An image-based metal artifact reduction technique utilizing forward projection in computed tomography.

The projection data generated via the forward projection of a computed tomography (CT) image (FP-data) have useful potentials in cases where only image data are available. However, there is a question of whether the FP-data generated from an image severely corrupted by metal artifacts can be used for the metal artifact reduction (MAR). The aim of this study was to investigate the feasibility of a MAR technique using FP-data by comparing its performance with that of a conventional robust MAR using projection data normalization (NMARconv). The NMARconv was modified to make use of FP-data (FPNMAR). A graphics processing unit was used to reduce the time required to generate FP-data and subsequent processes. The performances of FPNMAR and NMARconv were quantitatively compared using a normalized artifact index (AIn) for two cases each of hip prosthesis and dental fillings. Several clinical CT images with metal artifacts were processed by FPNMAR. The AIn values of FPNMAR and NMARconv were not significantly different from each other, showing almost the same performance between these two techniques. For all the clinical cases tested, FPNMAR significantly reduced the metal artifacts; thereby, the images of the soft tissues and bones obscured by the artifacts were notably recovered. The computation time per image was ~ 56 ms. FPNMAR, which can be applied to CT images without accessing the projection data, exhibited almost the same performance as that of NMARconv, while consuming significantly shorter processing time. This capability testifies the potential of FPNMAR for wider use in clinical settings.

Virtual monochromatic images of dual-energy CT as an alternative to single-energy CT: performance comparison using a detectability index for different acquisition techniques.

OBJECTIVES: To investigate the performance of virtual monochromatic (VM) images with the same dose and iodine contrast as those for single-energy (SE) images using five dual-energy (DE) scanners with DE techniques: two generations of fast kV switching (FKS), two generations of dual source (DS), and one split filter (SF). METHODS: A water-bath phantom with a diameter of 300 mm, which contains one rod-shaped phantom made of a material equivalent to soft-tissue and two rod-shaped phantoms made of diluted iodine (2 and 12 mg/mL), was scanned using both SE (120, 100, and 80 kV) and DE techniques with the same CT dose index in each scanner. The VM energy at which the CT number of the iodine rod is closest to that of each SE tube voltage was determined as the equivalent energy (Eeq). A detectability index (d') was calculated from the noise power spectrum, the task transfer functions, and a task function corresponding to each rod. The percentage of the d' value of the VM image to that of the corresponding SE image was calculated for performance comparison. RESULTS: The average percentages of d' of FKS1, FKS2, DS1, DS2, and SF were 84.6%, 96.2%, 94.3%, 107%, and 104% for 120 kV-Eeq; 75.9%, 91.2%, 88.2%, 99.2%, and 82.6% for 100 kV-Eeq; 71.6%, 88.9%, 82.6%, 85.2%, and 62.3% for 80 kV-Eeq, respectively. CONCLUSION: The performance of VM images was on the whole inferior to that of SE images especially at low equivalent energy levels, depending on the DE techniques and their generations. KEY POINTS: • This study evaluated the performance of VM images with the same dose and iodine contrast as those for SE images using five DE scanners. • The performance of VM images varied with the DE techniques and their generations and was mostly inferior at low equivalent energy levels. • The results highlight the importance of distribution of available dose over the two energy levels and spectral separation for the performance improvement of VM images.

Spatial analysis of acoustic noise transfer function with a human-body phantom in a clinical MRI scanner.

BACKGROUND: The acoustic noise in magnetic resonance imaging (MRI) potentially depends on the measurement position and presence of a patient inside the scanner bore. PURPOSE: To analyze the spatial characteristics of the acoustic noise by using the gradient-pulse-to-acoustic-noise transfer function (GPAN-TF) with and without a human-body phantom on the examination table. MATERIAL AND METHODS: Acoustic noise waveforms were acquired at 80 and 110 measurement positions with and without a phantom. The GPAN-TFs µPa/(mT/m) in the coils were calculated by deconvolution. The phantom effect on the spatial distribution of the acoustic noise was assessed using the peak sound pressure levels (SPLs), mean values, peak values, and peak frequencies of the GPAN-TFs. RESULTS: The peak SPLs in all positions for the X-, Y-, and Z-gradient coils were increased by 11.1 dB, 1.4 dB, and 6.1 dB, respectively, compared with the peak SPL of the magnetic isocenter. The maximum peak SPLs among all positions of the X-, Y-, and Z-gradient coils with the phantom were increased by 4.9 dB, 7.4 dB, and 6.9 dB, respectively, relative to those without the phantom. However, the peak SPLs decreased at some positions with the phantom placed on the table (X-gradient coil = 4.6 dB, Y-gradient coil = 5.0 dB, Z-gradient coil = 8.4 dB). The most common peak frequencies were in the range of 2000-3000 Hz. CONCLUSION: "Hotspot" areas with and without the phantom were associated with acoustic noise sources in the clinical MRI scanner and were enhanced by the phantom's presence.

