Levels Of Iterative Reconstruction Research Articles

Feasibility of lung imaging with a large field-of-view spectral photon-counting CT system

PurposeThe purpose of this study was to characterize the technical capabilities and feasibility of a large field-of-view clinical spectral photon-counting computed tomography (SPCCT) prototype for high-resolution (HR) lung imaging. Materials and methodsMeasurement of modulation transfer function (MTF) and acquisition of a line pairs phantom were performed. An anthropomorphic lung nodule phantom was scanned with standard (120kVp, 62mAs), low (120kVp, 11mAs), and ultra-low (80kVp, 3mAs) radiation doses. A human volunteer underwent standard (120kVp, 63mAs) and low (120kVp, 11mAs) dose scans after approval by the ethics committee. HR images were reconstructed with 1024 matrix, 300mm field of view and 0.25mm slice thickness using a filtered-back projection (FBP) and two levels of iterative reconstruction (iDose 5 and 9). The conspicuity and sharpness of various lung structures (distal airways, vessels, fissures and proximal bronchial wall), image noise, and overall image quality were independently analyzed by three radiologists and compared to a previous HR lung CT examination of the same volunteer performed with a conventional CT equipped with energy integrating detectors (120kVp, 10mAs, FBP). ResultsTen percent MTF was measured at 22.3lp/cm with a cut-off at 31lp/cm. Up to 28lp/cm were depicted. While mixed and solid nodules were easily depicted on standard and low-dose phantom images, higher iDose levels and slice thicknesses (1mm) were needed to visualize ground-glass components on ultra-low-dose images. Standard dose SPCCT images of in vivo lung structures were of greater conspicuity and sharpness, with greater overall image quality, and similar image noise (despite a flux reduction of 23%) to conventional CT images. Low-dose SPCCT images were of greater or similar conspicuity and sharpness, similar overall image quality, and lower but acceptable image noise (despite a flux reduction of 89%). ConclusionsA large field-of-view SPCCT prototype demonstrates HR technical capabilities and high image quality for high resolution lung CT in human.

Effect of CT reconstruction settings on the performance of a deep learning based lung nodule CAD system

PurposeTo study the effect of different reconstruction parameter settings on the performance of a commercially available deep learning based pulmonary nodule CAD system. Materials and methodsWe performed a retrospective analysis of 24 chest CT scans, reconstructed at 16 different reconstruction settings for two different iterative reconstruction algorithms (SAFIRE and ADMIRE) varying in slice thickness, kernel size and iterative reconstruction level strength using a commercially available deep learning pulmonary nodule CAD system. The DL-CAD software was evaluated at 25 different sensitivity threshold settings and nodules detected by the DL-CAD software were matched against a reference standard based on the consensus reading of three radiologists. ResultsA total of 384 CT reconstructions was analysed from 24 patients, resulting in a total of 5786 found nodules. We matched the detected nodules against the reference standard, defined by a team of thoracic radiologists, and showed a gradual drop in recall, and an improvement in precision when the iterative strength levels were increased for a constant kernel size. The optimal DL-CAD threshold setting for use in our clinical workflow was found to be 0.88 with an F2 of 0.73 ± 0.053. ConclusionsThe DL-CAD system behaves differently on IR data than on FBP data, there is a gradual drop in recall, and growth in precision when the iterative strength levels are increased. As a result, caution should be taken when implementing deep learning software in a hospital with multiple CT scanners and different reconstruction protocols. To the best of our knowledge, this is the first study that demonstrates this result from a DL-CAD system on clinical data.

Effect of an iterative reconstruction quantum noise reduction technique on computed tomography radiomic features.

