Reducing abdominal CT radiation dose with adaptive statistical iterative reconstruction technique. Invest Radiol. 2010;45:202-210. [PMID: 20177389 DOI: 10.1097/rli.ob013e3181dzfeec]

Observer performance for adaptive, image-based denoising and filtered back projection compared to scanner-based iterative reconstruction for lower dose CT enterography

PURPOSE

The purpose of this study was to compare observer performance for detection of intestinal inflammation for low-dose CT enterography (LD-CTE) using scanner-based iterative reconstruction (IR) vs. vendor-independent, adaptive image-based noise reduction (ANLM) or filtered back projection (FBP).

METHODS

Sixty-two LD-CTE exams were performed. LD-CTE images were reconstructed using IR, ANLM, and FBP. Three readers, blinded to image type, marked intestinal inflammation directly on patient images using a specialized workstation over three sessions, interpreting one image type/patient/session. Reference standard was created by a gastroenterologist and radiologist, who reviewed all available data including dismissal Gastroenterology records, and who marked all inflamed bowel segments on the same workstation. Reader and reference localizations were then compared. Non-inferiority was tested using Jackknife free-response ROC (JAFROC) figures of merit (FOM) for ANLM and FBP compared to IR. Patient-level analyses for the presence or absence of inflammation were also conducted.

RESULTS

There were 46 inflamed bowel segments in 24/62 patients (CTDIvol interquartile range 6.9-10.1 mGy). JAFROC FOM for ANLM and FBP were 0.84 (95% CI 0.75-0.92) and 0.84 (95% CI 0.75-0.92), and were statistically non-inferior to IR (FOM 0.84; 95% CI 0.76-0.93). Patient-level pooled confidence intervals for sensitivity widely overlapped, as did specificities. Image quality was rated as better with IR and AMLM compared to FBP (p < 0.0001), with no difference in reading times (p = 0.89).

CONCLUSIONS

Vendor-independent adaptive image-based noise reduction and FBP provided observer performance that was non-inferior to scanner-based IR methods. Adaptive image-based noise reduction maintained or improved upon image quality ratings compared to FBP when performing CTE at lower dose levels.

Improving Low-dose Cardiac CT Images based on 3D Sparse Representation

Cardiac computed tomography (CCT) is a reliable and accurate tool for diagnosis of coronary artery diseases and is also frequently used in surgery guidance. Low-dose scans should be considered in order to alleviate the harm to patients caused by X-ray radiation. However, low dose CT (LDCT) images tend to be degraded by quantum noise and streak artifacts. In order to improve the cardiac LDCT image quality, a 3D sparse representation-based processing (3D SR) is proposed by exploiting the sparsity and regularity of 3D anatomical features in CCT. The proposed method was evaluated by a clinical study of 14 patients. The performance of the proposed method was compared to the 2D spares representation-based processing (2D SR) and the state-of-the-art noise reduction algorithm BM4D. The visual assessment, quantitative assessment and qualitative assessment results show that the proposed approach can lead to effective noise/artifact suppression and detail preservation. Compared to the other two tested methods, 3D SR method can obtain results with image quality most close to the reference standard dose CT (SDCT) images.

Lowering radiation dose during dedicated colorectal cancer MDCT: comparison of low tube voltage and sinogram-affirmed iterative reconstruction at 80 kVp versus blended dual-energy images in a population of patients with low body mass index

PURPOSE

To assess the diagnostic accuracy, cancer staging, image quality, and radiation dose of 80-kVp computed tomography (CT) images for patients with colorectal cancers (CRCs) using sinogram-affirmed iterative reconstruction (SAFIRE).

METHODS

Sixty-four consecutive patients (mean weight 62.5  ±  11.3 kg, mean BMI 24.1  ±  3.3 kg/m(2)) with known CRC underwent dual-energy CT. Data were reconstructed as a weighted average (WA) 120-kVp dataset. Both filtered back projection (FBP) and SAFIRE were applied to reconstruct the WA 120-Kvp (Protocol A, B) and 80-kVp (Protocol C, D) image sets. The image noise, signal-to-noise ratio (SNR), and contrast-to-noise ratio (CNR) of the cancers, the normal reference tissues, and the effective dose for each protocol were assessed. The cancer detection, staging, and image quality were evaluated. Analysis of variance was used for statistical analysis.

RESULTS

Compared with the FBP datasets at WA 120-kVp (Protocol A) and 80-kVp (Protocol C), the SAFIRE-reconstructed images (Protocols B, D) demonstrated significantly lower image noise (P  <  0.0083). Protocol D yielded significantly higher CNRs and SNRs for the CRCs and normal reference tissues than did Protocols A and C (P  <  0.0083). Protocol D also exhibited a significantly higher CNR for the CRC and some normal reference tissues than did Protocol B (P  <  0.0083). For hypovascular liver metastases (n  =  10), Protocol D yielded better SNRs and significantly higher CNRs than did Protocol A (P  <  0.0083). Overall, accuracy for tumor staging and liver metastasis was 95.3% (61/64) and 100%, respectively, in all of the 4 protocols. The mean effective dose decreased 41% from the WA 120-kVp to the 80-kVp protocols (6.23 vs. 3.68 mSv).

CONCLUSIONS

The 80-kVp technique with SAFIRE provided high SNR, high CNR, and good accuracy for staging in nonobese patients with CRC. Our study results should be extrapolated to patient populations with a high BMI with caution. Further studies of high BMI patients are therefore warranted.

Optimal Adaptive Statistical Iterative Reconstruction Percentage in Dual-energy Monochromatic CT Portal Venography

RATIONALE AND OBJECTIVES

The aim of this article was to study the influence of different adaptive statistical iterative reconstruction (ASIR) percentages on the image quality of dual-energy computed tomography (DECT) portal venography in portal hypertension patients.

MATERIALS AND METHODS

DECT scans of 40 patients with cirrhosis (mean age, 56 years) at the portal venous phase were retrospectively analyzed. Monochromatic images at 60 and 70 keV were reconstructed with four ASIR percentages: 0%, 30%, 50%, and 70%. Computed tomography (CT) numbers of the portal veins (PVs), liver parenchyma, and subcutaneous fat tissue in the abdomen were measured. The standard deviation from the region of interest of the liver parenchyma was interpreted as the objective image noise (IN). The contrast-noise ratio (CNR) between PV and liver parenchyma was calculated. The diagnostic acceptability (DA) and sharpness of PV margins were obtained using a 5-point score. The IN, CNR, DA, and sharpness of PV were compared among the eight groups with different keV + ASIR level combinations.

RESULTS

The IN, CNR, DA, and sharpness of PV of different keV + ASIR groups were all statistically different (P < 0.05). In the eight groups, the best and worst CNR were obtained in the 60 keV + 70% ASIR and 70 keV + 0% ASIR (filtered back-projection [FBP]) combination, respectively, whereas the largest and smallest objective IN were obtained in the 60 keV + 0% ASIR (FBP) and 70 keV + 70% combination. The highest DA and sharpness values of PV were obtained at 50% ASIR for 60 keV.

CONCLUSIONS

An optimal ASIR percentage (50%) combined with an appropriate monochromatic energy level (60 keV) provides the highest DA in portal venography imaging, whereas for the higher monochromatic energy (70 keV) images, 30% ASIR provides the highest image quality, with less IN than 60 keV with 50% ASIR.

Model-Based Iterative Reconstruction in CT Enterography

OBJECTIVE

The purpose of this study was to analyze the performance of pure model-based iterative reconstruction (MBIR) in low-dose CT enterography.

SUBJECTS AND METHODS

Forty-four patients with Crohn disease referred for CT enterography were included. Low-dose modified-protocol and conventional-protocol CT datasets were contemporaneously acquired. Conventional-protocol image formation was performed with 40% adaptive statistical iterative reconstruction (ASIR). Modified-protocol data were reconstructed with 100% MBIR and 40% ASIR. Image quality was assessed subjectively and objectively at six levels. Independent clinical interpretations by two fully blinded radiologists were compared with reference standard consensus reviews by two nonblinded readers who had access to clinical information, previous imaging studies, and medical records.

