In contrast, a PET scan requires planning days in advance and the imaging performed only during specified working hours. Finally, multiple pharmaceuticals can be administered to the same subject simultaneously to investigate more than one molecular process in a single scan. For example, in breast cancer imaging, one pharmaceutical could be used to identify micro-calcifications, while a separate pharmaceutical could be used to identify HER-2 (human epidermal receptor 2) positive cells.In addition to not needing radioactive agents, SPCCT has higher spatial resolution compared to clinical molecular imaging. SPCCT can resolve objects down to 100 microns, whereas PET typically has a spatial resolution of 1–2 mm. Furthermore, some molecular processes provide enough intrinsic contrast to not need an exogenously administered agent. For example, while calcium and iron are close on the periodic table, the spectral nature of SPCCT enables the two atoms to be differentiated. Moreover, quantification of a molecular process is possible using SPCCT by measuring functionalized high-Z pharmaceuticals (mg/mL) targeted to that molecular process. Although PET has high sensitivity (ng/mL) due to the radioactive isotopes used, the most common PET pharmaceutical (FDG) only measures a single molecular process, thus it is difficult to directly quantify the underlying biological process.KeywordsMARS imagingSpectralPhoton-counting CTMolecular imagingPreclinical applicationsCardiovascular diseaseInfectious diseaseCancer imagingOrthopaedics
Photon-counting detectors are a promising new technology for medical computed tomography (CT). They enhance the clinical application spectrum of CT by providing energy-resolved CT data at very high spatial resolution without electronic noise and with improved tissue contrasts. This review article gives an overview of the principles of photon-counting detector CT, of potential clinical benefits and limitations, and of the experience gained so far in pre-clinical installations.KeywordsPhoton-counting detectorsMedical computed tomography (CT)Clinical applicationEnergy-resolved CT dataSpatial resolutionElectronic noiseTissue contrastsPrinciples of photon-counting detector CTClinical benefitsPre-clinical installationsCdTe
The recent technological developments in photon-counting detector computed tomography (PCD-CT) and the introduction of the first commercially available clinical PCD-CT unit open up new exciting opportunities for contrast media research. With PCD-CT, the efficacy of available iodine-based contrast media improves, allowing for a reduction of iodine dosage or, on the other hand, an improvement of image quality in low contrast indications. Virtual monoenergetic image reconstructions are routinely available and enable the virtual monoenergetic image energy to be adapted to the diagnostic task.
A key property of PCD-CT is the ability of spectral separation in combination with improved material decomposition. Thus, the discrimination of contrast media from intrinsic or pathological tissues and the discrimination of 2 or more contrasting elements that characterize different tissues are attractive fields for contrast media research. For these approaches, K-edge imaging in combination with high atomic number elements such as the lanthanides, tungsten, tantalum, or bismuth plays a central role.
The purpose of this article is to present an overview of innovative contrast media concepts that use high atomic number elements. The emphasis is on improving contrast enhancement for cardiovascular plaque imaging, stent visualization, and exploring new approaches using 2 contrasting elements. Along with the published research, new experimental findings with a contrast medium that incorporates tungsten are included.
Both the literature review and the new experimental data demonstrate the great potential and feasibility for new contrast media to significantly increase diagnostic performance and to enable new clinical fields and indications in combination with PCD-CT.
The latest evolutions in Computed Tomography (CT) technology have several applications in oncological imaging. The innovations in hardware and software allow for the optimization of the oncological protocol. Low-kV acquisitions are possible thanks to the new powerful tubes. Iterative reconstruction algorithms and artificial intelligence are helpful for the management of image noise during image reconstruction. Functional information is provided by spectral CT (dual-energy and photon counting CT) and perfusion CT.
Computed Tomography (CT) is a radiology technique capable of providing high resolution volumetric representations of the attenuation properties of tissue. The anatomical data provided by CT is complementary to the functional data provided by PET. Furthermore, CT data can be used to replace the time-consuming transmission scans typically used for PET attenuation correction. For this reasons, the commercialization of the first hybrid PET/CT scanners was a resounding success, to the point that standalone PET systems are seldom seen nowadays.
The main clinical indication of PET/CT is in oncology, as well as being instrumental in neurology and drug development research.
In this chapter we will review the challenges of designing hybrid, integrated PET and CT scanners. Both technical and practical considerations for clinical usage will be discussed. This chapter is complementary with the previous chapter on standalone PET technology.
We investigated the accuracy of the computed tomography (CT) numbers of virtual non-contrast (VNC) images for two different material decomposition algorithms using the same image data for different diluted contrast agent concentrations. A container filled with contrast agents was inserted into a cylindrical phantom and scanned with dual-energy protocols (80/Sn140 kV, 100/Sn140 kV) using a dual-source CT. VNC images were generated by the 2-material decomposition (MD) algorithm using the energy of each tube voltage and the linear attenuation coefficient, calculated from the theoretical spectral curve of the agent and the CT number of the image, respectively. Furthermore, VNC images using 3-material decomposition (3-MD) algorithm were produced by applying LiverVNC, an analysis parameter implemented in the scanner. The robustness of both the algorithms was verified by investigating the CT numbers of the agents in the VNC. The closer the CT number is to 0 HU, the more robust the algorithm. Without beam-hardening correction, the CT numbers increased with an increase in concentration in both the algorithms, maximal at 50 mg/ml concentration, with CT numbers of 38 HU for 2-MD, 86 HU for 3-MD. With correction, CT numbers were ± 10 HU or less for both the algorithms up to 30 mg/ml concentration, whereas, for concentrations above 40 mg/ml, the maximal averaged CT number was 12 HU for 2-MD, 22 HU for 3-MD. For both the algorithms, the accuracy of the CT numbers was maintained in the low-concentration range; parameter adjustment was necessary to maintain the accuracy at concentrations higher than clinically expected.
Objectives:
The aim of this study was to assess the accuracy and precision of a novel application of 3-material decomposition (3MD) with virtual monochromatic images (VMIs) in the dual-energy computed tomography (DECT) assessment of monosodium urate (MSU) and hydroxyapatite (HA) phantoms compared with a commercial 2-material decomposition (2MD) and dual-thresholding (DT) material decomposition methods.
Materials and methods:
Monosodium urate (0.0, 3.4, 13.3, 28.3, and 65.2 mg/dL tubes) and HA (100, 400, and 800 mg/cm3 tubes) phantoms were DECT scanned individually and together in the presence of the foot and ankle of 15 subjects. The raw data were decomposed with 3MD-VMI, 2MD, and DT to produce MSU-only and HA-only images. Mean values of 10 × 10 × 10-voxel volumes of interest (244 μm3) placed in each MSU and HA phantom well were obtained and compared with their known concentrations and across measurements with subjects' extremities to obtain accuracy and precision measures. A statistical difference was considered significant if P < 0.05.
Results:
Compared with known phantom standards, 3MD-VMI was accurate for the detection of MSU concentrations as low as 3.4 mg/dL (P = 0.75). In comparison, 2MD was limited to 13.3 mg/dL (P = 0.06) and DT was unable to detect MSU concentrations below 65.2 mg/L (P = 0.16). For the HA phantom, 3MD-VMI and 2MD were accurate for all concentrations including the lowest at 100 mg/cm3 (P = 0.63 and P = 0.55, respectively). Dual-thresholding was not useful for the decomposition of HA phantom. Precision was high for both 3MD-VMI and 2MD measurements for both MSU and HA phantoms. Qualitatively, 3MD-VMI MSU-only images demonstrated reduced beam-hardening artifact and voxel misclassification, compared with 2MD and DT.
