Human-observer receiver-operating-characteristic evaluation of attenuation, scatter, and resolution compensation strategies for (99m)Tc myocardial perfusion imaging.

Nonuniform attenuation, scatter, and distance-dependent resolution are confounding factors inherent in SPECT imaging. Iterative reconstruction algorithms permit modeling and compensation of these degradations. We investigated through human-observer receiver-operating-characteristic (ROC) studies which (if any) combination of such compensation strategies best improves the accuracy of detection of coronary artery disease (CAD) when expert readers have only stress images for diagnosis. METHODS: A 3-headed SPECT system fitted with a (153)Gd line source was used to acquire simultaneously (99m)Tc-methoxyisobutylisonitrile (MIBI) images and transmission data. With these acquisitions, the accuracy of detecting CAD was evaluated for the following reconstruction strategies: filtered backprojection (FBP); ordered-subset expectation maximization (OSEM) with attenuation correction (AC); OSEM with AC and scatter correction (SC) (AC + SC); and OSEM with AC, SC, and resolution compensation (RC) (AC + SC + RC). Reconstruction parameters for OSEM were optimized by use of human-observer ROC studies with hybrid images, whereas standard clinical parameters were used for FBP. A total of 100 patients, including 55 patients referred for angiography and 45 patients with <5% likelihood for CAD, were included in the ROC studies. Images reconstructed with the 4 methods were rated independently with regard to the presence of CAD by 7 observers using a continuous scale for certainty. RESULTS: With area under the ROC curve (A(z)) as the criterion, the iterative reconstructions with compensation strategies (AC, AC + SC, and AC + SC + RC) demonstrated better detection accuracy than did FBP reconstructions for the overall detection of CAD as well as for the localization of perfusion defects in the 3 vascular territories. In general, the trend was for an increase in the A(z) for the progression from FBP to OSEM with AC, to OSEM with AC + SC, and to OSEM with AC + SC + RC. Statistically, the combination strategy with AC + SC + RC provided significantly higher A(z) values than did FBP images for the overall detection of CAD and the localization of perfusion defects in the left anterior descending coronary artery and left circumflex coronary artery territories, whereas AC + SC provided significantly better performance in the right coronary artery territory. CONCLUSION: The results indicate that OSEM with AC + SC + RC outperforms FBP reconstructions, indicating that the modeling of physical degradations can improve the accuracy of detection of CAD with cardiac perfusion SPECT reconstructions.     

ROC and localization ROC analyses of lesion detection in whole-body FDG PET: effects of acquisition mode, attenuation correction and reconstruction algorithm.

Receiver operating characteristic (ROC) and localization ROC (LROC) studies were performed to compare lesion detection at the borderline of detectability on images reconstructed with two-dimensional filtered backprojection (FBP) without attenuation correction (a common clinical protocol), three-dimensional FBP without attenuation correction, two-dimensional FBP with segmented attenuation correction and a two-dimensional iterative maximum a posteriori (MAP) algorithm using attenuation correction. Lung cancer was the model for the study because of the prominent role of 18F-fluorodeoxyglucose PET in the staging of lung cancer and the importance of lesion detection for staging. METHODS: Simulated lung cancer lesions were added to two-dimensional and three-dimensional PET data from healthy volunteers. Data were reconstructed using the four methods. Four nuclear medicine physicians evaluated the images. Detection performance with each method was compared using ROC and LROC analysis. Jackknife analysis provided estimates of statistical significance for differences across all readers for the ROC results. RESULTS: ROC and LROC results indicated statistically significant degradation in detection performance with three-dimensional acquisition (average area under ROC curves [Az] 0.51; average area under LROC curves [A(z,LROC)] 0.13) and segmented attenuation correction (average Az 0.59; average Az,LROC 0.29) compared with two-dimensional FBP without attenuation correction (average Az 0.79; average A(z,LROC) 0.54). ROC and LROC results indicated an improvement in detection performance with iterative MAP reconstruction (average Az 0.83; average A(z,LROC) 0.64) compared with two-dimensional FBP reconstruction; this improvement was not statistically significant. CONCLUSION: Use of segmented attenuation correction or three-dimensional acquisition with FBP reconstruction is not expected to improve detection of lung lesions on whole-body PET images compared with images with two-dimensional FBP without attenuation correction. The potential improvement in detection obtained with an iterative MAP reconstruction method is small compared with that obtained with two-dimensional FBP without attenuation correction.     