Purpose: The goal of this study was to quantify the effects of iterative reconstruction on radiomics features of normal anatomic structures on head and neck computed tomography (CT) scans. Methods: Regions of interest (ROI) containing five different tissue types and an ROI containing only air were extracted from CT scans of the head and neck from 108 patients. Each scan was reconstructed using three different reconstruction levels (2, 4, and 6) in addition to bone, thin slice (1-mm slice thickness), and thin-bone reconstructions. From each ROI in all reconstructions, 142 radiomic features were calculated. For each of the six ROIs, features were compared between combinations of iDose levels (2v4, 4v6, and 2v6) with a threshold of after correcting for multiple comparisons ( ). Features from reconstructions were also compared to bone, thin slice, and thin-bone reconstructions. Spearman's rank correlation coefficient, , quantified the relative feature value agreement across reconstructions. Results: When comparing radiomics features across the three reconstruction levels, over half of all features reflected significant differences for all tissue types, while no features demonstrated significant differences when extracted from air ROIs. When assessing feature value agreement, at least 97% of features reflected excellent agreement ( ) when comparing the three iDose levels for all ROIs. When comparing to bone, thin slice, and thin-bone reconstructions, more than half of all features demonstrated significant differences for all ROIs and of features reflected excellent agreement for all ROIs. Conclusion: Many radiomics features are dependent on the iterative reconstruction level, and the magnitude of this dependency is affected by the tissue from which features are extracted. For studies using images reconstructed using varying reconstruction levels, features robust to these should be used.

Highly reduced-dose CT of the lumbar spine in a human cadaver model.

PurposeFeasibility of a highly reduced-dose lumbar spine CT protocol using iterative reconstruction (IR) in a human cadaver model.Materials and methodsThe lumbar spine of 20 human cadavers was repeatedly examined using three different reduced-dose protocols (RDCT) with decreasing reference tube current-exposure time products (RDCT-1: 50 mAs; RDCT-2: 30 mAs; RDCT-3: 10 mAs) at a constant tube voltage of 140 kV. A clinical standard-dose protocol (SDCT) served as the reference (reference tube current–exposure time product: 70 mAs; tube voltage: 140 kV). Images were reconstructed using filtered back projection (FBP) and two increasing levels of IR: IRL4 and IRL6. A five-point scale was used by two observers to assess the diagnostic quality of anatomical structures (cortical and trabecular bone, intervertebral foramina, pedicles and intervertebral joints, spinous and transverse processes). Objective image noise (OIN) was measured. Results were interpreted using a linear mixed-effects regression model.ResultsRDCT-2 with IRL6 (1.2 ± 0.5mSv) was the lowest reduced-dose protocol which provided diagnostically acceptable and equivalent image quality compared to the SDCT (2.3 ± 1.1mSV) with FBP (p > 0.05). All RDCT protocols achieved a significant reduction of the mean (±SD) effective radiation doses (RDCT-1: 1.7±0.9mSv; RDCT-2: 1.2±0.5mSv; RDCT-3: 0.4±0.2mSv; p < 0.05) compared to SDCT. OIN was lower in all RDCT protocols with the application of IRL4 and IRL6, compared to the SDCT with FBP (p < 0.05).ConclusionHighly reduced-dose lumbar spine CT providing diagnostically acceptable image quality is feasible using IR in this cadaver model and may be transferred into a clinical setting.

Spatial Distribution of Noise Reduction in Four Iterative Reconstruction Algorithms in CT-A Technical Evaluation.

Iterative reconstruction (IR) is a computed tomgraphy (CT) reconstruction algorithm aiming at improving image quality by reducing noise in the image. During this process, IR also changes the noise properties in the images. To assess how IR algorithms from four vendors affect the noise properties in CT images, an anthropomorphic phantom was scanned and images reconstructed with filtered back projection (FBP), and a medium and high level of IR. Each image acquisition was performed 30 times at the same slice position, to create noise maps showing the inter-image pixel standard deviation through the 30 images. We observed that IR changed the noise properties in the CT images by reducing noise more in homogeneous areas than at anatomical edges between structures of different densities. This difference increased with increasing IR level, and with increasing difference in density between two adjacent structures. Each vendor’s IR algorithm showed slightly different noise reduction properties in how much noise was reduced at different positions in the phantom. Users need to be aware of these differences when working with optimization of protocols using IR across scanners from different vendors.

Effect of iterative reconstruction algorithm levels on noise index and figure-of-merit in CT pulmonary angiography examinations.