Comparative assessment of noise properties for two deep learning CT image reconstruction techniques and filtered back projection.

BACKGROUND: Two deep learning image reconstruction (DLIR) techniques from two different computed tomography (CT) vendors have recently been introduced into clinical practice. PURPOSE: To characterize the noise properties of two DLIR techniques with different training methods, using a phantom containing a simple uniform and a complex non-uniform region. METHODS: A water-bath phantom with a diameter of 300 mm was used as a base phantom. A textured phantom with a diameter of 128 mm, which was made of two materials, one equivalent to water and the other being 12 mg/ml diluted iodine, irregularly mixed to create a complex texture (non-uniform region), was placed in the base phantom. Thirty repeated phantom scans were performed using two CT scanners (Revolution CT with Apex Edition, GE Healthcare; Aquilion One PRISM Edition, Canon Medical Systems) at two dose levels (CT dose index: 5 and 15 mGy). Images were reconstructed with each CT system's filtered back projection (FBP) and DLIR [TrueFidelity (TF), GE Healthcare; Advanced intelligent Clear-IQ Engine Body Sharp (AC), Canon Medical Systems] for three process strengths. For basic characteristics of noise, the standard deviation (SD) and noise power spectrum (NPS) were measured for the uniform (water) region. A noise magnitude map was generated by calculating the inter-image SD at each pixel position across the 30 images. Then, a noise reduction map (NRM), which visualizes the relative differences in noise magnitude between FBP and DLIR, was calculated. The NRM values ranged from 0.0 to 1.0. A low NRM value represents a less aggressive noise reduction. The histograms of the NRM value were analyzed for the uniform and non-uniform regions. RESULTS: The reduction in noise magnitude compared with FBP tended to be greater with AC (45%-85%) than with TF (32%-65%). The average NPS frequencies of TF and AC were almost comparable to those of FBP, except for the low-dose condition and the high noise reduction strength for AC. The NRM values of TF and AC were higher in the uniform region than in the non-uniform region. In the non-uniform region, TF's average NRM values (0.21-0.48) tended to be lower than AC's (0.39-0.78). The histograms for TF showed a small overlap between the uniform and the non-uniform regions; in contrast, those for AC showed a greater overlap. This difference seems to indicate that TF processes the uniform and non-uniform regions more differently than AC does. CONCLUSION: This study has revealed a distinct difference in characteristics between the two DLIR techniques: TF tends to offer less aggressive noise reduction in non-uniform regions and preserve the original signals, whereas AC tends to prioritize noise filtering over edge-preservation, especially at the low-dose condition and with the high noise reduction strength.

Reduction of streak artifacts caused by low photon counts utilizing an image-based forward projection in computed tomography.

BACKGROUND: The streak artifacts in computed tomography (CT) images caused by low photon counts are known to be effectively suppressed by raw-data-based techniques. This study aims to propose a technique to reduce the streak artifact without accessing the raw data. METHODS: The proposed streak artifact reduction (SAR) technique consists of three steps: numerical forward projection to a CT image, adaptive filtering of the generated sinogram, and image reconstruction from the processed sinogram. The authors have expanded the two-dimensional method (2D-SAR) to three dimensions (3D-SAR) by using consecutive CT images. The modulation transfer function (MTF), the image noise (standard deviation), and the visibility of comb-shaped objects were evaluated at a low dose of 5 mGy. Using anthropomorphic abdominal and chest phantoms, CT images and the artifact index (AI) were compared between 3D-SAR and two types of iterative reconstruction (IR). RESULTS: Sufficient artifact reductions associated with 54% and 61% reduction of noise for 2D- and 3D-SAR, respectively, were obtained in the phantom images, although the 50%MTF decreased by 28%. The visibility of the combs was improved with both the 2D- and 3D-SAR methods. The AI results of 3D-SAR were better than one type of IR and almost equal to the other type of IR, which was consistent with observed artifacts. CONCLUSION: Both 2D-SAR and 3D-SAR have turned out to be effective in reducing streak artifacts. The proposed technique will be an effective tool since it needs no raw data, and thus can be applied to any CT images produced by a wide variety of CT systems.