Receiver operating characteristic evaluation of iterative reconstruction with attenuation correction in 99mTc-sestamibi myocardial SPECT images.

The purpose of this study was to evaluate differences in myocardial defect detection between 99mTc-sestamibi myocardial SPECT images reconstructed using conventional filtered backprojection (FBP) without attenuation correction (AC) and those reconstructed using maximum-likelihood expectation maximization with nonuniform attenuation correction (MLAC). METHODS: An observer study and receiver operating characteristic (ROC) curve analysis were performed using simulated 99mTc-sestamibi SPECT data from a population of 24 mathematic anthropomorphic torso phantoms, which realistically modeled a wide range of anatomic variations. The phantoms modeled male patients with a flat diaphragm, male patients with a diaphragm raised to the level of the heart, and female patients with large breasts. Transmural, cold defects with a contrast of 0.25 were simulated in the left ventricular wall for 6 locations. Noisy projection data were generated from the phantoms and included the effects of nonuniform attenuation, collimator-detector response, and scatter. The data were then reconstructed using FBP and MLAC. Images were displayed in the short- and long-axis formats, as in clinical practice. Eight observers viewed blocks of FBP and MLAC images and, for each image, indicated on a continuous rating scale the probability that a defect was present. From the rating data, FBP and MLAC ROC curves were generated, and their areas (Az) were estimated and compared. RESULTS: In general, the FBP and MLAC ROC curves did not cross and the MLAC curve showed a higher Az than did the corresponding FBP curve. For male phantoms with a flat diaphragm, the average difference in Az was 0.04 and was not statistically significant (at the P = 0.05 level) for 6 of 8 observers. For male phantoms with a raised diaphragm, the average difference in Az was 0.22 and was statistically significant for 6 of 8 observers. For female phantoms with large breasts, the average difference in Az was 0.19 and was statistically significant for all 8 observers. CONCLUSION: This study showed an improvement in defect detection in myocardial SPECT images using MLAC in comparison with images using FBP without AC, particularly for patients with large breasts or with a diaphragm raised to the level of the heart.     

Improvement of brain perfusion SPET using iterative reconstruction with scatter and non-uniform attenuation correction

Filtered back-projection (FBP) is generally used as the reconstruction method for single-photon emission tomography although it produces noisy images with apparent streak artefacts. It is possible to improve the image quality by using an algorithm with iterative correction steps. The iterative reconstruction technique also has an additional benefit in that computation of attenuation correction can be included in the process. A commonly used iterative method, maximum-likelihood expectation maximisation (ML-EM), can be accelerated using ordered subsets (OS-EM). We have applied to the OS-EM algorithm a Bayesian one-step late correction method utilising median root prior (MRP). Methodological comparison was performed by means of measurements obtained with a brain perfusion phantom and using patient data. The aim of this work was to quantitate the accuracy of iterative reconstruction with scatter and non-uniform attenuation corrections and post-filtering in SPET brain perfusion imaging. SPET imaging was performed using a triple-head gamma camera with fan-beam collimators. Transmission and emission scans were acquired simultaneously. The brain phantom used was a high-resolution three-dimensional anthropomorphic JB003 phantom. Patient studies were performed in ten chronic pain syndrome patients. The images were reconstructed using conventional FBP and iterative OS-EM and MRP techniques including scatter and non-uniform attenuation corrections. Iterative reconstructions were individually post-filtered. The quantitative results obtained with the brain perfusion phantom were compared with the known actual contrast ratios. The calculated difference from the true values was largest with the FBP method; iteratively reconstructed images proved closer to the reality. Similar findings were obtained in the patient studies. The plain OS-EM method improved the contrast whereas in the case of the MRP technique the improvement in contrast was not so evident with post-filtering.     

