Validating computer applications for calculating spatial resolution and noise property in CT using simulated images with known properties.

The purpose of this study was to evaluate, using simulated images with known property values, how accurately some computer applications for calculating modulation transfer function (MTF), task transfer function (TTF), or noise power spectrum (NPS) in computed tomography (CT) based on widely known techniques produce their results. Specifically, they were three applications applicable to the wire method for MTF calculation, two applications corresponding to the circular edge (CE) and linear edge (LE) methods for TTF, and one application using a two-dimensional Fourier transform for NPS, which are collectively integrated with the software 'CTmeasure' provided by the Japanese Society of CT Technology. Images for the calculation with radial symmetry were generated based on a roll-off type filter function. The accuracy of each application was evaluated by comparing the calculated property with the true one. The calculated MTFs for the wire method accurately matched the true ones with percentage errors of smaller than 1.0%. In contrast, the CE and LE methods presented relatively large errors of up to 50% at high frequencies, whereas the NPS's errors were up to 30%. A closer investigation revealed, however, that these errors were attributable not to the applications but to the insufficiencies in the measurement techniques commonly employed. By improving the measurement conditions to minimize the effects of the insufficiencies, the errors notably decreased, whichvalidated the calculation techniques in the applications we used.

Effect of computed tomography value error on dose calculation in adaptive radiotherapy with Elekta X-ray volume imaging cone beam computed tomography.

PURPOSE: We evaluated the effect of changing the scan mode of the Elekta X-ray volume imaging cone beam computed tomography (CBCT) on the accuracy of dose calculation, which may be affected by computed tomography (CT) value errors in three dimensions. METHODS: We used the electron density phantom and measured the CT values in three dimensions. CT values were compared with planning computed tomography (pCT) values for various materials. The evaluated scan modes were for head and neck (S-scan), chest (M-scan), and pelvis (L-scan) with various collimators and filter systems. To evaluate the effects of the CT value error of the CBCT on dose error, Monte Carlo calculations of dosimetry were performed using pCT and CBCT images. RESULTS: The L-scan had a CT value error of approximately 800 HU at the isocenter compared with the pCT. Furthermore, inhomogeneity in the longitudinal CT value profile was observed in the bone material. The dose error for ±100 HU difference in CT values for the S-scan and M-scan was within ±2%. The center of the L-scan had a CT error of approximately 800 HU and a dose error of approximately 6%. The dose error of the L-scan occurred in the beam path in the case of both single field and two parallel opposed fields, and the maximum error occurred at the center of the phantom in the case of both the 4-field box and single-arc techniques. CONCLUSIONS: We demonstrated the three-dimensional CT value characteristics of the CBCT by evaluating the CT value error obtained under various imaging conditions. It was found that the L-scan is considerably affected by not having a unique bowtie filter, and the S-scan without the bowtie filter causes CT value errors in the longitudinal direction. Moreover, the CBCT dose errors for the 4-field box and single-arc irradiation techniques converge to the isocenter.

Deep learning-based reconstruction in ultra-high-resolution computed tomography: Can image noise caused by high definition detector and the miniaturization of matrix element size be improved?

PURPOSE: This study aimed to assess the noise characteristics of ultra-high-resolution computed tomography (UHRCT) with deep learning-based reconstruction (DLR). METHODS: Two different diameters of water phantom were scanned with three different resolution acquisition modes. Images were reconstructed by filtered back projection (FBP), hybrid iterative reconstruction (hybrid-IR), and DLR. Image noise analysis was performed with noise magnitude, peak frequency (fp) of the noise power spectrum (NPS), and the square root of the area under the curve (√AUCNPS) for the NPS curve. RESULTS: The noise magnitude was up to 3.30 times higher for the FBP acquired in SHR mode than that for the NR mode. The fp values of the FBP were 0.20-0.21, 0.34-0.36, and 0.34-0.37 cycles/mm for normal resolution (NR), high resolution (HR), and super high resolution (SHR) mode, respectively. The fp of hybrid-IR was 0.16-0.19, 0.21-0.26, and 0.23-0.26 cycles/mm for NR, HR, and SHR mode, respectively. The fp of DLR was 0.21-0.32 and 0.22-0.33 cycles/mm for HR and SHR mode, respectively. √AUCNPS showed that the highest value in FBP images of the SHR mode was up to 1.89 times that of the NR mode. DLR in the HR and SHR modes showed high noise reduction while suppressing fp shift with respect to FBP. CONCLUSIONS: The new DLR algorithm could be a solution to the noise increase due to the high-definition detector elements and the small reconstruction matrix element size.

