Filtered Back-projection Technique Research Articles

Ultra-low dose chest computed tomography: Effect of iterative reconstruction levels on image quality

PurposeTo optimize image quality and radiation dose of chest CT with respect to various iterative reconstruction levels, detector collimations and body sizes. MethodA Kyoto Kagaku Lungman with and without extensions was scanned using fixed ultra-low doses of 0.25, 0.49 and 0.74 mGy CTDIvol, and collimations of 40 and 80 mm. Images were reconstructed with the lung kernel, filtered back projection (FBP) and different ASIR-V levels (10–100%). Contrast-to-noise ratios (CNR) were calculated for 12 mm simulated lesions of different densities in the lung. Image noise, signal-to-noise ratios (SNR), variations in Hounsfield units (HU), noise power spectrum (NPS) and noise texture deviations (NTD) were evaluated for all reconstructions. NTD was calculated as percentage of pixels outside 3 standard deviations to evaluate IR-specific artefacts. ResultsCompared to the FBP, image noise reduced (5–55%) with ASIR-V levels irrespective of dose or collimation. SNR correlated positively (r ≥ 0.925, p ≤ 0.001) with ASIR-V levels at all doses, collimations, and phantom sizes. ASIR-V enhanced the CNR of the lesion with the lowest contrast from 12.7–42.1 (0–100% ASIR-V) at 0.74 mGy with 40 mm collimation. As expected, higher SNR and CNR were measured in the smaller phantom than the bigger phantom. Uniform HU were observed between FBP and ASIR-V levels at all doses, collimations, and phantom sizes. NPS curves left-shifted towards lower frequencies at increasing levels of ASIR-V irrespective of collimation. A positive correlation (r ≥ 0.946, p ≥ 0.001) was observed between NTD and ASIR-V levels. NTD of the FBP was not significantly (p ≤ 0.087) different from NTD of ASIR-V ≤ 20%. The data from the NPS and NTD indicates a blotchier and coarser noise texture at higher levels of ASIR-V, especially at 100% ASIR-V. ConclusionIn comparison with the FBP technique, ASIR-V enhanced quantitative image quality parameters at all ultra-low doses tested. Moreover, the use of ASIR-V showed consistency with body size and collimation. Hence, ASIR-V may be useful for improving image quality of chest CT at ultra-low doses.

Metallic artifact reduction by evaluation of the additional value of iterative reconstruction algorithms in hip prosthesis computed tomography imaging.

To evaluate iterative metal artifact reduction (iMAR) technique in images data of hip prosthesis on computed tomography (CT) and the added value of advanced modeled iterative reconstruction (ADMIRE) compared with standard filtered back projection (FBP).Twenty-eight patients addressed to CT examinations for hip prosthesis were included prospectively. Images were reconstructed with iMAR algorithm in addition to FBP and ADMIRE techniques. Measuring image noise assessed objective image quality and attenuation values with standardized region of interest (ROI) in 4 predefined anatomical structures (gluteus medius and rectus femoris muscles, inferior and anterior abdominal fat, and femoral vessels when contrast media was present). Subjective image quality was graded on a 5-point Likert scale, taking into account the size of artifacts, the metal–bone interface and the conspicuity of pelvic organs, and the diagnostic confidence.Improvement in overall image quality was statistically significant using iMAR (P<.001) compared with ADMIRE and FBP. ADMIRE did not show any impact in image noise, attenuation value, or global quality image. iMAR showed a significant decrease in image noise in all ROIs (Hounsfield Unit) as compared with FBP and ADMIRE. Interobserver agreement was high in all reconstructions (FBP, FBP+iMAR, ADMIRE, and ADMIRE + iMAR) more than 0.8. iMAR reconstructions showed emergence of new artifacts in bone–metal interface.iMAR algorithm allows a significant reduction of metal artifacts on CT images with unilateral or bilateral prostheses without additional value of ADMIRE. It improves the analysis of surrounding tissue but potentially generates new artifacts in bone–metal interface.