Performance of clinically available deep learning image reconstruction in computed tomography: a phantom study.

Purpose: To assess the physical performance of deep learning image reconstruction (DLIR) compared with those of filtered back projection (FBP) and iterative reconstruction (IR) and to estimate the dose reduction potential of the technique. Approach: A cylindrical water bath phantom with a diameter of 300 mm including two rods composed of acrylic and soft tissue-equivalent material was scanned using a clinical computed tomography (CT) scanner at four dose levels (CT dose index of 20, 15, 10, and 5 mGy). Phantom images were reconstructed using FBP, DLIR, and IR. The in-plane and z axis task transfer functions (TTFs) and in-plane noise power spectrum (NPS) were measured. The dose reduction potential was estimated by evaluating the system performance function calculated from TTF and NPS. The visibilities of a bar pattern phantom placed in the same water bath phantom were compared. Results: The use of DLIR resulted in a notable decrease in noise magnitude. The shift in peak NPS frequency was reduced compared with IR. Preservation of in-plane TTF was superior using DLIR than using IR. The estimated dose reduction potentials of DLIR and IR were 39% to 54% and 19% to 29%, respectively. However, the z axis resolution was decreased with DLIR by 6% to 21% compared with FBP. The bar pattern visibilities were approximately consistent with the TTF results in both planes. Conclusions: The in-plane edge-preserving noise reduction performance of DLIR is superior to that of IR. Moreover, DLIR enables approximately half-dose acquisitions with no deterioration in noise texture in cases that permit some z axis resolution reduction.

Technical Note: Performance comparison of ultra-high-resolution scan modes of two clinical computed tomography systems.

PURPOSE: Two ultra-high-resolution (UHR) computed tomography (CT) scanners are clinically available. One is achieved by a CT system with a 0.25 mm × 160 row detector (Detector-UHR), whereas the other is with a 0.6 mm × 32 row detector with in-plane comb filtering in a dual source CT (Comb-UHR). We compared radiation dose efficiencies of the two UHR modes to that of a routine scan mode (RS), using physical image quality measures for an assumed condition of abdominal CT angiography (CTA). METHODS: A wire phantom, a 300-mm cylindrical water bath phantom, and a microdisk phantom were used for measuring the modulation transfer function (MTF), noise power spectrum (NPS), and slice sensitivity profiles (SSP), respectively, at CTDIvol of 20 mGy. Images with minimal slice thicknesses were reconstructed by filtered back projection methods. System performance functions (SPF2 ) based on the prewhitening theorem were calculated by dividing MTF2 by NPS measurements. The ideal observer's detectability index (d'2 ) was also estimated for a task corresponding to a 1-mm diameter vessel. Furthermore, a bar-pattern phantom placed in a water phantom resembling an adult abdomen was scanned, and the visibility of the bars was observed. RESULTS: System performance function (SPF2 ) results showed that Comb-UHR has a 70% dose efficiency compared to RS and provides better than twofold SPF2 compared to Detector-UHR. The d'2 values of Detector-UHR, Comb-UHR, and RS were 6.5, 14.7, and 16.0, respectively. Although the bar-pattern phantom images were consistent with the SPF results, bar widths < 1.0 mm for Detector-UHR and < 0.75 mm for Comb-UHR and RS were not resolved. CONCLUSIONS: Though both the UHR modes exhibited system performances extending to 2.6 cycles/mm, they appeared not to be advantageous compared to RS in the conditions applicable to abdominal CTA, because of their insufficient dose efficiencies.

Algorithm-based artifact reduction in patients with arms-down positioning in computed tomography.