Assessment of longitudinal beam property and contrast uniformity for 256- and 320-row area detector computed tomography scanners in the 160-mm nonhelical volume-acquisition mode.

BACKGROUND: Because the x-ray property of patient longitudinal axis in area detector computed tomography (ADCT) depends on a heel effect, radiation dose and beam quality are not uniform along the long axis of the patient. OBJECTIVE: This study aimed to measure the longitudinal beam properties and contrast uniformity of ADCT scanners in the 160-mm nonhelical volume-acquisition (NVA) mode and provide useful datasets for the radiation dose reduction in ADCT examinations. MATERIALS AND METHODS: Two different types of ADCT scanners were used in this study. To assess the heel effect in 256- and 320-row ADCT scanners, we measured dose profile, half-value layer, and iodine contrast uniformity along longitudinal beam direction. RESULTS: The maximum effective energy difference within a 160-mm x-ray beam is approximately 4 keV. Maximum radiation dose on the anode side of the x-ray tube showed approximately 40%-45% reduction compared with that on the isocenter position; the heel effect properties longitudinally differed throughout the x-ray beam, and the decrease in the radiation dose in 256- and 320-row ADCT scanners was observed on the patient table side and gantry side respectively. The CT numbers of iodinated solutions for 256-row ADCT scanner were independent of the heel effect; nevertheless, the CT numbers of 320-row ADCT scanner tended to increase on the patient table (cathode) side. CONCLUSION: This study reveals that the radiation dose on the anode side of the x-ray tube shows approximately 40%-45% reduction compared with that on the isocenter position, and the heel effect properties for 256- and 320-row ADCT scanners longitudinally differ throughout the x-ray beam. The x-ray tube for individual ADCT scanners is mounted in an opposite direction along the long axis of the patient.

Overranging and overbeaming measurement in area detector computed tomography: A method for simultaneous measurement in volume helical acquisition.

PURPOSE: We propose a novel method to assess overbeaming and overranging, as well as the effect of reducing longitudinal exposure range, by using a dynamic z-collimator in area detector computed tomography. METHODS AND MATERIALS: A 500-mm diameter cylindrical imaging plate was exposed by helical scanning in a dark room. The beam collimation of the helical acquisitions was set at 32 and 80 mm. Overbeaming and overranging with the dynamic z-collimator were measured. RESULTS: The actual beam widths were approximately 39 and 88 mm at 32 and 80 mm collimation, respectively, and were relatively reduced owing to increased beam collimation. Overranging was 27.0 and 48.2 mm with a pitch of 0.83 and 1.49 at 32 mm collimation and 72.5 and 83.1 mm with a pitch of 0.87 and 0.99 at 80 mm collimation. The dynamic z-collimator relatively reduced the overranging by 17.3% and 17.1% for the 32 and 80 mm collimation, respectively. CONCLUSION: We devised a method to simultaneously measure overbeaming and overranging with only one helical acquisition. Although the dynamic z-collimator reduced the overranging by approximately 17%, wider collimation widths and higher pitch settings would increase the exposure dose outside the scan range.

Effective temporal resolution and image quality of volume scanning in 320-row detector CT.

To measure the effective temporal resolution (eTR) and image quality for three reconstruction modes for non-helical volume scanning in area detector CT. Temporal sensitivity profiles (TSPs) were obtained and the full width of the TSP at half maximum was used as an index of the eTR. Image quality was assessed by image noise and the corrected artifact index. The half reconstruction mode had a higher eTR than the full and automatic patient motion collection (APMC) reconstructions. Compared to full reconstruction, the image noise with APMC and half reconstruction were increased by 16% and 35%. The corrected artifact index was lowest with APMC. The square root of full width at tenth maximum of the TSP showed a high coefficient of determination (R2 = 0.934) for image noise. This study revealed the TSPs and eTRs for non-helical volume scanning in area detector CT. A high eTR resulted in higher image noise.

Comparative evaluation of image quality among different detector configurations using area detector computed tomography.

