Iterative Reconstruction Improves Image Quality Research Articles

Submillimeter balanced SSFP BOLD-functional MRI accelerated with 3D stack-of-spirals at 9.4 T.

This work aims to improve the speed of balanced SSFP (bSSFP) acquisition with segmented 3D stack-of-spirals for functional brain studies at ultrahigh field. Functional experiments were performed with an accelerated 3D stack-of-spirals sequence with water excitation for fat suppression. The resulting data were reconstructed using an iterative algorithm with corrections for system imperfections such as trajectory deviations and B0 inhomogeneity. In the first set of experiments, we evaluated the signal change and stability with respect to echo and TR for a full-field checkerboard stimulus. To demonstrate the high spatio-temporal resolution of the developed method, the results of three optimized protocols at submillimeter resolution (0.6-mm isotropic and 0.8-mm isotropic) and at 1.2 mm isotropic resolution for whole-brain coverage were shown. Water excitation and the model-based iterative reconstruction improved image quality. The BOLD-related signal changes increased with longer TE and longer TR. We observed an increase in thermal noise performance at lower TE and higher TR. However, signal stability deteriorates at higher TE and TR. Therefore, optimized protocols used shorter TE and moderately long TR to maximize the sensitivity and speed. Reproducible activations were detected along the gray-matter gyri in the submillimeter protocols with a median signal change of approximately 4% across subjects. Three-dimensional stack-of-spirals enables passband balanced SSFP functional imaging at a much higher spatial and temporal scale, compared with conventional spoiled gradient-echo train sequences.

Iterative reconstruction improves image quality and reduces radiation dose in trauma protocols; A human cadaver study.

BackgroundRadiation-related cancer risk is an object of concern in CT of trauma patients, as these represent a young population. Different radiation reducing methods, including iterative reconstruction (IR), and spilt bolus techniques have been introduced in the recent years in different large scale trauma centers.PurposeTo compare image quality in human cadaver exposed to thoracoabdominal computed tomography using IR and standard filtered back-projection (FBP) at different dose levels.Material and methodsTen cadavers were scanned at full dose and a dose reduction in CTDIvol of 5 mGy (low dose 1) and 7.5 mGy (low dose 2) on a Siemens Definition Flash 128-slice computed tomography scanner. Low dose images were reconstructed with FBP and Sinogram affirmed iterative reconstruction (SAFIRE) level 2 and 4. Quantitative image quality was analyzed by comparison of contrast-to-noise ratio (CNR) and signal-to-noise ratio (SNR). Qualitative image quality was evaluated by use of visual grading regression (VGR) by four radiologists.ResultsReaders preferred SAFIRE reconstructed images over FBP at a dose reduction of 40% (low dose 1) and 56% (low dose 2), with significant difference in overall impression of image quality. CNR and SNR showed significant improvement for images reconstructed with SAFIRE 2 and 4 compared to FBP at both low dose levels.ConclusionsIterative image reconstruction, SAFIRE 2 and 4, resulted in equal or improved image quality at a dose reduction of up to 56% compared to full dose FBP and may be used a strong radiation reduction tool in the young trauma population.

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.

Observer Performance with Varying Radiation Dose and Reconstruction Methods for Detection of Hepatic Metastases.

