Radiological Physics and Technology
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Publications in this journal
- [Show abstract] [Hide abstract] ABSTRACT: We investigated the underlying reasons for the occurrence of misalignment artifacts in prospectively ECG-triggered axial coronary CT angiography scans. In this study we analyzed 56 consecutive patients scanned in axial mode and 66 consecutive patients scanned in helical mode. Predictors for the occurrence of misalignment artifacts were evaluated by multivariable logistic regression analysis for those patients scanned in the axial mode; advanced age was identified as the sole independent predictor (odds ratio: 1.088; 95 % CI: 1.012-1.170; p = 0.0228). In a comparison with the patients scanned in the helical mode, the image quality score for patients aged 65 years or older was significantly higher in helical mode than in axial mode (2.6 ± 0.5 and 2.4 ± 0.7, respectively; p = 0.0313). Misalignment artifacts in the image are more common in the elderly than in younger. Helical mode should be preferred in this older patient population to allow for good diagnostic image quality.
- [Show abstract] [Hide abstract] ABSTRACT: Our purpose in this study was to reduce the noise in order to improve the SNR of Dw images with high b-value by using two correction schemes. This study was performed with use of phantoms made from water and sucrose at different concentrations, which were 10, 30, and 50 weight percent (wt%). In noise reduction for Dw imaging of the phantoms, we compared two correction schemes that are based on the Rician distribution and the Gaussian distribution. The highest error values for each concentration with use of the Rician distribution scheme were 7.3 % for 10 wt%, 2.4 % for 30 wt%, and 0.1 % for 50 wt%. The highest error values for each concentration with use of the Gaussian distribution scheme were 20.3 % for 10 wt%, 11.6 % for 30 wt%, and 3.4 % for 50 wt%. In Dw imaging, the noise reduction makes it possible to apply the correction scheme of Rician distribution.
- [Show abstract] [Hide abstract] ABSTRACT: The ordered subset expectation maximization with a point spread function (OSEM-PSF) was developed to improve the spatial resolution of reconstructed positron emission tomography (PET) images and has been reported to improve the contrast of hot spots in PET studies for oncology. However, in neuroreceptor imaging, the regional radioactivity concentration changes dynamically during the scan, and the effects of the PSF may differ among various radioligands or quantification methods. In this study, we investigated the effects of the PSF on quantification in PET studies with [(11)C]FLB 457 of dopamine D2 receptors, using both phantom and human data acquired by the Siemens Biograph 16 imaging platform. In the phantom studies, we evaluated the hot contrast recovery coefficient (HCRC) for variously sized hot spheres and the linearity between the measured and true radioactivities in OSEM-PSF images. Next, in the human studies with [(11)C]FLB 457, radioactivity concentrations and binding potentials for the cerebral cortex and thalamus were compared between images reconstructed with and without PSF. In the phantom studies, the OSEM-PSF images showed a better HCRC compared to images without PSF, and they showed a good linear correlation with true radioactivity. In the human studies, the radioactivity concentration increased especially in small regions with high accumulation of [(11)C]FLB 457 when the PSF was included. However, little difference in the binding potentials was observed for the target regions between both types of reconstructed images. In conclusion, PSF-based reconstruction reduced the spill-over phenomena in small hot regions; however, it caused no increase in the binding potentials in the [(11)C]FLB 457 studies.
- [Show abstract] [Hide abstract] ABSTRACT: Modern radiotherapy treatment techniques commonly include multi-leaf collimators (MLCs) to shape the treatment fields and to conform the radiation dose to the target volume. MLCs require accurate and frequent quality assurance (QA) to ensure spatial and temporal accuracy of the leaves in order to allow optimal dose delivery to the patient. In this study, the accuracy and efficacy of AutoCAL, a commercial software for MLC QA, were evaluated. The software was found to be reproducible to within 0.2 mm and to correspond with conventional QA methods (within 1.2 and 0.9 mm of film and water tank measurements, respectively.) Thus, AutoCAL was found to be an accurate and efficient tool for routine MLC QA and calibration.
- [Show abstract] [Hide abstract] ABSTRACT: This study verified the dose calculation accuracy of the analytical anisotropic algorithm (AAA), Acuros XB version 10 (AXB10), and version 11 (AXB11) installed in an Eclipse treatment planning system, by comparing with Monte Carlo (MC) simulations. First, the algorithms were compared in terms of dose distributions using four types of virtual heterogeneous multi-layer phantom for 6 and 15 MV photons. Next, the clinical head and neck intensity-modulated radiation therapy (IMRT) dose distributions for 6 MV photons were evaluated using dose volume histograms (DVHs) and three-dimensional gamma analysis. In percentage depth doses (PDDs) for virtual heterogeneous phantoms, AAA overestimated absorbed doses in the air cavity, bone, and aluminum in comparison with MC, AXB10, and AXB11. The PDDs of AXB10 almost agreed with those of MC and AXB11, except for the air cavity. The dose in the air cavity was higher for AXB10 than for AXB11, because their electron cutoff energies are set at 500 and 200 keV, respectively. For head and neck IMRT dose distributions, the D95 in the clinical target volume (CTV) for AAA was almost the same as that for AXB10 and was approximately 7 % larger than that for MC. Comparing each approach with MC using a criterion of 3 %/3 mm, the pass rates for AXB10, AXB11, and AAA were 92.4, 94.7, and 90.4 % in the CTV, respectively. In conclusion, AAA produces dose errors in heterogeneous regions, while AXB11 provides calculation accuracy comparable to MC. AXB10 overestimates the dose in regions that include an air cavity.