RESULTS

A 74.7% average radiation dose reduction was seen: low-dose modified-protocol effective dose, 1.61 ± 1.18 mSv (size-specific-dose-estimate, 2.47 ± 1.21 mGy); conventional-protocol effective dose, 6.05 ± 2.84 mSv (size-specific-dose-estimate, 9.25 ± 2.9 mGy). Image quality assessment yielded 9372 data points. Objective noise on modified-protocol MBIR images was superior (p < 0.05) to that with the conventional protocol at three of six levels and comparable at the other three levels. Modified-protocol images were superior to conventional-protocol ASIR images (p < 0.05 in all cases) for subjective noise, spatial resolution, contrast resolution, streak artifact, and diagnostic acceptability on coronal reconstructions. Axial diagnostic acceptability was superior for conventional-protocol ASIR (p = 0.76). For both readers, modified-protocol MBIR clinical readings agreed more closely with reference standard readings than did conventional-protocol ASIR readings with regard to bowel wall disease assessment (κ = 0.589 and 0.700 vs 0.583 and 0.564). Overall Crohn disease activity grade (κ = 0.549 and 0.441 vs 0.315 and 0.596) and detection of acute complications (κ = 1.0 and 0.689 vs 0.896 and 0.896) were comparable when evaluated on conventional-protocol ASIR and modified-protocol MBIR images.

CONCLUSION

Low-dose CT enterography with MBIR yields images that are comparable to or superior to conventional images.

Automated Tube Voltage Selection for Radiation Dose Reduction in CT Angiography Using Different Contrast Media Concentrations and a Constant Iodine Delivery Rate

OBJECTIVE

The purpose of this study was to systematically investigate radiation dose reduction using automated tube voltage selection during CT angiography (CTA) and to evaluate the impact of contrast medium (CM) injection protocols on dose reduction.

MATERIALS AND METHODS

A circulation phantom containing the thoracic and abdominal vasculature was used. Four different concentrations of CM (iopromide 300 and 370 mg I/mL and iomeprol 350 and 400 mg I/mL) were administered while maintaining an identical iodine delivery rate (1.8 g I/s) and total iodine load (20.0 g). Three different scanning protocols for CTA of the thoracoabdominal aorta were used: protocol A, no dose modulation; protocol B, automated tube current modulation (CARE Dose4D); and protocol C, automated tube voltage selection (CARE kV). The dose-length product was recorded to calculate the effective dose. Attenuation values (in Hounsfield units), image noise levels, and signal-to-noise ratios (SNRs) in six predefined intravascular sites (three thoracic and three abdominal) were measured by two readers. All values were analyzed using the Kruskal-Wallis test and two-way ANOVA.

RESULTS

There was a significant reduction in the effective dose (in millisieverts) for protocols B (mean ± SD, 2.03 ± 0.1 mSv) and C (1.00 ± 0.0 mSv) compared with protocol A (4.34 ± 0.0 mSv). The dose was reduced by 53% for protocol B and by 77% for protocol C. No significant differences were found in the effective dose among the different CM injection protocols within the scanning protocols; all p values were > 0.05. The attenuation values and SNRs were comparable among all the different CM injection protocols; all p values were > 0.05.

CONCLUSION

A large radiation dose reduction (77%) can be achieved using automated tube voltage selection independent of the CM injection protocol.

The use of adaptive statistical iterative reconstruction (ASiR) technique in evaluation of patients with cervical spine trauma: impact on radiation dose reduction and image quality

OBJECTIVE

The aim of this study was to evaluate the impact of adaptive statistical iterative reconstruction (ASiR) technique on the image quality and radiation dose reduction. The comparison was made with the traditional filtered back projection (FBP) technique.

METHODS

We retrospectively reviewed 78 patients, who underwent cervical spine CT for blunt cervical trauma between 1 June 2010 and 30 November 2010. 48 patients were imaged using traditional FBP technique and the remaining 30 patients were imaged using the ASiR technique. The patient demographics, radiation dose, objective image signal and noise were recorded; while subjective noise, sharpness, diagnostic acceptability and artefacts were graded by two radiologists blinded to the techniques.

RESULTS

We found that the ASiR technique was able to reduce the volume CT dose index, dose-length product and effective dose by 36%, 36.5% and 36.5%, respectively, compared with the FBP technique. There was no significant difference in the image noise (p = 0.39), signal (p = 0.82) and signal-to-noise ratio (p = 0.56) between the groups. The subjective image quality was minimally better in the ASiR group but not statistically significant. There was excellent interobserver agreement on the subjective image quality and diagnostic acceptability for both groups.

CONCLUSION

The use of ASiR technique allowed approximately 36% radiation dose reduction in the evaluation of cervical spine without degrading the image quality.

ADVANCES IN KNOWLEDGE

The present study highlights that the ASiR technique is extremely helpful in reducing the patient radiation exposure while maintaining the image quality. It is highly recommended to utilize this novel technique in CT imaging of different body regions.

Dose reduction in 64-row whole-body CT in multiple trauma: an optimized CT protocol with iterative image reconstruction on a gemstone-based scintillator

OBJECTIVE

Evaluation of potential dose savings by implementing adaptive statistical iterative reconstruction (ASiR) on a gemstone-based scintillator in a clinical 64-row whole-body CT (WBCT) protocol after multiple trauma.

METHODS

Dose reports of 152 WBCT scans were analysed for two 64-row multidetector CT scanners (Scanners A and B); the main scanning parameters were kept constant. ASiR and a gemstone-based scintillator were used in Scanner B, and the noise index was adjusted (head: 5.2 vs 6.0; thorax/abdomen: 29.0 vs 46.0). The scan length, CT dose index (CTDI) and dose-length product (DLP) were analysed. The estimated mean effective dose was calculated using normalized conversion factors. Student's t-test was used for statistics.

RESULTS

Both the mean CTDI (mGy) (Scanner A: 53.8 ± 2.0, 10.3 ± 2.5, 14.4 ± 3.7; Scanner B: 48.7 ± 2.2, 7.1 ± 2.3, 9.1 ± 3.6; p < 0.001, respectively) and the mean DLP (mGy cm) (Scanner A: 1318.9 ± 167.8, 509.3 ± 134.7, 848.8 ± 254.0; Scanner B: 1190.6 ± 172.6, 354.6 ± 128.3, 561.0 ± 246.7; p < 0.001, respectively) for the head, thorax and abdomen were significantly reduced with Scanner B. There was no relevant difference in scan length. The total mean effective dose (mSv) was significantly decreased with Scanner B (24.4 ± 6.0, 17.2 ± 5.8; p < 0.001).

CONCLUSION

The implementation of ASiR and a gemstone-based scintillator allows for significant dose savings in a clinical WBCT protocol.

ADVANCES IN KNOWLEDGE

Recent technical developments can significantly reduce radiation dose of WBCT in multiple trauma. Dose reductions of 10-34% can be achieved.

Model-based iterative reconstruction and adaptive statistical iterative reconstruction: dose-reduced CT for detecting pancreatic calcification

Background

Iterative reconstruction methods have attracted attention for reducing radiation doses in computed tomography (CT).

Purpose

To investigate the detectability of pancreatic calcification using dose-reduced CT reconstructed with model-based iterative construction (MBIR) and adaptive statistical iterative reconstruction (ASIR).

Material and Methods

This prospective study approved by Institutional Review Board included 85 patients (57 men, 28 women; mean age, 69.9 years; mean body weight, 61.2 kg). Unenhanced CT was performed three times with different radiation doses (reference-dose CT [RDCT], low-dose CT [LDCT], ultralow-dose CT [ULDCT]). From RDCT, LDCT, and ULDCT, images were reconstructed with filtered-back projection (R-FBP, used for establishing reference standard), ASIR (L-ASIR), and MBIR and ASIR (UL-MBIR and UL-ASIR), respectively. A lesion (pancreatic calcification) detection test was performed by two blinded radiologists with a five-point certainty level scale.

Results

Dose-length products of RDCT, LDCT, and ULDCT were 410, 97, and 36 mGy-cm, respectively. Nine patients had pancreatic calcification. The sensitivity for detecting pancreatic calcification with UL-MBIR was high (0.67–0.89) compared to L-ASIR or UL-ASIR (0.11–0.44), and a significant difference was seen between UL-MBIR and UL-ASIR for one reader (P = 0.014). The area under the receiver-operating characteristic curve for UL-MBIR (0.818–0.860) was comparable to that for L-ASIR (0.696–0.844). The specificity was lower with UL-MBIR (0.79–0.92) than with L-ASIR or UL-ASIR (0.96–0.99), and a significant difference was seen for one reader (P < 0.01).