Purpose To estimate the ability of lower dose levels and iterative reconstruction (IR) to display hepatic metastases that can be detected by radiologists. Materials and Methods Projection data from 83 contrast agent-enhanced CT examinations were collected. Metastases were defined by histopathologic analysis or progression and regression. Lower radiation dose configurations were reconstructed at five dose levels with filtered back projection (FBP) and IR (automatic exposure control settings: 80, 100, 120, 160, and 200 quality reference mAs [QRM]). Three abdominal radiologists circumscribed metastases, indicating confidence (confidence range, 0-100) and image quality. Noninferiority was assessed by using jackknife alternative free-response receiver operating characteristic (JAFROC) analysis (noninferiority limit, -0.10) and reader agreement rules, which required identification of metastases identified at routine dose, and no nonlesion localizations in patients negative for metastases, in 71 or more patient CT examinations (of 83), for each configuration. Results There were 123 hepatic metastases (mean size, 1.4 cm; median volume CT dose index and size-specific dose estimate, 11.0 and 13.4 mGy, respectively). By using JAFROC figure of merit, 100 QRM FBP did not meet noninferiority criteria and had estimated performance difference from routine dose of -0.08 (95% confidence interval: -0.11, -0.04). Preset reader agreement rules were not met for 100 QRM IR or 80 QRM IR, but were met for doses 120 QRM or higher (ie, size-specific dose estimate ≥ 8.0 mGy). IR improved image quality (P < .05) but not reader performance. Other than 160 QRM IR, lower dose levels were associated with reduced confidence in metastasis detection (P < .001). Conclusion For detection of hepatic metastases by using contrast-enhanced CT, dose levels that corresponded to 120 quality reference mAs (size-specific dose estimate, 8.0 mGy) and higher performed similarly to 200 quality reference mAs with filtered back projection. © RSNA, 2018 Online supplemental material is available for this article.

Simultaneous multi slice (SMS) balanced steady state free precession first-pass myocardial perfusion cardiovascular magnetic resonance with iterative reconstruction at 1.5\xa0T

BackgroundSimultaneous-Multi-Slice (SMS) perfusion imaging has the potential to acquire multiple slices, increasing myocardial coverage without sacrificing in-plane spatial resolution. To maximise signal-to-noise ratio (SNR), SMS can be combined with a balanced steady state free precession (bSSFP) readout. Furthermore, application of gradient-controlled local Larmor adjustment (GC-LOLA) can ensure robustness against off-resonance artifacts and SNR loss can be mitigated by applying iterative reconstruction with spatial and temporal regularisation. The objective of this study was to compare cardiovascular magnetic resonance (CMR) myocardial perfusion imaging using SMS bSSFP imaging with GC-LOLA and iterative reconstruction to 3 slice bSSFP.MethodsTwo contrast-enhanced rest perfusion sequences were acquired in random order in 8 patients: 6-slice SMS bSSFP and 3 slice bSSFP. All images were reconstructed with TGRAPPA. SMS images were also reconstructed using a non-linear iterative reconstruction with L1 regularisation in wavelet space (SMS-iter) with 7 different combinations for spatial (λσ) and temporal (λτ) regularisation parameters. Qualitative ratings of overall image quality (0 = poor image quality, 1 = major artifact, 2 = minor artifact, 3 = excellent), perceived SNR (0 = poor SNR, 1 = major noise, 2 = minor noise, 3 = high SNR), frequency of sequence related artifacts and patient related artifacts were undertaken. Quantitative analysis of contrast ratio (CR) and percentage of dark rim artifact (DRA) was performed.ResultsAmong all SMS-iter reconstructions, SMS-iter 6 (λσ 0.001 λτ 0.005) was identified as the optimal reconstruction with the highest overall image quality, least sequence related artifact and higher perceived SNR. SMS-iter 6 had superior overall image quality (2.50 ± 0.53 vs 1.50 ± 0.53, p = 0.005) and perceived SNR (2.25 ± 0.46 vs 0.75 ± 0.46, p = 0.010) compared to 3 slice bSSFP. There were no significant differences in sequence related artifact, CR (3.62 ± 0.39 vs 3.66 ± 0.65, p = 0.88) or percentage of DRA (5.25 ± 6.56 vs 4.25 ± 4.30, p = 0.64) with SMS-iter 6 compared to 3 slice bSSFP.ConclusionsSMS bSSFP with GC-LOLA and iterative reconstruction improved image quality compared to a 3 slice bSSFP with doubled spatial coverage and preserved in-plane spatial resolution. Future evaluation in patients with coronary artery disease is warranted.