PURPOSE: Arm-artifact, a type of streak artifact frequently observed in computed tomography (CT) images obtained at arms-down positioning in polytrauma patients, is known to degrade image quality. This study aimed to develop a novel arm-artifact reduction algorithm (AAR) applied to projection data. METHODS: A phantom resembling an adult abdomen with two arms was scanned using a 16-row CT scanner. The projection data were processed by AAR, and CT images were reconstructed. The artifact reduction for the same phantom was compared with that achieved by two latest iterative reconstruction (IR) techniques (IR1 and IR2) using a normalized artifact index (nAI) at two locations (ventral and dorsal side). Image blurring as a processing side effect was compared with IR2 of the model-based IR using a plastic needle phantom. Additionally, the projection data of two clinical cases were processed using AAR, and the image noise was evaluated. RESULTS: AAR and IR2 significantly reduced nAI by 87.5% and 74.0%, respectively at the ventral side and 84.2% and 69.6%, respectively, at the dorsal side compared with each filtered back projection (P < 0.01), whereas IR1 did not. The proposed algorithm mostly maintained the original spatial resolution, compared with IR2, which yielded apparent image blurring. The image noise in the clinical cases was also reduced significantly (P < 0.01). CONCLUSIONS: AAR was more effective and superior than the latest IR techniques and is expected to improve the image quality of polytrauma CT imaging with arms-down positioning.

A three-dimensional cross-directional bilateral filter for edge-preserving noise reduction of low-dose computed tomography images.

BACKGROUND: Image-based noise reduction techniques are useful because they can be applied across various computed tomography (CT) scanner models from different vendors, regardless of the iterative reconstruction availability. The purpose of this study was to propose a 3-dimensional cross-directional bilateral filter (3D-CDBF) and compare the edge-preserving noise reduction on low-dose CT images to a model-based iterative reconstruction (MBIR). METHODS: The 3D-CDBF comprises a bilateral filter and a smoothing filter applied in range filtering. The filtering process was applied with four iterations using empirically determined parameters that yielded the best tradeoff between noise reduction and edge preservation for a very low radiation dose of 2.5 mGy. In-plane and z-directional edge preservation performances for low-contrast rod phantoms (60 Hounsfield units) were compared to a clinically available MBIR and a conventional 3D bilateral filter (3D-BF), using task-based spatial resolution (task-based transfer function: TTF) and slice thickness. Moreover, the noise power spectra (NPS) were compared. Furthermore, performance was compared on abdominal CT images acquired from volunteers at 2.5 mGy (approved by our institutional review board). RESULTS: In phantom tests, 3D-CDBF provided 28.5% higher spatial resolution at 50%TTF compared to MBIR. Moreover, total NPS was lower, while the slice thickness (z-axis resolution) was slightly broader than that achieved by MBIR (0.99 mm vs. 0.92 mm). 3D-BF was inferior to both 3D-CDBF and MBIR in all measurements. Consistent with phantom results, 3D-CDBF significantly reduced noise on abdominal images compared to MBIR (P < 0.001), exhibiting better preservation of organ edges. CONCLUSION: This 3D-CDBF may provide superior edge preserving noise reduction of low-dose CT images compared to currently available MBIR.

Quality evaluation of image-based iterative reconstruction for CT: Comparison with hybrid iterative reconstruction.

The purpose of this study is to evaluate the physical image quality of a commercially available image-based iterative reconstruction (IIR) system for two object contrasts to resemble a soft tissue (60 HU) and an enhanced vessel (270 HU), and compare the results with those of filtered back projection (FBP) and iterative reconstruction (IR). A 192-slice computed tomography (CT) scanner was used for data acquisitions. IIR images were processed from the FBP images. Task-based in-plane transfer function (TTF) and slice sensitivity profile (SSPtask ) were measured from rod objects inside of a 25-cm diameter water phantom at four dose levels (2.5, 5, 10, and 20 mGy). Noise power spectrum (NPS) was measured from the water-only part. System performance (SP) function was calculated as TTF2 /NPS over FBP, IR, and IIR for comparison. In addition, an image subtraction was performed using images of rod objects, a bar-pattern phantom, and a clinical abdomen case to observe the noise reduction performance of IIR. As a results, IIR mostly preserved TTF and SSPtask of FBP, whereas IR exhibited enhanced TTF at 10 and 20 mGy for 60 HU contrast and at all doses for 270 HU contrast. SP of IIR at 2.5, 5, 10 mGy (half doses) were similar to those of FBP at 5, 10, 20 mGy, respectively. IR exhibited enhanced SP at medium to high frequencies. The subtracted images showed weak remained edge signals in the bar-pattern and abdominal images. In conclusion, IIR uniformly improved the task-based image quality of FBP over the entire frequency range, whereas IR improved the characteristics over medium to high frequencies. The dose reduction potential of IIR estimated from SP is approximately 50%, when allowing the slight signal reductions.