The 320-detector row computed tomography (CT) system, i.e., the area detector CT (ADCT), can perform helical scanning with detector configurations of 4-, 16-, 32-, 64-, 80-, 100-, and 160-detector rows for routine CT examinations. This phantom study aimed to compare the quality of images obtained using helical scan mode with different detector configurations. The image quality was measured using modulation transfer function (MTF) and noise power spectrum (NPS). The system performance function (SP), based on the pre-whitening theorem, was calculated as MTF2/NPS, and compared between configurations. Five detector configurations, i.e., 0.5 × 16 mm (16 row), 0.5 × 64 mm (64 row), 0.5 × 80 mm (80 row), 0.5 × 100 mm (100 row), and 0.5 × 160 mm (160 row), were compared using a constant volume CT dose index (CTDIvol) of 25 mGy, simulating the scan of an adult abdomen, and with a constant effective mAs value. The MTF was measured using the wire method, and the NPS was measured from images of a 20-cm diameter phantom with uniform content. The SP of 80-row configuration was the best, for the constant CTDIvol, followed by the 64-, 160-, 16-, and 100-row configurations. The decrease in the rate of the 100- and 160-row configurations from the 80-row configuration was approximately 30%. For the constant effective mAs, the SPs of the 100-row and 160-row configurations were significantly lower, compared with the other three detector configurations. The 80- and 64-row configurations were adequate in cases that required dose efficiency rather than scan speed.

Influence of Acquisition Mode of Cone-beam Computed Tomography on Accuracy of Image Registration for Image-guided Radiotherapy.

Half scan can acquire images at the 200° rotation in image-guided radiation treatment using cone-beam CT and is useful to evaluate the influence of the half-scan-imaging start angle and imaging direction on image registration accuracy. The half-scan-imaging start angle is changed from 180° to 340° in the clockwise direction and from 180° to 20° in the counter clockwise direction to calculate the registration error. As a result, registration errors between -0.37 mm and 0.27 mm in the left and right directions occur because of the difference in the imaging start angle and approximately 0.3° in the gantry rotation direction because of the difference in the imaging direction. Because half scan does not have data for 360° rotation, depending on the subject structure, inconsistency of opposing data can lower reconstruction accuracy and cause a verification error. In addition, in image acquisition during rotation, the slower the shutter speed is, the more the actual gantry angle and angle information of the image are apart, which is considered the cause of rotation errors. Although these errors are very minute, it is thought that there is no influence on the treatment effect, but these errors are considered an evaluation item indispensable for ensuring the accuracy of high-precision radiation treatment. In addition, these errors need to be considered for ensuring the quality of high-precision radiation treatment.

Objective assessment of low-contrast computed tomography images with iterative reconstruction.

OBJECTIVE: This study aims to assess low-contrast image quality using a low-contrast object specific contrast-to-noise ratio (CNRLO) analysis for iterative reconstruction (IR) computed tomography (CT) images. METHODS: A phantom composed of low-contrast rods placed in a uniform material was used in this study. Images were reconstructed using filtered back projection (FBP) and IR (Adaptive Iterative Dose Reduction 3D). Scans were performed at six dose levels: 1.0, 1.8, 3.1, 4.6, 7.1 and 13.3mGy. Objective image quality was assessed by comparing CNRLO with CNR using a human observer test. RESULTS: Compared with FBP, IR yielded increased CNR at the same dose levels. The results of CNRLO and observer tests showed similarities or only marginal differences between FBP and IR at the same dose levels. The coefficient of determination for CNRLO was significantly better (R(2)=0.86) than that of CNR (R(2)=0.47). CONCLUSION: For IR, CNRLO could potentially serve as an objective index reflective of a human observer assessment. The results of CNRLO test indicated that the IR algorithm was not superior to FBP in terms of low-contrast detectability at the same radiation doses.

Temporal resolution measurement of 128-slice dual source and 320-row area detector computed tomography scanners in helical acquisition mode using the impulse method.

