Depth of interaction effect on timing resolution in PET block detectors

We have investigated the effect of the depth of interaction (DOI) on the coincidence timing distribution of a PET block detector excited with 511 keV photons. Measurements were performed with a detector consisting of a quad photomultiplier tube (PMT) optically coupled to a 6times6 array of mixed lutetium silicate (MLS) scintillator crystals with different surface treatments. The PET detector was side-illuminated with an electronically collimated beam and the coincidence timing distribution was recorded at different DOI as the detector block was stepped through the beam. The shift of the timing distribution peak measured the variation of the average propagation time of the scintillation photons within the block. From top to bottom (30 mm distance), the average delay ranged from 120 ps for polished crystals up to 350 ps for crystals with all sides roughened. The results of this study allowed modeling the effect of DOI delay on timing performance for front-end irradiation with 511 keV photons, assuming a range of different values for the intrinsic timing resolution of the detector. The relative contribution from DOI effects to the total time coincidence resolution was found to be significant only for detectors with an intrinsic time resolution less than 250 ps FWHM