Conclusion

In UL-MBIR, pancreatic calcification can be detected with high sensitivity, however, we should pay attention to the slightly lower specificity.

Comparison of Knowledge-based Iterative Model Reconstruction and Hybrid Reconstruction Techniques for Liver CT Evaluation of Hypervascular Hepatocellular Carcinoma

OBJECTIVE

The purpose of this work was to evaluate the image quality, lesion conspicuity, and dose reduction provided by knowledge-based iterative model reconstruction (IMR) in computed tomography (CT) of the liver compared with hybrid iterative reconstruction (IR) and filtered back projection (FBP) in patients with hepatocellular carcinoma (HCC).

METHODS

Fifty-six patients with 61 HCCs who underwent multiphasic reduced-dose CT (RDCT; n = 33) or standard-dose CT (SDCT; n = 28) were retrospectively evaluated. Reconstructed images with FBP, hybrid IR (iDose), IMR were evaluated for image quality using CT attenuation and image noise. Objective and subjective image quality of RDCT and SDCT sets were independently assessed by 2 observers in a blinded manner.

RESULTS

Image quality and lesion conspicuity were better with IMR for both RDCT and SDCT than either FBP or IR (P < 0.001). Contrast-to-noise ratio of HCCs in IMR-RDCT was significantly higher on delayed phase (DP) (P < 0.001), and comparable on arterial phase, than with IR-SDCT (P = 0.501). Iterative model reconstruction RDCT was significantly superior to FBP-SDCT (P < 0.001). Compared with IR-SDCT, IMR-RDCT was comparable in image sharpness and tumor conspicuity on arterial phase, and superior in image quality, noise, and lesion conspicuity on DP. With the use of IMR, a 27% reduction of effective dose was achieved with RDCT (12.7 ± 0.6 mSv) compared with SDCT (17.4 ± 1.1 mSv) without loss of image quality (P < 0.001).

CONCLUSIONS

Iterative model reconstruction provides better image quality and tumor conspicuity than FBP and IR with considerable noise reduction. In addition, more than comparable results were achieved with IMR-RDCT to IR-SDCT for the evaluation of HCCs.

Methods for clinical evaluation of noise reduction techniques in abdominopelvic CT

Most noise reduction methods involve nonlinear processes, and objective evaluation of image quality can be challenging, since image noise cannot be fully characterized on the sole basis of the noise level at computed tomography (CT). Noise spatial correlation (or noise texture) is closely related to the detection and characterization of low-contrast objects and may be quantified by analyzing the noise power spectrum. High-contrast spatial resolution can be measured using the modulation transfer function and section sensitivity profile and is generally unaffected by noise reduction. Detectability of low-contrast lesions can be evaluated subjectively at varying dose levels using phantoms containing low-contrast objects. Clinical applications with inherent high-contrast abnormalities (eg, CT for renal calculi, CT enterography) permit larger dose reductions with denoising techniques. In low-contrast tasks such as detection of metastases in solid organs, dose reduction is substantially more limited by loss of lesion conspicuity due to loss of low-contrast spatial resolution and coarsening of noise texture. Existing noise reduction strategies for dose reduction have a substantial impact on lowering the radiation dose at CT. To preserve the diagnostic benefit of CT examination, thoughtful utilization of these strategies must be based on the inherent lesion-to-background contrast and the anatomy of interest. The authors provide an overview of existing noise reduction strategies for low-dose abdominopelvic CT, including analytic reconstruction, image and projection space denoising, and iterative reconstruction; review qualitative and quantitative tools for evaluating these strategies; and discuss the strengths and limitations of individual noise reduction methods.

Comparison of the image qualities of filtered back-projection, adaptive statistical iterative reconstruction, and model-based iterative reconstruction for CT venography at 80 kVp

PURPOSE

To evaluate the subjective and objective qualities of computed tomography (CT) venography images at 80 kVp using model-based iterative reconstruction (MBIR) and to compare these with those of filtered back projection (FBP) and adaptive statistical iterative reconstruction (ASIR) using the same CT data sets.

MATERIALS AND METHODS

Forty-four patients (mean age: 56.1 ± 18.1) who underwent 80 kVp CT venography (CTV) for the evaluation of deep vein thrombosis (DVT) during 4 months were enrolled in this retrospective study. The same raw data were reconstructed using FBP, ASIR, and MBIR. Objective and subjective image analysis were performed at the inferior vena cava (IVC), femoral vein, and popliteal vein.

RESULTS

The mean CNR of MBIR was significantly greater than those of FBP and ASIR and images reconstructed using MBIR had significantly lower objective image noise (p < .001). Subjective image quality and confidence of detecting DVT by MBIR group were significantly greater than those of FBP and ASIR (p < .005), and MBIR had the lowest score for subjective image noise (p < .001).

CONCLUSION

CTV at 80 kVp with MBIR was superior to FBP and ASIR regarding subjective and objective image qualities.

KEY POINTS

• MBIR provides superior image quality compared with FBP and ASIR • CTV at 80kVp with MBIR improves diagnostic confidence in diagnosing DVT • CTV at 80kVp with MBIR presents better image quality with low radiation.

The role of pure iterative reconstruction in conventional dose CT enterography

PURPOSE

Pure iterative reconstruction (Pure IR) has been proposed as a solution to improve diagnostic quality of low dose CT images. We assess the performance of model based iterative reconstruction (MBIR) in improving conventional dose CT enterography (CTE) images.

METHODS

43 Crohn's patients (27 female) (38.5 ± 12.98 years) referred for CTE were included. Images were reconstructed with pure IR (MBIR, General Electric Healthcare) in addition to standard department protocol (reconstructed with hybrid iterative reconstruction (Hybrid IR) [60% filtered back projection/40% adaptive statistical IR (General Electric Healthcare)]. Image quality was assessed objectively and subjectively at 6 anatomical levels. Clinical interpretation was undertaken in consensus by 2 blinded radiologists along with 2 non-blinded readers ('gold standard'). Results were analyzed using Statistical Package for Social Scientists.

RESULTS

Mean effective radiation dose was 6.05 ± 2.84 mSv (size specific dose estimates 9.25 ± 2.9 mGy). Objective and subjective assessment yielded 6106 data points. Pure IR images significantly outperformed those using standard reconstruction techniques across all subjective (p < 0.001 for all comparisons) (noise, contrast resolution, spatial resolution, streak artifact, axial diagnostic acceptability, coronal diagnostic acceptability) and objective (p < 0.004) (noise, signal-to-noise ratio) parameters. Clinical reads of the pure IR images agreed more closely with the gold standard reads than the hybrid IR image reads in terms of overall Crohn's activity grade (κ = 0.630, 0.308) and detection of acute complications (κ = 1.0, 0.896). Results were comparable for bowel wall disease severity assessment (κ = 0.523, 0.593).

CONCLUSIONS

Pure IR considerably improves image quality of conventional dose CTE images and therefore its use should be expanded beyond low dose protocols to improving image quality at conventional dose CT imaging.

Prospective evaluation of prior image constrained compressed sensing (PICCS) algorithm in abdominal CT: a comparison of reduced dose with standard dose imaging

PURPOSE

To prospectively study CT dose reduction using the "prior image constrained compressed sensing" (PICCS) reconstruction technique.

METHODS

Immediately following routine standard dose (SD) abdominal MDCT, 50 patients (mean age, 57.7 years; mean BMI, 28.8) underwent a second reduced dose (RD) scan (targeted dose reduction, 70%-90%). DLP, CTDIvol, and SSDE were compared. Several reconstruction algorithms (FBP, ASIR, and PICCS) were applied to the RD series. SD images with FBP served as reference standard. Two blinded readers evaluated each series for subjective image quality and focal lesion detection.

RESULTS

Mean DLP, CTDIvol, and SSDE for RD series were 140.3 mGy cm (median 79.4), 3.7 mGy (median 1.8), and 4.2 mGy (median 2.3) compared with 493.7 mGy cm (median 345.8), 12.9 mGy (median 7.9 mGy), and 14.6 mGy (median 10.1) for SD series, respectively. Mean effective patient diameter was 30.1 cm (median 30), which translates to a mean SSDE reduction of 72% (P < 0.001). RD-PICCS image quality score was 2.8 ± 0.5, improved over the RD-FBP (1.7 ± 0.7) and RD-ASIR (1.9 ± 0.8) (P < 0.001), but lower than SD (3.5 ± 0.5) (P < 0.001). Readers detected 81% (184/228) of focal lesions on RD-PICCS series, vs. 67% (153/228) and 65% (149/228) for RD-FBP and RD-ASIR, respectively. Mean image noise was significantly reduced on RD-PICCS series (13.9 HU) compared with RD-FBP (57.2) and RD-ASIR (44.1) (P < 0.001).

