Improving CTR With the FastIC ASIC for TOF-PET by Overcoming SiPM Noise With Baseline Correction

Time resolution in time-of-flight positron emission tomography (TOF-PET) has improved significantly over the last decade due to advancements in scintillation materials, photodetectors, and readout electronics, which has increased the signal-to-noise ratio (SNR) compared to conventional positron emission tomography. Silicon photomultipliers (SiPMs) in TOF-PET detectors are often operated at high bias voltage to improve the time performance at the expense of increasing signal noise. SiPM noise, both correlated and uncorrelated, can cause baseline fluctuations, leading to time-walk effects when a leading edge trigger strategy is used, and thus limiting timing performance. We examined the effect of SiPM baseline fluctuations using the FastIC ASIC, a scalable multichannel readout for fast timing applications. We flagged noisy events by using a comparator signal triggered by dark counts before the actual scintillation event. We tested different classification and correction methods with scintillating crystals and Cherenkov radiators, coupled to analog SiPMs from Broadcom (NUV-MT) and Hamamatsu Photonics. We reduced the coincidence time resolution (CTR) in bismuth germanate $2\times 2\times $ 3 mm3 (BGO) crystals from $410~\pm ~10$ to $388~\pm ~10$ ps FWHM (5%) by correcting the time-walk on the noisy events. We measured an improvement from $107~\pm 2$ to $93.5~\pm ~0.6$ ps (11%) for LYSO $2\times 2\times $ 3 mm3 crystals by filtering the noisy events. An improvement of 9% on the CTR of the EJ232 plastic scintillator was also achieved by filtering noisy events, reducing it from $82.2~\pm ~0.5$ to $75~\pm ~1$ ps. This study presents a scalable method for flagging undesired events in a full TOF-PET system and discusses the impact of SiPM noise on the FastIC readout.