Relationship between size-specific dose estimates and image quality in computed tomography depending on patient size.

This study investigates the relationship between contrast-to-noise ratio (CNR) and size-specific dose estimate (SSDE) in computed tomography (CT) depending on patient size. In addition, the relationship to the auto exposure control (AEC) techniques is examined. A tissue-equivalent material having human-liver energy dependence is developed and used to evaluate these relationships. Three exposure dose levels (constant CT dose index, constant SSDE, and with AEC) are tested using four different phantom sizes (diameter: 15, 20, 25 and 30 cm) in two different CT scanners (SOMATOM Definition Flash, Siemens, and LightSpeed VCT, GE). The contrast-to-noise ratios (CNRs) are measured using the developed phantom. It is found that the CNR increases with decreasing phantom size at constant SSDE, although the increase ratio is smaller than that of the constant CT dose index. This result indicates that the image characteristics differ even when the patient dose received from the CT examination is equivalent for each patient size. In the case of AEC use, the CNR results of the Siemens scanner exhibit a similar trend to those obtained for constant SSDE, for each phantom size. This suggests that the AEC technique that maintains a constant image quality (CARE Dose 4D) for each patient size corresponds well to the image quality obtained for constant SSDE. These findings facilitate further understanding of the relationship between image quality and exposure CT dose depending on patient size.

Aortic Root Calcification Score as an Independent Factor for Predicting Major Adverse Cardiac Events in Familial Hypercholesterolemia.

AIM: The aims of this study were: 1) to determine whether the accumulation of aortic root calcification (ARC) assessed using coronary computed tomography angiography (CCTA) can predict future cardiovascular events, and 2) to estimate the onset and progression of ARC in patients with familial hypercholesterolemia (FH). METHODS: One hundred thirteen consecutive Japanese patients with heterozygous FH (male=54, mean age=52.1±15.6 years, mean LDL-C=299.0±94.6 mg/dL), without known coronary artery disease, who underwent 64-detector row CCTA were retrospectively evaluated. ARC was defined as the presence of calcium at the aortic root. The extent of ARC was expressed in Agatston units as the ARC-score. Major adverse cardiac events (MACE) were defined as either cardiac death, ST elevated myocardial infarction (STEMI), non-ST elevated myocardial infarction (NSTEMI), unstable angina pectoris (UAP), planned percutaneous coronary intervention (PCI), coronary artery bypass grafting (CABG), or stroke. The periods to MACE were estimated using multivariate logistic regression analysis. RESULTS: During the follow-up period (median 1635 days), 19 instances of MACE occurred. Multivariate logistic regression analysis revealed that ARC was a significant independent predictor of MACE (OR=1.48; 95% CI 1.11-1.87, p＜0.001, respectively). The regression equations were Y=0.09X- 1.59 (R2=0.34, p＜0.001) in males and Y=0.08X-1.60 (R2=0.13, p＜0. 05) in females. CONCLUSIONS: ARC was significantly associated with future MACE in Japanese patients with heterozygous FH. ARC may start to develop, on average, at 17.4 and 19.7 years of age in males and females, respectively, with heterozygous FH.

Method for determining the half-value layer in computed tomography scans using a real-time dosimeter: Application to dual-source dual-energy acquisition.