PURPOSE: To analyse the temporal resolution (TR) of modern computed tomography (CT) scanners using the impulse method, and assess the actual maximum TR at respective helical acquisition modes. METHODS: To assess the actual TR of helical acquisition modes of a 128-slice dual source CT (DSCT) scanner and a 320-row area detector CT (ADCT) scanner, we assessed the TRs of various acquisition combinations of a pitch factor (P) and gantry rotation time (R). RESULTS: The TR of the helical acquisition modes for the 128-slice DSCT scanner continuously improved with a shorter gantry rotation time and greater pitch factor. However, for the 320-row ADCT scanner, the TR with a pitch factor of <1.0 was almost equal to the gantry rotation time, whereas with pitch factor of >1.0, it was approximately one half of the gantry rotation time. The maximum TR values of single- and dual-source helical acquisition modes for the 128-slice DSCT scanner were 0.138 (R/P=0.285/1.5) and 0.074s (R/P=0.285/3.2), and the maximum TR values of the 64×0.5- and 160×0.5-mm detector configurations of the helical acquisition modes for the 320-row ADCT scanner were 0.120 (R/P=0.275/1.375) and 0.195s (R/P=0.3/0.6), respectively. CONCLUSION: Because the TR of a CT scanner is not accurately depicted in the specifications of the individual scanner, appropriate acquisition conditions should be determined based on the actual TR measurement.

Optimization of scan timing for aortic computed tomographic angiography using the test bolus injection technique.

BACKGROUND: With fast computed tomography (CT), it is possible for the scanning to outpace the contrast medium bolus during aortic CT angiography (CTA). PURPOSE: To evaluate the effectiveness of a new method for reducing the risk of outpacing in which the scan start timing (ST) and speed can be estimated from the peak enhancement time measured at the femoral artery using a single test-bolus injection (femoral artery test injection method [FTI method]). MATERIAL AND METHODS: In 30 cases of aortic CTA, we measured the time to peak enhancement at the femoral artery (TPF) and the ascending aorta (TPA) with test-bolus injection performed twice in each examination. From the resultant linear relationship between TPF and transit time (TT = TPF - TPA), we developed a method for determining the ST and TT from TPF. One hundred patients were assigned to two groups: FTI and bolus tracking (BT), each with 50 patients. CT values were measured in main vessels (ascending aorta, descending aorta, femoral artery). The CT values of the vessels and the rate of cases with more than 300 HU (good cases) were compared between the two groups. RESULTS: The enhancement in the FTI method was significantly higher than that of the BT method (average CT values: FTI, 388.3 ± 52.4; BT, 281.2 ± 59.1; P < 0.001). The rates of good cases for FTI and BT were 86.0% and 46.0%, respectively. CONCLUSION: The FTI method was very effective in reducing the risk of outpacing of the contrast medium transit in aortic CTA without the need for an additional contrast medium dose.

Assessment of temporal resolution of multi-detector row computed tomography in helical acquisition mode using the impulse method.

The purpose of this study was to propose a method for assessing the temporal resolution (TR) of multi-detector row computed tomography (CT) (MDCT) in the helical acquisition mode using temporal impulse signals generated by a metal ball passing through the acquisition plane. An 11-mm diameter metal ball was shot along the central axis at approximately 5 m/s during a helical acquisition, and the temporal sensitivity profile (TSP) was measured from the streak image intensities in the reconstructed helical CT images. To assess the validity, we compared the measured and theoretical TSPs for the 4-channel modes of two MDCT systems. A 64-channel MDCT system was used to compare TSPs and image quality of a motion phantom for the pitch factors P of 0.6, 0.8, 1.0 and 1.2 with a rotation time R of 0.5 s, and for two R/P combinations of 0.5/1.2 and 0.33/0.8. Moreover, the temporal transfer functions (TFs) were calculated from the obtained TSPs. The measured and theoretical TSPs showed perfect agreement. The TSP narrowed with an increase in the pitch factor. The image sharpness of the 0.33/0.8 combination was inferior to that of the 0.5/1.2 combination, despite their almost identical full width at tenth maximum values. The temporal TFs quantitatively confirmed these differences. The TSP results demonstrated that the TR in the helical acquisition mode significantly depended on the pitch factor as well as the rotation time, and the pitch factor and reconstruction algorithm affected the TSP shape.

[Performance evaluation of CT automatic exposure control on fast dual spiral scan].

