Crystal Delay and Time Walk Correction Methods for Coincidence Resolving Time Improvements of a Digital-Silicon-Photomultiplier-Based PET/MRI Insert

Our group has built the MRI compatible PET insert Hyperion II
^D
, which is based on digital silicon photomultipliers [digital photon counters (DPCs)]. We characterized its performance for two different scintillator configurations already in previous publications. For this paper, we reused the raw detector data that were used in previous publications and stored for offline analysis. We reprocessed these data in order to improve the timing performance of the PET scanner. Different crystal delay and time walk calibration and correction methods are evaluated with respect to the coincidence resolving time (CRT) of the scanner. For all applied correction methods, we report CRTs both for a preclinical and a clinical scintillator configuration for all DPC trigger schemes and several energy windows. The newly obtained results were compared with the previously published values, and the additional benefit of using a time walk correction was evaluated. Previously published CRTs could be improved using a refined version of the original crystal delay calibration method without applying a walk correction by 6.3%-10.9% for a wide energy window ranging from 250-625 keV, and we could show an additional improvement of up to 2.6% by adding a walk correction. Using trigger scheme 1 and a very narrow energy window around the photopeak (500-520 keV), we could reach CRT values on system level of 208/240 ps for the clinical/preclinical scintillator configuration.