CONCLUSION

PICCS allows for marked dose reduction at abdominal CT with improved image quality and diagnostic performance over reduced dose FBP and ASIR. Further study is needed to determine indication-specific dose reduction levels that preserve acceptable diagnostic accuracy relative to higher dose protocols.

Computational and human observer image quality evaluation of low dose, knowledge-based CT iterative reconstruction

PURPOSE

Aims in this study are to (1) develop a computational model observer which reliably tracks the detectability of human observers in low dose computed tomography (CT) images reconstructed with knowledge-based iterative reconstruction (IMR™, Philips Healthcare) and filtered back projection (FBP) across a range of independent variables, (2) use the model to evaluate detectability trends across reconstructions and make predictions of human observer detectability, and (3) perform human observer studies based on model predictions to demonstrate applications of the model in CT imaging.

METHODS

Detectability (d') was evaluated in phantom studies across a range of conditions. Images were generated using a numerical CT simulator. Trained observers performed 4-alternative forced choice (4-AFC) experiments across dose (1.3, 2.7, 4.0 mGy), pin size (4, 6, 8 mm), contrast (0.3%, 0.5%, 1.0%), and reconstruction (FBP, IMR), at fixed display window. A five-channel Laguerre-Gauss channelized Hotelling observer (CHO) was developed with internal noise added to the decision variable and/or to channel outputs, creating six different internal noise models. Semianalytic internal noise computation was tested against Monte Carlo and used to accelerate internal noise parameter optimization. Model parameters were estimated from all experiments at once using maximum likelihood on the probability correct, PC. Akaike information criterion (AIC) was used to compare models of different orders. The best model was selected according to AIC and used to predict detectability in blended FBP-IMR images, analyze trends in IMR detectability improvements, and predict dose savings with IMR. Predicted dose savings were compared against 4-AFC study results using physical CT phantom images.

RESULTS

Detection in IMR was greater than FBP in all tested conditions. The CHO with internal noise proportional to channel output standard deviations, Model-k4, showed the best trade-off between fit and model complexity according to AICc. With parameters fixed, the model reasonably predicted detectability of human observers in blended FBP-IMR images. Semianalytic internal noise computation gave results equivalent to Monte Carlo, greatly speeding parameter estimation. Using Model-k4, the authors found an average detectability improvement of 2.7 ± 0.4 times that of FBP. IMR showed greater improvements in detectability with larger signals and relatively consistent improvements across signal contrast and x-ray dose. In the phantom tested, Model-k4 predicted an 82% dose reduction compared to FBP, verified with physical CT scans at 80% reduced dose.

CONCLUSIONS

IMR improves detectability over FBP and may enable significant dose reductions. A channelized Hotelling observer with internal noise proportional to channel output standard deviation agreed well with human observers across a wide range of variables, even across reconstructions with drastically different image characteristics. Utility of the model observer was demonstrated by predicting the effect of image processing (blending), analyzing detectability improvements with IMR across dose, size, and contrast, and in guiding real CT scan dose reduction experiments. Such a model observer can be applied in optimizing parameters in advanced iterative reconstruction algorithms as well as guiding dose reduction protocols in physical CT experiments.

Ultralow-Dose Abdominal Computed Tomography: Comparison of 2 Iterative Reconstruction Techniques in a Prospective Clinical Study

PURPOSE

To assess lesion detection and image quality of ultralow-dose (ULD) abdominal computed tomography (CT) reconstructed with filtered back projection (FBP) and 2 iterative reconstruction techniques: hybrid-based iDose, and image-based SafeCT.

MATERIALS AND METHODS

In this institutional review board-approved ongoing prospective clinical study, 41 adult patients provided written informed consent for an additional ULD abdominal CT examination immediately after standard dose (SD) CT exam on a 256-slice multidetector computed tomography (iCT, Philips-Healthcare). The SD examination (size-specific dose estimate, 10 ± 3 mGy) was performed at 120 kV with automatic exposure control, and reconstructed with FBP. The ULD examination (1.5 ± 0.4 mGy) was performed at 120 kV and fixed tube current of 17 to 20 mAs/slice to achieve ULD radiation dose, with the rest of the scan parameters same as SD examination. The ULD data were reconstructed with (a) FBP, (b) iDose, and (c) SafeCT. Lesions were detected on ULD FBP series and compared to SD FBP "reference-standard" series. True lesions, pseudolesions, and missed lesions were recorded. Four abdominal radiologists independently blindly performed subjective image quality. Objective image quality included image noise calculation and noise spectral density plots.

RESULTS

All true lesions (n, 52: liver metastases, renal cysts, diverticulosis) in SD FBP images were detected in ULD images. Although there were no missed or pseudolesions on ULD iDose and ULD SafeCT images, appearance of small low-contrast hepatic lesions was suboptimal. The ULD FBP images were unacceptable across all patients for both lesion detection and image quality. In patients with a body mass index (BMI) of 25 kg/m or less, ULD iDose and ULD SafeCT images were acceptable for image quality that was close to SD FBP for both normal and abnormal abdominal and pelvic structures. With increasing BMI, the image quality of ULD images was deemed unacceptable due to photo starvation. Evaluation of kidney stones with ULD iDose/SafeCT images was found acceptable regardless of patient size. Image noise levels were significantly lower in ULD iDose and ULD SafeCT images compared to ULD FBP (P < 0.01).

CONCLUSIONS

Preliminary results show that ULD abdominal CT reconstructed with iterative reconstruction techniques is achievable in smaller patients (BMI ≤ 25 kg/m) but remains a challenge for overweight to obese patients. Lesion detection is similar in full-dose SD FBP and ULD iDose/SafeCT images, with suboptimal visibility of low-contrast lesions in ULD images.

Effects of a high-pitch protocol and a hybrid iterative reconstruction algorithm on image quality of cerebral subtracted 3D CT angiography

PURPOSE

To evaluate the image quality and the radiation dose of 3D-computed tomography angiography (3D-CTA) with a high-pitch protocol and a hybrid iterative reconstruction (HIR).

MATERIALS AND METHODS

This was a prospective study and thirty patients were scanned at a 0.51-helical pitch with filtered back-projection (FBP, protocol-A), and 30 patients were scanned at a 0.91-helical pitch with FBP and HIR (protocol-B and C). The mean volume CT dose index (CTDI(vol)), image noise, and mean cerebral arterial and venous attenuation were compared between the three protocols. Two readers assessed image noise, arterial contrast and venous overlap.

RESULTS

The mean CTDI(vol) of protocol-B/C (38.9 mGy) was lower than that of protocol-A (49.7 mGy). Mean image noise of protocol-B [12.6 ± 1.3 Hounsfield units (HU)] was higher than that of protocol-A (10.3 ± 1.2 HU). There was no significant difference in arterial attenuation between protocol-A (327.5 ± 57.5 HU) and C (327.7 ± 59.4 HU). Venous attenuation of protocol-C (148.5 ± 50.4 HU) was lower than that of protocol-A (185.9 ± 50.6 HU). In qualitative analysis, the image noise of protocol-B was higher than that of protocol-A/C. Venous enhancement of protocol-B/C was more inconspicuous than that of protocol-A.

CONCLUSIONS

3D-CTA with a high-pitch protocol and HIR can reduce radiation dose while decreasing venous enhancement and image noise to an adequate level for diagnosis.

Automated tube voltage adaptation in combination with advanced modeled iterative reconstruction in thoracoabdominal third-generation 192-slice dual-source computed tomography: effects on image quality and radiation dose

RATIONALE AND OBJECTIVES

To evaluate image quality and radiation exposure of portal venous-phase thoracoabdominal third-generation 192-slice dual-source computed tomography (DSCT) with automated tube voltage adaptation (TVA) in combination with advanced modeled iterative reconstruction (ADMIRE).