Conclusions:
Three-material decomposition-VMI DECT is accurate for quantification of MSU and HA concentrations in phantoms and accurately detects a lower concentration of MSU than either 2MD or DT. For concentration measurements of both MSU and HA phantoms, 3MD-VMI and 2MD have high precision, but DT had limitations. Clinical implementation of 3MD-VMI DECT promises to improve the performance of this imaging modality for diagnosis and treatment monitoring of gout.
Rapid system and hardware development of X-ray computed tomography (CT) technologies has been accompanied by equally exciting advances in image reconstruction algorithms. Of the two reconstruction algorithms, analytical and iterative, iterative reconstruction (IR) algorithms have become a clinically viable option in CT imaging. The first CT scanners in the early 1970s used IR algorithms, but lack of computation power prevented their clinical use. In 2009, the first IR algorithms became commercially available and replaced conventionally established analytical algorithms as filtered back projection. Since then, IR has played a vital role in the field of radiology. Although all available IR algorithms share the common mechanism of artifact reduction and/or potential for radiation dose reduction, the magnitude of these effects depends upon specific IR algorithms. IR reconstructs images by iteratively optimizing an objective function. The objective function typically consists of a data integrity term and a regularization term. Therefore, different regularization priors are used in IR algorithms. This paper will briefly look at the overall evolution of CT image reconstruction and the regularization priors used in IR algorithms. Finally, a discussion is presented based on the reality of various reconstruction methodologies at a glance to find the preferred one. Consequently, we will present anticipation towards future advancements in this domain.
The decomposition of the linear attenuation coefficient into photoelectric absorption and Compton scattering provides virtual monochromatic images (VMIs). The accuracy of the computed tomography (CT) number of VMI, which is obtained by decomposing the linear attenuation coefficient into photoelectric absorption and Compton scattering, was verified in the energy range of 40-200 keV. The possibility of improving the accuracy of CT numbers by using pre-energy-calibrated images as input was also investigated. The VMIs were generated in two groups of images: (i) dual-energy scanned images and (ii) high- and low-energy images generated by two-material decomposition (i.e., pre-energy-calibrated images). The object for analysis was solid iodine rods inserted in the center of the multi-energy CT phantom. The VMIs were generated from the dual-energy scanned images and pre-energy-calibrated images, and the theoretical and measured CT numbers of solid iodine rods were compared. Furthermore, the absolute error (AE) and relative error (RE) were calculated. With both images, the accuracy of the CT numbers was extremely high for regions close to the high- and low-tube-voltage X-ray energy or the high and low energy of the input images. By using the pre-energy-calibrated images, the maximum AE was reduced from 133 to 96 HU at an energy of 40 keV. Similarly, the maximum RE was reduced from 325 to 50% at an energy of 200 keV. The pre-energy-calibrated images reduced the overall error of the CT numbers and controlled the energy region where accurate CT numbers could be obtained.
Background:
The distinction between solitary inflammatory lesion and solitary lung cancer remains a challenge because of their considerable overlapping computed tomography (CT) imaging features.
Purpose:
This study aimed to verify whether spectral CT parameters can differentiate solitary lung cancer from solitary inflammatory lesions and to find their correlations with lesion size.
Methods:
A total of 78 patients with solitary lung lesions were included in our study. All of them underwent enhanced CT scans with Gemstone Spectral Imaging (GSI) mode, which was one of the dual-energy imaging technologies. According to maximum diameter (Dmax) of the lesion, regions of interest were collected and divided into inflammatory (group I: <3 cm [IA], n = 17; ≥3 cm [IB], n = 14) and cancer groups (group II: <3 cm [IIA], n = 20; ≥3 cm [IIB], n = 27). Computed tomography values (HU40keV, HU70keV), effective atomic number (Zeff), iodine concentration (IC), normalized IC (NIC), and spectral curve slopes (λ30, λ40) of each region of interest were calculated. The NIC was defined as the IC ratio of the lesion to the descending aorta. Mann-Whitney U test was used for intergroup (I vs II, IA vs IIA, IB vs IIB) and intragroup (IA vs IB, IIA vs IIB) comparisons, and receiver operating characteristic curve analysis was performed. Correlation analysis was applied to find the relationship between Dmax and GSI parameters.
Results:
No significant correlation was found between GSI parameters and Dmax in the inflammatory group, whereas inverse correlations were found in the cancer group. Gemstone spectral imaging parameters (except HU70keV) of group IIA were significantly higher than those of group IIB. There were significant differences in HU40keV, IC, NIC, λ30, and λ40 between groups IB and IIB under both arterial and venous phase (P values < 0.05), whereas the area under the curve for λ30 under venous phase was largest, and sensitivity and specificity were 96.32% and 85.71%, respectively. However, only HU40keV and HU70keV values under the arterial phase of IIA were significantly higher than those of IA.
Conclusions:
Quantitative parameters of GSI demonstrated an inverse correlation with the lesion size of solitary lung cancer, and GSI parameters can be new ways to differentiate solitary lung cancer from solitary inflammatory lesions.
Pulmonary embolism (PE) associated with Mycoplasma pneumoniae pneumonia (MPP) in children has already attracted more attention. CT pulmonary angiography (CTPA) has been the preferred method for diagnosing PE, but it has some limitations, especially for children. Dual-energy spectral CT has been used in diagnosing PE in adults.
To evaluate the application of dual-energy spectral CT in diagnosing PE in children with MPP.
Eighty-three children with MPP and highly suspected PE, underwent CTPA with spectral imaging mode, 25 children were diagnosis with PE. Noise, clot-to-artery contrast-to-noise ratio, image quality and diagnosis confidence were calculated and assessed on nine monochromatic image sets (40 to 80 keV). CTPA images were observed for the presence, localization and embolic degrees of PE. Clots were divided into intra- and extra-consolidation clots. For extra-consolidation clots, iodine concentration (IC) of perfusion defects and normal lung, perfusion defects of four children before and after the treatment were measured and compared. For intra-consolidation clots, IC of consolidation areas with clots and consolidation areas without clot were measured and compared.
The optimal energy level for detecting PE in children was 55 keV. 116 clots (29 extra-consolidations) were found, IC of defect regions was 0.69 ± 0.28 mg/mL (extra-consolidations) and 0.90 ± 0.23 mg/mL (intra-consolidations), both significantly lower than the 2.76 ± 0.45 mg/mL in normal lungs and 10.25 ± 1.76 mg/mL in consolidations without clots (P < 0.001). Significant difference was found in the presence or absence of perfusion defects between occlusive clots and nonocclusive clots (P < 0.001). IC of the perfusion defects significantly increased after treatment (P < 0.001).
In dual-energy spectral CTPA, 55 keV images optimize PE detection for children, and MD images quantify pulmonary blood flow of PE, and may help to detect small clots and quantify embolic degrees.
Photon-counting detectors are a promising new technology for medical computed tomography (CT). They enhance the clinical application spectrum of CT by providing energy-resolved CT data at very high spatial resolution without electronic noise and with improved tissue contrasts. This review article gives an overview of the principles of photon-counting detector CT, of potential clinical benefits and limitations, and of the experience gained so far in preclinical installations.
Aim:
Noninvasive evaluation of hypoxia in rabbit VX2 lung transplant tumors using spectral CT parameters and texture analysis.
