Exploring μSiPMs with BGO for Cherenkov time-of-flight in PET

Good timing resolution in time-of-flight positron emission tomography (ToF-PET) scanners is a much researched way to effectively increase sensitivity. Recently, Cherenkov radiation in bismuth germanate (BGO) has been considered as an option for fast timing and high sensitivity, since Cherenkov radiation is prompt and BGO offers a high stopping power. However, digitizing BGO’s Cherenkov radiation and benefiting from its timing is challenging due to low abundance of Cherenkov photons, its mixture with scintillation and the electronic noise. To address these issues, in this work, we investigate SiPMs segmented into μSiPMs with BGO in simulation. We developed a simulation framework starting with an optical Geant4 simulation of a 3mm or 20mm long BGO crystal and modeled the SiPM response including dark counts, external cross-talk, single photon time resolution (SPTR), dead time, a low pass filter to represent electronic bandwidth limitations and analog noise. After testing segmentations from a full SiPM down to an array of 20 × 20 μSiPMs, we found that segmentation could artificially increase the bandwidth of the detector. In our simulation including noise factors, a segmentation into a 20 × 20 μSiPM array allows to reach coincidence time resolutions (CTRs, FWHM) of 68 ps for a 3mm and 118 ps for a 20mm crystal compared to 104 ps for a 3mm and 183 ps for a 20mm crystal with a full SiPM. A segmentation lower than 10 × 10 showed a significantly worse CTR. Overall, sufficiently high segmentation allows an effectively better detection of Cherenkov photons leading to better timing performance.