PURPOSE: We have proposed a method for determining the half-value layers (HVL) in dual-source dual-energy computed tomography (DS-DECT) scans without the need for the X-ray tubes to be fixed. METHODS: A custom-made lead-covered case and an ionizing chamber connected with a multi-function digitizer module (a real-time dosimeter) were used. The chamber was placed in the center of the case, and aluminum or copper filters were placed in front of the aperture. The HVL was measured using aperture widths of 1.0, 2.0, and 3.0 cm for tube potentials of 80, 120, and 150 kV in single-source single-energy CT (SS-SECT) scans and was calculated from the peak air kerma rate (peak method) and the integrated air kerma rate (integrating method); the obtained values were compared with those from a conventional non-rotating method performed using the same procedure. The HVL was then measured using an aperture width of 1.0 cm for tube potential combinations of 70/Sn150 kV and 100/Sn150 kV in DS-DECT scans using the peak method. RESULTS: In the SS-SECT scans, the combination of a 1.0-cm aperture and the peak method was adequate due to the small differences in the HVL values obtained for the conventional non-rotating method. The method was also found to be applicable for the DS-DECT scans. CONCLUSIONS: Our proposed method can determine the HVL in SS-SE and DS-DECT scans to a good level of accuracy without the need for the X-ray tubes to be fixed. The combination of a 1.0-cm aperture and the peak method was adequate.

Object shape dependency of in-plane resolution for iterative reconstruction of computed tomography.

The present study aimed to investigate whether the in-plane resolution property of iterative reconstruction (IR) of computed tomography (CT) data is object shape-dependent by testing columnar shapes with diameters of 3, 7, and 10cm (circular edge method) and a cubic shape with 5-cm side lengths (linear edge method). For each shape, objects were constructed of acrylic (contrast in Hounsfield units [ΔHU]=120) as well as a soft tissue equivalent material (ΔHU=50). For each shape, we measured the modulation transfer functions (MTFs) of IR and filtered back projection (FBP) using two multi-slice CT scanners at scan doses of 5 and 10mGy. In addition, we evaluated a thin metal wire using the conventional method at 10mGy. For FBP images, the MTF results of the tested objects and the wire method showed substantial agreement, thus demonstrating the validity of our analysis technique. For IR images, the MTF results of different shapes were nearly identical for each object contrast and dose combination, and we did not observe shape-dependent effects of the resolution properties of either tested IR. We conclude that both the circular edge method and linear edge method are equally useful for evaluating the resolution properties of IRs.

Tube Current Modulation Between Single- and Dual-Energy CT With a Second-Generation Dual-Source Scanner: Radiation Dose and Image Quality.

OBJECTIVE: The purpose of this study was to compare the effects of tube current modulation between single- and dual-energy CT with a second-generation dual-source scanner. MATERIALS AND METHODS: Custom-made elliptic polymethylmethacrylate phantoms for slim and large patients were used. Absorbed radiation dose at the central point of the phantoms was measured with a solid-state detector while the phantoms were scanned in single-energy (120 kV) and dual-energy (100/Sn140, 80/Sn140, and 140/80 kV) modes with a second-generation dual-source CT scanner. Tube current modulation was activated in both modes, and quality reference tube current-time settings of 150, 300, 450, and 600 mAs were selected. Scanning was performed three times under the same conditions, and image noise was evaluated by measuring the SD of CT numbers in four separate regions of three adjacent images of the phantoms. RESULTS: Absorbed dose increased and image noise decreased with an increase in quality reference tube current-time setting when the slim phantom was scanned. For the large phantom, the radiation dose and noise level reached a plateau above quality reference tube current-time settings of 300 mAs for 100/Sn140 kV and 450 mAs for 120 kV. The radiation dose was small and the noise level was large with 80/Sn140 kV compared with that obtained with 120 and 100/Sn140 kV at all quality reference tube current-time settings. CONCLUSION: When a large phantom is scanned with 100/Sn140 kV, exposure demand for tube current modulation exceeds system limits at a lower quality reference tube current-time setting than for scanning 120 kV.

128-slice dual-source CT coronary angiography with prospectively electrocardiography-triggered high-pitch spiral mode: radiation dose, image quality, and diagnostic acceptability.