Improvement of brain perfusion SPET using iterative reconstruction with scatter and non-uniform attenuation correction

Filtered back-projection (FBP) is generally used as the reconstruction method for single-photon emission tomography although it produces noisy images with apparent streak artefacts. It is possible to improve the image quality by using an algorithm with iterative correction steps. The iterative reconstruction technique also has an additional benefit in that computation of attenuation correction can be included in the process. A commonly used iterative method, maximum-likelihood expectation maximisation (ML-EM), can be accelerated using ordered subsets (OS-EM). We have applied to the OS-EM algorithm a Bayesian one-step late correction method utilising median root prior (MRP). Methodological comparison was performed by means of measurements obtained with a brain perfusion phantom and using patient data. The aim of this work was to quantitate the accuracy of iterative reconstruction with scatter and non-uniform attenuation corrections and post-filtering in SPET brain perfusion imaging. SPET imaging was performed using a triple-head gamma camera with fan-beam collimators. Transmission and emission scans were acquired simultaneously. The brain phantom used was a high-resolution three-dimensional anthropomorphic JB003 phantom. Patient studies were performed in ten chronic pain syndrome patients. The images were reconstructed using conventional FBP and iterative OS-EM and MRP techniques including scatter and nonuniform attenuation corrections. Iterative reconstructions were individually post-filtered. The quantitative results obtained with the brain perfusion phantom were compared with the known actual contrast ratios. The calculated difference from the true values was largest with the FBP method; iteratively reconstructed images proved closer to the reality. Similar findings were obtained in the patient studies. The plain OS-EM method improved the contrast whereas in the case of the MRP technique the improvement in contrast was not so evident with post-filtering.     

Iterative reconstruction reduces abdominal CT dose

Objective

In medical imaging, lowering radiation dose from computed tomography scanning, without reducing diagnostic performance is a desired achievement. Iterative image reconstruction may be one tool to achieve dose reduction. This study reports the diagnostic performance using a blending of 50% statistical iterative reconstruction (ASIR) and filtered back projection reconstruction (FBP) compared to standard FBP image reconstruction at different dose levels for liver phantom examinations.

Methods

An anthropomorphic liver phantom was scanned at 250, 185, 155, 140, 120 and 100 mA s, on a 64-slice GE Lightspeed VCT scanner. All scans were reconstructed with ASIR and FBP. Four readers evaluated independently on a 5-point scale 21 images, each containing 32 test sectors. In total 672 areas were assessed. ROC analysis was used to evaluate the differences.

Results

There was a difference in AUC between the 250 mA s FBP images and the 120 and 100 mA s FBP images. ASIR reconstruction gave a significantly higher diagnostic performance compared to standard reconstruction at 100 mA s.

Conclusion

A blending of 50–90% ASIR and FBP may improve image quality of low dose CT examinations of the liver, and thus give a potential for reducing radiation dose.     

Quantitative SPECT reconstruction of iodine-123 data

Many clinical and research studies in nuclear medicine require quantitation of iodine-123 (123I) distribution for the determination of kinetics or localization. The objective of this study was to implement several reconstruction methods designed for single-photon emission computed tomography (SPECT) using 123I and to evaluate their performance in terms of quantitative accuracy, image artifacts, and noise. The methods consisted of four attenuation and scatter compensation schemes incorporated into both the filtered backprojection/Chang (FBP) and maximum likelihood-expectation maximization (ML-EM) reconstruction algorithms. The methods were evaluated on data acquired of a phantom containing a hot sphere of 123I activity in a lower level background 123I distribution and nonuniform density media. For both reconstruction algorithms, nonuniform attenuation compensation combined with either scatter subtraction or Metz filtering produced images that were quantitatively accurate to within 15% of the true value. The ML-EM algorithm demonstrated quantitative accuracy comparable to FBP and smaller relative noise magnitude for all compensation schemes.     

The effect of the number of iterative restorations on tuned aperture computed tomography for approximal caries detection

OBJECTIVES: To determine whether the number of iterative restorations performed on TACT images affects observers' ability to detect dental caries. METHODS: Eight TACT basis images of 40 extracted human posterior teeth were acquired using a CCD sensor. TACT slices of each tooth were generated and subsequently submitted to 1, 2 or 3 iterative restorations. Stacks of images from the three experimental conditions were presented to six observers in a balanced order, on a high-resolution 21' color monitor. Observers scored the presence or absence of approximal caries using a 5-point confidence scale. Observers' assessments were compared with the results of histological examination. Receiver operating characteristic (ROC) curves were generated and possible significant differences between observers and between modalities tested by ANOVA. The level of significance was set at alpha=0.05. Interobserver reliability was calculated as intraclass correlation. RESULTS: Mean areas under the ROC curves (A(z)) for the three experimental conditions were 0.791 (one iterative restoration), 0.81 (two iterative restorations), and 0.778 (three iterative restorations). ANOVA did not demonstrate any significant difference between modalities (P=0.25) but did so between observers (P=0.031). Interobserver reliability was similar for all experimental conditions tested. CONCLUSIONS: Varying the number of iterative restorations from one to three does not affect observers' ability to detect approximal caries with TACT slices.     