The performance of individual computed tomography automatic exposure control (CT-AEC) is very important for radiation dose reduction and image quality equalization in CT examinations. The purpose of this study was to evaluate the performance of CT-AEC in conventional pitch mode (Normal spiral) and fast dual spiral scan (Flash spiral) in a 128-slice dual-source CT scanner. To evaluate the response properties of CT-AEC in the 128-slice DSCT scanner, a chest phantom was placed on the patient table and was fixed at the center of the field of view (FOV). The phantom scan was performed using Normal spiral and Flash spiral scanning. We measured the effective tube current time product (Eff. mAs) of simulated organs in the chest phantom along the longitudinal (z) direction, and the dose dependence (distribution) of in-plane locations for the respective scan modes was also evaluated by using a 100-mm-long pencil-type ionization chamber. The dose length product (DLP) was evaluated using the value displayed on the console after scanning. It was revealed that the response properties of CT-AEC in Normal spiral scanning depend on the respective pitches and Flash spiral scanning is independent of the respective pitches. In-plane radiation dose of Flash spiral was lower than that of Normal spiral. The DLP values showed a difference of approximately 1.7 times at the maximum. The results of our experiments provide information for adjustments for appropriate scanning parameters using CT-AEC in a 128-slice DSCT scanner.

Spatial resolution measurement for iterative reconstruction by use of image-averaging techniques in computed tomography.

The purpose of our study was to investigate the validity of a spatial resolution measuring method that uses a combination of a bar-pattern phantom and an image-averaging technique, and to evaluate the spatial resolution property of iterative reconstruction (IR) images with middle-contrast (50 HU) objects. We used computed tomography (CT) images of the bar-pattern phantom reconstructed by the IR technology Adaptive Iterative Dose Reduction 3D (AIDR 3D), which was installed in the multidetector CT system Aquilion ONE (Toshiba Medical Systems, Otawara, Japan). The contrast of the bar-pattern image was set to 50 HU, which is considered to be a middle contrast that requires higher spatial resolution clinically. We employed an image-averaging technique to eliminate the influence of image noise, and we obtained averaged images of the bar-pattern phantom with sufficiently low noise. Modulation transfer functions (MTFs) were measured from the images. The conventional wire method was also used for comparison; in this method, AIDR 3D showed MTF values equivalent to those of filtered back projection. For the middle-contrast condition, the results showed that the MTF of AIDR 3D decreased with the strength of IR processing. Further, the MTF of AIDR 3D decreased with dose reduction. The image-averaging technique used was effective for correct evaluation of the spatial resolution for middle-contrast objects in IR images. The results obtained by our method clarified that the resolution preservation of AIDR 3D was not sufficient for middle-contrast objects.

[Investigation of vessel visibility in the iterative reconstruction method in coronary computed tomography angiography using simulated vessel phantom].

Iterative reconstruction methods can reduce the noise of computed tomography (CT) images, which are expected to contribute to the reduction of patient dose CT examinations. The purpose of this study was to investigate impact of an iterative reconstruction method (iDose(4), Philips Healthcare) on vessel visibility in coronary CT angiography (CTA) by using phantom studies. A simulated phantom was scanned by a CT system (iCT, Philips Healthcare), and the axial images were reconstructed by filtered back projection (FBP) and given a level of 1 to 7 (L1-L7) of the iterative reconstruction (IR). The vessel visibility was evaluated by a quantitative analysis using profiles across a 1.5-mm diameter simulated vessel as well as visual evaluation for multi planar reformation (MPR) images and volume rendering (VR) images in terms of the normalized-rank method with analysis of variance. The peak CT value of the profiles decreased with IR level and full width at half maximum of the profile also decreased with the IR level. For normalized-rank method, there was no statistical difference between FBP and L1 (20% dose reduction) for both MPR and VR images. The IR levels higher than L1 sacrificed the spatial resolution for the 1.5-mm simulated vessel, and their visual vessel visibilities were significantly inferior to that of the FBP.

[Novel method of noise power spectrum measurement for computed tomography images with adaptive iterative reconstruction method].

