Simultaneous measurement of transmission and emission contamination using a collimated 137Cs point source for the HRRT

The high resolution research tomograph (HRRT) is a brain-dedicated scanner manufactured by CPS Innovations using LSO panel detectors. Transmission is measured using a ¹³⁷Cs point source, which is moved axially and rotated to cover the FOV. The point source is collimated axially and transaxially to illuminate only a few planes on the heads opposite to the point source location. Pseudo-coincidence events are generated using a given crystal and the source location. The transmission system was previously validated for cold transmissions. For post-injection (hot) transmission, it is not possible to eliminate the emission contamination by raising the lower energy threshold. Since real mock scan is unpractical on HRRT and fake mock scan requires additional data, we developed a new technique to simultaneously measure the transmission and the mock scans. The technique uses a virtual source, axially located at a distance equal to the half of the axial FOV and illuminates a fan separated from the real transmission fan. We validated the shifted-mock scan technique by comparing it to real mock scan one with a ⁶⁸Ge phantom and examined its effectiveness with a hot 20 cm phantom filled with ¹⁸F decaying over several half-lives. Local residual bias in μ-map was attributed to transmission scatter and corrected by using partial segmentation in the MAP-TR algorithm, μ-maps from cold and hot transmissions were compared on several clinical patients and a Hoffman brain phantom for which their influence on emission quantification was studied.