Local detection of prostate cancer by positron emission tomography with 2-fluorodeoxyglucose: comparison of filtered back projection and iterative reconstruction with segmented attenuation correction.

BACKGROUND: To compare filtered back projection (FBP) and iterative reconstruction with segmented attenuation correction (IRSAC) in the local imaging of prostate cancer by positron emission tomography with 2-fluorodeoxyglucose (FDG-PET). METHODS: We retrospectively identified 13 patients with primary (n=7) or recurrent (n=6) prostate cancer who had increased uptake in the prostate on FDG-PET performed without urinary catheterization, contemporaneous biopsy confirming the presence of active tumor in the prostate, and correlative cross-sectional imaging by MRI (n=8) or CT (n=5). FDG-PET images were reconstructed by FBP and IRSAC. Two independent nuclear medicine physicians separately rated FBP and IRSAC images for visualization of prostatic activity on a 4-point scale. Results were compared using biopsy and cross-sectional imaging findings as the standard of reference. RESULTS: IRSAC images were significantly better that FBP in terms of visualization of prostatic activity in 12 of 13 patients, and were equivalent in 1 patient (p<0.001, Wilcoxon signed ranks test). In particular, 2 foci of tumor activity in 2 different patients seen on IRSAC images were not visible on FBP images. In 11 patients who had a gross tumor mass evident on cross-sectional imaging, there was good agreement between PET and cross-sectional anatomic imaging with respect to tumor localization. CONCLUSIONS: In selected patients, cancer can be imaged within the prostate using FDG-PET, and IRSAC is superior to FBP in image reconstruction for local tumor visualization.     

Improving spatial resolution in SPECT with the combination of PSPMT based detector and iterative reconstruction algorithms.

This paper investigates the possibility of developing a SPECT system that combines the high spatial resolution of position sensitive photomultiplier tubes (PSPMTs) with the excellent performance of iterative reconstruction algorithms. A small field of view (FOV) camera based on a PSPMT and a pixelized scintillation crystal made of CsI(Tl) have been used for the acquisition of the projections. With the use of maximum likelihood expectation maximization (ML-EM) and ordered subsets expectation maximization (OSEM) slices of the object are obtained while three-dimensional (3D) reconstruction of the object is carried out using a modified marching cubes (MMC) algorithm. The spatial resolution of tomographic images obtained with the system was 2-3mm. The spatial resolution of a conventional system that uses filtered backprojection (FBP) for slices reconstruction was more than 9 mm.     

Comparing filtered backprojection and ordered-subsets expectation maximization for small-lesion detection and localization in 67Ga SPECT.

Iterative reconstruction of SPECT images has recently become clinically available as an alternative to filtered backprojection (FBP). However, there is conflicting evidence on whether iterative reconstruction, such as with the ordered-subsets expectation maximization (OSEM) algorithm, improves diagnostic performance over FBP. The study objective was to determine if the detection and localization of small lesions in simulated thoracic gallium SPECT images are better with OSEM reconstruction than with FBP, both with and without attenuation correction (AC). METHODS: Images were simulated using an analytic projector acting on the mathematic cardiac torso computer phantom. Perfect scatter rejection was assumed. Lesion detection accuracy was assessed using localization receiver operating characteristic methodology. The images were read by 5 nuclear medicine physicians. For each reconstruction strategy and for each observer, data were collected in 2 viewing sessions of 100 images. Two-way ANOVA and, when indicated, the Scheffé multiple comparisons test were applied to check for significant differences. RESULTS: Little difference in the accuracy of detection or localization was seen between FBP with and without AC. OSEM with AC extended the contrast range for accurate lesion detection and localization over that of the other methods investigated. Without AC, no significant difference between OSEM and FBP reconstruction was detected. CONCLUSION: OSEM with AC may improve the detection and localization of thoracic gallium-labeled lesions over FBP reconstruction.     