Materials and methods:
Twenty-five VX2 lung transplant tumors of twenty-two rabbits were included in the study. Contrast-enhanced spectral CT scanning in the arterial phase (AP) and venous phase (VP) was performed. Tumors were divided into strong and weak hypoxic groups by hypoxic probe staining results. Spectral CT image-related parameters [70 keV CT value, normalized iodine concentration (NIC), slope of spectral HU curve (λHU)] were measured and the texture analysis on the monochromatic images was performed. Imaging parameters and texture features between tumors with different hypoxic degrees were compared and their diagnostic efficacies for predicting hypoxia in lung cancers were analyzed using receiver operating characteristic (ROC) curve.
Results:
NIC in VP and λHU in VP of the strong hypoxic group were significantly higher than those in the weak hypoxic group (p < 0.05). For the texture features, entropy in VP and kurtosis in AP were significantly different between the two hypoxic groups. According to ROC analysis, λHU in VP had a better diagnostic ability for predicting hypoxia in tumors [Area Under Curve (AUC): 0.883, sensitivity: 85.7%, specificity: 100%]. The combination of four features improved AUC to 0.955.
Conclusion:
NIC in VP, λHU in VP, entropy in VP and kurtosis in AP have certain values in predicting tumor hypoxia and a combination of image parameters and texture features improves diagnostic efficiency.
Objectives
To investigate the robustness of radiomic features between three dual-energy CT (DECT) systems.Methods
An anthropomorphic body phantom was scanned on three different DECT scanners, a dual-source (dsDECT), a rapid kV-switching (rsDECT), and a dual-layer detector DECT (dlDECT). Twenty-four patients who underwent abdominal DECT examinations on each of the scanner types during clinical follow-up were retrospectively included (n = 72 examinations). Radiomic features were extracted after standardized image processing, following ROI placement in phantom tissues and healthy appearing hepatic, splenic and muscular tissue of patients using virtual monoenergetic images at 65 keV (VMI65keV) and virtual unenhanced images (VUE). In total, 774 radiomic features were extracted including 86 original features and 8 wavelet transformations hereof. Concordance correlation coefficients (CCC) and analysis of variances (ANOVA) were calculated to determine inter-scanner robustness of radiomic features with a CCC of ≥ 0.9 deeming a feature robust.ResultsNone of the phantom-derived features attained the threshold for high feature robustness for any inter-scanner comparison. The proportion of robust features obtained from patients scanned on all three scanners was low both in VMI65keV (dsDECT vs. rsDECT:16.1% (125/774), dlDECT vs. rsDECT:2.5% (19/774), dsDECT vs. dlDECT:2.6% (20/774)) and VUE (dsDECT vs. rsDECT:11.1% (86/774), dlDECT vs. rsDECT:2.8% (22/774), dsDECT vs. dlDECT:2.7% (21/774)). The proportion of features without significant differences as per ANOVA was higher both in patients (51.4–71.1%) and in the phantom (60.6–73.4%).Conclusions
The robustness of radiomic features across different DECT scanners in patients was low and the few robust patient-derived features were not reflected in the phantom experiment. Future efforts should aim to improve the cross-platform generalizability of DECT-derived radiomics.Key Points• Inter-scanner robustness of dual-energy CT-derived radiomic features was on a low level in patients who underwent clinical examinations on three DECT platforms.• The few robust patient-derived features were not confirmed in our phantom experiment.• Limited inter-scanner robustness of dual-energy CT derived radiomic features may impact the generalizability of models built with features from one particular dual-energy CT scanner type
Dual energy CT (DECT) imaging is a technique that extends the capabilities of CT beyond that of established densitometric evaluations. CT pulmonary angiography (CTPA) performed with dual energy technique benefits from both the availability of low kVp CT data and also the concurrent ability to quantify iodine enhancement in the lung parenchyma. Parenchymal enhancement, presented as pulmonary perfused blood volume maps, may be considered a surrogate of pulmonary perfusion. These distinct capabilities have led to new opportunities in the evaluation of pulmonary vascular diseases. Dual energy CTPA offers the potential for improvements in pulmonary emboli detection, diagnostic confidence, and most notably severity stratification. Furthermore, the appreciated insights of pulmonary vascular physiology conferred by DECT have resulted in increased use for the assessment of pulmonary hypertension, with particular utility in the subset of patients with chronic thromboembolic pulmonary hypertension. With the increasing availability of dual energy-capable CT systems, dual energy CTPA is becoming a standard-of-care protocol for CTPA acquisition in acute PE. Furthermore, qualitative and quantitative pulmonary vascular DECT data heralds promise for the technique as a “one-stop shop” for diagnosis and surveillance assessment in patients with pulmonary hypertension. This review explores the current application, clinical value, and limitations of DECT imaging in acute and chronic pulmonary vascular conditions. It should be noted that certain manufacturers and investigators prefer alternative terms, such as spectral or multi energy CT imaging. In this review the term dual energy is utilised, although readers can consider these terms synonymous for purposes of the principles explained.
Background: X-ray computed tomography is the first imaging technology that supports accurate nondestructive interior image reconstruction of an object from sufficient projection data. Low-dose computed tomography (LDCT) has been considered to relieve the harm to patients caused by X-ray radiation. However, LDCT images can be degraded by quantum noise and streak artifacts. Methods: The objective of the authors’ study is to evaluate the optimal level of the hybrid iterative reconstruction (HIR) that generates images with the best diagnostic quality on different dose and noise levels. HIR with optimizations is proposed to reduce image noise and provide better performance at a low dose. The Catphan R 504 phantom is employed to assess various image qualities (IQ). Results: For any given scanning protocols, there is linear noise reduction and linear increase of contrast-to- noise ratio (CNR) using optimal HIR. The evidence from various module tests demonstrates that the shape of the noise power spectrum is continuously shifted to low frequency with increasing HIR levels compared with that of filtered-back-projection (FBP). This may describe the difference between the human observer performance and features of the ideal low-contrast objects. Conclusion: Optimal HIR is clearly demonstrated to be a superior method for reducing image noise and improving CNR compared to FBP. Optimal HIR also inhibits texture change or spectrum shift compared with the pure IR method. Even though there are continuous noise reduction and CNR increase with HIR at increasing levels, the human observer performance does not seem to improve simultaneously due to coarser noise (low-frequency noise). HIR level 3 to 5 is optimal for their study. It is possible for the optimal HIR to offer equivalent diagnostic IQ at a lower dose compared with FBP at a routine dose.
Purpose:
The goal is to provide a sufficient condition for the invertibility of a multi-energy (ME) X-ray transform. The energy-dependent X-ray attenuation profiles can be represented by a set of coefficients using the Alvarez-Macovski (AM) method. An ME X-ray transform is a mapping from N AM coefficients to N noise-free energy-weighted measurements, where N ⩾ 2.
Methods:
We apply a general invertibility theorem to prove the equivalence of global and local invertibility for an ME X-ray transform. We explore the global invertibility through testing whether the Jacobian of the mapping J(A) has zero values over the support of the mapping. The Jacobian of an arbitrary ME X-ray transform is an integration over all spectral measurements. A sufficient condition for J(A)≠0 for all A is that the integrand of J(A) is ⩾0 (or ⩽0) everywhere. Note that the trivial case of the integrand equals 0 everywhere is ignored. Using symmetry, we simplified the integrand of the Jacobian to three factors that are determined by the total attenuation, the basis functions, and the energy-weighting functions, respectively. The factor related to the total attenuation is always positive, hence the invertibility of the X-ray transform can be determined by testing the signs of the other two factors. Furthermore, we use the Cramér-Rao lower bound (CRLB) to characterize the noise-induced estimation uncertainty and provide a maximum-likelihood (ML) estimator.