MATERIALS AND METHODS

Fifty-one patients underwent oncologic portal venous-phase thoracoabdominal follow-up CT twice within 7 months. The initial examination was performed on second-generation 128-slice DSCT with fixed tube voltage of 120 kV in combination with filtered back projection reconstruction. The second examination was performed on a third-generation 192-slice DSCT using automated TVA in combination with ADMIRE. Attenuation and image noise of liver, spleen, renal cortex, aorta, vena cava inferior, portal vein, psoas muscle, and perinephric fat were measured. Signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were calculated. Radiation dose was assessed as size-specific dose estimates (SSDE). Subjective image quality was assessed by two observers using five-point Likert scales. Interobserver agreement was calculated using intraclass correlation coefficients (ICC).

RESULTS

Automated TVA set tube voltage to 90 kV (n = 8), 100 kV (n = 31), 110 kV (n = 11), or 120 kV (n = 1). Average SSDE was decreased by 34.9% using 192-slice DSCT compared to 128-slice 120-kV DSCT (7.8 ± 2.4 vs. 12.1 ± 3.2 mGy; P < .001). Image noise was substantially lower; SNR and CNR were significantly increased in 192-slice DSCT compared to 128-slice DSCT (all P < .005). Image quality was voted excellent for both acquisition techniques (5.00 vs. 4.93; P = .083).

CONCLUSIONS

Automated TVA in combination with ADMIRE on third-generation 192-slice DSCT in portal venous-phase thoracoabdominal CT provides excellent image quality with reduced image noise and increased SNR and CNR, whereas average radiation dose is reduced by 34.9% compared to 128-slice DSCT.

Feasibility Study of Radiation Dose Reduction in Adult Female Pelvic CT Scan with Low Tube-Voltage and Adaptive Statistical Iterative Reconstruction

OBJECTIVE

To evaluate image quality of female pelvic computed tomography (CT) scans reconstructed with the adaptive statistical iterative reconstruction (ASIR) technique combined with low tube-voltage and to explore the feasibility of its clinical application.

MATERIALS AND METHODS

Ninety-four patients were divided into two groups. The study group used 100 kVp, and images were reconstructed with 30%, 50%, 70%, and 90% ASIR. The control group used 120 kVp, and images were reconstructed with 30% ASIR. The noise index was 15 for the study group and 11 for the control group. The CT values and noise levels of different tissues were measured. The contrast to noise ratio (CNR) was calculated. A subjective evaluation was carried out by two experienced radiologists. The CT dose index volume (CTDIvol) was recorded.

RESULTS

A 44.7% reduction in CTDIvol was observed in the study group (8.18 ± 3.58 mGy) compared with that in the control group (14.78 ± 6.15 mGy). No significant differences were observed in the tissue noise levels and CNR values between the 70% ASIR group and the control group (p = 0.068-1.000). The subjective scores indicated that visibility of small structures, diagnostic confidence, and the overall image quality score in the 70% ASIR group was the best, and were similar to those in the control group (1.87 vs. 1.79, 1.26 vs. 1.28, and 4.53 vs. 4.57; p = 0.122-0.585). No significant difference in diagnostic accuracy was detected between the study group and the control group (42/47 vs. 43/47, p = 1.000).

CONCLUSION

Low tube-voltage combined with automatic tube current modulation and 70% ASIR allowed the low CT radiation dose to be reduced by 44.7% without losing image quality on female pelvic scan.

The adaptive statistical iterative reconstruction-V technique for radiation dose reduction in abdominal CT: comparison with the adaptive statistical iterative reconstruction technique

OBJECTIVE

To investigate whether reduced radiation dose abdominal CT images reconstructed with adaptive statistical iterative reconstruction V (ASIR-V) compromise the depiction of clinically competent features when compared with the currently used routine radiation dose CT images reconstructed with ASIR.

METHODS

27 consecutive patients (mean body mass index: 23.55 kg m(-2) underwent CT of the abdomen at two time points. At the first time point, abdominal CT was scanned at 21.45 noise index levels of automatic current modulation at 120 kV. Images were reconstructed with 40% ASIR, the routine protocol of Dong-A University Hospital. At the second time point, follow-up scans were performed at 30 noise index levels. Images were reconstructed with filtered back projection (FBP), 40% ASIR, 30% ASIR-V, 50% ASIR-V and 70% ASIR-V for the reduced radiation dose. Both quantitative and qualitative analyses of image quality were conducted. The CT dose index was also recorded.

RESULTS

At the follow-up study, the mean dose reduction relative to the currently used common radiation dose was 35.37% (range: 19-49%). The overall subjective image quality and diagnostic acceptability of the 50% ASIR-V scores at the reduced radiation dose were nearly identical to those recorded when using the initial routine-dose CT with 40% ASIR. Subjective ratings of the qualitative analysis revealed that of all reduced radiation dose CT series reconstructed, 30% ASIR-V and 50% ASIR-V were associated with higher image quality with lower noise and artefacts as well as good sharpness when compared with 40% ASIR and FBP. However, the sharpness score at 70% ASIR-V was considered to be worse than that at 40% ASIR. Objective image noise for 50% ASIR-V was 34.24% and 46.34% which was lower than 40% ASIR and FBP.

CONCLUSION

Abdominal CT images reconstructed with ASIR-V facilitate radiation dose reductions of to 35% when compared with the ASIR.

ADVANCES IN KNOWLEDGE

This study represents the first clinical research experiment to use ASIR-V, the newest version of iterative reconstruction. Use of the ASIR-V algorithm decreased image noise and increased image quality when compared with the ASIR and FBP methods. These results suggest that high-quality low-dose CT may represent a new clinical option.

Paediatric CT protocol optimisation: a design of experiments to support the modelling and optimisation process

In the last decade, several studies have emphasised the need to understand and optimise the computed tomography (CT) procedures in order to reduce the radiation dose applied to paediatric patients. To evaluate the influence of the technical parameters on the radiation dose and the image quality, a statistical model has been developed using the design of experiments (DOE) method that has been successfully used in various fields (industry, biology and finance) applied to CT procedures for the abdomen of paediatric patients. A Box-Behnken DOE was used in this study. Three mathematical models (contrast-to-noise ratio, noise and CTDI vol) depending on three factors (tube current, tube voltage and level of iterative reconstruction) were developed and validated. They will serve as a basis for the development of a CT protocol optimisation model.

CT imaging of congenital lung lesions: effect of iterative reconstruction on diagnostic performance and radiation dose

BACKGROUND

Different iterative reconstruction techniques are available for use in pediatric computed tomography (CT), but these techniques have not been systematically evaluated in infants.

OBJECTIVE

To determine the effect of iterative reconstruction on diagnostic performance, image quality and radiation dose in infants undergoing CT evaluation for congenital lung lesions.

MATERIALS AND METHODS

A retrospective review of contrast-enhanced chest CT in infants (<1 year) with congenital lung lesions was performed. CT examinations were reviewed to document the type of lung lesion, vascular anatomy, image noise measurements and image reconstruction method. CTDIvol was used to calculate size-specific dose estimates (SSDE). CT findings were correlated with intraoperative and histopathological findings. Analysis of variance and the Student's t-test were used to compare image noise measurements and radiation dose estimates between groups.

RESULTS

Fifteen CT examinations used filtered back projection (FBP; mean age: 84 days), 15 used adaptive statistical iterative reconstruction (ASiR; mean age: 93 days), and 11 used model-based iterative reconstruction (MBIR; mean age: 98 days). Compared to operative findings, 13/15 (87%), 14/15 (93%) and 11/11 (100%) lesions were correctly characterized using FBP, ASiR and MBIR, respectively. Arterial anatomy was correctly identified in 12/15 (80%) using FBP, 13/15 (87%) using ASiR and 11/11 (100%) using MBIR. Image noise was less for MBIR vs. ASiR (P < 0.0001). Mean SSDE was different among groups (P = 0.003; FBP = 7.35 mGy, ASiR = 1.89 mGy, MBIR = 1.49 mGy).

CONCLUSION

Congenital lung lesions can be adequately characterized in infants using iterative CT reconstruction techniques while maintaining image quality and lowering radiation dose.