Comparison of 180° and 360° acquisition for myocardial perfusion SPECT with compensation for attenuation, detector response, and scatter: Monte Carlo and mathematical observer results

BACKGROUND: The optimal projection data acquisition strategy for myocardial perfusion (MP) single photon emission computed tomography (SPECT) remains controversial. METHODS: We compared MP SPECT using 180 degrees and 360 degrees projection data obtained with the same acquisition time, reconstructed either with filtered back projection (FBP) or the iterative ordered-subsets expectation maximization (OS-EM) algorithm with various combinations of attenuation, detector response, and scatter compensation using mathematical observers and a myocardial defect detection task. We used Monte Carlo-simulated projection data from a population of 3-dimensional nurbs-based cardiac-torso (NCAT) phantoms with ranges of variability in patient anatomy, organ uptake, defect location, defect size, and noise level based on clinical data. Projection data from 180 degrees and 360 degrees acquisitions were generated by assuming the same acquisition time. After iterative or FBP reconstruction, standard postprocessing methods were applied. For each acquisition and reconstruction method, we optimized the number of iterations and cut-off frequency of the Butterworth filter using the Channelized Hotelling Observer methodology. The optimum set of parameters was that which gave the maximum area under the curve. RESULTS: For both acquisition protocols, OS-EM with compensations provided better performance than FBP or OS-EM without compensation. For FBP, the optimized 180 degrees acquisition provided a statistically significant increase in AUC as compared with optimized 360 degrees acquisition. For OS-EM, the AUCs for 180 degrees were slightly larger than for 360 degrees acquisitions when comparing images reconstructed with the same compensations. However, the differences were smaller and not statistically significant. CONCLUSION: With optimized reconstruction and filtering parameters, 180 degrees acquisition provided a statistically significant improvement over 360 degrees acquisition for FBP reconstruction. However, for OS-EM the differences were small and not statistically significant.     

Iterative reconstruction based on median root prior in quantification of myocardial blood flow and oxygen metabolism.

The aim of this study was to compare reproducibility and accuracy of two reconstruction methods in quantification of myocardial blood flow and oxygen metabolism with 15O-labeled tracers and PET. A new iterative Bayesian reconstruction method based on median root prior (MRP) was compared with filtered backprojection (FBP) reconstruction method, which is traditionally used for image reconstruction in PET studies. METHODS: Regional myocardial blood flow (rMBF), oxygen extraction fraction (rOEF) and myocardial metabolic rate of oxygen consumption (rMMRO2) were quantified from images reconstructed in 27 subjects using both MRP and FBP methods. For each subject, regions of interest (ROIs) were drawn on the lateral, anterior and septal regions on four planes. To test reproducibility, the ROI drawing procedure was repeated. By using two sets of ROIs, variability was evaluated from images reconstructed with the MRP and the FBP methods. RESULTS: Correlation coefficients of mean values of rMBF, rOEF and rMMRO2 were significantly higher in the images reconstructed with the MRP reconstruction method compared with the images reconstructed with the FBP method (rMBF: MRP r = 0.896 versus FBP r = 0.737, P < 0.001; rOEF: 0.915 versus 0.855, P < 0.001; rMMRO2: 0.954 versus 0.885, P < 0.001). Coefficient of variation for each parameter was significantly lower in MRP images than in FBP images (rMBF: MRP 23.5% +/- 11.3% versus FBP 30.1% +/- 14.7%, P < 0.001; rOEF: 21.0% +/- 11.1% versus 32.1% +/- 19.8%, P < 0.001; rMMRO2: 23.1% +/- 13.2% versus 30.3% +/- 19.1%, P < 0.001). CONCLUSION: The MRP reconstruction method provides higher reproducibility and lower variability in the quantitative myocardial parameters when compared with the FBP method. This study shows that the new MRP reconstruction method improves accuracy and stability of clinical quantification of myocardial blood flow and oxygen metabolism with 15O and PET.     

CT angiography after carotid artery stenting: assessment of the utility of adaptive statistical iterative reconstruction and model-based iterative reconstruction

Introduction

Follow-up CT angiography (CTA) is routinely performed for post-procedure management after carotid artery stenting (CAS). However, the stent lumen tends to be underestimated because of stent artifacts on CTA reconstructed with the filtered back projection (FBP) technique. We assessed the utility of new iterative reconstruction techniques, such as adaptive statistical iterative reconstruction (ASIR) and model-based iterative reconstruction (MBIR), for CTA after CAS in comparison with FBP.

Methods

In a phantom study, we evaluated the differences among the three reconstruction techniques with regard to the relationship between the stent luminal diameter and the degree of underestimation of stent luminal diameter. In a clinical study, 34 patients who underwent follow-up CTA after CAS were included. We compared the stent luminal diameters among FBP, ASIR, and MBIR, and performed visual assessment of low attenuation area (LAA) in the stent lumen using a three-point scale.

