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.