Adaptive statistical iterative reconstruction improves image quality without affecting perfusion CT quantitation in primary colorectal cancer

ObjectivesTo determine the effect of Adaptive Statistical Iterative Reconstruction (ASIR) on perfusion CT (pCT) parameter quantitation and image quality in primary colorectal cancer. MethodsProspective observational study. Following institutional review board approval and informed consent, 32 patients with colorectal adenocarcinoma underwent pCT (100kV, 150mA, 120s acquisition, axial mode). Tumour regional blood flow (BF), blood volume (BV), mean transit time (MTT) and permeability surface area product (PS) were determined using identical regions-of-interests for ASIR percentages of 0%, 20%, 40%, 60%, 80% and 100%. Image noise, contrast-to-noise ratio (CNR) and pCT parameters were assessed across ASIR percentages. Coefficients of variation (CV), repeated measures analysis of variance (rANOVA) and Spearman’ rank order correlation were performed with statistical significance at 5%. ResultsWith increasing ASIR percentages, image noise decreased by 33% while CNR increased by 61%; peak tumour CNR was greater than 1.5 with 60% ASIR and above. Mean BF, BV, MTT and PS differed by less than 1.8%, 2.9%, 2.5% and 2.6% across ASIR percentages. CV were 4.9%, 4.2%, 3.3% and 7.9%; rANOVA P values: 0.85, 0.62, 0.02 and 0.81 respectively. ConclusionsASIR improves image noise and CNR without altering pCT parameters substantially.

Evaluation of Iterative Reconstruction Method and Attenuation Correction in Brain Dopamine Transporter SPECT Using an Anthropomorphic Striatal Phantom.

Objective(s):The aim of this study was to determine the optimal reconstruction parameters for iterative reconstruction in different devices and collimators for dopamine transporter (DaT) single-photon emission computed tomography (SPECT). The results were compared between filtered back projection (FBP) and different attenuation correction (AC) methods.Methods:An anthropomorphic striatal phantom was filled with 123I solutions at different striatum-to-background radioactivity ratios. Data were acquired using two SPECT/CT devices, equipped with a low-to-medium-energy general-purpose collimator (cameras A-1 and B-1) and a low-energy high-resolution (LEHR) collimator (cameras A-2 and B-2). The SPECT images were once reconstructed by FBP using Chang’s AC and once by ordered subset expectation maximization (OSEM) using both CTAC and Chang’s AC; moreover, scatter correction was performed. OSEM on cameras A-1 and A-2 included resolution recovery (RR). The images were analyzed, using the specific binding ratio (SBR). Regions of interest for the background were placed on both frontal and occipital regions.Results:The optimal number of iterations and subsets was 10i10s on camera A-1, 10i5s on camera A-2, and 7i6s on cameras B-1 and B-2. The optimal full width at half maximum of the Gaussian filter was 2.5 times the pixel size. In the comparison between FBP and OSEM, the quality was superior on OSEM-reconstructed images, although edge artifacts were observed in cameras A-1 and A-2. The SBR recovery of OSEM was higher than that of FBP on cameras A-1 and A-2, while no significant difference was detected on cameras B-1 and B-2. Good linearity of SBR was observed in all cameras. In the comparison between Chang’s AC and CTAC, a significant correlation was observed on all cameras. The difference in the background region influenced SBR differently in Chang’s AC and CTAC on cameras A-1 and B-1.Conclusion:Iterative reconstruction improved image quality on all cameras, although edge artifacts were observed in images captured by cameras with RR. The SBR of OSEM with RR was higher than that of FBP, while the SBR of OSEM without RR was equal to that of FBP. Also, the SBR of Chang’s AC varied with different background regions in cameras A-1 and B-1.

Pure Iterative Reconstruction Improves Image Quality in Computed Tomography of the Abdomen and Pelvis Acquired at Substantially Reduced Radiation Doses in Patients With Active Crohn Disease.