Technological advances in hybrid imaging and impact on dose

New imaging technologies utilising X-rays and radiopharmaceuticals have developed rapidly. Clinical application of computed tomography (CT) has revolutionised medical imaging and plays an enormous role in medical care. Due to technical improvements, spatial, contrast and temporal resolutions have continuously improved. In spite of significant reduction of CT doses during recent years, CT is still a dominating source of radiation exposure to the population. Combinations with single photon emission computed tomography (SPECT) and positron emission tomography (PET) and especially the use of SPECT/CT and PET/CT, provide important additional information about physiology as well as cellular and molecular events. However, significant dose contributions from SPECT and PET occur, making PET/CT and SPECT/CT truly high dose procedures. More research should be done to find optimal activities of radiopharmaceuticals for various patient groups and investigations. The implementation of simple protocol adjustments, including individually based administration, encouraged hydration, forced diuresis and use of optimised voiding intervals, laxatives, etc., can reduce the radiation exposure to the patients. New data about staff doses to fingers, hands and eye lenses indicate that finger doses could be a problem, but not doses to the eye lenses and to the whole body.

Virtual single-source computed tomography using dual-source acquisition: a new technique for the dose-neutral intraindividual comparison of different scan protocols

OBJECTIVES

The objective of this study was to compare the image quality of a standard single-source (SSS) computed tomography (CT) with that of a virtual single-source CT (VSS-CT) data set reconstructed from 2 raw data sets obtained by dual-source CT acquisition in abdominal CT to establish a radiation dose-neutral approach for the intraindividual comparison of 3 acquisition protocols at different radiation dose levels (RDLs).

MATERIALS AND METHODS

An abdominal phantom representing an 80-kg male was imaged using dual-source CT (SOMATOM Definition; Siemens Healthcare) at 3 RDLs with 120 kV(p) and different tube currents (low, standard, and high milliampere-second protocol). For each RDL, raw data were obtained once in single-source mode using x-ray tube A only and 5 times in dual-source mode using different ratios for tube current of x-ray tubes A and B (same total radiation dose; A/B: 90%/10%, 80%/20%, 70%/30%, 60%/40%, 50%/50%). For each RDL, SSS-CT and 5 virtual single-source image data sets (VSS-CT50 - 90) were reconstructed. To compare SSS-CT and VSS-CT data sets, image quality was assessed in terms of high- and low-contrast performance by calculating the modulation transfer function, image noise, noise power spectrum, and, for low contrast lesion detectability, the modified multiscale structural similarity index (MS-SSIM*). A maximum decrease of Δ = 5% of image quality compared with SSS-CT was defined as acceptable, and a noninferiority analysis with Δ was performed.

RESULTS

For modulation transfer function, noninferiority was observed for all VSS-CT data sets and RDL (P < 0.05). Image noise demonstrated an acceptable increase (<3.2%, P < 0.05) for each RDL and noise power spectrum showed only minor differences in the midfrequency range. The MS-SSIM* index demonstrated for the high RDL protocol a minor decrease for VSS-CT data sets (<2%, P < 0.05). For the standard and low RDL, the relative differences of the MS-SSIM* index increased and were only in 1 case above Δ (standard RDL, mean VSS-CT80 5.1%, P > 0.05).

CONCLUSIONS

The image quality obtained by virtual and SSS reconstruction using equivalent total radiation exposure to the patient showed only negligible differences in image quality. Therefore, this technique might allow an intraindividual comparison of full and reduced radiation dose protocols within 1 image acquisition step by simply splitting the radiation dose between the 2 x-ray tubes of a dual-source CT.

Improving pulmonary vessel image quality with a full model-based iterative reconstruction algorithm in 80kVp low-dose chest CT for pediatric patients aged 0-6 years

BACKGROUND

Full model-based iterative reconstruction (MBIR) algorithm decreasing image noise and improving spatial resolution significantly, combined with low voltage scan may improve image and vessels quality.

PURPOSE

To evaluate the image quality improvement of pulmonary vessels using a full MBIR in low-dose chest computed tomography (CT) for children.

MATERIAL AND METHODS

This study was institutional review board approved. Forty-one children (age range, 28 days-6 years, mean age, 2.0 years) who underwent 80 kVp low-dose CT scans were included. Age-dependent noise index (NI) for a 5-mm slice thickness image was used for the acquisition: NI = 11 for 0-12 months old, NI = 13 for 1-2 years old, and NI = 15 for 3-6 years old. Images were retrospectively reconstructed into thin slice thickness of 0.625 mm using the MBIR and a conventional filtered back projection (FBP) algorithm. Two radiologists independently evaluated images subjectively focusing on the ability to display small arteries and diagnosis confidence on a 5-point scale with 3 being clinically acceptable. CT value and image noise in the descending aorta, muscle and fat were measured and statistically compared between the two reconstruction groups.

RESULTS

The ability to display small vessels was significantly improved with the MBIR reconstruction. The subjective scores of displaying small vessels were 5.0 and 3.7 with MBIR and FBP, respectively, while the respective diagnosis confidence scores were 5.0 and 3.8. Quantitative image noise for the 0.625 mm slice thickness images in the descending aorta was 15.8 ± 3.8 HU in MBIR group, 57.3% lower than the 37.0 ± 7.3 HU in FBP group. The signal-to-noise ratio and contrast-to-noise ratio for the descending aorta were 28.3 ± 7.9 and 24.05 ± 7.5 in MBIR group, and 12.1 ± 3.7 and 10.6 ± 3.5 in FBP group, respectively. These values were improved by 133.9% and 132.1%, respectively, with MBIR reconstruction compared to FBP reconstruction.

CONCLUSION

Compared to the conventional FBP reconstruction, the image quality and diagnostic performance for pulmonary vessels was significantly improved by the MBIR reconstruction in low-dose chest CT in children.

Observer Performance in the Detection and Classification of Malignant Hepatic Nodules and Masses with CT Image-Space Denoising and Iterative Reconstruction

PURPOSE

To determine if lower-dose computed tomographic (CT) scans obtained with adaptive image-based noise reduction (adaptive nonlocal means [ANLM]) or iterative reconstruction (sinogram-affirmed iterative reconstruction [SAFIRE]) result in reduced observer performance in the detection of malignant hepatic nodules and masses compared with routine-dose scans obtained with filtered back projection (FBP).

MATERIALS AND METHODS

This study was approved by the institutional review board and was compliant with HIPAA. Informed consent was obtained from patients for the retrospective use of medical records for research purposes. CT projection data from 33 abdominal and 27 liver or pancreas CT examinations were collected (median volume CT dose index, 13.8 and 24.0 mGy, respectively). Hepatic malignancy was defined by progression or regression or with histopathologic findings. Lower-dose data were created by using a validated noise insertion method (10.4 mGy for abdominal CT and 14.6 mGy for liver or pancreas CT) and images reconstructed with FBP, ANLM, and SAFIRE. Four readers evaluated routine-dose FBP images and all lower-dose images, circumscribing liver lesions and selecting diagnosis. The jackknife free-response receiver operating characteristic figure of merit (FOM) was calculated on a per-malignant nodule or per-mass basis. Noninferiority was defined by the lower limit of the 95% confidence interval (CI) of the difference between lower-dose and routine-dose FOMs being less than -0.10.

RESULTS

Twenty-nine patients had 62 malignant hepatic nodules and masses. Estimated FOM differences between lower-dose FBP and lower-dose ANLM versus routine-dose FBP were noninferior (difference: -0.041 [95% CI: -0.090, 0.009] and -0.003 [95% CI: -0.052, 0.047], respectively). In patients with dedicated liver scans, lower-dose ANLM images were noninferior (difference: +0.015 [95% CI: -0.077, 0.106]), whereas lower-dose FBP images were not (difference -0.049 [95% CI: -0.140, 0.043]). In 37 patients with SAFIRE reconstructions, the three lower-dose alternatives were found to be noninferior to the routine-dose FBP.

CONCLUSION

At moderate levels of dose reduction, lower-dose FBP images without ANLM or SAFIRE were noninferior to routine-dose images for abdominal CT but not for liver or pancreas CT.

Technical Note: Measuring contrast- and noise-dependent spatial resolution of an iterative reconstruction method in CT using ensemble averaging

PURPOSE

The spatial resolution of iterative reconstruction (IR) in computed tomography (CT) is contrast- and noise-dependent because of the nonlinear regularization. Due to the severe noise contamination, it is challenging to perform precise spatial-resolution measurements at very low-contrast levels. The purpose of this study was to measure the spatial resolution of a commercially available IR method using ensemble-averaged images acquired from repeated scans.

