An all-digital coincidence-selection and coincidence-trigger generation for a small animal RPC-PET camera

The time resolution for gamma photon pairs of a detector based on resistive plate chamber technology is close to 300 ps FWHM. This allows a tight time window for coincidence detection, reducing the number of random coincidences observed and the overall noise of the acquired image. We implemented, inside a Xilinx Virtex-5 field programmable gate array, a fully programmable coincidence-trigger system with sub-nanosecond resolution and low latency for a small animal RPC-PET camera being built. An automatic procedure to construct a histogram of coincidences was implemented that helps characterizing the camera and allows compensating for different delays in the time channels. The time window for coincidences can then be placed at the optimal position helping rejecting coincidences from undesired sources.