Optimizing timing resolution for TOF PET detectors based on monolithic scintillation crystals using fast photosensor arrays

We have investigated the time-of-flight (TOF) capability of a monolithic 20 mm × 20 mm × 12 mm LYSO crystal coupled to a Hamamatsu position-sensitive H8711-03 4×4 multi-anode photomultiplier tube. The x-, y-, and z-coordinates of the photoconversion location inside the crystal are determined using a maximum likelihood estimation algorithm, based on a calibration scan with a pencil beam of 511 keV photons across the front and one of the side surfaces of the crystal. A FWHM resolution in x- and y-direction of 2.7 mm is obtained. In the z-direction, i.e. the depth-of-interaction (DOI), the FWHM resolution ranges from 2.5 mm near the photomultiplier tube to 4 mm at a distance of 10 mm from the photomultiplier tube. Artifacts are observed at all edges of the crystal. A timing resolution of 349 ps FWHM is obtained in coincidence with a fast BaF2 detector. DOI time walk is observed to strongly depend on the region of the crystal selected. In the present setup, a software correction of the DOI time walk of less than 100 ps across the detector does not significantly improve the coincidence timing resolution, as the walk effect is much smaller than the system timing resolution. A simulation of the DOI time walk effect in the coincidence of two 30 mm thick LaBr3 crystals indicates that a DOI time walk correction may lead to a significant improvement in timing resolution.