Results:
The factor related to the basis functions is always negative when the photoelectric/Compton/Rayleigh basis functions are used and K-edge materials are not considered. The sign of the energy-weighting factor depends on the system source spectra and the detector response functions. For four special types of X-ray detectors, the sign of this factor stays the same over the integration range. Therefore, when these four types of detectors are used for imaging non-K-edge materials, the ME X-ray transform is globally invertible. The same framework can be used to study an arbitrary ME X-ray imaging system, e,g, when K-edge materials are present. Furthermore, the ML estimator we presented is an unbiased, efficient estimator and can be used for a wide range of scenes.
Conclusions:
We have provided a framework to study the invertibility of an arbitrary ME X-ray transform and proved the global invertibility for four types of systems.
Purpose
To assess the task-based performance of images obtained under different focal spot size and acquisition mode on a dual-energy CT scanner.
Methods
Axial CT image series of the Catphan phantom were obtained using a tube focus at different sizes. Acquisitions were performed in standard single-energy, high resolution (HR) and dual-energy modes. Images were reconstructed using conventional and high definition (HD) kernels. Task-based transfer function at the 50% level (TTF50%) for teflon, delrin, low density polyethylene (LDPE) and acrylic, as well as image noise and noise texture, were assessed across all focal spots and acquisition modes using Noise Power Spectrum (NPS) analysis. A non-prewhitening mathematical observer model was used to calculate detectability index (dNPW′).
Results
TTF50% degraded with increasing focal spot size. TTF50% ranged from 0.67 mm⁻¹ for teflon to 0.25 mm⁻¹ for acrylic. For standard kernel, image noise and NPS-determined average spatial frequency were 8.3 HU and 0.29 mm⁻¹, respectively in single-energy, 12.0 HU and 0.37 mm⁻¹ in HR, and 7.9 HU and 0.26 mm⁻¹ in dual-energy mode. For standard kernel, dNPW′ was 61 in single-energy and HR mode and reduced to 56 in dual-energy mode.
Conclusions
The task-based image quality assessment metrics have shown that spatial resolution is higher for higher image contrast materials and detectability is higher in the standard single-energy mode compared to HR and dual-energy mode. The results of the current study provide CT operators the required knowledge to characterize their CT system towards the optimization of its clinical performance.
Purpose:
This study developed methods to quantify and improve the accuracy of dual-energy CT (DECT)-based bone marrow edema imaging using a clinical CT system. Objectives were: 1.) to quantitatively compare DECT with gold-standard, fluid-sensitive MRI for imaging of edema-like marrow signal intensity (EMSI) and 2.) to identify image analysis parameters that improve delineation of EMSI associated with acute knee injury on DECT images.
Methods:
DECT images from ten participants with acute knee injury were decomposed into estimated fractions of bone, healthy marrow, and edema based on energy-dependent differences in tissue attenuation. Fluid-sensitive MR images were registered to DECT for quantitative, voxel-by-voxel comparison between the two modalities. An optimization scheme was developed to find attenuation coefficients for healthy marrow and edema that improved EMSI delineation, compared to MRI. DECT method accuracy was evaluated by measuring dice coefficients, mutual information, and normalized cross correlation between the DECT result and registered MRI.
Results:
When applying the optimized three-material decomposition method, dice coefficients for EMSI identified through DECT versus MRI were 0.32 at the tibia and 0.13 at the femur. Optimization of attenuation coefficients improved dice coefficient, mutual information, and cross-correlation between DECT and gold-standard MRI by 48-107% compared to three-material decomposition using non-optimized parameters, and improved mutual information and cross-correlation by 39-58% compared to the manufacturer-provided two-material decomposition.
Conclusions:
This study quantitatively evaluated the performance of DECT in imaging knee injury-associated EMSI and identified a method to optimize DECT-based visualization of complex tissues (marrow and edema) whose attenuation parameters cannot be easily characterized. Further studies are needed to improve DECT-based EMSI imaging at the femur.
Purpose
To assess the quality of images obtained on a dual energy computed tomography (CT) scanner.
Methods
Image quality was assessed on a 64 detector-row fast kVp-switching dual energy CT scanner (Revolution GSI, GE Medical Systems). The Catphan phantom and a low contrast resolution phantom were employed. Acquisitions were performed at eight different radiation dose levels that ranged from 9 mGy to 32 mGy. Virtual monochromatic spectral images (VMI) were reconstructed in the 40–140 keV range using all available kernels and iterative reconstruction (IR) at four different blending levels. Modulation Transfer Function (MTF) curves, image noise, image contrast, noise power spectrum and contrast to noise ratio were assessed.
Results
In-plane spatial resolution at the 10% of the MTF curve was 0.60 mm⁻¹. In-plane spatial resolution was not modified with VMI energy and IR blending level. Image noise was reduced from 16.6 at 9 mGy to 6.7 at 32 mGy, while peak frequency remained within 0.14 ± 0.01 mm⁻¹. Image noise was reduced from 14.3 at IR 10% to 11.5 at IR 50% at a constant peak frequency. The lowest image noise and maximum peak frequency were recorded at 70 keV.
Conclusions
Our results have shown how objective image quality is varied when different levels of radiation dose and different settings in IR are applied. These results provide CT operators an in depth understanding of the imaging performance characteristics in dual energy CT.
To validate the accuracy of spectral curves obtained by an image-data-based algorithm and clarify the error factors that reduce accuracy. Iodine rods of known composition and different concentrations were inserted into a cylinder or elliptic-cylinder phantom and scanned according to the dual-energy protocol. Spectral curves were obtained by (i) theoretical calculation, (ii) image-data-based 2-material decomposition, and (iii) using a dedicated workstation. Accuracy was verified by comparing the spectral curve obtained by theoretical calculations with those obtained by the image-data-based algorithms or the dedicated workstations. For a quantitative evaluation, the error and relative error (RE) were calculated. In the image-data-based calculation, the errors with respect to the theoretical CT number ranged from − 8.3 to 71.1 HU. For all 192 combinations, 80.7% of the errors were under ± 15 HU, and 97.9% of the REs were under 10%. In the dedicated workstation, the errors ranged from − 94.7 to 26.8 HU. For all combinations, 68.8% of the errors were under ± 15 HU, and 68.2% of the REs were under 10%. By appropriately setting the effective energy corresponding to the CT number of the basis materials, an accurate spectral curve can be obtained. The beam-hardening effect is canceled by the 2-material decomposition process even without beam-hardening correction. Accuracy is primarily reduced by scattered radiation rather than the beam-hardening effect.
Objectives
To investigate inter-scan and inter-scanner variation of iodine concentration (IC) and attenuation in virtual monoenergetic images at 65 keV (HU65keV) in patients with repeated abdominal examinations on dual-source (dsDECT), rapid kV switching (rsDECT), and dual-layer detector DECT (dlDECT).Methods
We retrospectively included 131 patients who underwent two abdominal DECT examinations on the same scanner (dsDECT: n = 46, rsDECT: n = 45, dlDECT: n = 40). IC and HU65keV were measured by placing regions of interest in the liver, spleen, kidneys, aorta, portal vein, and inferior vena cava. Overall IC and HU65keV for each scanner, their inter-scan differences and proportional variation were calculated and compared between scanner types.ResultsThe three scanner-specific cohorts showed similar weight, body diameter, age, sex, and contrast media injection parameters as well as inter-scan differences hereof (p range: 0.23–0.99). Absolute inter-scan differences of HU65keV and IC were comparable between scanners (p range: 0.08–1.0). Overall inter-scan variation was significantly higher in IC than HU65keV (p < 0.05). For the liver, rsDECT showed significantly lower inter-scan variation of IC compared to dsDECT/dlDECT (p = 0.005/0.01), while for the spleen, this difference was only significant compared to dsDECT (p = 0.015). Normalizing IC of the liver to the portal vein and of the spleen to the aorta did not significantly reduce inter-scan variation (p = 0.97 and 0.50).Conclusions
Iodine measurements across different DECT scanners show inter-scan variation which is higher compared to variation of attenuation values. Inter-scanner differences in longitudinal variation and overall iodine concentration depend on the scanner pairs and organs assessed and should be acknowledged in clinical and scientific DECT applications.Key Points
• All scanner types showed comparable inter-scan variation of attenuation, while for iodine, the rapid kV switching DECT showed lower variability in the liver and spleen.
