Takanaga Niimi

Statistical characteristics of streak artifacts on CT images: Relationship between streak artifacts and mA s values

The purpose of this study is to investigate how streak artifacts on computed tomography (CT) images vary with reduction in radiation doses by assessing the quantitative relationship between the streak artifacts and milliampere-time product (mA s) values. A commercially available chest phantom was used to measure the streak artifacts on the CT images obtained using a 4- and 16-multidetector-row helical CT scanners with various mA s values at a constant tube voltage of 120 kVp. The cardiac slice image was employed as a target image for evaluating the streak artifacts on the CT image. Eighty parallel line segments with a length of 20 pixels were placed perpendicular to numerous streak artifacts on the cardiac slice image, and the largest difference between adjacent CT values in each of the 80 CT-value profiles of these line segments was employed as a feature variable of streak artifacts; these feature variables have been analyzed by the extreme value theory. The largest difference between adjacent CT values in each CT-value profile can be statistically modeled by a Gumbel distribution. Further, the maximum level of streak artifacts on CT images that will be tolerated for clinical use and low-dose CT screening examination was expected to be estimated using the location parameter in the Gumbel distribution.

A detection method for streak artifacts and radiological noise in a non-uniform region in a CT image.

By using the CT images obtained by subtracting two CT images acquired under the same conditions and slice locations, we have devised a method for detecting streak artifacts in non-uniform regions and only radiological noise components in CT images. A chest phantom was scanned using 16- and 64-multidetector row helical CT scanners with various mAs values at 120kVp. The upper lung slice image was employed as a target image for evaluating the streak artifacts and radiological noise. One hundred parallel line segments with a length of 80 pixels were placed on the subtracted CT image, and the largest CT value in each CT value profile was employed as a feature variable of the streak artifacts; these feature variables were analyzed with the extreme value theory (Gumbel distribution). To detect only the radiological noise, all CT values contained in the 100 line profile were plotted on normal probability paper and the standard deviation was estimated from the inclination of its fitted line for the CT value plots. The two detection methods devised in this study were able to evaluate the streak artifacts and radiological noise in the CT images with high accuracy.

Quantitative assessment of image noise and streak artifact on CT image: Comparison of z-axis automatic tube current modulation technique with fixed tube current technique

The purpose of our study is to quantitatively assess the effects of z-axis automatic tube current modulation technique on image noise and streak artifact, by comparing with fixed tube current technique. Standard deviation of CT-values was employed as a physical index for evaluating image noise, and streak artifact was quantitatively evaluated using our devised Gumbel evaluation method. z-Axis automatic tube current modulation technique will improve image noise and streak artifact, compared with fixed tube current technique, and will make it possible to significantly reduce radiation doses at lung levels while maintaining the same image quality as fixed tube current technique.

Emerging Myocardial SPECT with 201 Tl Using Semiconductor Detectors: Clinical Application to Stress Myocardial Perfusion Scintigraphy

Novel single photon emission computed tomography (D-SPECT) scanners with solid-state semiconductor detectors using cadmium zinc-telluride (CZT) have been clinically applied. D-SPECT is a second scanner in Japan in which CZT detectors offer higher sensitivity and better spatial resolution than conventional Anger cameras. Myocardial counts are important to assure image quality that might be improved due to the favorable physical performance of the CZT detectors. Different physical properties between CZT detectors and Anger cameras alter the quality of myocardial SPECT images. The image quality of 201 Tl-SPECT improved after introducing CZT detectors to achieve that of 99m Tc. In addition, results from ECG-gated myocardial SPECT in clinical practice such as left ventricular volumes and ejection fraction were similar. These results could be applied to patients in the same way as those of conventional Anger cameras. However, reverse redistribution appears myocardial 201 Tl-SPECT images of apparently normal persons. Uptake in apical areas is sometimes reduced. In conclusion, myocardial perfusion scintigraphy with 201 Tl and CZT detectors has been clinically applied in Japan. Some 201 Tl data from gamma cameras and CZT detectors are identical, but others differ. Further investigation is needed to optimize myocardial perfusion scintigraphy with 201 Tl.

Evaluation of Cadmium-Zinc-Telluride Detector-based Single-Photon Emission Computed Tomography for Nuclear Cardiology: a Comparison with Conventional Anger Single-Photon Emission Computed Tomography

PurposeThe differences in performance between the cadmium-zinc-telluride (CZT) camera or collimation systems and conventional Anger single-photon emission computed tomography (A-SPECT) remain insufficient from the viewpoint of the user. We evaluated the performance of the D-SPECT (Spectrum Dynamics, Israel) system to provide more information to the cardiologist or radiological technologist about its use in the clinical field.Materials and MethodsThis study evaluated the performance of the D-SPECT system in terms of energy resolution, detector sensitivity, spatial resolution, modulation transfer function (MTF), and collimator resolution in comparison with that of A-SPECT (Bright-View, Philips, Japan). Energy resolution and detector sensitivity were measured for Tc-99m, I-123, and Tl-201. The SPECT images produced by both systems were evaluated visually using the anthropomorphic torso phantom.ResultsThe energy resolution of D-SPECT with Tc-99m and I-123 was approximately two times higher than that of A-SPECT. The detector sensitivity of D-SPECT was higher than that of A-SPECT (Tc-99m: 4.2 times, I-123: 2.2 times, and Tl-201: 5.9 times). The mean spatial resolution of D-SPECT was two times higher than that of A-SPECT. The MTF of D-SPECT was superior to that of the A-SPECT system for all frequencies. The collimator resolution of D-SPECT was lower than that of A-SPECT; however, the D-SPECT images clearly indicated better spatial resolution than the A-SPECT images.ConclusionThe energy resolution, detector sensitivity, spatial resolution, and MTF of D-SPECT were superior to those of A-SPECT. Although the collimator resolution was lower than that of A-SPECT, the D-SPECT images were clearly of better quality.

Fractal-feature distance analysis of contrast-detail phantom image and meaning of pseudo fractal dimension and complexity*

The purposes of our studies are to examine whether or not fractal-feature distance deduced from virtual volume method can simulate observer performance indices and to investigate the physical meaning of pseudo fractal dimension and complexity. Contrast-detail (C-D) phantom radiographs were obtained at various mAs values (0.5-4.0 mAs) and 140 kVp with a computed radiography system, and the reference image was acquired at 13 mAs. For all C-D images, fractal analysis was conducted using the virtual volume method that was devised with a fractional Brownian motion model. The fractal-feature distances between the considered and reference images were calculated using pseudo fractal dimension and complexity. Further, we have performed the C-D analysis in which ten radiologists participated, and compared the fractal-feature distances with the image quality figures (IQF). To clarify the physical meaning of the pseudo fractal dimension and complexity, contrast-to-noise ratio (CNR) and standard deviation (SD) of images noise were calculated for each mAs and compared with the pseudo fractal dimension and complexity, respectively. A strong linear correlation was found between the fractal-feature distance and IQF. The pseudo fractal dimensions became large as CNR increased. Further, a linear correlation was found between the exponential complexity and image noise SD.