METHODS

A low-contrast phantom containing three rods (7, 14, and 21 HU below background) was scanned on a 128-slice CT scanner at three dose levels (CTDIvol = 16, 8, and 4 mGy). Images were reconstructed using two filtered-backprojection (FBP) kernels (B40 and B20) and a commercial IR method (sinogram affirmed iterative reconstruction, SAFIRE, Siemens Healthcare) with two strength settings (I40-3 and I40-5). The same scan was repeated 100 times at each dose level. The modulation transfer function (MTF) was calculated based on the edge profile measured on the ensemble-averaged images.

RESULTS

The spatial resolution of the two FBP kernels, B40 and B20, remained relatively constant across contrast and dose levels. However, the spatial resolution of the two IR kernels degraded relative to FBP as contrast or dose level decreased. For a given dose level at 16 mGy, the MTF50% value normalized to the B40 kernel decreased from 98.4% at 21 HU to 88.5% at 7 HU for I40-3 and from 97.6% to 82.1% for I40-5. At 21 HU, the relative MTF50% value decreased from 98.4% at 16 mGy to 90.7% at 4 mGy for I40-3 and from 97.6% to 85.6% for I40-5.

CONCLUSIONS

A simple technique using ensemble averaging from repeated CT scans can be used to measure the spatial resolution of IR techniques in CT at very low contrast levels. The evaluated IR method degraded the spatial resolution at low contrast and high noise levels.

Hybrid and model-based iterative reconstruction techniques for pediatric CT

OBJECTIVE. Radiation exposure from CT examinations should be reduced to a minimum in children. Iterative reconstruction (IR) is a method to reduce image noise that can be used to improve CT image quality, thereby allowing radiation dose reduction. This article reviews the use of hybrid and model-based IRs in pediatric CT and discusses the possibilities, advantages, and disadvantages of IR in pediatric CT and the importance of radiation dose reduction for CT of children. CONCLUSION. IR is a promising and potentially highly valuable technique that can be used to substantially reduce the amount of radiation in pediatric imaging. Future research should determine the maximum achievable radiation dose reduction in pediatric CT that is possible without a loss of diagnostic image quality.

Sub-milliSievert (sub-mSv) CT colonography: a prospective comparison of image quality and polyp conspicuity at reduced-dose versus standard-dose imaging

OBJECTIVE

To prospectively compare reduced-dose (RD) CT colonography (CTC) with standard-dose (SD) imaging using several reconstruction algorithms.

METHODS

Following SD supine CTC, 40 patients (mean age, 57.3 years; 17 M/23 F; mean BMI, 27.2) underwent an additional RD supine examination (targeted dose reduction, 70-90%). DLP, CTDI(vol), effective dose, and SSDE were compared. Several reconstruction algorithms were applied to RD series. SD-FBP served as reference standard. Objective image noise, subjective image quality and polyp conspicuity were assessed.

RESULTS

Mean CTDI(vol) and effective dose for RD series was 0.89 mGy (median 0.65) and 0.6 mSv (median 0.44), compared with 3.8 mGy (median 3.1) and 2.8 mSv (median 2.3) for SD series, respectively. Mean dose reduction was 78%. Mean image noise was significantly reduced on RD-PICCS (24.3 ± 19HU) and RD-MBIR (19 ± 18HU) compared with RD-FBP (90 ± 33), RD-ASIR (72 ± 27) and SD-FBP (47 ± 14 HU). 2D image quality score was higher with RD-PICCS, RD-MBIR, and SD-FBP (2.7 ± 0.4/2.8 ± 0.4/2.9 ± 0.6) compared with RD-FBP (1.5 ± 0.4) and RD-ASIR (1.8 ± 0.44). A similar trend was seen with 3D image quality scores. Polyp conspicuity scores were similar between SD-FBP/RD-PICCS/RD-MBIR (3.5 ± 0.6/3.2 ± 0.8/3.3 ± 0.6).

CONCLUSION

Sub-milliSievert CTC performed with iterative reconstruction techniques demonstrate decreased image quality compared to SD, but improved image quality compared to RD images reconstructed with FBP.

KEY POINTS

• CT colonography dose can be substantially lowered using advanced iterative reconstruction techniques. • Iterative reconstruction techniques (MBIR/PICCS) reduce image noise and improve image quality. • The PICCS/MBIR-reconstructed, reduced-dose series shows decreased 2D/3D image quality compared to the standard-dose series. • Polyp conspicuity was similar on standard-dose images compared to reduced-dose images reconstructed with MBIR/PICCS.

Lung nodule detection performance in five observers on computed tomography (CT) with adaptive iterative dose reduction using three-dimensional processing (AIDR 3D) in a Japanese multicenter study: Comparison between ultra-low-dose CT and low-dose CT by receiver-operating characteristic analysis

PURPOSE

To compare lung nodule detection performance (LNDP) in computed tomography (CT) with adaptive iterative dose reduction using three dimensional processing (AIDR3D) between ultra-low dose CT (ULDCT) and low dose CT (LDCT).

MATERIALS AND METHODS

This was part of the Area-detector Computed Tomography for the Investigation of Thoracic Diseases (ACTIve) Study, a multicenter research project being conducted in Japan. Institutional Review Board approved this study and informed consent was obtained. Eighty-three subjects (body mass index, 23.3 ± 3.2) underwent chest CT at 6 institutions using identical scanners and protocols. In a single visit, each subject was scanned using different tube currents: 240, 120 and 20 mA (3.52, 1.74 and 0.29 mSv, respectively). Axial CT images with 2-mm thickness/increment were reconstructed using AIDR3D. Standard of reference (SOR) was determined based on CT images at 240 mA by consensus reading of 2 board-certificated radiologists as to the presence of lung nodules with the longest diameter (LD) of more than 3mm. Another 5 radiologists independently assessed and recorded presence/absence of lung nodules and their locations by continuously-distributed rating in CT images at 20 mA (ULDCT) and 120 mA (LDCT). Receiver-operating characteristic (ROC) analysis was used to evaluate LNDP of both methods in total and also in subgroups classified by LD (>4, 6 and 8 mm) and nodular characteristics (solid and ground glass nodules).

RESULTS

For SOR, 161 solid and 60 ground glass nodules were identified. No significant difference in LNDP for entire solid nodules was demonstrated between both methods, as area under ROC curve (AUC) was 0.844 ± 0.017 in ULDCT and 0.876 ± 0.026 in LDCT (p=0.057). For ground glass nodules with LD 8mm or more, LNDP was similar between both methods, as AUC 0.899 ± 0.038 in ULDCT and 0.941 ± 0.030 in LDCT. (p=0.144).

CONCLUSION

ULDCT using AIDR3D with an equivalent radiation dose to chest x-ray could have comparable LNDP to LDCT with AIDR3D except for smaller ground glass nodules in cases with normal range body habitus.

Usefulness of low-dose nonenhanced computed tomography with iterative reconstruction for evaluation of urolithiasis: diagnostic performance and agreement between the urologist and the radiologist

OBJECTIVE

To evaluate the efficacy of low-dose nonenhanced computed tomography (LDCT) with iterative reconstruction (IR) technique for urologists to detect urolithiasis by comparing diagnostic performance and interobserver agreement between the urologist and the uroradiologist.

PATIENTS AND METHODS

We evaluated the 116 patients with urinary stones (n = 197) using both conventional-dose nonenhanced computed tomography (CT) using filtered back projection (CDCT-FBP) and LDCT-IR. Scans were interpreted for stone characteristics, objective image noise, and subjective image assessment. Diagnostic performance and interobserver agreement of LDCT-IR were assessed between 1 urologist and 1 radiologist.

RESULTS

There were no significant differences in all stones. The average effective dose (mSV) in the all size groups was 5.92 (CDCT-FBP) and 1.39 (LDCT-IR), respectively (P <.001). The average effective dose reduction rate was 76.6%, allowing minimal additional radiation exposure from simultaneous CT. Objective image noise was higher in LDCT-IR (20.0-26.2; P <.01), but there was no significant difference in the Hounsfield unit between both CT protocols (52.3 and 56.7; P = .103). There were no cases of any unacceptable images in subjective image assessment. The sensitivity and specificity of LDCT-IR were 99.1%-100.0% with a diagnostic accuracy of 99.1%-100% for stones ≥3 mm. Diagnostic performance was similar between the urologist and the radiologist. Interobserver agreement of LDCT-IR between the 2 reviewers was high with kappa values (0.901-1.000).

CONCLUSION

LDCT-IR provided an excellent diagnostic performance and interobserver agreement between the urologist and the uroradiologist, reducing radiation exposure significantly; in real settings, the urologist should consider replacing LDCT-IR as the standard examination for detecting urolithiasis.