Results

In the phantom study, stent luminal diameter was increasingly underestimated as luminal diameter became smaller in all CTA images. Stent luminal diameter was larger with MBIR than with the other reconstruction techniques. Similarly, in the clinical study, stent luminal diameter was larger with MBIR than with the other reconstruction techniques. LAA detectability scores of MBIR were greater than or equal to those of FBP and ASIR in all cases.

Conclusion

MBIR improved the accuracy of assessment of stent luminal diameter and LAA detectability in the stent lumen when compared with FBP and ASIR. We conclude that MBIR is a useful reconstruction technique for CTA after CAS.     

Improved tolerance to missing data in myocardial perfusion SPET using OSEM reconstruction

When projection data are incomplete for various technical reasons, artefacts may occur in the reconstructed images. This study examines whether an iterative reconstruction method, the ordered subsets implementation of the EM algorithm (OSEM), can improve reconstruction and minimise the artefacts compared to filtered back-projection (FBP). We varied the number and location of projections removed to investigate when significant artefacts occur, and whether diagnosis is affected. Phantom studies were analysed with sequential orthogonal pairs of projection angles removed (as would typically occur when either data loss or severe motion is detected during acquisition with a right-angled, dual-head cardiac single-photon emission tomography system) and reconstructed with both FBP and OSEM. Twelve normal myocardial perfusion studies were also assessed to study the effect of missing projections on clinical diagnosis. Differences between reconstructions with intact versus missing data were measured. Also, reconstructed images were clinically assessed and scored on a five-point scale based on whether the artefacts would alter clinical interpretation. Although both reconstruction methods showed artefacts, the absolute differences between reconstructed phantom data with intact and missing projection sets were significantly greater (P<0.005) for FBP than for OSEM for all numbers of missing projections. The clinical data showed similar differences between FBP and OSEM reconstructions. The three observers noted superiority of OSEM compared to FBP, with reduced incidence of clinically significant artefacts. However, neither reconstruction method could tolerate six or more missing pairs from 32 projections. There was no significant dependence on the angular location of missing projections. In the absence of any attempt to correct for missing projections, OSEM reduced the influence of artefacts compared to FBP.     

Improvement in automated quantitation of myocardial perfusion abnormality by using iterative reconstruction image in combination with resolution recovery,attenuation and scatter corrections for the detection of coronary artery disease

Objective

An iterative reconstruction method in combination with resolution recovery, attenuation and scatter corrections (IR-RASC) can improve image quality. It, however, is undetermined whether this technique can improve the detection of coronary artery disease (CAD) when automated quantitative analysis is used. This study evaluated diagnostic values of IR-RASC in combination with automated quantitative analysis in stress myocardial perfusion imaging (MPI) in the CAD detection.

Methods

This study enrolled consecutive 64 patients (mean age 66.2 ± 17.3 years, 39 males) who had undergone both ^99mTc-labeled tetrofosmin stress MPI and coronary angiography within 3 months. Stress MPI abnormalities quantified as summed stress score (SSS), summed rest score (SRS) and summed difference score (SDS) by Heart Risk View-S (HRV-S) and Quantitative Perfusion SPECT (QPS) softwares using IR-RASC images were compared with those by using conventional filtered back-projection method (FBP) images and angiographic findings.

Results

Based on expert visual assessment, SSS and SRS by HRV-S/QPS softwares with IR-RASC were significantly lower than those by HRV-S/QPS softwares with FBP at mid- and basal left ventricular segments. Receiver-operating characteristics analysis showed that areas under the curve assessed by HRV-S (0.687) and QPS (0.678) with IR-RASC were nearly identical to those (0.717–0.724) by expert assessment with FBP, and were significantly (P < 0.05) greater than those by HRV-S (0.505) and QPS (0.522) with FBP. When HRV-S was used, the specificity and diagnostic accuracy of IR-RASC in the CAD detection were significantly greater than those of FBP: 90.3 versus 51.6%, P < 0.0001 and 79.7 versus 54.7%, P = 0.0027, respectively. Likewise, when QPS was used, the specificity and diagnostic accuracy of IR-RASC in the CAD detection were significantly greater than those of FBP: 80.6 versus 41.9%, P < 0.0001, and 78.1 versus 51.6%, P = 0.0018, respectively. There, however, were no significant differences in sensitivity between IR-RASC and FBP images.