An investigation of spatial resolution and noise in reconstructed CT images using iterative reconstruction (IR) and filtered back-projection (FBP)

This study compared the spatial resolution and the amount of noise in CT images reconstructed using the iterative reconstruction (IR) and filtered back-projection (FBP) techniques. We used a circular homogeneous phantom (pixel value of 0.5) containing a small circle at its center, with a diameter of 0.53-cm and a pixel value of 1.0. In order to reconstruct the image, we used the simultaneous iterative reconstruction technique (SIRT) with 5, 15, 25, 35 and 45 iterations respectively, and FBP with a Ram-Lak filter. The spatial resolution of the reconstructed image was characterized by the modulation transfer function (MTF) and image noise was characterized by the standard deviation within the homogeneous area. We found that the spatial resolution using SIRT with 35 iterations is similar to using FBP. The spatial resolution using SIRT tends to be worse using less than 35 iterations, and it tends to be better using more than 35 iterations. Using SIRT, image noise decreased with increasing iterations. The standard deviation for the SIRT technique using 45 iterations was about 0.02 similar to the FBP technique. Thus, SIRT could potentially produce better spatial resolution than the FBP technique with comparable image noise.

Application of Vendor-Neutral Iterative Reconstruction Technique to Pediatric Abdominal Computed Tomography.

ObjectiveTo compare image qualities between vendor-neutral and vendor-specific hybrid iterative reconstruction (IR) techniques for abdominopelvic computed tomography (CT) in young patients.Materials and MethodsIn phantom study, we used an anthropomorphic pediatric phantom, age-equivalent to 5-year-old, and reconstructed CT data using traditional filtered back projection (FBP), vendor-specific and vendor-neutral IR techniques (ClariCT; ClariPI) in various radiation doses. Noise, low-contrast detectability and subjective spatial resolution were compared between FBP, vendor-specific (i.e., iDose1 to 5; Philips Healthcare), and vendor-neutral (i.e., ClariCT1 to 5) IR techniques in phantom. In 43 patients (median, 14 years; age range 1–19 years), noise, contrast-to-noise ratio (CNR), and qualitative image quality scores of abdominopelvic CT were compared between FBP, iDose level 4 (iDose4), and ClariCT level 2 (ClariCT2), which showed most similar image quality to clinically used vendor-specific IR images (i.e., iDose4) in phantom study. Noise, CNR, and qualitative imaging scores were compared using one-way repeated measure analysis of variance.ResultsIn phantom study, ClariCT2 showed noise level similar to iDose4 (14.68–7.66 Hounsfield unit [HU] vs. 14.78–6.99 HU at CT dose index volume range of 0.8–3.8 mGy). Subjective low-contrast detectability and spatial resolution were similar between ClariCT2 and iDose4. In clinical study, ClariCT2 was equivalent to iDose4 for noise (14.26–17.33 vs. 16.01–18.90) and CNR (3.55–5.24 vs. 3.20–4.60) (p > 0.05). For qualitative imaging scores, the overall image quality ([reader 1, reader 2]; 2.74 vs. 2.07, 3.02 vs. 2.28) and noise (2.88 vs. 2.23, 2.93 vs. 2.33) of ClariCT2 were superior to those of FBP (p < 0.05), and not different from those of iDose4 (2.74 vs. 2.72, 3.02 vs. 2.98; 2.88 vs. 2.77, 2.93 vs. 2.86) (p > 0.05).ConclusionVendor-neutral IR technique shows image quality similar to that of clinically used vendor-specific hybrid IR technique for abdominopelvic CT in young patients.

Simulated Dose Reduction for Abdominal CT With Filtered Back Projection Technique: Effect on Liver Lesion Detection and Characterization.

Previous studies have shown the possibility to reduce radiation dose in abdominal CT by 25-50% without negatively affecting detection of liver lesions. How radiation dose reduction affects characterization of liver metastases is not as well known. The objective of this study was to investigate how different levels of simulated dose reduction affect the detection and characterization of liver lesions, primarily hypovascular metastases. A secondary objective was to analyze the relationship between the lesion size and contrast-to-noise ratio (CNR) and the detection rate. Thirty-nine patients (19 with metastases and 20 without) were retrospectively selected. The following radiation dose levels (DLs) were simulated: 100% (reference level), 75%, 50%, and 25%. Five readers were asked to mark liver lesions and rate the probability of malignancy on a 5-grade Likert scale. Noninferiority analysis using the jackknife free-response ROC (JAFROC) method was performed as well as direct comparison of detection rates and grades. JAFROC analysis showed noninferior detection and characterization of metastases at DL75 as compared with DL100. However, the number of benign lesions and false-positive localizations rated as "suspected malignancy" was significantly higher at DL75. Radiation dose can be reduced by 25% without negatively affecting diagnosis of hypovascular liver metastases. Characterization of benign lesions, however, is impaired at DL75, which may lead to unnecessary follow-up examinations. Finally, increased image noise seems to affect the detection of small lesions to a degree that cannot be explained solely by the reduction in CNR.