United iterative reconstruction for spectral computed tomography

Spectral computed tomography (CT) has attracted considerable attention because of its energy-resolving capability in identifying and discriminating materials. The use of a narrow energy bin can improve energy resolution. However, a narrow energy bin has high noise ratio, which degrades the imaging quality of spectral CT. To address this problem, this study exploits the structure correlations of images in the energy domain and proposed two types of united iterative reconstruction (UIR) algorithms. One type uses the well-reconstructed broad-spectrum image, with all available photons, as a constraint, whereas the other type uses a pseudo narrow-energy image, which is estimated with the use of our proposed structure-coupling (SC) method, as a constraint. The SC method utilizes local structures to connect images that are reconstructed with broad-spectrum and narrow-energy CT datasets. Given a broad-spectrum image, the SC method can accurately estimate its corresponding narrow-energy image. Results show that UIR algorithms significantly outperform conventional iterative reconstruction algorithms for narrow-energy image reconstruction in spectral CT. Among the UIR algorithms, SC-UIR yields the best results.

Microsimulation model of CT versus MRI surveillance of Bosniak IIF renal cystic lesions: should effects of radiation exposure affect selection of imaging strategy?

OBJECTIVE

The objective of this study was to quantify the effects of radiation-induced cancer risks in patients with Bosniak category IIF lesions undergoing CT versus MRI surveillance.

MATERIALS AND METHODS

We developed a Markov-Monte Carlo model to determine life expectancy losses attributable to radiation-induced cancers in hypothetical patients undergoing CT versus MRI surveillance of Bosniak IIF lesions. Our model tracked hypothetical patients as they underwent imaging surveillance for up to 5 years, accounting for potential lesion progression and treatment. Estimates of radiation-induced cancer mortality were generated using a published organ-specific radiation-risk model based on Biological Effects of Ionizing Radiation VII methods. The model also incorporated surgical mortality and renal cancer-specific mortality. Our primary outcome was life expectancy loss attributable to radiation-induced cancers. A sensitivity analysis was performed to assess the stability of the results with variability in key parameters.

RESULTS

The mean number of examinations per patient was 6.3. In the base case, assuming 13 mSv per multiphase CT examination, 64-year-old men experienced an average life expectancy decrease of 5.5 days attributable to radiation-induced cancers from CT; 64-year-old women experienced a corresponding life expectancy loss of 6.9 days. The results were most sensitive to patient age: Life expectancy loss attributable to radiation-induced cancers increased to 21.6 days in 20-year-old women and 20.0 days in 20-year-old men. Varied assumptions of each modality's (CT vs MRI) depiction of lesion complexity also impacted life expectancy losses.

CONCLUSION

Microsimulation modeling shows that radiation-induced cancer risks from CT surveillance for Bosniak IIF lesions minimally affect life expectancy. However, as progressively younger patients are considered, increasing radiation risks merit stronger consideration of MRI surveillance.

CT of the pancreas: comparison of image quality and pancreatic duct depiction among model-based iterative, adaptive statistical iterative, and filtered back projection reconstruction techniques

The purpose of this study is to compare CT images of the pancreas reconstructed with model-based iterative reconstruction (MBIR), adaptive statistical iterative reconstruction (ASiR), and filtered back projection (FBP) techniques for image quality and pancreatic duct (PD) depiction. Data from 40 patients with contrast-enhanced abdominal CT [CTDIvol: 10.3 ± 3.0 (mGy)] during the late arterial phase were reconstructed with FBP, 40% ASiR-FBP blending, and MBIR. Two radiologists assessed the depiction of the main PD, image noise, and overall image quality using 5-point scale independently. Objective CT value and noise were measured in the pancreatic parenchyma, and the contrast-to-noise ratio (CNR) of the PD was calculated. The Friedman test and post-hoc multiple comparisons with Bonferroni test following one-way ANOVA were used for qualitative and quantitative assessment, respectively. For the subjective assessment, scores for MBIR were significantly higher than those for FBP and 40% ASiR (all P < 0.001). No significant differences in CT values of the pancreatic parenchyma were noted among FBP, 40% ASiR, and MBIR images (P > 0.05). Objective image noise was significantly lower and CNR of the PD was higher with MBIR than with FBP and 40% ASiR (all P < 0.05). Our results suggest that pancreatic CT images reconstructed with MBIR have lower image noise, better image quality, and higher conspicuity and CNR of the PD compared with FBP and ASiR.

High-pitch, low-voltage and low-iodine-concentration CT angiography of aorta: assessment of image quality and radiation dose with iterative reconstruction

Objective

To assess the image quality of aorta obtained by dual-source computed tomography angiography (DSCTA), performed with high pitch, low tube voltage, and low iodine concentration contrast medium (CM) with images reconstructed using iterative reconstruction (IR).

Methods

One hundred patients randomly allocated to receive one of two types of CM underwent DSCTA with the electrocardiogram-triggered Flash protocol. In the low-iodine group, 50 patients received CM containing 270 mg I/mL and were scanned at low tube voltage (100 kVp). In the high-iodine CM group, 50 patients received CM containing 370 mg I/mL and were scanned at the tube voltage (120 kVp). The filtered back projection (FBP) algorithm was used for reconstruction in both groups. In addition, the IR algorithm was used in the low-iodine group. Image quality of the aorta was analyzed subjectively by a 3-point grading scale and objectively by measuring the CT attenuation in terms of the signal- and contrast-to-noise ratios (SNR and CNR, respectively). Radiation and CM doses were compared.

Results

The CT attenuation, subjective image quality assessment, SNR, and CNR of various aortic regions of interest did not differ significantly between two groups. In the low-iodine group, images reconstructed by FBP and IR demonstrated significant differences in image noise, SNR, and CNR (p<0.05). The low-iodine group resulted in 34.3% less radiation (4.4 ± 0.5 mSv) than the high-iodine group (6.7 ± 0.6 mSv), and 27.3% less iodine weight (20.36 ± 2.65 g) than the high-iodine group (28 ± 1.98 g). Observers exhibited excellent agreement on the aortic image quality scores (κ = 0.904).

Conclusions

CT images of aorta could be obtained within 2 s by using a DSCT Flash protocol with low tube voltage, IR, and low-iodine-concentration CM. Appropriate contrast enhancement was achieved while maintaining good image quality and decreasing the radiation and iodine doses.

A quantitative comparison of noise reduction across five commercial (hybrid and model-based) iterative reconstruction techniques: an anthropomorphic phantom study

OBJECTIVE. The objective of our study was to compare the performance of three hybrid iterative reconstruction techniques (IRTs) (ASiR, iDose4, SAFIRE) and their respective strengths for image noise reduction on low-dose CT examinations using filtered back projection (FBP) as the standard reference. Also, we compared the performance of these three hybrid IRTs with two model-based IRTs (Veo and IMR) for image noise reduction on low-dose examinations. MATERIALS AND METHODS. An anthropomorphic abdomen phantom was scanned at 100 and 120 kVp and different tube current-exposure time products (25-100 mAs) on three CT systems (for ASiR and Veo, Discovery CT750 HD; for iDose4 and IMR, Brilliance iCT; and for SAFIRE, Somatom Definition Flash). Images were reconstructed using FBP and using IRTs at various strengths. Nine noise measurements (mean ROI size, 423 mm(2)) on extracolonic fat for the different strengths of IRTs were recorded and compared with FBP using ANOVA. Radiation dose, which was measured as the volume CT dose index and dose-length product, was also compared. RESULTS. There were no significant differences in radiation dose and image noise among the scanners when FBP was used (p > 0.05). Gradual image noise reduction was observed with each increasing increment of hybrid IRT strength, with a maximum noise suppression of approximately 50% (48.2-53.9%). Similar noise reduction was achieved on the scanners by applying specific hybrid IRT strengths. Maximum noise reduction was higher on model-based IRTs (68.3-81.1%) than hybrid IRTs (48.2-53.9%) (p < 0.05). CONCLUSION. When constant scanning parameters are used, radiation dose and image noise on FBP are similar for CT scanners made by different manufacturers. Significant image noise reduction is achieved on low-dose CT examinations rendered with IRTs. The image noise on various scanners can be matched by applying specific hybrid IRT strengths. Model-based IRTs attain substantially higher noise reduction than hybrid IRTs irrespective of the radiation dose.