Conclusions

IR-RASC can improve diagnostic accuracy of the CAD detection using an automated scoring system compared to FBP, by reducing false positivity due to artefactual appearance.

     

Assessment of geometrical distortion and activity distribution after attenuation correction: A SPECT phantom study

BACKGROUND: If single photon emission computed tomography (SPECT) images are reconstructed with filtered backprojection (FBP), not accounting for photon attenuation, artifacts can occur related to geometrical distortion and inaccurate estimation of regional distribution of radioactivity. By reconstructing the images with an iterative algorithm such as the maximum likelihood-expectation maximization (ML-EM) that incorporates the attenuation distribution information, it is possible to compensate for nonuniform attenuation. The aim of this study was to assess whether correction for nonuniform attenuation in SPECT can reduce the geometrical distortion and improve the activity quantitation. METHODS AND RESULTS: Three capillary sources containing the same amount of technetium 99m were imaged by a dual-headed SPECT system provided with two gadolinium 153 scanning transmission line sources, in nonuniform attenuation conditions. The images were reconstructed (1) with the use of FBP, (2) with the iterative ML-EM algorithm, and (3) with the iterative ML-EM algorithm incorporating attenuation maps. The geometrical distortion was estimated by comparing the spread that occurred in 2 orthogonal directions in the reconstructed transverse slices, expressed by full width at half maximum related to the x-axis and y-axis line spread functions. The accuracy of activity quantitation was analyzed by comparing the counts in regions of interest placed over the transverse slices of the 3 sources, located in different attenuating areas. The FBP-reconstructed slices showed a spread of image intensity toward the direction of minor attenuation; the source shape improved in the iterative ML-EM images, as well as in the iterative attenuation-corrected ML-EM images. The sources located deep in the phantom showed an apparent decrease in image intensity in both FBP and ML-EM images, which became less evident in the iterative attenuation-corrected ML-EM images. CONCLUSIONS: Image reconstruction with the iterative ML-EM algorithm, without the use of attenuation maps, can reduce geometrical distortion and eliminate streak artifacts, leading to an improvement in the object's shape and size, but does not reduce activity underestimation and inaccurate quantitation. In the iterative attenuation-corrected ML-EM images, there was a significant improvement in the accurate quantitation of activity distribution and a further reduction in geometrical distortion. In conclusion, nonisotropic attenuation correction with iterative ML-EM reduced the geometrical distortion of images and improved the accuracy of activity quantitation.     

Brain computed tomography using iterative reconstruction to diagnose acute middle cerebral artery stroke: usefulness in combination of narrow window setting and thin slice reconstruction

Purpose

The purpose of this study is to determine whether iterative model reconstruction (IMR) optimized for brain CT could improve the detection of acute stroke in the setting of thin image slices and narrow window settings.

Methods

We retrospectively reviewed 27 patients who presented acute middle cerebral artery (MCA) stroke. Images were reconstructed using filtered back projection (FBP; 1- and 5-mm slice thickness) and IMR (1 mm thickness), and contrast-to-noise ratios (CNRs) of infarcted and non-infarcted areas were compared. To analyze the performance of acute MCA stroke detection, we used receiver operating characteristic (ROC) curve techniques and compared 5-mm FBP with standard and narrow window settings, and 1-mm FBP and IMR with narrow window settings.

Results

The CNR in 1-mm IMR (1.1?±?1.0) was significantly higher than in 5- (0.8?±?0.7) and 1-mm FBP (0.4?±?0.4) (p?<?0.001). Furthermore, the average area under the ROC curve was significantly higher with 1-mm IMR with narrow window settings (0.90, 95% CI: 0.86, 0.94) than it was with 5-mm FBP (0.78, 95% CI: 0.72, 0.83).

Conclusion

The combination of thin image slices and narrow window settings under IMR reconstruction provide better diagnostic performance for acute MCA stroke than conventional reconstruction methods.

     

Evaluating image reconstruction methods for tumor detection in 3-dimensional whole-body PET oncology imaging.