Ultrafast X-ray tomographic imaging of multiphase flow in bubble columns - Part 1: Image processing and reconstruction comparison

Ultrafast X-ray tomography is a recently developed imaging technique for multiphase flows. As with conventional X-ray tomography it involves reconstruction of images from X-ray projection data. If used for multiphase flow measurements, it moreover needs to be complemented with automated image processing algorithms for the extraction of flow features, such as gas holdup profiles, bubble/particle size distributions or disperse phase velocities. So far, image reconstruction is carried out with the standard filtered back-projection technique, which is fast but may not be optimal in the presence of noisy or corrupted data. As the latter is a frequent issue, search for optimal image reconstruction and data processing algorithms is continuously ongoing. This paper serves as a foundation of the image reconstruction and processing framework for the application to multiphase flow. A description is given of the procedure from reconstruction to thresholding to property extraction of ultrafast X-ray tomographic images. Two reconstruction techniques, FBP and SART are employed on the basis of phantom measurements. Each technique is evaluated separately, for FBP regarding the choice of filter and for SART regarding the termination criteria. Image reconstruction resolution, computational costs and sensitivity to the threshold value are investigated. Based on the analysis, FBP with the Ram-Lak filter is selected for image processing purposes. Furthermore, it is shown that from experiments with moving objects, there is fair agreement between measurements and the phantom dimensions. The described imaging process can be applied to different attenuation materials, simulating gas-solid and gas-liquid properties.

Could new reconstruction CT techniques challenge MRI for the detection of brain metastases in the context of initial lung cancer staging?

To evaluate the diagnostic performance of brain CT images reconstructed with a model-based iterative algorithm performed at usual and reduced dose. 115 patients with histologically proven lung cancer were prospectively included over 15 months. Patients underwent two CT acquisitions at the initial staging, performed on a 256-slice MDCT, at standard (CTDIvol: 41.4 mGy) and half dose (CTDIvol: 20.7 mGy). Both image datasets were reconstructed with filtered back projection (FBP) and iterative model-based reconstruction (IMR) algorithms. Brain MRI was considered as the reference. Two blinded independent readers analysed the images. Ninety-three patients underwent all examinations. At the standard dose, eight patients presented 17 and 15 lesions on IMR and FBP CT images, respectively. At half-dose, seven patients presented 15 and 13 lesions on IMR and FBP CT images, respectively. The test could not highlight any significant difference between the standard dose IMR and the half-dose FBP techniques (p-value = 0.12). MRI showed 46 metastases on 11 patients. Specificity, negative and positive predictive values were calculated (98.9-100 %, 93.6-94.6 %, 75-100 %, respectively, for all CT techniques). No significant difference could be demonstrated between the two CT reconstruction techniques. • No significant difference between IMR100 and FBP50 was shown. • Compared to FBP, IMR increased the image quality without diagnostic impairment. • A 50 % dose reduction combined with IMR reconstructions could be achieved. • Brain MRI remains the best tool in lung cancer staging.