To evaluate the influence of iterative reconstruction (IR) levels on Computed Tomography (CT) image quality and to establish Figure of Merit (FOM) value for CT Pulmonary Angiography (CTPA) examinations. Images of 31 adult patients who underwent CTPA examinations in our institution from March to April 2019 were retrospectively collected. Other data, such as scanning parameters, radiation dose and body habitus information from the subjects were also recorded. Six different levels of IR were applied to the volume data of the subjects. Five circles of the region of interest (ROI) were drawn in five different arteries namely, pulmonary trunk, right pulmonary artery, left pulmonary artery, ascending aorta and descending aorta. The mean Signal-to-noise ratio (SNR) was obtained, and the FOM was calculated in a fraction of the SNR2 divided by volume-weighted CT dose index (CTDIvol) and SNR2 divided by the size-specific dose estimates (SSDE). Overall, we observed that the mean value of CTDIvol and SSDE were 13.79±7.72 mGy and 17.25±8.92 mGy, respectively. Notably, SNR values significantly increase with increase of the IR level (p < 0.05). There are also significant differences (p < 0.05) in the FOM for both SNR2/SSDE and SNR2/CTDIvol attained in different IR levels. We successfully evaluate the value of radiation dose and image quality performance and set up a figure of merit for both parameters to further verify scanning protocols by radiology personnel.

The influence of iterative reconstruction level on image quality and radiation dose in CT pulmonary angiography examinations

Computed Tomography Pulmonary Angiography (CTPA) examination is one of the essential imaging procedures to detect pulmonary embolism (PE). Despite its advantages, it may contribute high dose exposure to the patients if unoptimized parameters were applied. Numbers of optimization techniques have been introduced to reduce dose while preserving the diagnostic quality of the procedure. Hence, this study aimed to evaluate the effects of using iterative reconstruction (IR) algorithm with adoption of different tube potentials and establish the Figure of Merit (FOM) in the relationship between radiation exposure and image quality. Thirty adult patients (n = 30) were retrospectively recruited. Data, such as patient demography, scanning protocols and radiation dose information, were collected and analyzed. Each CT image was reconstructed with different levels of IR algorithm. A 150 mm2-wide circular region of interest (ROI) was drawn inside five consecutive areas surrounding the pulmonary artery in each reconstructed image. Diagnostic performance was evaluated based on signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR). Meanwhile, FOM was evaluated in four different indicators, which were SNR2/Cvol, SNR2/SSDE, CNR2/Cvol and CNR2/SSDE. Volume weighted CT Dose Index (Cvol) and Size-specific Dose Estimate (SSDE) were estimated for each subject and utilized for the denominator in calculating FOM. Cvol and SSDE increased up to 170% and 154%, respectively, with higher tube voltage. They also increased up to 33% and 16%, respectively, as the patients' effective diameter increased. SNR, CNR and FOM values were comparable with different IR levels, tube voltage and effective diameter. In conclusion, CTPA would yield better results with increasing IR levels, decreasing tube voltage and on patients with small body size.

Spatial resolution compensation by adjusting the reconstruction kernels for iterative reconstruction images of computed tomography

PurposeHybrid iterative reconstruction (IR) is useful to reduce noise in computed tomography (CT) images. However, it often decreases the spatial resolution. The ability of high spatial resolution kernels (harder kernels) to compensate for the decrease in the spatial resolution of hybrid IRs was investigated. MethodsAn elliptic cylindrical phantom simulating an adult abdomen was used. Two types of rod-shaped objects with ~330 and ~130 HU were inserted to simulate contrasts of arteries in CT angiography. Two multi-slice CT systems were used to scan the phantoms with 120 kVp and scan doses of 20 and 10 mGy. The task transfer functions (TTFs) were measured from the circular edges of the rod images. The noise power spectrum (NPS) was measured from the images of the water-only section. The CT images were reconstructed using a filtered back projection (FBP) with baseline kernels and two levels of hybrid IRs with harder kernels. The profiles of the clinical images across the aortic dissection flaps were measured to evaluate actual spatial resolutions. ResultsThe TTF degradation of each hybrid IR was recovered by the harder kernels, whereas the noise reduction effect was retained, for both the 20 and 10 mGy. The profiles of the dissection flaps for the FBP were maintained by using the harder kernels. Even with the best combination of hybrid IR and harder kernel, the noise level at 10 mGy was not reduced to the level of FBP at 20 mGy, suggesting no capability of a 50% dose reduction while maintaining noise.

