Preliminary characterization of ASIC-based detectors for TOF-PET applications

In PET systems with a geometry different from a closed ring, the Time of Flight (TOF) of the annihilation photons appears to be relevant. Such a system capability helps to compensate the missing angular information that generates artifacts on the reconstructed images. In this work, two ASIC designs are evaluated for their use in PET systems having accurate coincidence resolving times (CRT) while preserving both energy and spatial resolutions. One of the ASIC options is the TOFPET1 from PETsys and the other the so-called PETIROC2 from Weeroc. Detection blocks based on scintillation LYSO crystal arrays have been used to first optimize the experiments. Parameters such as spatial, energy and time resolution have been obtained. Promising CRT results were found nearing 350 ps FWHM for a pair of photosensor channels (PETsys) and roughly 800 ps for all 16 against 16 channels (Weeroc). In terms of spatial resolution, 2 mm pixels size are well resolved with energy values approaching 15% have preliminary been measured for both ASICs. With the TOFPET1 ASIC, also tests with monolithic crystals were carried out. Those showed the system to provide depth of interaction (DOI) information. A DOI resolution (FWHM) of about 3 mm was preliminary estimated in a 25×25×10 mm
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LYSO scintillator. A calibration process for time alignment of the signal paths was carried out to provide accurate CRT results. The time walk error between channels was also compensated. Several methods for the timestamp averaging have been tried, resulting on preliminary CRT values in the 1 ns FWHM range.