BACKGROUND: Dual-source computed tomography (CT) enables CT coronary angiography (CTCA) with a prospectively electrocardiography (ECG)-triggered high-pitch spiral (HPS) mode. PURPOSE: To evaluate the radiation dose, image quality, and diagnostic acceptability of the HPS mode in CTCA and to compare HPS with the step-and-shoot (SAS) and low-pitch spiral (LPS) modes. MATERIAL AND METHODS: One hundred and thirty-eight patients who underwent CTCA with a 128-slice dual-source CT scanner were retrospectively included in this study. Seventeen patients (average heart rate of ≤65 beats per minute [bpm] prior to acquisition) were evaluated in the HPS mode, 88 (average heart rate of >65 and ≤80 bpm prior to acquisition) in the SAS mode, and 33 (average heart rate of >80 bpm prior to acquisition or patients with an unstable heart rhythm) in the LPS mode. Radiation dose and image noise were recorded for each patient. Diagnostic acceptability was graded using a four-point scale (1, unacceptable; 2, suboptimal; 3, acceptable; 4, fully acceptable). RESULTS: The effective dose in the HPS mode was 1.5 ± 0.2 mSv, which was lower than that in SAS (8.9 ± 2.7 mSv) and LPS (21.5 ± 4.3 mSv) modes. There were no significant differences in the image noise levels in the descending aorta and left atrium. The average per-patient diagnostic acceptability was 3.2, 3.6, and 3.7 in HPS, SAS, and LPS modes, respectively. CONCLUSION: The radiation dose is lower with HPS than with other modes, and the HPS mode-acquired images of patients with heart rates of ≤65 bpm are nearly acceptable for diagnostic image interpretation.

[Phantom Study on Dose Reduction Using Iterative Reconstruction in Low-dose Computed Tomography for Lung Cancer Screening].

We investigated dose reduction ability of an iterative reconstruction technology for low-dose computed tomography (CT) for lung cancer screening. The Sinogram Affirmed Iterative Reconstruction (SAFIRE) provided in a multi slice CT system, Somatom Definition Flash (Siemens Healthcare) was used. An anthropomorphic chest phantom (N-1, Kyoto Kagaku) was scanned at volume CT dose index (CTDIvol) of 0.50-11.86 mGy with 120 kV. For noise (standard deviation) and contrast-to-noise ratio (CNR) measurements, CTP486 and CTP515 modules in the Catphan (The Phantom Laboratory) were scanned. Radiological technologists were participated in the perceptual comparison. SAFIRE reduced the SD values by approximately 50% compared with filter back projection (FBP). The estimated dose reduction rates by SAFIRE determined from the perceptual comparison was approximately 23%, while 75% dose reduction rate was expected from the SD value reduction of 50%.

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.

[Evaluation of dose output according to the objects using organ-based tube-current modulation in thoracic computed tomography].

The purpose of this study was to evaluate the dose output according to the object using organ-based tube-current modulation in thoracic CT. The output doses with elliptical and circular shaped phantoms were measured using an ionizing CT chamber. The image noise was quantitatively measured in images obtained from the elliptical phantom. Although total dose outputs with and without the modulation were almost the same, dose outputs at a frontal angle of 120° decreased and those at another angle of 240° increased with the modulation. When the same-shaped phantoms were used, the differences in variation of dose outputs due to the difference in phantom size were small and those due to the difference in the percentage ratio of long- to short-axis diameter of the cross-section were large. There was no significant difference in the amount of noise with and without the dose modulation except for the case of overdose to the small phantom. Therefore, organ-based tube current modulation does not change the total dose output and it maintains the amount of noise by controlling the dose output for each projection angle. Additionally, this dose control is independent of the object size.

Radiation dose and physical image quality in 128-section dual-source computed tomographic coronary angiography: a phantom study.

One-hundred-and-twenty-eight-section dual X-ray source computed tomography (CT) systems have been introduced into clinical practice and have been shown to increase temporal resolution. Higher temporal resolution allows low-dose spiral mode at a high pitch factor during CT coronary angiography. We evaluated radiation dose and physical image qualities in CT coronary angiography by applying high-pitch spiral, step-and-shoot, and low-pitch spiral modes to determine the optimal acquisition mode for clinical situations. An anthropomorphic phantom, small dosimeters, a calibration phantom, and a microdisc phantom were used to evaluate the radiation doses absorbed by thoracic organs, noise power spectrums, in-plane and z-axis modulation transfer functions, slice sensitivity profiles, and number of artifacts for the three acquisition modes. The high-pitch spiral mode had the advantage of a small absorbed radiation dose, but provided low image quality. The low-pitch spiral mode resulted in a high absorbed radiation dose of approximately 200 mGy for the heart. Although the absorbed radiation dose was lower in the step-and-shoot mode than in the low-pitch spiral mode, the noise power spectrum was inferior. The quality of the in-plane modulation transfer function differed, depending on spatial frequency. Therefore, the step-and-shoot mode should be applied initially because of its low absorbed radiation dose and superior image quality.