Effect of Iterative Reconstruction Algorithm Associated with Low Contrast Detectability Performance from CT Pulmonary Angiography Examinations

Iterative reconstruction (IR) in Computed Tomography (CT) scan examinations are known for noise reduction to achieve a higher level of diagnostic. This study aims to evaluate the radiation dose and image quality performance in different iterative reconstruction levels on CT Pulmonary Angiography (CTPA) examination and to establish a figure of merit (FOM) between both parameters. Thirty adult patients who underwent CTPA examinations were retrospectively included in the present study. Scanning acquisition protocol and radiation dose information, together with patient habitus information, was collected and recorded. The vessel attenuation was measured in five consecutive areas surrounding the pulmonary artery for each iterative reconstruction image. Results of radiation dose were presented in terms of Computed Tomography Dose Index (CTDIvol) obtained from CT-console and SSDE that was calculated by the AAPM report 204 recommendation method. Meanwhile, image quality was estimated by contrast-to-noise ratio (CNR) on average from five consecutive areas of CNR. The mean values of CTDIvol and SSDE were 13.79 ± 7.72 mGy and 17.25 ± 8.92 mGy, respectively. The amount of CNR was significantly different within the iterative reconstruction level (p <0.001). There was a significant difference in the figure of merit for both CNR2/SSDE and CNR2/CTDIvol attained in different iterative reconstruction levels, (p <0.001) and (p <0.001), respectively. In conclusion, we successfully evaluate the value of radiation dose and image quality performance and establish a figure of merit for both parameters to aid further verification of scanning protocols by radiology staff.

Image quality and dose reduction opportunity of deep learning image reconstruction algorithm for CT: a phantom study.

To assess the impact on image quality and dose reduction of a new deep learning image reconstruction (DLIR) algorithm compared with a hybrid iterative reconstruction (IR) algorithm. Data acquisitions were performed at seven dose levels (CTDIvol :15/10/7.5/5/2.5/1/0.5mGy) using a standard phantom designed for image quality assessment. Raw data were reconstructed using the filtered back projection (FBP), two levels of IR (ASiR-V50% (AV50); ASiR-V100% (AV100)), and three levels of DLIR (TrueFidelity™ low, medium, high). Noise power spectrum (NPS) and task-based transfer function (TTF) were computed. Detectability index (d') was computed to model a large mass in the liver, a small calcification, and a small subtle lesion with low contrast. NPS peaks were higher with AV50 than with all DLIR levels and only higher with DLIR-H than with AV100. The average NPS spatial frequencies were higher with DLIR than with IR. For all DLIR levels, TTF50% obtained with DLIR was higher than that with IR. d' was higher with DLIR than with AV50 but lower with DLIR-L and DLIR-M than with AV100. d' values were higher with DLIR-H than with AV100 for the small low-contrast lesion (10 ± 4%) and in the same range for the other simulated lesions. New DLIR algorithm reduced noise and improved spatial resolution and detectability without changing the noise texture. Images obtained with DLIR seem to indicate a greater potential for dose optimization than those with hybrid IR. • This study assessed the impact on image quality and radiation dose of a new deep learning image reconstruction (DLIR) algorithm as compared with hybrid iterative reconstruction (IR) algorithm. • The new DLIR algorithm reduced noise and improved spatial resolution and detectability without perceived alteration of the texture, commonly reported with IR. • As compared with IR, DLIR seems to open further possibility of dose optimization.

Improving Low-contrast Detectability and Noise Texture Pattern for Computed Tomography Using Iterative Reconstruction Accelerated with Machine Learning Method: A Phantom Study

