An Investigation of Projection Sampling for Ga-67 Tumor Detection

Psychophysical studies were used to investigate the effect of number of projection angles $\Phi$ on images reconstructed from simultaneous emission-transmission scans of fixed duration. The specific task was detection and localization of Ga-67 tumors in simulated images of the thorax, and a Tc-99m transmission source was modeled in the data acquisition. Reconstructions with the rescaled block-iterative (RBI) expectation-maximization algorithm included corrections for nonuniform attenuation and collimator response, but not scatter. A combination of channelized Hotelling observer (CHO) ROC and human observer localization ROC (LROC) studies were conducted. The CHO was used to optimize the number of RBI iterations and the post-reconstruction filtering level for reconstruction strategies based on $\Phi\in\{20,30,40,60,90,120\}$ angles. The LROC study compared these optimized strategies. No significant differences in human detection-task performance were found for $\Phi\geq 30$. This finding is attributed largely to collimator effects, the presence of uncorrected scatter in the data, and the post-filtering of the reconstructed images.