Evaluation of SiPM photodetectors for use in phoswich detectors

MR compatible PET is an attractive hybrid modality for preclinical small animal studies. Small ring diameter PET systems, such as those needed to fit inside an MR bore, tend to suffer from radial blurring due to parallax error away from the center of the FOV. To reduce this error a phoswich design can be adopted where layers of different scintillation materials are identified by decay time characteristics. This design requires a photo-detector and electronics capable of discriminating between different scintillator decay times. The benefits of a phoswich design in a PET/MR system can only be realized using MR compatible photo detectors capable of decay time identification. Photomultiplier tubes (PMTs) and avalanche photo diodes (APDs) have traditionally been used in phoswich detectors, however PMTs are unsuitable for use in a magnetic field and APDs pose a challenge due to their low gain and relatively poor timing characteristics. Silicon photomultipliers (SiPMs) are MR compatible but have not been extensively studied for use in phoswich detectors. In this work, we compare the scintillation decay time identification capability of two SiPM models, the SensL MicroFB-SMA-30035 and the Hamamatsu S10985-050C, for two scintillation pairs LGSO/LYSO and LSO:Ce,Ca(0.1%)/LSO:Ce,Ca(0.3%). The two SiPMs were compared to a Hamamatsu H3178-51 PMT. Data was acquired by capturing waveforms using a DRS4 evaluation board. The fall time histograms of the SiPMs from 80-20% of the signal shows that the Hamamatsu MPPC has better decay time separation and thus, better phoswich potential than the SensL SiPM. However, the Hamamatsu PMT shows better decay time separation than both of the SiPMs.