A Comprehensive Study on the Timing Limits of the TOFPET2 ASIC and on Approaches for Improvements

Novel electronic readout schemes of analog silicon photomultipliers (SiPMs) have shown impressive timing performance of $\gamma $ -detectors in positron emission tomography (PET). However, transferring these novel concepts to the system level is a key to exploit the improved coincidence time resolution (CTR) in (pre-)clinical imaging. In this study, the commercially available TOFPET2 application-specific integrated circuits (ASICs) from PETsys Electronics S.A. is tested in terms of the best achievable CTR. The measurable CTR limits will be obtained by state-of-the-art high-frequency (HF) readout, minimizing the impact of the electronic front end on the time resolution. We achieved (73 ± 1) ps with the HF readout and (134 ± 10) ps with the TOFPET2 ASIC for a Ce-, and Ca-doped lutetium-oxyorthosilcate (LYSO) crystal of 2 $\times $ 2 $\times $ 3 mm2 size. We show that SiPM signal amplification and an effectively reduced TOFPET2 input stage impedance boost the CTR of a 20-mm high LYSO:Ce crystal to 187 ± 8 ps. Our studies also lead to the observation of side peaks in the coincidence time difference spectrum. These peaks are studied in depth and a conclusion on the ASIC for 100-ps PET applications is drawn.