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

Vanessa Nadig,Harald Radermacher,Volkmar Schulz,Stefan Gundacker,Bjoern Weissler,Malika Yusopova,David Schug

doi:10.1109/trpms.2022.3158704

Abstract

Novel electronic readout schemes of analog silicon photomultipliers (SiPMs) have shown impressive timing performance of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\gamma $ </tex-math></inline-formula> -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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times $ </tex-math></inline-formula> 2 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times $ </tex-math></inline-formula> 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.

Highlights

In time-of-flight positron emission tomography (TOFPET), digitizing coincident γ-events with high-precision timestamps is key to improve the signal-to-noise ratio (SNR) of PET images [1]–[5]
We further established a combination of the HF readout with the TOFPET2 application-specific integrated circuits (ASICs) with the aim to boost the performance of the ASIC and to take the essential step to investigate improved signal formation on system level
We have shown the feasibility of applying a similar highspeed amplification before the TOFPET2 ASIC, which leads to improved coincidence time resolution (CTR) and single-photon resolution in corresponding staircase plots for all tested silicon photomultipliers (SiPMs)

Summary

Introduction

In time-of-flight positron emission tomography (TOFPET), digitizing coincident γ-events with high-precision timestamps is key to improve the signal-to-noise ratio (SNR) of PET images [1]–[5]. The TOFPET2 ASIC consumes a maximum power of 8.2 mW per channel (including signal digitization) and has shown promising performance results in prior studies [14]–[16]. This ASIC is under investigation by many research groups, making limitations of its electronic front end a subject with a large impact [17]–[21]. We investigate the timing limits of the ASIC front end by the means of comparing the TOFPET2 performance to a HF readout approach. We will exploit the possibility of encoding the signals of a single SiPM with multiple timestamps

Methods

Results

Conclusion