Optimal abdominal CT protocol for obese patients

IntroductionThis study investigated the impact of different protocols on radiation dose and image quality for obese patients undergoing abdominal CT examinations. MethodsFive abdominal/pelvis CT protocols employed across three scanners from a single manufacturer in a single centre used a variety of parameters (kV: 100/120, reference mAs: 150/190/218/250/300, image reconstruction: filtered back projection (FBP)/iterative (IR)). The routine protocol employed 300 reference mAs and 120 kV. Data sets resulting from obese patient examinations (n = 42) were assessed for image quality using visual grading analysis by three experienced radiologists. Objective assessment (noise, signal/contrast-noise ratios) and radiation dose was compared to determine optimal protocols for prospective testing on a further sample of patients (n = 47) for scanners using FBP and IR techniques. ResultsCompared to the routine protocol, mean radiation dose was reduced by 60% when using 100 kV and SAFIRE technique strength 3 (p = 0.001). Reduction of up to 30% in radiation dose was noted for the FBP protocol: 120 kV and 190 reference mAs (p = 0.008). Subjective and objective image quality for both protocols were comparable to that of the routine protocol (p > 0.05). An overall improvement in image quality with increasing strength of SAFIRE was noted. Upon clinical implementation of the optimal dose protocols, local radiology consensus deemed image quality to be acceptable for the participating obese patient cohort. ConclusionRadiation dose for obese patients can be optimised whilst maintaining image quality. Where iterative reconstruction is available relatively low kV and quality reference mAs are also viable for imaging obese patients at 30–60% lower radiation doses.

Image quality improvement using model-based iterative reconstruction in low dose chest CT for children with necrotizing pneumonia

BackgroundModel-based iterative reconstruction (MBIR) is a promising reconstruction method which could improve CT image quality with low radiation dose. The purpose of this study was to demonstrate the advantage of using MBIR for noise reduction and image quality improvement in low dose chest CT for children with necrotizing pneumonia, over the adaptive statistical iterative reconstruction (ASIR) and conventional filtered back-projection (FBP) technique.MethodsTwenty-six children with necrotizing pneumonia (aged 2 months to 11 years) who underwent standard of care low dose CT scans were included. Thinner-slice (0.625 mm) images were retrospectively reconstructed using MBIR, ASIR and conventional FBP techniques. Image noise and signal-to-noise ratio (SNR) for these thin-slice images were measured and statistically analyzed using ANOVA. Two radiologists independently analyzed the image quality for detecting necrotic lesions, and results were compared using a Friedman’s test.ResultsRadiation dose for the overall patient population was 0.59 mSv. There was a significant improvement in the high-density and low-contrast resolution of the MBIR reconstruction resulting in more detection and better identification of necrotic lesions (38 lesions in 0.625 mm MBIR images vs. 29 lesions in 0.625 mm FBP images). The subjective display scores (mean ± standard deviation) for the detection of necrotic lesions were 5.0 ± 0.0, 2.8 ± 0.4 and 2.5 ± 0.5 with MBIR, ASIR and FBP reconstruction, respectively, and the respective objective image noise was 13.9 ± 4.0HU, 24.9 ± 6.6HU and 33.8 ± 8.7HU. The image noise decreased by 58.9 and 26.3% in MBIR images as compared to FBP and ASIR images. Additionally, the SNR of MBIR images was significantly higher than FBP images and ASIR images.ConclusionsThe quality of chest CT images obtained by MBIR in children with necrotizing pneumonia was significantly improved by the MBIR technique as compared to the ASIR and FBP reconstruction, to provide a more confident and accurate diagnosis for necrotizing pneumonia.

Efficacy of model-based iterative reconstruction technique in non-enhanced CT of the renal tracts for ureteric calculi.

The purpose of this study was to assess the efficacy of model-based iterative reconstruction (MBIR), statistical iterative reconstruction (SIR), and filtered back projection (FBP) image reconstruction algorithms in the delineation of ureters and overall image quality on non-enhanced computed tomography of the renal tracts (NECT-KUB). This was a prospective study of 40 adult patients who underwent NECT-KUB for investigation of ureteric colic. Images were reconstructed using FBP, SIR, and MBIR techniques and individually and randomly assessed by two blinded radiologists. Parameters measured were overall image quality, presence of ureteric calculus, presence of hydronephrosis or hydroureters, image quality of each ureteric segment, total length of ureters unable to be visualized, attenuation values of image noise, and retroperitoneal fat content for each patient. There were no diagnostic discrepancies between image reconstruction modalities for urolithiasis. Overall image qualities and for each ureteric segment were superior using MBIR (67.5% rated as 'Good to Excellent' vs. 25% in SIR and 2.5% in FBP). The lengths of non-visualized ureteric segments were shortest using MBIR (55.0% measured 'less than 5cm' vs. ASIR 33.8% and FBP 10%). MBIR was able to reduce overall image noise by up to 49.36% over SIR and 71.02% over FBP. MBIR technique improves overall image quality and visualization of ureters over FBP and SIR.

