Digital delay line shaping-zero crossing algorithm for timestamp extraction in PET

Timing resolution is a key parameter in PET scanners in order to reduce random events. However, fully digital DAQ systems working in free-running sampling mode with conversion rates ≪ 100 MHz have to rely on interpolation or model fitting techniques in order to achieve a tight coincidence window. This paper presents a simple but accurate method for on-line timestamp extraction in PET detectors. The proposed algorithm is based on a simple bipolar shaping using digital delays and a subsequent zero-crossing detection by linear interpolation. It can be easily integrated in an FPGA and it operates synchronously with the data processing chain. Experimental results show that this method produces a gaussian time difference spectrum with good timing resolution of σ = 2.20 ns (5.17 ns FWHM) obtained with a fairly moderate conversion rate of 40 MHz.