To evaluate the performance of iterative reconstruction (IR) and filtered back projection (FBP) images in terms of low-contrast detectability at different radiation doses, IR levels, and slice thickness using the mathematical model observer with a focus on low-contrast detectability. The CCT189 MITA CT IQ Low-Contrast Phantom was used and helical scans were performed using a 64-detector CT scanner. Tube voltage was set at 120 kVp and tube current was adjusted from 45 to 600 mA. Images were reconstructed at slice thicknesses of 0.625 and 5.0 mm with FBP and five types of iterative progressive reconstruction with visual modeling (IPV) algorithms. The noise power spectrum (NPS) and normalized NPS were calculated. To evaluate low-contrast detectability, the model observer with the channelized Hotelling observer model was applied using low-contrast modules in the phantom. The NPS and normalized NPS for IPV images had similar curves as that for FBP images. At a slice thickness of 0.625 mm and equivalent radiation dose level, the mean improvement of low-contrast detectability for IPV images was 1.19-2.15-fold greater than FBP images with corresponding noise reduction levels. At equivalent noise levels of 5.0-8.0 HU, low-contrast detectability of the IPVstd2 to IPVstr2 images as almost the same or better than that of the FBP images. However, the detectability of the IPVstr4 image was lower than that of the FBP image (p = 0.02). Low-contrast detectability of the IPV images was improved with a similar normalized NPS as with FBP images. Furthermore, a radiation reduction of >50% was achieved for the IPV images, while maintaining similar low-contrast detectability.

Feasibility of extremely reduced-dose CT of the thoracic spine in human cadavers.

According to the as low as reasonably achievable (ALARA) principle, radiation exposure in computed tomography (CT) should be minimized while maintaining adequate image quality. Dedicated CT protocols combined with iterative reconstruction (IR) can reduce radiation dose and/or improve image quality. To investigate the feasibility of extremely reduced-dose (RD) CT of the thoracic spine in human cadavers using a standard-dose (SD) and three different RDCT protocols reconstructed with filtered back projection (FBP) and IR. The thoracic spines of 11 cadavers were examined using different RDCT protocols with decreasing reference tube currents (RDCT-1: 50 mAs; RDCT-2: 30 mAs; RDCT-3: 10 mAs) at 140 kV. A clinical SDCT (70 mAs, 140 kV) served as reference. Raw data were reconstructed using FBP and two increasing levels of IR (IRL4 and IRL6). Images were evaluated for image quality, diagnostic acceptability, and visibility of anatomical structures according to a 5-point-scale. Regardless of the reconstruction technique, image quality was rated as diagnostically acceptable for all cadavers in SDCT and RDCT-1. Image quality of reconstructions with FBP were generally rated lower. Application of IR improved image quality ratings in SDCT and RDCT. RDCT-2 with IR was the most reduced-dose CT protocol which enabled diagnostically acceptable image quality in all cadavers. Compared to SDCT, RDCT protocols resulted in significantly reduced effective radiation doses (SDCT: 4.1 ± 1.5 mSv; RDCT-1: 2.9 ± 1.1 mSv; 2:1.7 ± 0.6 mSv; 3:0.6 ± 0.1 mSv; P = 0.001). Diagnostically acceptable RDCT of the thoracic spine with 1.7 mSv is feasible using IR.

Systematic Evaluation of Low-dose MDCT for Planning Purposes of Lumbosacral Periradicular Infiltrations.

To evaluate image quality and confidence for planning of periradicular infiltrations using virtually lowered tube currents and in-house developed iterative reconstruction (IR) for multidetector computed tomography (MDCT). A total of 20 patients (mean age 54.9 ± 13.1 years) underwent MDCT for planning purposes of periradicular infiltrations at the lumbosacral spine (120 kVp and 100 mAs). Planning scans were simulated as if they were performed at 50% (D50), 10% (D10), 5% (D5), and 1% (D1) of the tube current of original scanning. Image reconstruction was achieved with two levels of IR (A: similar in appearance to clinical reconstructions, B: 10 times stronger noise reduction). Qualitative image evaluation was performed by two readers (R1 and R2) considering overall image quality and artifacts, image contrast, determination of nerve root, and confidence for intervention planning (scoring: 1 high, 2 medium, and 3 low confidence). LevelA of IR was favorable regarding overall image quality, artifacts, image contrast, and nerve root depiction according to both readers, with preserved good to excellent scores down to D10 scans. The confidence for intervention planning was not significantly different (p > 0.05) between scans with tube currents virtually lowered down to 10% as compared to the original scans when using levelA of IR (R1: 1.2 ± 0.4, R2: 1.1 ± 0.3). Inter-reader agreement for planning confidence was good to excellent (range of weighted Cohen's kappa: 0.62-1.00). The use of MDCT for planning purposes of lumbosacral periradicular infiltrations may be possible with tube currents lowered down to 10% of standard dose (equal to 10 mAs) without limitations in planning confidence.