Submillisievert Radiation Dose Coronary CT Angiography: Clinical Impact of the Knowledge-Based Iterative Model Reconstruction

The purpose of this study was to evaluate the noise and image quality of images reconstructed with a knowledge-based iterative model reconstruction (knowledge-based IMR) in ultra-low dose cardiac computed tomography (CT). We performed submillisievert radiation dose coronary CT angiography on 43 patients. We also performed a phantom study to evaluate the influence of object size with the automatic exposure control phantom. We reconstructed clinical and phantom studies with filtered back projection (FBP), hybrid iterative reconstruction (hybrid IR), and knowledge-based IMR. We measured effective dose of patients and compared CT number, image noise, and contrast noise ratio in ascending aorta of each reconstruction technique. We compared the relationship between image noise and body mass index for the clinical study, and object size for phantom study. The mean effective dose was 0.98 ± 0.25 mSv. The image noise of knowledge-based IMR images was significantly lower than those of FBP and hybrid IR images (knowledge-based IMR: 19.4 ± 2.8; FBP: 126.7 ± 35.0; hybrid IR: 48.8 ± 12.8, respectively) (P < .01). The contrast noise ratio of knowledge-based IMR images was significantly higher than those of FBP and hybrid IR images (knowledge-based IMR: 29.1 ± 5.4; FBP: 4.6 ± 1.3; hybrid IR: 13.1 ± 3.5, respectively) (P < .01). There were moderate correlations between image noise and body mass index in FBP (r = 0.57, P < .01) and hybrid IR techniques (r = 0.42, P < .01); however, these correlations were weak in knowledge-based IMR (r = 0.27, P < .01). Compared to FBP and hybrid IR, the knowledge-based IMR offers significant noise reduction and improvement in image quality in submillisievert radiation dose cardiac CT.

Effect of Cut-Off Frequency of Butterworth Filter on Detectability and Contrast of Hot and Cold Regions in Tc-99m SPECT

In SPECT, noise is one of the major limitations that degrade image quality. To suppress the noisy signals in an image, digital filters are most commonly applied. However, in SPECT image reconstruction, selection of an appropriate filter and its functions has always remained a difficult task. In this work an attempt was made to investigate the effects of varying cut-off frequencies and in keeping the order of Butterworth filter constant on detectability and contrast of hot and cold re-gions images. A new insert simulating hot and cold regions which provides similar views in a reconstructed image was placed in the phantom’s cylindrical source tank and imaged. Tc-99m radionuclide was distributed uniformly in the phantom. SPECT data were collected in a 20% energy window centered at 140 keV by a Philips ADAC Forte dual head gamma camera mounted with a LEHR collimator. Images were generated by using the filtered backprojection technique. A Butterworth filter of order 5 with cut-off frequencies 0.35 and 0.45 cycles·cm-1 was applied. Images were examined in terms of hot and cold regions, detectability and contrast. Results show that the hot and cold regions’ detectability and contrast vary with the change of cut-off frequency. With a 0.45 cycles·cm-1 cut-off frequency, a significant enhancement in contrast of cold regions was achieved as compared to a 0.35 cycles·cm-1 cut-off frequency. Furthermore, the detectability of hot and cold regions improved with the use of a 0.45 cycles·cm-1 cut-off frequency. In conclusion, image quality of hot and cold regions affected in a different way with a change of cut-off frequency. Thus, care should be taken in selecting the filter cut-off frequency prior to reconstruction of images; particularly, when both types of regions are expected in the reconstructed image.

Tomosynthesis Imaging

<i>Tomosynthesis Imaging</i>

TU-CD-207-05: A Novel Digital Tomosynthesis System Using Orthogonal Scanning Technique: A Feasibility Study

