Initial implementation of all-digital PET DAQ system

An all-digital data acquisition (DAQ) system for positron emission tomography (PET) events is presented. The advantages of digital DAQ for PET system have been recognized recently. Sampling rate faster than 200Mhz is required when LSO scintillation pulses are directly digitized. That make it unaffordable to implement a PET system with all digital DAQs, due to numerous channels in PET system and costly price of fast analog-to-digital convertors (ADCs). The multi-voltage thresholds (MVT) method, whose timing performance for PET events has been proven, can be easily implemented with comparators and time-to-digital convertors (TDCs). However the position determination of PET events is still a challenge for MVT, due to the huge difference of corner pulses´ amplitude and limited range of threshold voltages for high timing resolution. In this work, we present the implementation of the all-digital PET DAQ system based on the improved MVT method. To improve MVT method, several extra thresholds at low voltages are set for position determination. The time when pulse voltage arrives at pre-set threshold is determined by use of a high resolution multi-channel TDC (60ps/bin). Depending on the highest voltage of trigged thresholds, a part of obtained digital samples are chosen out to reconstruct the original pulse. The time, energy and position of event can be derived from the reconstructed pulse. The process of calibration and coincidence are performed by software in computer. By using multi-channel TDC modules, which can be easily implemented in field programmable gates array (FPGA), the all-digital DAQ system can digitize scintillation pulses with several pico-seconds interval. The preliminary implementation and results on coincidence timing resolution, energy spectrum and flood map obtained from the all-digital DAQ system with Hamamatsu R8900 PMT and LYSO crystal array will be presented.