We compare 3 image reconstruction algorithms for use in 3-dimensional (3D) whole-body PET oncology imaging. We have previously shown that combining Fourier rebinning (FORE) with 2-dimensional (2D) statistical image reconstruction via the ordered-subsets expectation-maximization (OSEM) and attenuation-weighted OSEM (AWOSEM) algorithms demonstrates improvements in image signal-to-noise ratios compared with the commonly used analytic 3D reprojection (3DRP) or FORE+FBP (2D filtered backprojection) reconstruction methods. To assess the impact of these reconstruction methods on detecting and localizing small lesions, we performed a human observer study comparing the different reconstruction methods. The observer study used the same volumetric visualization software tool that is used in clinical practice, instead of a planar viewing mode as is generally used with the standard receiver operating characteristic (ROC) methodology. This change in the human evaluation strategy disallowed the use of a ROC analysis, so instead we compared the fraction of actual targets found and reported (fraction-found) and also investigated the use of an alternative free-response operating characteristic (AFROC) analysis. METHODS: We used a non-Monte Carlo technique to generate 50 statistically accurate realizations of 3D whole-body PET data based on an extended mathematic cardiac torso (MCAT) phantom and with noise levels typical of clinical scans performed on a PET scanner. To each realization, we added 7 randomly located 1-cm-diameter lesions (targets) whose contrasts were varied to sample the range of detectability. These targets were inserted in 3 organs of interest: lungs, liver, and soft tissues. The images were reconstructed with 3 reconstruction strategies (FORE+OSEM, FORE+AWOSEM, and FORE+FBP). Five human observers reported (localized and rated) 7 targets within each volume image. An observer's performance accuracy with each algorithm was measured, as a function of the lesion contrast and organ type, by the fraction of those targets reported and by the area below the AFROC curve. This AFROC curve plots the fraction of reported targets at each rating threshold against the fraction of cases with (> or =1) similarly rated false reports. RESULTS: Images reconstructed with FORE+AWOSEM yielded the best overall target detection as compared with FORE+FBP and FORE+OSEM, although these differences in detectability were region specific. The FORE+FBP and FORE+AWOSEM algorithms had similar performances for liver targets. The FORE+OSEM algorithm performed significantly worse at target detection, especially in the liver. We speculate that this is the result of using an incorrect statistical model for OSEM and that the incorporation of attenuation weighting in AWOSEM largely compensates for this model inaccuracy. These results were consistent for both the fraction of actual targets found and the AFROC analysis. CONCLUSION: We demonstrated the efficacy of performing observer detection studies using the same visualization tools as those used in clinical PET oncology imaging. These studies demonstrated that the FORE+AWOSEM algorithm led to the best overall detection and localization performance for 1-cm-diameter targets compared with the FORE+OSEM and FORE+FBP algorithms.     

Rectal cancer: MR imaging in local staging--is gadolinium-based contrast material helpful?

PURPOSE: To determine retrospectively whether addition of gadolinium-enhanced T1-weighted magnetic resonance (MR) sequence to T2-weighted turbo spin-echo (SE) MR imaging is valuable for preoperative assessment of T stage and circumferential resection margin in patients with primary rectal cancer. MATERIALS AND METHODS: Local institutional review board approved study and waived informed patient consent. Eighty-three patients with operable primary rectal cancer underwent preoperative MR imaging. Retrospectively, two observers independently scored T2-weighted turbo SE MR images and, in a second reading, T2-weighted images combined with gadolinium-enhanced T1-weighted turbo SE MR images for tumor penetration through rectal wall and tumor extension into mesorectal fascia. A confidence level scoring system was used, and receiver operating characteristic (ROC) curves were generated. Histologic findings were standard of reference. Difference in performance of T2-weighted and combined T2-weighted plus gadolinium-enhanced T1-weighted sequences was analyzed by comparing corresponding areas under ROC curves (A(z)) for each observer. Interobserver agreement was calculated by using linear weighted kappa statistics. RESULTS: Addition of contrast-enhanced T1-weighted to T2-weighted MR imaging did not significantly improve diagnostic accuracy for prediction of tumor penetration through rectal wall (A(z) of T2-weighted vs T2-weighted plus T1-weighted images for observer 1, 0.740 vs 0.764; observer 2, 0.856 vs 0.768) and tumor extension into mesorectal fascia (A(z) for observer 1, 0.962 vs 0.902; observer 2, 0.902 vs 0.911). Diagnostic performance (A(z)) of MR and interobserver agreement were high for prediction of tumor extension into mesorectal fascia (kappa = 0.61, 0.74) but only moderate for penetration through rectal wall (kappa = 0.47, 0.45). CONCLUSION: Gadolinium-enhanced MR sequences did not improve diagnostic accuracy for assessment of tumor penetration through rectal wall and tumor extension into mesorectal fascia.