Purpose: As an alternative to full tomographic imaging technique such as cone-beam computed tomography (CBCT), there is growing interest to adopt digital tomosynthesis (DTS) for the use of diagnostic as well as therapeutic applications. The aim of this study is to propose a new DTS system using novel orthogonal scanning technique, which can provide superior image quality DTS images compared to the conventional DTS scanning system. Methods: Unlike conventional DTS scanning system, the proposed DTS is reconstructed with two sets of orthogonal patient scans. 1) X-ray projections that are acquired along transverse trajectory and 2) an additional sets of X-ray projections acquired along the vertical direction at the mid angle of the previous transverse scan. To reconstruct DTS, we have used modified filtered backprojection technique to account for the different scanning directions of each projection set. We have evaluated the performance of our method using numerical planning CT data of liver cancer patient and a physical pelvis phantom experiment. The results were compared with conventional DTS techniques with single transverse and vertical scanning. Results: The experiments on both numerical simulation as well as physical experiment showed that the resolution as well as contrast of anatomical structures was much clearer using our method. Specifically, the image quality comparing with transversely scanned DTS showed that the edge and contrast of anatomical structures along Left-Right (LR) directions was comparable however, considerable discrepancy and enhancement could be observed along Superior-Inferior (SI) direction using our method. The opposite was observed when vertically scanned DTS was compared. Conclusion: In this study, we propose a novel DTS system using orthogonal scanning technique. The results indicated that the image quality of our novel DTS system was superior compared to conventional DTS system. This makes our DTS system potentially useful in various on-line clinical applications.

Knowledge-based iterative reconstruction technique: effect of different reconstruction settings on noise reduction and image quality in hepatic contrast-enhanced CT at different radiation dose levels

Objective To evaluate the effects of different reconstruction settings of knowledge-based iterative reconstruction technique (IMR) on noise reduction and image quality in hepatic contrast-enhanced CT(CECT) at different radiation dose levels. Methods Patients who underwent hepatic CECT because of their diseases were enrolled in this prospective study. According to random number table, patients were randomly assigned to two groups (standard dose group, SD-group, n=29; low dose group, LD-group, n=25).All patients underwent both non-enhanced CT and triphasic CECT scan including hepatic arterial phase (HAP), portal-venous phase (PVP) and delayed phase. PVP images of SD-group were acquired with tube voltage of 120 kVp and tube current-time products of 250 mAs, and 80 kVp and 500 mAs for LD-group. PVP images were reconstructed with both filtered back projection (FBP) and IMR techniques. Settings applied in IMR reconstructions consisted of body routine (BR) and body soft tissue (BST) with three levels (L1 to L3), and image series included: L1BR, L2BR, L3BR, L1BST, L2BST and L3BST. Subjective assessment of image quality including low contrast detectability (LCD), image distortion (ID) and diagnostic confidence (DC) as well as objective image quality including image noise, signal to noise ratio (SNR) and contrast to noise ratio (CNR) were compared between groups. Effective radiation dose was recorded. Objective image quality and subjective image quality were compared by using Friedman H test and ANOVA, respectively. In addition, Student t test was used to compare effective radiation doses between groups. Results In SD-group, IMR settings of L2BST, L3BST scored highest in LCD with median score of 4; L3BST and L2BST scored lowest in ID with median score of 3, while FBP and L1BR scored highest with median score of 4; L1BR, L2BR and L1BST scored highest in DC with median score of 4, while L3BST scored lowest with median score of 3. In LD-group, the distribution of all reconstruction settings scores in LCD and ID were similar to those in SD-group; however, L1BST scored highest with median score of 4, and FBP scored lowest with median score of 2 in DC. There were statistical differences in LCD, ID and DC among all the reconstruction settings for both groups (P<0.05). In terms of FBP technique, the image noise, SNR and CNR were (9.8±2.0)HU, 13.3±3.3 and 6.0±1.9 in SD-group, and (16.2±4.1)HU, 12.9±3.3 and 6.6± 2.3 in LD-group, respectively. In SD-group, image noise exhibited a trend of decrease from L1BR to L3BST[from (4.5±0.9) HU to (3.2±0.2)HU], while SNR, CNR trended to increase (SNR: from 21.4±1.4 to 24.6±4.6; CNR: from 9.4±2.2 to 10.9±2.9, respectively). The image noise, SNR and CNR in LD-group showed the same trends as SD-group [image noise: from (5.8±0.2)HU to (3.9±0.6)HU; SNR: from 26.3±4.5 to 33.1±6.9; CNR: from 13.6±4.0 to 17.4±5.1, respectively]. In both groups, there were statistically differences in image noise, SNR and CNR among all IMR settings and FBP (F values were 15.50 to 131.39, P<0.01). The effective radiation dose of the LD-group was significantly lower than that of the SD-group [(4.19±0.22) versus (7.32±0.58) mSv, t=15.27, P<0.01). Conclusions IMR can reduce image noise and improve image quality in hepatic CECT at both standard and low dose levels. L1BR, L2BR and L1BST are the most optimized reconstruction settings for the standard dose protocol, while L1BST performs best for the low dose protocol. Key words: Liver; Tomography, X-ray computed; Radiation dosage