• Iodine concentration showed higher inter-scan variation than attenuation measurements; normalization to vessels did not significantly improve inter-scan reproducibility of iodine concentration in parenchymal organs.
• Differences between the three scanner types regarding overall iodine concentration and attenuation obtained from both timepoints were within the range of average intra-patient, inter-scan differences for most assessed organs and vessels.
As the access of radiology practices to dual-energy CT (DECT) has increased worldwide, seamless integration into clinical workflows and optimized use of this technology are desirable. In this article, we provide basic concepts of commercially available DECT hardware implementations, discuss financial and logistical aspects, provide tips for protocol building and image routing strategies, and review radiation dose considerations to establish a DECT service line in abdominal imaging. • Tube-based and detector-based DECT implementations with varying features and strengths are available on the imaging market. • Thorough assessment of financial and logistical aspects is key to successful implementation of a DECT service line. • Optimized protocol building and image routing strategies are of critical importance for effective use and seamless inception of DECT in routine clinical workflows.
More and more clinical CT scanners have Dual Energy CT capabilities, and routine usage of this technique, as well as the number of scientific publications on this topic, is increasing rapidly. At the moment, several technical approaches are commercially available that have different strengths and weaknesses. Also, post-processing features differ considerably between different software products. However, the need for a certain degree of standardization has already been recognized by the community. For cardiovascular imaging, the main focus of Dual Energy CT is on the visualization and quantification of iodinated contrast media in vessels. Virtual monoenergetic images allow for an interactive optimization of iodine contrast relative to soft tissue, calcium, or stent material. Virtual monoenergetic CT number values are independent of the patient or the prefiltration of the used scanner; they are in this way more objective and reproducible. Additional diagnostic information is available through the calculation of iodine maps for the heart and lungs. While some of the discussed applications are at the very early stage of feasibility studies, others, such as bone removal, virtual monoenergetic imaging, or visualization of iodine uptake in the lung parenchyma, are already now widely used.
Fast kV-switching is a dual energy acquisition technique in CT in which alternating views correspond to the low and high tube voltages. Its high temporal resolution and its suitability to a variety of source trajectories make it an attractive option for dual energy data acquisition. Its disadvantages include a one view mis-registration between the data for high and low voltages, the potential for poor spectrum separation because the fast kV-switching waveform may be more like a sine wave than the desired square wave, and the higher noise in the low voltage data because of the technical difficulty of swinging the tube current to counter the loss of x-ray production efficiency and loss of penetration at lower tube voltages. These issues are investigated with a recently developed pre-reconstruction decomposition method by the authors. Results include that symmetric view matching eliminates streaks from the view mis-registration, a sinusoidal waveform swinging between 80 and 135 kV gives sufficient spectrum separation, and that contrast-to-noise for the simulated imaging task maximizes at monochromatic energy of 75 keV.
With the advancement of Computed Tomography technology, improving image quality while reducing patient dose has been a big technical challenge. The recent CT750 HD system from GE Healthcare provides significantly improved spatial resolution and the capability to reduce dose during routine clinical imaging. This paper evaluates the image quality of this system. Spatial resolution, dose reduction, noise, and low contrast detectability have been quantitatively characterized. Results show a quantifiable and visually discernable higher spatial resolution for both body and cardiac scanning modes without compromise of image noise. Further, equivalent image quality performance with up to 50% lower dose has been achieved.
Dual energy CT (DECT) measures the object of interest using two different x-ray spectra in order to provide energy-selective CT images or in order to get the material decomposition of the object. Today, two decomposition techniques are known. Image-based DECT treats the rawdata sets as being independent until they are reconstructed. Prior to reconstruction the rawdata undergoes the typical water precorrection. The reconstructed images are then linearly combined to obtain material-selective DECT images.
The second decomposition technique is rawdata-based. Rawdata-based DECT is passing the rawdata through a decomposition function D(q1, q2) followed by image reconstruction. Rawdata-based decomposition is exact and the final image will not show beam hardening artifacts. However, rawadata-based decomposition requires consistent rawdata sets.
Our new imaged-based method uses generalized precorrection functions \(P^c_j(q_j)\) that do not aim at delivering cupping artifact-free single energy images but that aim at delivering an improved DECT image by using the linear combination P1(q1) + P2(q2), which can also be carried out in image domain.
This work compares the ability of the three methods to perform material decomposition and to provide energy-selective monochromatic CT images. Due to its increased degrees of freedom the generalized precorrection functions significantly outperform the image quality achievable with conventional image-based DECT decomposition. Nevertheless, the artifact content is still higher than when using rawdata-based decomposition techniques. Since our method can be realized as a polynomial function or as a look-up table, it can easily be used to substitute the water precorrection functions built into today’s scanners. Thereby the approach is a practicable way to improve image-based DECT without changing the scanner software or hardware.
Dual energy CT (DECT) is a new technique that allows differentiation of materials and tissues based on CT density values derived from two synchronous CT acquisitions at different tube potentials. With the introduction of a new dual source CT system, this technique can now be used routinely in abdominal imaging. Potential clinical applications include evaluation of renal masses, liver lesions, urinary calculi, small bowel, pancreas, and adrenal glands. In CT angiography of abdominal aortic aneurysms, dual energy CT techniques can be used to remove bones from the datasets, and virtual unenhanced images allow differentiation of contrast agent from calcifying thrombus in patients with endovascular stents. This review describes potential applications, practical guidelines, and limitations of dual energy CT in the abdomen.
The detection limitations inherent in statistically limited computed tomographic (CT) images are described through the application of signal detection theory. The detectability of large-area, low-contrast objects is shown to be chiefly dependent upon the low-frequency content of the noise power spectral density. For projection data containing uncorrelated noise, the resulting ramp-like, low-frequency behavior of the noise power spectrum of CT reconstructions may be conveniently characterized by the number of noise-equivalent x-ray quanta (NEQ) detected in the projection measurements. The NEQ for a given image may be determined either from a measurement of the noise power spectrum or from the noise granularity computed with an appropriate weighting function. A measure of the efficiency of scanner dose utilization is proposed which compares the average dose to that required by an ideal scanner to obtain the same NEQ.
An expression is derived showing that the two-dimensional noise power spectrum of computed X-ray tomography is proportional to [G(k)]2/k where k is the radial spatial frequency and G(k) is the one-dimensional corrective filter used in the filtered back-projection reconstuction technique. It is shown that predicted noise power spectra compare well with those estimated from CT reconstructions of simulated noise for both the ramp filter and the Hanning-weighted ramp filter. A consequence of the non-uniform shape of the noise power spectrum is that statistical noise in CT reconstructions is correlated from point to point. Because of this correlation when the reconstructed CT values are averaged over some region, the uncertainty of the average depends on the shape of the region as well as its area. This dependence is confirmed by computer simulations.
