Waveform-sampling electronics for time-of-flight PET scanner

Abstract

Waveform sampling (WFS) is an appealing technique for instruments requiring precision time and pulse-height measurements. Recent advances in switched-capacitor-array ASICs such as the Domino Ring Sampler (DRS4) have made WFS affordable for large systems. LAPET is a whole-body time-of-flight PET scanner using 38880 LaBr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (5% Ce) scintillator crystals of dimension 4 × 4 × 30 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> , imaged by 432 Photonis XP20D0 PMTs, grouped into 24 identical detector modules. High light yield (61000 photons/MeV) and fast decay time (20 ns) make LaBr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> an excellent scintillator for TOF PET. Our group previously reported coincidence timing resolution 315-330 ps (fwhm) in benchtop measurements and 375 ps in full-system measurements using semi-custom electronics. This contribution reports on a complete redesign of the LAPET electronics, trigger, and data acquisition system. Our design uses 240 DRS4 chips to obtain oscilloscope-quality sampling of each PMT waveform at 2 GSPS. The 7 PMTs with which each crystal's scintillation light is collected map cleanly into the 8 analog inputs of a DRS4 chip, facilitating a redundant and nearly deadtime-free (at clinical rates) trigger design, in spite of the ~ 3 μs required for DRS4 readout. An FPGA-based trigger using analog pulse shaping and 100 MSPS sampling provides coarse energy and timing measurements used to detect coincident pairs and to select DRS4 chips for readout. Simulation studies show that oscilloscope-quality readout of each PMT signal will permit more flexible handling of detector calibrations, PMT waveform baseline offsets, and pulse pile-up effects. We thus expect the upgraded electronics to permit system-level performance that more closely approximates single-module benchtop results and to preserve that performance at clinical count rates. Our goals are both to explore the feasibility of WFS for a large scanner and to improve the overall performance of the LAPET research scanner. We present initial tests using prototype units of our redesigned electronics.