A real time coincidence system for high count-rate TOF or non-TOF PET cameras using hybrid method combining AND-logic and Time-mark technology

A fully digital FPGA-based high count-rate coincidence system has been developed for TOF and non-TOF PET cameras. Using hybrid method combining AND-logic and Time-mark technology can gain both excellent timing resolution and high processing speed. In this hybrid architecture, every gamma event is synchronized by a 125 MHz system clock and generating a trigger associated with a time-mark given by an 8-bit high-resolution TDC (68.3 ps/bin). AND-logic is applied to the synchronized triggers for real-time raw sorting of coincident events. An efficient FPGA based Time-mark fine-sort algorithm is used to select all the possible coincidence events within the preset coincidence time window. This FPGA-based coincidence system for modular PET camera offers reprogrammable flexibility and expandability, so the coincidence system is easily employed for different scale of PET camera detector setup. Distributed processing method and pipeline technology is adopted in the design to obtain very high processing speed. Both prompt and time-delayed accidental coincidences are simultaneously processed in real time. The real-time digital coincidence system supports coincidence between 2 to 12 detector modules setup, capable of processing 72 millions single events per second with no dead-time and captures triple events coincidence for better imaging performance evaluation. The coincidence time window-size and time-offset of each coincidence event pair can be programmed independently in the step of 68.3 ps (TDC LSB) during the data acquisition in different applications to optimize the signal-to-noise ratio. The complex coincidence system is integrated in one (A4 size) circuit board with 1.5 Gbps fiber optical interface. We have evaluated the system performance using the actual circuit and Monte Carlo simulations.