All X-ray computerized tomography systems that are available or proposed base their reconstructions on measurements that integrate over energy. X-ray tubes produce a broad spectrum of photon energies and a great deal of information can be derived by measuring changes in the transmitted spectrum. We show that for any material, complete energy spectral information may be summarized by a few constants which are independent of energy. A technique is presented which uses simple, low-resolution, energy spectrum measurements and conventional computerized tomography techniques to calculate these constants at every point within a cross-section of an object. For comparable accuracy, patient dose is shown to be approximately the same as that produced by conventional systems. Possible uses of energy spectral information for diagnosis are presented.
We present a performance evaluation of a recently introduced dual-source computed tomography (DSCT) system equipped with two X-ray tubes and two corresponding detectors, mounted onto the rotating gantry with an angular offset of 90 degrees . We introduce the system concept and derive its consequences and potential benefits for electrocardiograph [corrected] (ECG)-controlled cardiac CT and for general radiology applications. We evaluate both temporal and spatial resolution by means of phantom scans. We present first patient scans to illustrate the performance of DSCT for ECG-gated cardiac imaging, and we demonstrate first results using a dual-energy acquisition mode. Using ECG-gated single-segment reconstruction, the DSCT system provides 83 ms temporal resolution independent of the patient's heart rate for coronary CT angiography (CTA) and evaluation of basic functional parameters. With dual-segment reconstruction, the mean temporal resolution is 60 ms (minimum temporal resolution 42 ms) for advanced functional evaluation. The z-flying focal spot technique implemented in the evaluated DSCT system allows 0.4 mm cylinders to be resolved at all heart rates. First clinical experience shows a considerably increased robustness for the imaging of patients with high heart rates. As a potential application of the dual-energy acquisition mode, the automatic separation of bones and iodine-filled vessels is demonstrated.
The aim of this study was to assess the feasibility of a differentiation of iodine from other materials and of different body tissues using dual energy CT. Ten patients were scanned on a SOMATOM Definition Dual Source CT (DSCT; Siemens, Forchheim, Germany) system in dual energy mode at tube voltages of 140 and 80 kVp and a ratio of 1:3 between tube currents. Weighted CT Dose Index ranged between 7 and 8 mGy, remaining markedly below reference dose values for the respective body regions. Image post-processing with three-material decomposition was applied to differentiate iodine or collagen from other tissue. The results showed that a differentiation and depiction of contrast material distribution is possible in the brain, the lung, the liver and the kidneys with or without the underlying tissue of the organ. In angiographies, bone structures can be removed from the dataset to ease the evaluation of the vessels. The differentiation of collagen makes it possible to depict tendons and ligaments. Dual energy CT offers a more specific tissue characterization in CT and can improve the assessment of vascular disease. Further studies are required to draw conclusions on the diagnostic value of the individual applications.
The objective of our study was to evaluate the feasibility of virtual unenhanced images reconstructed from a dual-energy CT scan to depict urinary stones in an iodine solution in a phantom study.
Twenty urinary stones of different sizes (1.4-4.2 mm in short-axis diameter) were placed in plastic containers. The containers were consecutively filled with different concentrations of iodine solution (21, 43, 64, 85, and 107 mg/dL; CT attenuation value range, 510-2,310 H at 120 kVp). Dual-energy CT was repeated with 80-140 and 100-140 kVp pairs, two collimation-slice thickness combinations, and the presence or absence of a 4-cm-thick oil gel around the phantom. The iodine-subtraction virtual unenhanced images were reconstructed using commercial software. The images were evaluated by three radiologists in consensus for the visibility of the stones and the presence of residual nonsubtracted iodine. Stone visibility rates were compared between the 80-140 and 100-140 kVp pairs and the five different iodine concentrations.
Stone visibility rates with the 80-140 kVp pair were 99%, 93%, 96%, 94%, and 3% and those with the 100-140 kVp pair were 98%, 95%, 99%, 94%, and 99% for an iodine concentration of 21, 43, 64, 85, and 107 mg/dL, respectively. The poor visibility rate with 80-140 kVp and 107 mg/dL iodine concentration was due to the failure of iodine subtraction.
Dual-energy CT iodine-subtraction virtual unenhanced technique is capable of depicting urinary stones in iodine solutions of a diverse range of concentrations in a phantom study.
We compare the noise properties of conventional and energy selective X-ray computed tomography. The images produced by the systems are not directly comparable so we discuss their relationship and show that the conventional image is a subset of the energy selective data. We describe how to form a conventional image from the energy selective images and demonstrate that, if this is done at the optimal display energy, the resulting conventional image will have the same noise as that produced by a conventional system with the same dose. But the energy selective system also extracts all the energy dependent information so it produces more information for the same dose than a conventional system.
Background:
Dual-energy computed tomography (DECT) is a widely used and actively researched imaging modality that can estimate the physical properties of an object more accurately than single-energy CT (SECT). Recently, iterative reconstruction methods called one-step methods have received attention among various approaches since they can resolve the intermingled limitations of the conventional methods. However, the one-step methods typically have expensive computational costs, and their material decomposition performance is largely affected by the accuracy in the spectral coefficients estimation.
Objective:
In this study, we aim to develop an efficient one-step algorithm that can effectively decompose into the basis material maps and is less sensitive to the accuracy of the spectral coefficients.
Methods:
By use of a new loss function that employs the non-linear forward model and the weighted squared errors, we propose a one-step reconstruction algorithm named generalized simultaneous algebraic reconstruction technique (GSART). The proposed algorithm was compared with the image-domain material decomposition and other existing one-step reconstruction algorithm.
Results:
In both simulation and experimental studies, we demonstrated that the proposed algorithm effectively reduced the beam-hardening artifacts thereby increasing the accuracy in the material decomposition.
Conclusions:
The proposed one-step reconstruction for material decomposition in dual-energy CT outperformed the image-domain approach and the existing one-step algorithm. We believe that the proposed method is a practically very useful addition to the material-selective image reconstruction field.
Dual energy CT (DECT) measures the object of interest using two different x-ray spectra in order to provide energy-selective CT images or in order to get the material decomposition of the object. Today, two decomposition techniques are known. Image-based DECT uses linear combinations of reconstructed images to get an image that contains material-selective DECT information. Rawdata-based DECT correctly treats the available information by passing the rawdata through a decomposition function that uses information from both rawdata sets to create DECT specific (e.g., material-selective) rawdata. Then the image reconstruction yields material-selective images. Rawdata-based image decomposition generally obtains better image quality; however, it needs matched rawdata sets. This means that physically the same lines need to be measured for each spectrum. In today's CT scanners, this is not the case. The authors propose a new image-based method to combine mismatched rawdata sets for DECT information. The method allows for implementation in a scanner's rawdata precorrection pipeline or may be used in image domain. They compare the ability of the three methods (image-based standard method, proposed method, and rawdata-based standard method) to perform material decomposition and to provide monochromatic images. Thereby they use typical clinical and preclinical scanner arrangements including circular cone-beam CT and spiral CT. The proposed method is found to perform better than the image-based standard method and is inferior to the rawdata-based method. However, the proposed method can be used with the frequent case of mismatched data sets that exclude rawdata-based methods.
