Publication
Fast timing in ToF-PET improves the signal-to-noise ratio for better patient comfort through either a lower dose or a shorter scan time. Recent clinical PET scanners using lutetium-based crystals, like LYSO, reach a coincidence time resolution (CTR) of around 200 ps. Their CTR is mainly limited by the scintillation process. Thus, crystals with a significant emission of Cherenkov light, most prominently BGO, are being investigated as an alternative. BGO cannot reach its full potential due to a too low bandwidth of the electronics and too low time resolution of the photosensors. We investigate the segmentation of the photosensor into an array of individually-read-out μSiPMs to allow a better use of the Cherenkov photon’s prompt time information through an effectively higher bandwidth. In this work, we simulated the optical photon production with Geant4 and applied a signal model with a leading-edge threshold to determine timestamps and calculate the CTR from the first μ SiPM timestamp. Detectors with either 3 mm or 20 mm BGO or LYSO crystals were examined with varying photosensor segmentations. Segmenting the photosensor resulted in a significant improvement of the CTR for BGO crystals of both lengths, reducing it to 29±15ps (FWHM) for 3 mm and 80±21ps (FWHM) for 20 mm. BGO benefits from the segmentation due to its higher Cherenkov-to-scintillation ratio, while the same does not apply to LYSO with its lower Cherenkov and higher scintillation light yield. Adding noise factors, we validated the simulation of a full SiPM with measurements. For the 20 mm BGO crystal, we found that the CTR of the highest segmentation deteriorated slightly, but stayed in the range of 100 ps. For a more realistic model, further simulations with more noise factors are planned. Finally, first measurements with test structures will be used to verify our simulation results.