We assessed diagnostic accuracy and image quality of modified protocol (MP) computed tomography (CT) of the abdomen and pelvis reconstructed using pure iterative reconstruction (IR) in patients with Crohn disease (CD). Thirty-four consecutive patients with CD were referred with suspected extramural complications. Two contemporaneous CT datasets were acquired in all patients: standard protocol (SP) and MP. The MP and SP protocols were designed to impart radiation exposures of 10% to 20% and 80% to 90% of routine abdominopelvic CT, respectively. The MP images were reconstructed with model-based IR (MBIR) and adaptive statistical IR (ASIR). The MP-CT and SP-CT dose length product were 88 (58) mGy.cm (1.27 [0.87] mSv) and 303 [204] mGy.cm (4.8 [2.99] mSv), respectively (P < 0.001). Median diagnostic acceptability, spatial resolution, and contrast resolution were significantly higher and subjective noise scores were significantly lower on SP-ASIR 40 compared with all MP datasets. There was perfect clinical agreement between MP-MBIR and SP-ASIR 40 images for detection of extramural complications. Modified protocol CT using pure IR is feasible for assessment of active CD.

State of the Art: Iterative CT Reconstruction Techniques.

Owing to recent advances in computing power, iterative reconstruction (IR) algorithms have become a clinically viable option in computed tomographic (CT) imaging. Substantial evidence is accumulating about the advantages of IR algorithms over established analytical methods, such as filtered back projection. IR improves image quality through cyclic image processing. Although all available solutions share the common mechanism of artifact reduction and/or potential for radiation dose savings, chiefly due to image noise suppression, the magnitude of these effects depends on the specific IR algorithm. In the first section of this contribution, the technical bases of IR are briefly reviewed and the currently available algorithms released by the major CT manufacturers are described. In the second part, the current status of their clinical implementation is surveyed. Regardless of the applied IR algorithm, the available evidence attests to the substantial potential of IR algorithms for overcoming traditional limitations in CT imaging.

Iterative reconstruction improves image quality and preserves diagnostic accuracy in the setting of blunt solid organ injuries.

This study aims to investigate the effect of iterative reconstruction (IR) on MDCT image quality and radiologists' ability to diagnose and grade blunt solid organ injuries. One hundred (100) patients without and 52 patients with solid organ injuries were scanned on a 64-slice MDCT scanner using reference 300 mAs, 120 kVp, and fixed 75 s delay. Raw data was reconstructed using filtered back projection (FBP) and three levels of iterative reconstruction (Philips iDose levels 2, 4, and 6). Four emergency radiologists, blinded to the reconstruction parameters and original interpretation, independently reviewed each case, assessed image quality, and assigned injury grades. Each reader was then asked to determine if they thought that IR was used and, if so, what level. There was no significant difference in diagnostic accuracy between FBP and the various IR levels or effect on the detection and grading of solid organ injuries (p > 0.8). Images reconstructed using iDose level 2 were judged to have the best overall image quality (p < 0.01). The radiologists had high sensitivity in detecting if IR was used (80 %, 95 % CI 76-84 %). IR performed comparably to FBP with no effect on radiologist ability to accurately detect and grade blunt solid organ injuries.

Iterative Reconstructions versus Filtered Back-Projection for Urinary Stone Detection in Low-Dose CT