Feasibility of sub-milliSievert CT of the cervical spine: Initial results in fresh human cadavers

PurposeTo investigate the feasibility of sub-milliSievert CT of the cervical spine in fresh human cadavers using a standard-dose (SD) and four different reduced-dose (RD) protocols reconstructed with filtered back projection (FBP) and iterative reconstruction (IR). MethodsThe cervical spine of 29 cadavers was examined using different RDCT protocols with decreasing reference tube currents (RDCT-1:70 mAs; RDCT-2:50 mAs; RDCT-3:30 mAs; RDCT-4:10 mAs) at 140 kV. A clinical SDCT (160 mAs, 120 kV) served as reference. Raw data were reconstructed using FBP and two increasing levels of IR (IRL4&6). Images of the upper (C1–4) and lower (C5–7) cervical spine were evaluated for image quality, diagnostic acceptability and visibility of anatomical structures according to a 5-point-scale. ResultsImage quality of the upper cervical spine was diagnostically acceptable for all protocols using FBP and IR except for RDCT-4 with FBP. Image quality of the lower cervical spine was rated as non-diagnostic in RDCT-3 with FBP and RDCT-4 with FBP and IR. RDCT-3 with IR was the most reduced dose CT protocol allowing diagnostically acceptable image quality for both upper and lower cervical spine in all cadavers. RDCT protocols achieved significantly reduced effective radiation doses (SDCT: 1.5 ± 0.7 mSv; RDCT-1:1 ± 0.6 mSv; RDCT-2:0.7 ± 0.4 mSv; RDCT-3:0.4 ± 0.2 mSv; RDCT-4:0.2 ± 0.1 mSv; p < 0.001). ConclusionDiagnostically acceptable sub-milliSievert CT of the cervical spine is feasible with a low reference tube current at 140 kV using iterative reconstruction and could be suitable for isolated cervical trauma in cooperative patients.

Optimization of radiation dose for CT detection of lytic and sclerotic bone lesions: a phantom study

To determine the best compromise between low radiation dose and suitable image quality for the detection of lytic and sclerotic bone lesions of the lumbar spine and pelvis. A phantom was scanned using the routine protocol (STD, 13mGy) and six decreasing dose levels. Raw data were reconstructed using level 3 of iterative reconstruction (IR3) with 1-mm slice thickness for the STD protocol and highest IR levels with 3-mm slice thickness for the others. CTDIvol was used for radiation dose assessment. Quantitative criteria (noise power spectrum [NPS], task-based transfer function [TTF], and the detectability index [d']), as well as qualitative analysis, were used to compare protocols. NPS and TTF were computed using specific software (imQuest). d' was computed for two imaging tasks: lytic and sclerotic bone lesions. A subjective analysis was performed to validate the image quality obtained on the anthropomorphic phantom with the different dose values. Similar d' values were found for CTDIvol from 3 to 4mGy with IR4 and from 1 to 2mGy for IR5 compared with d' values using the STD protocol. Image quality was validated subjectively for IR4 but rejected for IR5 (image smoothing). Finally, for the same d', the dose was reduced by 74% compared with the STD protocol, with the CTDIvol being 3.4mGy for the lumbar spine and for the pelvis. A dose level as low as 3.4mGy, in association with high levels of IR, provides suitable image quality for the detection of lytic and sclerotic bone lesions of the lumbar spine and pelvis. • A CTDI volof 3.4mGy, in association with high iterative reconstruction level, provides suitable image quality for the detection of lytic and sclerotic bone lesions, both at objective and subjective analysis. • Compared with the standard protocol, radiation dose can be reduced up to 74% for the lumbar spine and pelvis. • A task-based image quality assessment using the detectability index represents an objective method for the assessment of image quality and bridgesthe gap between complex physical metrics and subjective image analysis.

Feasibility of Submillisievert CT of the Skeletal Pelvis Using Iterative Reconstruction: A Human Cadaver Study.

