[Paper Review] Evaluation Of The PETsys TOFPET2 ASIC In Multi-Channel Coincidence Experiments

Vanessa Nadig, David Schug+2 more|arXiv (Cornell University)|Nov 19, 2019

Radiation Detection and Scintillator Technologies7 citations

TL;DR

This study evaluates the PETsys TOFPET2 ASIC for time-of-flight PET applications, testing its performance across multiple SiPM types and scintillator configurations. It achieves a coincidence resolution time (CRT) as low as 216.1 ps with Broadcom SiPMs and 219.9 ps with Hamamatsu SiPMs, demonstrating stable, multi-channel readout suitable for next-generation ToF-PET/MRI systems.

ABSTRACT

Aiming to measure the difference in arrival times of two coincident gamma-photons with an accuracy in the order of 200 ps, time-of-flight positron emission tomography systems commonly employ silicon photomultipliers (SiPMs) and high-resolution digitization electronics, application specific integrated circuits (ASICs). This work evaluates the performance of the TOFPET2 ASIC, released by PETsys Electronics S.A. in 2017, dependent on its configuration parameters in multi-channel coincidence measurements. SiPM arrays fabricated by different vendors (KETEK, SensL, Hamamatsu, Broadcom) were tested in combination with the ASIC. Scintillator arrays featuring different reflector designs and different configurations of the TOFPET2 ASIC software parameters were evaluated. The benchtop setup used is provided with the TOFPET2 ASIC evaluation kit by PETsys Electronics S.A.. Compared to existing studies featuring the TOFPET2 ASIC, multi-channel performance results dependent on a larger set of ASIC configuration parameters were obtained that have not been reported to this extend so far. The ASIC shows promising CRTs down to 219.9 ps in combination with two Hamamatsu S14161-3050-HS-08 SiPM arrays (128 channels read out, energy resolution 13.08 %) and 216.1 ps in combination with two Broadcom AFBR-S4N44P643S SiPM arrays (32 channels read out, energy resolution 9.46 %). The length of the trigger delay of the dark count suppression scheme has an impact on the ASIC performance. The integrator gain configuration has been investigated and allows an improvement of the energy resolution. Measuring up to the time-of-flight performance of state-of-the-art positron emission tomography (ToF-PET) systems while providing a uniform and stable readout for multiple channels at the same time, the TOFPET2 ASIC is treated as promising candidate for the integration in future ToF-PET systems.

Motivation & Objective

• To evaluate the performance of the TOFPET2 ASIC in multi-channel coincidence measurements for time-of-flight PET systems.
• To assess the impact of ASIC configuration parameters—such as discriminator thresholds, integrator gain, and trigger delay—on coincidence resolution time (CRT) and energy resolution.
• To test the ASIC’s compatibility and performance with various SiPM types (KETEK, SensL, Hamamatsu, Broadcom) and scintillator topologies.
• To determine the feasibility of achieving sub-200 ps CRTs with the ASIC in a benchtop setup while maintaining stable, uniform multi-channel readout.
• To investigate the influence of dark count suppression and data rate on system performance.

Proposed method

• Used the TOFPET2 ASIC evaluation kit from PETsys Electronics S.A. to conduct multi-channel coincidence measurements.
• Tested four SiPM types (KETEK, SensL, Hamamatsu, Broadcom) in combination with scintillator arrays featuring different reflector designs and configurations.
• Measured coincidence resolution time (CRT) and energy resolution using a 22Na source in a benchtop setup with varying source-to-detector distances to control data rates.
• Varied key ASIC configuration parameters including discriminator threshold (vth_t1), trigger delay length in the dark count suppression scheme, and integrator gain.
• Acquired data in both time-to-digital converter (TDC) and charge-to-digital converter (QDC) modes to evaluate timing and energy performance.
• Analyzed time difference spectra for satellite peaks and rate-dependent performance effects.

Experimental results

Research questions

• RQ1 What is the minimum coincidence resolution time (CRT) achievable with the TOFPET2 ASIC across different SiPM types and scintillator configurations?
• RQ2 How do ASIC configuration parameters—such as discriminator threshold, trigger delay, and integrator gain—affect CRT and energy resolution?
• RQ3 To what extent does the dark count suppression scheme’s trigger delay impact timing performance?
• RQ4 Can the integrator gain be tuned to improve energy resolution at the cost of energy spectrum linearity?
• RQ5 Does the data rate (source distance) influence the ASIC’s timing or energy resolution performance?

Key findings

• The TOFPET2 ASIC achieved a coincidence resolution time (CRT) of 216.1 ps when paired with two Broadcom AFBR-S4N44P643S SiPM arrays (32 channels, energy resolution 9.46%).
• A CRT of 219.9 ps was achieved with two Hamamatsu S14161-3050-HS-08 SiPM arrays (128 channels, energy resolution 13.08%).
• Increasing the discriminator threshold (vth_t1) to 40–50 improved both CRT and energy resolution, indicating optimal threshold settings exist.
• The trigger delay length in the dark count suppression scheme significantly influenced performance and could be tuned to further improve CRT.
• Adjusting the integrator gain improved energy resolution by up to 1% in absolute terms, though it slightly reduced linearity in the energy spectrum.
• Satellite peaks were observed in multi-channel time difference spectra, indicating potential crosstalk or signal shaping effects in high-channel configurations.