Adaptive iterative reconstruction techniques (IRs) can decrease image noise in computed tomography (CT) and are expected to contribute to reduction of the radiation dose. To evaluate the performance of IRs, the conventional two-dimensional (2D) noise power spectrum (NPS) is widely used. However, when an IR provides an NPS value drop at all spatial frequency (which is similar to NPS changes by dose increase), the conventional method cannot evaluate the correct noise property because the conventional method does not correspond to the volume data natures of CT images. The purpose of our study was to develop a new method for NPS measurements that can be adapted to IRs. Our method utilized thick multi-planar reconstruction (MPR) images. The thick images are generally made by averaging CT volume data in a direction perpendicular to a MPR plane (e.g. z-direction for axial MPR plane). By using this averaging technique as a cutter for 3D-NPS, we can obtain adequate 2D-extracted NPS (eNPS) from 3D NPS. We applied this method to IR images generated with adaptive iterative dose reduction 3D (AIDR-3D, Toshiba) to investigate the validity of our method. A water phantom with 24 cm-diameters was scanned at 120 kV and 200 mAs with a 320-row CT (Acquilion One, Toshiba). From the results of study, the adequate thickness of MPR images for eNPS was more than 25.0 mm. Our new NPS measurement method utilizing thick MPR images was accurate and effective for evaluating noise reduction effects of IRs.

[Evaluation of low-contrast detectability in low-dose chest computed tomography].

We investigated whether low-contrast resolution evaluation can be applied to chest (lung field) computed tomography (CT) images that are commonly reconstructed using filter kernels with strong frequency emphasis and displayed at wide window widths. We assumed low dose chest CT examinations and set the current-time product to 5 and 10 mAs. The visual detection study was performed by five radiological technologists using water phantom images in which the simulated low contrast objects (disc objects) were implanted. In addition, values of the low contrast detectability index (LCDI) based on a signal-to-noise ratio theorem were calculated from the modulation transfer factor, noise power spectrum, and the object's spectrum for each combination of object size, contrast, current-time product, and kernel. The detectability results correlated well with the LCDI values and correlated less well with the contrast-to-noise ratio results. These results were consistent with the results of past reports on abdomen images. Therefore, our results indicated that low contrast sensitivity can be applied to chest CT images as well as abdominal ones.

[Examination of motion artifacts for helical and non-helical scanning modes in head CT].

For head computed tomography (CT), non-helical scanning has been recommended even in the widely used multi-slice CT (MSCT). Also, an acute stroke imaging standardization group has recommended the non-helical mode in Japan. However, no detailed comparison has been reported for current MSCT with more than 16 slices. In this study, we compared the non-helical and helical modes for head CT, focusing on temporal resolution and motion artifacts. The temporal resolution was evaluated by using temporal sensitivity profiles (TSPs) measured using a temporal impulse method. In both modes, the TSPs and temporal modulation transfer factors (MTFs) were measured for various pitch factors using 64-slice CT (Aquilion 64, Toshiba). Two motion phantoms were scanned to evaluate motion artifacts, and then quantitative analyses for motion artifacts and helical artifacts were performed by measuring multiple regions of interest (ROIs) in the phantom images. In addition, the rates of artifact occurrence for retrospective clinical cases were compared. The temporal resolution increased as the pitch factor was increased. Remarkable streak artifacts appeared in the non-helical images of the motion phantom, in spite of the equivalent effective temporal resolution. In clinical analysis, results consistent with the phantom studies were shown. These results indicated that the low pitch helical mode would be effective for emergency head CT with patient movement.

Image quality dependence on in-plane positions and directions for MDCT images.

OBJECTIVE: The present study was performed to examine the dependence of image quality on in-plane position and direction in computed tomography (CT) imaging using the modulation transfer function (MTF), noise power spectrum (NPS) and analysis of signal-to-noise ratio (SNR). For detailed analysis of SNR, the low-contrast detectability was compared using simulated small low-contrast objects. MATERIALS AND METHODS: Three models of multidetector-row CT (MDCT) were employed. The measurement positions for MTF were set to the isocentre and several peripheral areas, and NPS and SNR were calculated for the isocentre and 128 mm off-centre. To evaluate directional dependence, the one-dimensional physical properties were measured separately in the radial and azimuthal directions. Seven radiological technologists also performed a perceptual detection study at the different in-plane positions using computer-simulated low-contrast images. RESULTS: The results of MTF and SNR differed between the isocentre and the peripheral area. The MTF values also tended to decrease with distance from the isocentre, and the SNR values in the low frequency range for the peripheral area were superior to those for the isocentre. In the detection study, the low-contrast detectability in the peripheral area was 13-40% higher than the value in the isocentre. CONCLUSION: The results of the present study indicated that clinical CT images have remarkable non-uniformity of image quality. Therefore, the detailed analysis performed in this study will provide useful information for the development of advanced image processing applications, such as computer-aided diagnosis (CAD) and de-noising of CT images.