- [Show abstract] [Hide abstract] ABSTRACT: At the quantum-mechanical level, all substances (not merely electromagnetic waves such as light and X-rays) exhibit wave-particle duality. Whereas students of radiation science can easily understand the wave nature of electromagnetic waves, the particle (photon) nature may elude them. Therefore, to assist students in understanding the wave-particle duality of electromagnetic waves, we have developed a photon-counting camera that captures single photons in two-dimensional images. As an image intensifier, this camera has a triple-stacked micro-channel plate (MCP) with an amplification factor of 10(6). The ultra-low light of a single photon entering the camera is first converted to an electron through the photoelectric effect on the photocathode. The electron is intensified by the triple-stacked MCP and then converted to a visible light distribution, which is measured by a high-sensitivity complementary metal oxide semiconductor image sensor. Because it detects individual photons, the photon-counting camera is expected to provide students with a complete understanding of the particle nature of electromagnetic waves. Moreover, it measures ultra-weak light that cannot be detected by ordinary low-sensitivity cameras. Therefore, it is suitable for experimental research on scintillator luminescence, biophoton detection, and similar topics.
- [Show abstract] [Hide abstract] ABSTRACT: Studies show that the radiation dose received during a micro-CT examination may have adverse effects on living subjects. However, the correlations between the biological effects and the radiation doses have never been thoroughly evaluated in the majority of cases. In this study, we evaluated the biological radiation effects of measured radiation doses in ICR mice using cone-beam micro-CT scans. Long-term in vivo whole-body micro-CT scans of ICR mice were performed for a duration of 4 weeks. Although a scanning frequency of three scans per week is higher than that necessary for conventional studies, this study represents particular cases where the subjects may undergo an extreme number of examinations. The average X-ray dose of a CT scan measures 16.19 mGy at the center of a phantom and 16.24 mGy at an offset position of 7.5 mm from the center of the phantom. The total average dose at the center of the phantom during the 4-week scanning period was 194.3 mGy. No significant radiation effects were observed in the weight gain curves, organ weights, blood analyses, litter sizes, reared offspring sizes, and the histopathologic results. Therefore, it is unlikely that the measured doses for the CT scans caused any radiation damage in the mice.
- [Show abstract] [Hide abstract] ABSTRACT: Practical simulations of low-dose CT images have a possibility of being helpful means for optimization of the CT exposure dose. Because current methods reported by several researchers are limited to specific vendor platforms and generally rely on raw sinogram data that are difficult to access, we have developed a new computerized scheme for producing simulated low-dose CT images from real high-dose images without use of raw sinogram data or of a particular phantom. Our computerized scheme for low-dose CT simulation was based on the addition of a simulated noise image to a real high-dose CT image reconstructed by the filtered back-projection algorithm. First, a sinogram was generated from the forward projection of a high-dose CT image. Then, an additional noise sinogram resulting from use of a reduced exposure dose was estimated from a predetermined noise model. Finally, a noise CT image was reconstructed with a predetermined filter and was added to the real high-dose CT image to create a simulated low-dose CT image. The noise power spectrum and modulation transfer function of the simulated low-dose images were very close to those of the real low-dose images. In order to confirm the feasibility of our method, we applied this method to clinical cases which were examined with the high dose initially and then followed with a low-dose CT. In conclusion, our proposed method could simulate the low-dose CT images from their real high-dose images with sufficient accuracy and could be used for determining the optimal dose setting for various clinical CT examinations.
- [Show abstract] [Hide abstract] ABSTRACT: Our purpose in this study was to construct a 3-dimensional (3D) region of interest (ROI) for analyzing the time-signal intensity curve (TIC) semi-automatically in dynamic contrast-enhanced magnetic resonance (DCE-MR) imaging of the breast. DCE-MR breast imaging datasets were acquired by a 3.0-Tesla MR system with the use of a 3D fast gradient echo sequence. The essential idea in the new method was to analyze each pixel and to construct an ROI made up of pixels with similar TICs. First, an analyst selected a starting point in the contrast media-enhanced tumor. Second, we calculated Pearson's correlation coefficients (CCs) between the TIC in the starting coordinate selected by the analyst and the TIC in the other coordinates. Third, ROI pixels were selected if their CC threshold satisfied a level of coefficient variation of the ROI determined by prior research performed in our institution. We made a retrospective review of patients who underwent breast DCE-MR examination for pre-operative diagnosis. To confirm the feasibility of the resulting 3D-ROI from TIC analysis, we compared Fischer's score obtained from 3D-ROI by applying a new method to a score obtained from a manually selected 2-dimensional (2D) ROI which was used during routine clinical examination. The Fischer's scores obtained from both the automatically selected 3D-ROI and the manually selected 2D-ROI showed almost equivalent results. Thus, we considered that the new method was comparable to the conventional method. Furthermore, the new method has the potential to be used for evaluation of the extent of tumors.
