Optimization of regularization of attenuation and scatter-corrected 99mTc cardiac SPECT studies for defect detection using hybrid images

Through means of a receiver-operating-characteristics study, we optimize the iteration number and three-dimensional (3-D) Gaussian postfiltering of Tc-99m cardiac emission ordered-subset expectation-maximization (OSEM) reconstructions that implement corrections for both attenuation and scatter. Hybrid images, wherein artificial perfusion defects were added to clinical patient studies that were read as being normally perfused, were used for this optimization. The test conditions included three different iteration numbers of OSEM (1, 5, and 10) using four angles per subset, followed by 3-D Gaussian low-pass filtering at each iteration level. The level of Gaussian low-pass filtering was varied using standard deviations (a) of 0.6, 0.75, 1, and 1.25 pixels, in addition to a case where no postfiltering was applied. Four observers read 80 images for each of the 15 test conditions being investigated, providing confidence ratings as to the presence or absence of perfusion defects. Results indicate that at all iterations, optimum detection performance is obtained for a broad plateau or range of postfilters (sigma = 0.6 to 1 pixel). As expected, a gradual reduction in performance is seen at either end of this broad maximum where the images either have been very heavily smoothed or have very little postfiltering. Finally, one iteration of OSEM appears to be the appropriate choice since no significant improvement in detection accuracy was observed with increasing iteration number as long as the reconstructions are postfiltered with or in the range of 0.6 to I pixel.