Multi-CFD Timing Estimators for PET Block Detectors

In a conventional PET system with block detectors, a timing estimator is created by generating the analog sum of the signals from the four photomultiplier tubes (PMT) in a module and discriminating the sum with a single constant fraction discriminator (CFD). The differences in the propagation time between the PMTs in the module can potentially degrade the timing resolution of the module. While this degradation is probably too small to affect performance in conventional PET imaging, it may impact the timing inaccuracy for time-of-flight PET systems (which have higher timing resolution requirements). Using a separate CFD for each PMT might allow for propagation time differences to be compensated through calibration and correction in software. In this paper we investigate and quantify the timing resolution achievable when the signal from each of the 4 PMTs is digitized by a separate CFD. Several methods are explored for both obtaining values for the propagation time differences between the PMTs and combining the four arrival times to form a single timing estimator. We find that the propagation time correction offsets are best derived through an exhaustive search, and that the "weighted average" method provides the best timing estimator. Using these methods, the timing resolution achieved with 4 CFDs (1052plusmn82 ps) is equivalent to the timing resolution with the conventional single CFD setup (1064plusmn216 ps)