Dual energy CT scanning (tomochemistry) has been proposed as a method for determining various parameters relating to the elemental composition of the tissues. In this paper, our aim is to study the relative noise inherent in two proposed techniques for dual energy scanning: a ‘‘two crystal’’ technique and a ‘‘two kV’’ technique. In the two crystal technique, a split crystal detector is used to simultaneously obtain the high and low energy data during one scan at high kV. The two kV technique requires two scans taken with widely different kV settings. We first review three commonly used approaches for utilizing the scan data to compute the relevant parameters. A theoretical formalism is constructed which aids in understanding these methods. Then this formalism is used to study the influence of CT image noise on measurement precision in the case where the unknown parameters are densities. It is shown that, (1) the unavoidable overlap in the spectral data obtained by the two crystal technique results in a much lower signal‐to‐noise ratio than can be obtained by using the two kV technique, (2) the necessity for hard filtration of the high energy beam in the two kV technique has not heretofore been appreciated, and (3) the dose for a given x‐ray tube heat load is also lower with the two kV technique.
i) A review has been made of spectral analysis and its relation with other branches of time‐series analysis. A detailed account has been given of the methods available for estimating the spectral density, the band spectrum and the integrated spectrum; suggestions have also been made for confidence intervals for the latter two estimators.
(ii) Emphasis has been laid throughout upon the difficulties which are met in practice and gaps in the theoretical structure have been indicated. In applications to physical problems, it is suggested that an empirical statistical approach is not enough by itself and that more realistic descriptions of each particular phenomenon should be attempted.
(iii) Reference has already been made in Section 9 to a sampling investigation in which it is proposed to apply the techniques mentioned in this paper to a large number of artificially constructed series of the type given by (1).
Spectral Computed Tomography (Spectral CT), and in particular fast kVp switching dual-energy computed tomography, is an imaging modality that extends the capabilities of conventional computed tomography (CT). Spectral CT enables the estimation of the full linear attenuation curve of the imaged subject at each voxel in the CT volume, instead of a scalar image in Hounsfield units. Because the space of linear attenuation curves in the energy ranges of medical applications can be accurately described through a two-dimensional manifold, this decomposition procedure would be, in principle, limited to two materials. This paper describes an algorithm that overcomes this limitation, allowing for the estimation of N-tuples of material-decomposed images. The algorithm works by assuming that the mixing of substances and tissue types in the human body has the physicochemical properties of an ideal solution, which yields a model for the density of the imaged material mix. Under this model the mass attenuation curve of each voxel in the image can be estimated, immediately resulting in a material-decomposed image triplet. Decomposition into an arbitrary number of pre-selected materials can be achieved by automatically selecting adequate triplets from an application-specific material library. The decomposition is expressed in terms of the volume fractions of each constituent material in the mix; this provides for a straightforward, physically meaningful interpretation of the data. One important application of this technique is in the digital removal of contrast agent from a dual-energy exam, producing a virtual nonenhanced image, as well as in the quantification of the concentration of contrast observed in a targeted region, thus providing an accurate measure of tissue perfusion.
Recent publications in the field of Computed Tomography (CT) demonstrate the rising interest in applying dual-energy methods for material classification during clinical routine examinations. Based on today's standard of technology, dual-energy CT can be realized by either scanning with different X-ray spectra or by deployment of energy selective detector technologies. The list of so-called dual-kVp methods contains sequential scans, fast kVp-switching and dual-source CT. Examples of energy selective detectors are scintillator-based energyintegrating dual-layer devices or direct converter with quantum counting electronics. The general difference of the approaches lies in the shape of the effectively detected X-ray energy spectra and in the presence of crossscatter radiation in the case of dual-source devices. This leads to different material classification capabilities for the various techniques. In this work, we present detector response simulations of realistic CT scans with subsequent CT image reconstruction. Analysis of the image data allows direct and objective comparison of the dual-kVp, dual-layer, and quantum counting CT system concepts. The dual-energy performance is benchmarked in terms of image noise and Iodine-bone separation power at given image sharpness and dose exposure. For the case of dual-source devices the effect of cross-scatter radiation, as well as the benefit of additional filtering are taken into account.
We present the analysis of the accuracy and precision of dual energy material basis decomposition for the quantification of tissue fat content in computed tomography. We compare the benefits of a pre-reconstruction (sinogram-based) dual energy imaging technique versus a post-reconstruction (image) based dual energy decomposition technique using a numerical simulation. A phantom containing plastics of known composition is measured to validate the technique. The accuracy of the image based dual energy decomposition technique is contingent on the amount of beam hardening encountered in the phantom. The accuracy of the pre-reconstruction dual energy technique depends on how accurately the system spectral response can be modeled. In both cases the precision of the dual energy imaging is determined by the photon flux.
Stimulated by the introduction of clinical dual source CT, the interest in dual energy methods has been increasing in the past years. Whereas the potential of material decomposition by dual energy methods is known since the early 1980ies, the realization of dual energy methods is a wide field of today's research. Energy separation can be achieved with energy selective detectors or by varying X-ray source spectra. This paper focuses on dual energy techniques with varying X-ray spectra. These can be provided by dual source CT devices, operated with different kVp settings on each tube. Excellent spectral separation is the key property for use in clinical routine. The drawback of higher cost for two tubes and two detectors leads to an alternative realization, where a single source CT yields different spectra by fast kVp switching from reading to reading. This provides access to dual-energy methods in single source CT. However, this technique comes with some intrinsic limitations. The maximum X-ray flux is reduced in comparison to the dual source system. The kVp rise and fall time between each reading reduces the spectral separation. In comparison to dual source CT, for a constant number of projections per energy spectrum the temporal resolution is reduced; a reasonable trade of between reduced numbers of projection and limited temporal resolution has to be found. The overall dual energy performance is the guiding line for our investigations. We present simulations and measurements which benchmark both solutions in terms of spectral behavior, especially of spectral separation.
In a conventional X-ray CT system, where an object is scanned with a selected incident x-ray spectrum, or kVp, the reconstructed images only approximate the linear X-ray attenuation coefficients of the imaged object at an effective energy of the incident X-ray beam. The errors are primarily the result of beam hardening due to the polychromatic nature of the X-ray spectrum. Modem clinical CT scanners can reduce this error by a process commonly referred to as spectral calibration. Spectral calibration linearizes the measured projection value to the thickness of water. However, beam hardening from bone and contrast agents can still induce shading and streaking artifacts and cause CT number inaccuracies in the image. In this paper, we present a dual kVp scanning method, where during the scan, the kVp is alternately switching between target low and high preset values, typically 80kVp and 140 kVp, with a period less than 1ms. The measured projection pairs are decomposed into the density integrals of two basis materials in projection space. The reconstructed density images are further processed to obtain monochromatic attenuation coefficients of the object at any desired energy. Energy levels yielding optimized monochromatic images are explored, and their analytical representations are derived.
Dual-energy CT has attracted much attention in recent years. Most recently, a fast-kVp switching (FKS) dual-energy method has been presented with clinical and phantom results to demonstrate its efficacy. The purpose of our study was to quantitatively compare the CTDIW of FKS and routine CT exams under the body and head conditions. For a fair comparison, the low contrast detectability (LCD) was matched before measuring dose. In FKS protocols, an x-ray generator switch rapidly between 140kVp and 80kVp in adjacent views, and the effective tube current is around 600mA. In addition to the tube voltage and current, the flux ratio between high and low kVp is optimized by asymmetric sampling of 35%-65%. The head and body protocols further differ by the gantry speed (0.9sec/1.0sec) and type of bowtie filter (head/body). For baseline single-energy, we followed the IEC standard head and body protocols (120kV, 1sec, 5mm) but iteratively adjusted the tube current (mA) in order to match the LCD. CTDIW was measured using either a 16 cm (for head scanning) or a 32 cm (for body scanning) PMMA phantom of at least 14 cm in length. The LCD was measured using the water section of Catphan 600. To make the study repeatable, the automated statistical LCD measurement tool available on GE Discovery CT750 scanner was used in this work. The mean CTDIW for the head and body single-energy acquisitions were 57.5mGy and 29.2mGy, respectively. The LCD was measured at 0.45% and 0.42% (slice thickness=5mm, object size=3mm, central 4 images), respectively. The average CTDIW for FKS head and body scans was 70.4mGy and 33.4mGy, respectively, at the optimal monochromatic energy of 65 keV. The corresponding LCD was measured at 0.45% and 0.43%, respectively. This demonstrates that, with matching LCD, CTDIW of FKS is comparable to that of routine CT exams under head and body conditions.