- [Show abstract] [Hide abstract] ABSTRACT: Our purpose in this study was to evaluate the clinical usefulness of a new skyline-view imaging technique for axial projection of the patella with use of the anterior border of the patella and tibial tuberosity as position indicators. Our database consisted of pairs of axial images of the patella of the same patients, obtained with use of conventional and new techniques for the radiographic diagnosis of knee-joint diseases. A total of 118 pairs of knee images were obtained from 103 patients ranging in age from 16 to 86 years (mean age 49.7 years). The patellar axial positioning errors were determined in each of the images obtained with the two techniques. The relative error according to the patellar tilt was determined from each of the axial images of the patellas of the same patients obtained with the conventional and new techniques for the radiographic diagnosis of knee-joint diseases. The patellar axial positioning error was 0.40 with the conventional technique, whereas that with the new technique was significantly different at 0.30. This clinical study confirmed that the new skyline-view imaging technique, which uses the anterior border of the patella and the tibial tuberosity as position markers that can be confirmed by palpation, provides more accurate axial images than the conventional imaging technique.
- [Show abstract] [Hide abstract] ABSTRACT: We devised a new noise filtering method to reduce the noise in the line spread function (LSF) for presampled modulation transfer function (MTF) analysis with the edge method. A filter was designed to reduce noise effectively using a position-dependent filter controlled by the boundary frequency b for low-pass filtering, which is calculated by 1/2d (d: distance from the LSF center). In this filtering process, strong filters with very low b can be applied to regions distant from the LSF center, and the region near the LSF center can be maintained simultaneously by a correspondingly high b. Presampled MTF accuracies derived by use of the proposed method and an edge spread function (ESF)-fitting method were compared by use of simulated ESFs with and without noise, resembling a computed radiography (CR) and an indirect-type flat panel detector (FPD), respectively. In addition, the edge images of clinical CR, indirect-type FPD, and direct-type FPD systems were examined. For a simulated ESF without noise, the calculated MTFs of the variable filtering method agreed precisely with the true MTFs. The excellent noise-reduction ability of the variable filter was demonstrated for all simulated noisy ESFs and those of three clinical systems. Although the ESF-fitting method provided excellent noise reduction only for the CR-like simulated ESF with noise, its noise elimination performance could not be demonstrated due to the lesser robustness of the fitting.
- [Show abstract] [Hide abstract] ABSTRACT: The signal-to-noise ratio (SNR) and parallel imaging (PI) performance of two commercial phased-array coils (PACs) were examined in magnetic resonance imaging (MRI) of the brain. All measurements were performed on a 3.0 T MRI instrument. The SNR and PI performance were evaluated with 32-channel and 15-channel PACs. A gradient echo sequence was used for obtaining images of a phantom. SNR and geometry factor (g-factor) maps were calculated from two images with identical parameters. Horizontal and vertical profiles were taken through the SNR maps in the axial plane. The average g-factor was measured in a circular region of interest in the g-factor maps for the axial plane. The SNR map of the 32-channel coil showed a higher SNR than that of the 15-channel coil at the phantom's posterior and lateral surfaces. The SNR profiles for the 32-channel coil also showed a 1.3-fold increase at the phantom's center. The average g-factor of the 32-channel coil was lower than that of the 15-channel coil at the same acceleration factor. These results indicate that the 32-channel coil can provide a higher spatial resolution and/or a faster imaging speed. Horizontal and vertical profiles are useful for evaluation of the performance of commercially available PACs.
- [Show abstract] [Hide abstract] ABSTRACT: This paper presents an iterative image reconstruction method for radial encodings in MRI based on a total variation (TV) regularization. The algebraic reconstruction method combined with total variation regularization (ART_TV) is implemented with a regularization parameter specifying the weight of the TV term in the optimization process. We used numerical simulations of a Shepp-Logan phantom, as well as experimental imaging of a phantom that included a rectangular-wave chart, to evaluate the performance of ART_TV, and to compare it with that of the Fourier transform (FT) method. The trade-off between spatial resolution and signal-to-noise ratio (SNR) was investigated for different values of the regularization parameter by experiments on a phantom and a commercially available MRI system. ART_TV was inferior to the FT with respect to the evaluation of the modulation transfer function (MTF), especially at high frequencies; however, it outperformed the FT with regard to the SNR. In accordance with the results of SNR measurement, visual impression suggested that the image quality of ART_TV was better than that of the FT for reconstruction of a noisy image of a kiwi fruit. In conclusion, ART_TV provides radial MRI with improved image quality for low-SNR data; however, the regularization parameter in ART_TV is a critical factor for obtaining improvement over the FT.
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