OBJECTIVE. The purpose of this study is to investigate the feasibility of submillisievert CT of the skeletal pelvis of human cadavers using a standard-dose protocol and four different reduced-dose protocols reconstructed with filtered back projection (FBP) and iterative reconstruction (IR). MATERIALS AND METHODS. The pelvis of 25 human cadavers was repeatedly examined using different reduced-dose CT (RDCT) protocols with decreasing reference tube current-exposure time products (RDCT protocol 1, 80 mAs; RDCT protocol 2, 60 mAs; RDCT protocol 3, 40 mAs; and RDCT protocol 4, 10 mAs) and a tube voltage of 120 kV. A standard-dose CT (SDCT) protocol (reference tube current-exposure time product, 100 mAs; tube voltage, 120 kV) used for the same cadavers served as the reference. Raw data were reconstructed using FBP and two increasing levels of IR (IR levels 4 and 6). The image quality and diagnostic acceptability of images of the anterior pelvic ring, acetabulum, and posterior pelvic ring including the sacroiliac joints were evaluated on a 5-point scale. A mixed-effects model for repeated measures was performed. RESULTS. The image quality of all anatomic structures was rated as diagnostically acceptable for all protocols reconstructed with IR, except for 11 cadavers that were imaged using RDCT protocol 4. For reconstructions with FBP, image quality was generally rated lower and was diagnostically acceptable only for images obtained using SDCT and RDCT protocol 1 and 2. RDCT protocol 3 with IR was the RDCT protocol with the largest reduced dose still allowing diagnostically acceptable image quality for all anatomic structures in all cadavers. Compared with SDCT, the RDCT protocols resulted in significantly reduced mean (± SD) effective radiation doses (SDCT, 2.0 ± 0.7 mSv; RDCT protocol 1, 1.6 ± 0.6 mSv; RDCT protocol 2, 1.2 ± 0.4 mSv; RDCT protocol 3, 0.8 ± 0.3 mSv; and RDCT protocol 4, 0.3 ± 0.1 mSv; p = 0.001). CONCLUSION. Diagnostically acceptable submillisievert CT of the skeletal pelvis is feasible using IR. To adhere to the ALARA (as low as reasonably achievable) principle, submillisievert pelvic CT protocols combined with IR should be implemented as part of routine clinical practice.

Tube Current Reduction in CT Angiography: How Low Can We Go in Imaging of Patients With Suspected Acute Stroke?

OBJECTIVE. The purpose of this study was to systematically evaluate image quality, detectability of large-vessel occlusion or dissection, and diagnostic confidence in CT angiography (CTA) with virtually lowered tube current and iterative reconstruction in patients with suspected acute stroke. MATERIALS AND METHODS. Thirty patients (15 with large-vessel occlusion or dissection) underwent CTA of the supraaortal up to the intracranial arterial vessels. CTA scans were simulated as if they were made at 50% (D50), 25% (D25), and 10% (D10) of the original tube current. Image reconstruction was achieved with two levels of iterative reconstruction (A, similar to clinical reconstructions; B, two times stronger regularization). Two readers performed qualitative image evaluation considering overall image quality, artifacts, vessel contrast, detection of vessel abnormalities, and diagnostic confidence. RESULTS. Level B of iterative reconstruction was favorable regarding overall image quality and artifacts for D10, whereas level A was favorable for D100 and D50. CTA scans at D25 and both levels of iterative reconstruction still showed good vessel contrast, with even peripheral arterial branches of the anterior, middle, and posterior cerebral arteries being clearly detectable. Furthermore, CTA scans at D25 and level A of iterative reconstruction showed an adequate level of diagnostic confidence without any missed large-vessel occlusion or dissection according to evaluations by both readers. CONCLUSION. CTA with iterative reconstruction and tube currents decreased to 25% of that for original imaging is feasible without limitations in vessel contrast or detection of vessel abnormalities in patients with suspected acute stroke. Thus, the approach evaluated enables substantial reductions in radiation exposure for patients undergoing head and neck CTA.

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.

Effects of radiation dose levels and spectral iterative reconstruction levels on the accuracy of iodine quantification and virtual monochromatic CT numbers in dual-layer spectral detector CT: an iodine phantom study.

