Photon interaction position-dependent time walk measurement for the PET detectors with a ontinuous crystal coupled to a multi-anode PMT

Relying on the single-end scintillation light detection for PET detectors based on continuous crystal, the annihilation photon interaction position inside of crystal can be determined by our 3D maximum likelihood position estimation algorithm, which provides a possibility to investigate the correlation between the time walk and interaction position to optimize the detector timing performance. Measurements were performed with a test detector consisting of a continuous LYSO crystal (18.1 mm × 18.1 mm × 10 mm) optically coupled to a multi-anode PMT (H7546B) and a coincidence detector constructed by a block of LaBr3 crystal coupled to a R9800 PMT. The time walk, referring to the shift of the timing distribution peak, was experimentally measured as a function of the 3D interaction position. The maximum time walk for events in a (x, y) plane is measured as 140 ps, meanwhile it is 50 ps for events with different depth-of-interaction (DOI). The (x, y)-dependent time walk is rooted in the timing signal readout structure of the MA-PMT, and the DOI-dependent time walk is consistent with the variation of the average propagation time from the interaction position to the surface of PMT. The time walk was corrected by the measured function, which improved the time resolution of detector from 374.5 ps to 342.3 ps.