To evaluate prospectively, in patients with suspected or known urinary stone disease, the image quality anddiagnostic confidence of nonenhanced abdominal low-dose computed tomography (CT) with iterative reconstruction (IR) compared to filtered back-projection (FBP). Fifty consecutive patients with suspected (n=33) or known (n=17) urinary stone disease underwent nonenhanced abdominal low-dose CT (120 kVp, 30 effective mAs, 1.6±0.5 mSv). Reconstructions were performed with sinogram-affirmed IR and with FBP. Attenuation (in Hounsfield units) was measured in subcutaneous fat and urinary bladder; image noise was determined. Two readers assessed image quality, number and location of urinary calculi were recorded, and diagnostic confidence was assessed. Statistical analyses included Mann-Whitney, Friedman's two-way, Wilcoxon signed rank, Pearson's, and Spearman's rank order correction tests. Attenuation of urinary bladder (P=.208, reader 1; P=.123, reader 2) and fat (P=.568, reader 1; P=.834, reader 2) was similar among FBP and IR datasets. Image noise was reduced in IR datasets by 40.1% (P<.001). IR improved image quality (P<.01), and obesity as factor impairing image quality was noted in FBP but not in IR images (P<.05). There was no significant difference in number of calculi in datasets reconstructed with IR and FBP (P=.102, reader 1; P=.059, reader 2). Diagnostic confidence regarding identification of urinary calculi improved with IR (P<.05, reader 1; P<.01, reader 2). IR improves image quality and confidence for diagnosing urinary stone disease in abdominal low-dose CT.

Iterative reconstruction techniques for computed tomography part 2: initial results in dose reduction and image quality

To present the results of a systematic literature search aimed at determining to what extent the radiation dose can be reduced with iterative reconstruction (IR) for cardiopulmonary and body imaging with computed tomography (CT) in the clinical setting and what the effects on image quality are with IR versus filtered back-projection (FBP) and to provide recommendations for future research on IR. We searched Medline and Embase from January 2006 to January 2012 and included original research papers concerning IR for CT. The systematic search yielded 380 articles. Forty-nine relevant studies were included. These studies concerned: the chest(n = 26), abdomen(n = 16), both chest and abdomen(n = 1), head(n = 4), spine(n = 1), and no specific area (n = 1). IR reduced noise and artefacts, and it improved subjective and objective image quality compared to FBP at the same dose. Conversely, low-dose IR and normal-dose FBP showed similar noise, artefacts, and subjective and objective image quality. Reported dose reductions ranged from 23 to 76 % compared to locally used default FBP settings. However, IR has not yet been investigated for ultra-low-dose acquisitions with clinical diagnosis and accuracy as endpoints. Benefits of IR include improved subjective and objective image quality as well as radiation dose reduction while preserving image quality. Future studies need to address the value of IR in ultra-low-dose CT with clinically relevant endpoints. • Iterative reconstruction improves image quality of CT images at equal acquisition parameters. • IR preserves image quality compared to normal-dose filtered back-projection. • The reduced radiation dose made possible by IR is advantageous for patients. • IR has not yet been investigated with clinical diagnosis and accuracy as endpoints.

Pilot Study of Detection, Radiologist Confidence and Image Quality With Sinogram-Affirmed Iterative Reconstruction at Half–Routine Dose Level

The objective of this study was to determine the effect of Sinogram-Affirmed Iterative Reconstruction (SAFIRE) on radiological detection, diagnostic confidence, and image quality at half-dose, contrast-enhanced abdominopelvic computed tomography. Forty dual-source examinations were reconstructed using routine-dose with filtered back projection, half-dose filtered back projection, and half-dose SAFIRE. Three radiologists detected lesions in abdominopelvic organs, reporting findings of potential medical significance, diagnostic confidence, and image quality. There was greater than 78% concordance between full- and half-dose images ± SAFIRE, and no difference in the detection of lesions within organs between half-dose images ± SAFIRE (P = 0.22 - 1.0). Detection of potentially important findings varied by reader, but not between dose/reconstruction methods. Diagnostic confidence varied widely (P < 0.001 to P > 0.91). Sinogram-Affirmed Iterative Reconstruction significantly improved image quality in the pelvis (P ≤ 0.04). Half-dose images ± SAFIRE had organ-specific detections similar to routine-dose images. Sinogram-Affirmed Iterative Reconstruction improved image quality in the pelvis, but diagnostic confidence and image quality scores in the abdomen depended on the reader.