Over recent decades a quiet revolution has taken place in the application of modern imaging theory to many fields of applied imaging. Nowhere has this movement been more dramatic than within the field of diagnostic medical x-ray imaging, to the extent that there is now a grow- ing consensus around a universal imaging language for the description and inter-comparison of the increasingly diverse range of technologies. This common language owes much to the basic quantum-limited approach pioneered by Rose and his contemporaries. It embodies the funda- mentally statistical nature of image signals, and enables scientists and engineers to develop new system designs optimized for the detection of small signals while constraining patient x-ray ex- posures to tolerable levels. In this paper we attempt to provide a summary of some of the more salient features of progress being made in the understanding of the signal-to-noise limitations of medical imaging systems, and to place this progress within historical context. Reflecting the ex- periences of both authors, emphasis will be given to medical diagnostics based on x-ray imaging techniques.
Tables and graphs of the photon mass attenuation coefficient mu/rho and the mass energy-absorption coefficient mu(en)/rho are presented for all of the elements Z=1 to 92, and for 48 compounds and mixtures of radiological interest. The tables cover energies of the photon (x ray, gamma ray, bremsstrahlung) from 1 keV to 20 MeV. The mu/rho values are taken from the current photon interaction database at the National Institute of Standards and Technology, and the mu(en)/rho values are based on the new calculations by Seltzer described in Radiation Research. These tables of mu/rho and mu(en)/rho replace and extend the tables given by Hubbell in the International Journal of Applied Radiation and Isotopes.
The spatial resolution and noise properties of a computed tomography (CT) image may be altered by two-dimensional linear filtering of the initial image. In this paper, we derive filters that minimize the noise variance subject to a constraint on the spatial resolution. The resulting filter functions can reduce the noise variance by 17% in comparison with conventional filters. The method for obtaining these filters requires knowledge of the noise and imaging properties of the system. We derive theoretical expressions for these properties and introduce experimental techniques for their measurement. The statistical characteristics are shown to be anisotropic, spatially variant, and object dependent. We discuss the implications of this result both for optimal filtering and for the general problem of CT image noise property measurement.
Dual energy CT scanning (tomochemistry) has been proposed as a method for determining various parameters relating to the elemental composition of the tissues. In this paper, our aim is to study the relative noise inherent in two proposed techniques for dual energy scanning; a "two crystal" technique and a "two kV" technique. In the two crystal technique, a split crystal detector is used to simultaneously obtain the high and low energy data during one scan at high kV. The two kV technique requires two scans taken with widely different kV settings. We first review three commonly used approaches for utilizing the scan data to compute the relevant parameters. A theoretical formalism is constructed which aids in understanding these methods. Then this formalism is used to study the influence of CT image noise on measurement precision in the case where the unknown parameters are densities. It is shown that, (1) the unavoidable overlap in the spectral data obtained by the two crystal technique results in a much lower signal-to-noise ratio than can be obtained by using the two kV technique, (2) the necessity for hard filtration of the high energy beam in the two kV technique has not heretofore been appreciated, and (3) the dose for a given x-ray tube heat load is also lower with the two kV technique.
The two-dimensional modulation transfer function [MTF(vx, vy)] has been determined for the EMI CT head scanner by measuring the point spread function (PSF) in different locations in the field of view. This PSF was obtained by scanning a fine wire supported perpendicular to the tomographic slice. Based on these MTFs, the resolving power of the EMI scanner was found to be 3.1 line pairs/cm. Our results also verify the symmetry of the system response and the uniformity of the system resolution.
We report the evaluation of a prototype dual-energy implementation using rapid kVp switching on a clinical computed tomographic scanner. The method employs prereconstruction basis material decomposition of the dual-energy projection data. Each dual-energy scan can be processed into conventional single-kVp images, basis material density images, and monoenergetic images. Phantom studies were carried out to qualitatively and quantitatively evaluate and validate the approach.
The theoretical background of the dual-energy method in CT is discussed in detail, especially the meaning of a measured linear attenuation coefficient of a polyenergetic X-ray spectrum in the presence of beam hardening. Using exact values for the incident spectra the iteration procedure shown here allows one to obtain the true values of the effective atomic number and electron density of an absorber material. The applicability of the method in CT is illustrated by computer simulation of the image reconstruction for a homogeneous and an inhomogeneous phantom.
This article describes a technique in which X-ray transmission readings are taken through the head at a multitude of angles: from these data, absorption values of the material contained within the head are calculated on a computer and presented as a series of pictures of slices of the cranium. The system is approximately 100 times more sensitive than conventional X-ray systems to such an extent that variations in soft tissues of nearly similar density can be displayed.
Dose reduction efforts in diagnostic CT have brought the tradeoff of dose versus image quality to the forefront. The need for meaningful characterization of image noise beyond that offered by pixel standard deviation is becoming increasingly important. This work aims to study the implementation of the noise power spectrum (NPS) and noise equivalent quanta (NEQ) on modern, multislice diagnostic CT scanners. The details of NPS and NEQ measurement are outlined and special attention is paid to issues unique to multislice CT. Aliasing, filter design and effects of acquisition geometry are investigated. While it was found that both metrics can be implemented in modern CT, it was discovered that NEQ cannot be aptly applied with certain non-traditional reconstruction filters or in helical mode. NPS and NEQ under a variety of conditions are examined. Extensions of NPS and NEQ to uses in protocol standardization are also discussed.
Dual-energy material density images obtained by
prereconstruction-basis material decomposition techniques offer specific
tissue information, but they exhibit relatively high pixel noise. It is
shown that noise in the material density images is negatively correlated
and that this can be exploited for noise reduction in the two-basis
material density images. The algorithm minimizes noise-related
differences between pixels and their local mean values, with the
constraint that monoenergetic CT values, which can be calculated from
the density images, remain unchanged. Applied to the material density
images, a noise reduction by factors of 2 to 5 is achieved. While
quantitative results for regions of interest remain unchanged, edge
effects can occur in the processed images. To suppress these, locally
adaptive algorithms are presented and discussed. Results are documented
by both phantom measurements and clinical examples
Several techniques to increase the accuracy and to reduce the variance of the noise power spectrum (NPS) measurement for digital X-ray imaging systems are investigated. These techniques include: (1) averaging the outputs from subblocks of the entire image; (2) averaging the two-dimensional NPS data along a circular route centered on the origin of spectral domain; and (3) masking a window function on each subblock before Fourier transforms. Techniques (1) and (2) are used mainly to reduce the variance of the NPS measurement. Technique (3) serves to improve the accuracy of the final result. Experiments with two different charge-coupled device-based X-ray imaging systems demonstrated that the precision and accuracy of the NPS measurement could be significantly improved using these techniques. The impact of the image partition for averaging is discussed and the corresponding NPS estimations are presented for the number of subblocks ranging from 4 to 64. The effect of masking on the NPS is also studied using four different window functions.
First performance evaluation of a dual-source CT ͑DSCT͒ system Application of the noise power spectrum in modern diagnostic MDCT: Part I. Measure-ment of noise power spectra and noise equivalent quanta The noise power spectrum in computed x-ray tomography
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