The purpose of this study is to investigate the accuracy of iodine quantification and virtual monochromatic CT numbers obtained with the dual-layer spectral CT (DLCT) using a phantom at different radiation dose levels and spectral iterative reconstruction (IR) levels. An abdomen phantom with seven iodine inserts (2.0, 2.5, 5.0, 7.5, 10.0, 15.0, 20.0 mg/mL) was imaged using a DLCT scanner. Five repeated scans were performed at computed tomography dose index volume (CTDIvol) levels of 5, 10, 15, 20, 25 mGy at tube voltages of 120 and 140 kVp, respectively. Spectral-based images were reconstructed using four spectral IR levels (spectral level of 0, 2, 4, 6). Iodine density images and virtual monochromatic images (VMI) at energy levels of 50, 70 and 120 keV were created. The absolute percentage bias (APB) of the measured iodine concentration and the true iodine concentration, and the measured VMI CT numbers and the theoretical VMI CT numbers were compared to determine the difference of radiation dose levels and different spectral IR levels. At CTDIvol levels of 25, 20, 15, 10 mGy, radiation dose levels had no effect on the accuracy of iodine quantitation; at CTDIvol level of 5 mGy, the accuracy of iodine quantification was the poorest, with the mean APBiodine of 4.33% (P<0.05). There was no significant difference in the accuracy of iodine quantitation between 120 and 140 kVp (P=0.648). At energy levels of 50, 70 and 120 keV, there was no significant difference in the accuracy of the VMI CT numbers among the CTDIvol levels of 25, 20 and 15 mGy. However, the accuracy of VMI CT numbers was significantly degraded at the CTDIvol levels of 10 and 5 mGy (P<0.05). At energy level of 50 keV, the accuracy of VMI CT numbers was not affected by tube voltages (kVps) used (P=0.125). At the energy levels of 70 and 120 keV, 140 kVp produced a smaller bias than 120 kVp, with the mean APBHU at 120 and 140 kVp being of 3.62% vs. 2.99% for 70 keV (P<0.01), and 11.65% vs. 9.28% for 120 keV (P<0.01), respectively. Spectral IR levels did not affect the accuracy of iodine quantification and VMI CT numbers (P=0.998, P=0.963). The accuracy of iodine quantification and VMI CT numbers was only affected by very low radiation dose levels. At the clinically applied radiation dose levels of >10 mGy, the accuracy of both iodine quantification and VMI CT numbers is relatively stable and high.

Reproducibility of FFR-CT at High Levels of Iterative Reconstruction

Introduction: FFR-CT has a developing presence as a non-invasive means of assessing myocardial ischaemia. Reproducibility of workstation-based FFR-CT is crucial, and data in this area are limited, particularly with differing reconstruction algorithms. Iterative reconstruction algorithms are used to lower radiation dose while maintaining image quality. While these techniques are widely applied to CTCA, there are limited data on the accuracy of FFR-CT at high levels of iterative reconstruction. Methods: FFR-CT was retrospectively performed on 20 patients who underwent both CTCA and stress MIBI within 60 days at a single centre between January 2014 and December 2018. Two radiologists independently calculated the FFR at three points along the RCA, LAD and LCx onsite using prototype software (cFFR version 3.1, Siemens GmbH, Forchheim, Germany) on either ADMIRE 4 (n = 11) or SAFIRE 4 (n = 9) i26. Intraclass correlation coefficients (ICC) were calculated using a two-way consistency model.Tabled 1Observer 1 mean FFRObserver 2 mean FFRICC (95% CI)Proximal0.99 (+/− 0.01)0.99 (+/− 0.01)0.82 (0.70–0.89)Mid0.93 (+/− 0.05)0.94 (+/− 0.04)0.94 (0.90–0.97)Distal0.84 (+/− 0.08)0.85 (+/− 0.09)0.93 (0.79–0.92)Total0.92 (+/− 0.04)0.93 (+/− 0.04)0.97 (0.96–0.98) Open table in a new tab Results: 8 patients had lesions in proximal or mid vessels that were thought to be of at least moderate severity on CTCA. Conclusion: Onsite FFR-CT has excellent levels of reproducibility and this is maintained despite high levels of iterative reconstruction. This may allow CTCA to be acquired at lower radiation doses even if CT-FFR is planned.