Coronary CT Angiography with Use of Iterative Reconstruction Algorithm in Coronary Stenting: A Systematic Review of Image Quality, Diagnostic Value and Radiation Dose

The aim of this study was to perform a systematic review of the image quality, diagnostic value and radiation dose of coronary CT angiography with use of iterative reconstruction (IR) in the assessment of coronary stents when compared to the standard filtered back-projection (FBP) techniques. A search of medical databases of English literature was performed to identify studies comparing coronary CT angiography with use of IR and FBP techniques in coronary stenting. Qualitative and quantitative assessment of image quality, diagnostic accuracy and radiation dose associated with coronary CT angiography were analyzed and compared between the two reconstruction algorithms. Ten studies met selection criteria and were included in the analysis. In comparison with FBP, coronary CT angiography with use of IR showed improvement in image quality through both qualitative and quantitative analysis. The IR technique resulted in higher diagnostic value than that of FBP with the mean sensitivity, specificity and accuracy being 98%, 77.4% and 80.1% for IR technique, and 90.5%, 68.2%, and 68.6% for FBP approach, respectively, with significant difference reached in the specificity and accuracy ( p< 0� 05). Furthermore, there was significantly lower dose with use of IR compared to FBP technique (4.4 mSv vs. 7.0 mSv). Coronary CT angiography with use of IR algorithm leads to significant improvements in the assessment of coronary stents with much lower radiation dose.

A high-resolution photon-counting breast CT system with tensor-framelet based iterative image reconstruction for radiation dose reduction

Both computer simulations and experimental phantom studies were carried out to investigate the radiation dose reduction with tensor framelet based iterative image reconstruction (TFIR) for a dedicated high-resolution spectral breast computed tomography (CT) based on a silicon strip photon-counting detector. The simulation was performed with a 10 cm-diameter water phantom including three contrast materials (polyethylene, 8 mg ml−1 iodine and B-100 bone-equivalent plastic). In the experimental study, the data were acquired with a 1.3 cm-diameter polymethylmethacrylate (PMMA) phantom containing iodine in three concentrations (8, 16 and 32 mg ml−1) at various radiation doses (1.2, 2.4 and 3.6 mGy) and then CT images were reconstructed using the filtered-back-projection (FBP) technique and the TFIR technique, respectively. The image quality between these two techniques was evaluated by the quantitative analysis on contrast-to-noise ratio (CNR) and spatial resolution that was evaluated using the task-based modulation transfer function (MTF). Both the simulation and experimental results indicated that the task-based MTF obtained from TFIR reconstruction with one-third of the radiation dose was comparable to that from the FBP reconstruction for low contrast target. For high contrast target, the TFIR was substantially superior to the FBP reconstruction in terms of spatial resolution. In addition, TFIR was able to achieve a factor of 1.6–1.8 increase in CNR, depending on the target contrast level. This study demonstrates that the TFIR can reduce the required radiation dose by a factor of two-thirds for a CT image reconstruction compared to the FBP technique. It achieves much better CNR and spatial resolution for high contrast target in addition to retaining similar spatial resolution for low contrast target. This TFIR technique has been implemented with a graphic processing unit system and it takes approximately 10 s to reconstruct a single-slice CT image, which can potentially be used in a future multi-slit multi-slice spiral CT system.

