Reducing Dynamic Bladder Artifact in Pelvic Bone SPECT: An Assessment of Lesion Detectability Using Numerical and Human Observers

In pelvic bone SPECT using Tc-99m labelled com-pounds, the accumulation of activity into the bladder during the data acquisition process often results in data inconsistencies which, when reconstrusted with filtered backprojection, results in streak artifacts. If the uptake rate is sufficient, these streaks may be significant enough to impair lesion detection. We have investigated various reconstruction methods in an effort to reduce this artifact. Pelvic SPECT imaging was simulated using the Zubal voxelized phantom, with provisions for a changing activity distribution within the bladder. Reconstructions were performed using filtered backprojection, ordered subset-expectation maximization and dynamic expectation maximization. Each method was first optimized for post-reconstruction smoothing parameters using a channelized, non-prewhitening (CNPW) numerical observer model. The numerical observer used is based on human observer LROC methodology whereby both a likely lesion location and a confidence rating is supplied by the observer for each image. Based on the results of the CNPW observer, a human LROC observer study was performed in order to assess the various reconstruction methods in terms of lesion detectability. Three human observers were used in this test. The results of this test indicate that filtered backprojection performs significantly worse than static OSEM iterative reconstruction with attenuation correction when assessed using the area under the LROC curve $(A_{LROC}=0.47\; {\rm vs} \;0.71)$. Results comparing OSEM with dEM indicate that the dEM algorithm is able to further reduce streak artifacts compared to OSEM, but this improvement was not reflected in improved $A_{LROC}$ scores. In fact, detectability actually decreased slightly when using ${\rm dEM}(A_{LROC}=0.71\;{\rm vs }\;0.66)$, although this reduction was not seen to be statistically significant. It is possible that the slightly reduced performance of the dEM algorithm may be due, in part, to not performing an optimization in the number of reconstruction iterations as was performed for the OSEM method.