Computed Tomography Coronary Stent Imaging With Iterative Reconstruction

To evaluate the improvement of iterative reconstruction in image space (IRIS) technique in computed tomographic (CT) coronary stent imaging with sharp kernel, and to make a trade-off analysis. Fifty-six patients with 105 stents were examined by 128-slice dual-source CT coronary angiography (CTCA). Images were reconstructed using standard filtered back projection (FBP) and IRIS with both medium kernel and sharp kernel applied. Image noise and the stent diameter were investigated. Image noise was measured both in background vessel and in-stent lumen as objective image evaluation. Image noise score and stent score were performed as subjective image evaluation. The CTCA images reconstructed with IRIS were associated with significant noise reduction compared to that of CTCA images reconstructed using FBP technique in both of background vessel and in-stent lumen (the background noise decreased by approximately 25.4% ± 8.2% in medium kernel (P <C; 0.0001) and 30.3% ± 3.4% in sharp kernel (P <C; 0.0001). The in-stent lumen noise decreased by approximately 14.2% ± 19.2% in medium kernel (P <C; 0.0001) and 27.0% ± 17.8% in sharp kernel (P <C; 0.0001)). Subjective image assessment showed that the noise of the images reconstructed with IRIS decreased compared to that with FBP. Moreover, the images with sharp kernel showed better visualization of the stent struts and in-stent lumen than that with medium kernel. Iterative reconstruction in image space reconstruction can effectively reduce the image noise and improve image quality. The sharp kernel images constructed with iterative reconstruction are considered the optimal images to observe coronary stents in this study.

A Knowledge-based Iterative Model Reconstruction Algorithm: Can Super-Low-Dose Cardiac CT Be Applicable in Clinical Settings?

To investigate whether "full" iterative reconstruction, a knowledge-based iterative model reconstruction (IMR),enables radiation dose reduction by 80% at cardiac computed tomography (CT). A total of 23 patients (15 men, eight women; mean age 64.3 ± 13.4 years) who underwent retrospectively electrocardiography-gated cardiac CT with dose modulation were evaluated. We compared full-dose (FD; 730 mAs) images reconstructed with filtered back projection (FBP) technique and the low-dose (LD; 146 mAs) images reconstructed with FBP and IMR techniques. Objective and subjective image quality parameters were compared among the three different CT images. There was no significant difference in the CT attenuation among the three reconstructions. The mean image noise of LD-IMR (18.3± 10.6 Hounsfield units [HU]) was significantly lowest among the three reconstructions (41.9 ± 15.3 HU for FD-FBP and 109.9 ± 42.6 HU for LD-FBP; P < .01). The contrast-to-noise ratio of LD-IMR was better than that of FD-FBP and LD-FBP (P < .01). Visual evaluation score was also highest for LD-IMR. The IMR can provide improved image quality at super-low-dose cardiac CT with 20% of the standard tube current.

Clinical Implementation of Intrafraction Cone Beam Computed Tomography Imaging During Lung Tumor Stereotactic Ablative Radiation Therapy

To develop and clinically evaluate a volumetric imaging technique for assessing intrafraction geometric and dosimetric accuracy of stereotactic ablative radiation therapy (SABR). Twenty patients received SABR for lung tumors using volumetric modulated arc therapy (VMAT). At the beginning of each fraction, pretreatment cone beam computed tomography (CBCT) was used to align the soft-tissue tumor position with that in the planning CT. Concurrent with dose delivery, we acquired fluoroscopic radiograph projections during VMAT using the Varian on-board imaging system. Those kilovolt projections acquired during millivolt beam-on were automatically extracted, and intrafraction CBCT images were reconstructed using the filtered backprojection technique. We determined the time-averaged target shift during VMAT by calculating the center of mass of the tumor target in the intrafraction CBCT relative to the planning CT. To estimate the dosimetric impact of the target shift during treatment, we recalculated the dose to the GTV after shifting the entire patient anatomy according to the time-averaged target shift determined earlier. The mean target shift from intrafraction CBCT to planning CT was 1.6, 1.0, and 1.5 mm; the 95th percentile shift was 5.2, 3.1, 3.6 mm; and the maximum shift was 5.7, 3.6, and 4.9 mm along the anterior-posterior, left-right, and superior-inferior directions. Thus, the time-averaged intrafraction gross tumor volume (GTV) position was always within the planning target volume. We observed some degree of target blurring in the intrafraction CBCT, indicating imperfect breath-hold reproducibility or residual motion of the GTV during treatment. By our estimated dose recalculation, the GTV was consistently covered by the prescription dose (PD), that is, V100% above 0.97 for all patients, and minimum dose to GTV >100% PD for 18 patients and >95% PD for all patients. Intrafraction CBCT during VMAT can provide geometric and dosimetric verification of SABR valuable for quality assurance and potentially for treatment adaptation.