A time driven readout scheme for PET and CT using APDs and SiPMs

In this paper we present the time-driven readout scheme for photodetectors in the domain of PET/CT applications. In our first test period as partner of the BioCare Consortium supported by the European Commission's FP6 framework program, we put emphasis on a scheme to be used with a LSO-APD detector suitable for both CT and PET, using exclusively electronics circuits developed for the LHC program at CERN. Compared to standard PMT-based systems, the proposed time-based readout method together with CERN ASICs produced comparable performance in terms of energy resolution, i.e. 15% FWHM, but had a markedly inferior timing response of 1.6 ns FWHM in a dual APD system. This is not attributed to the readout scheme itself but to crystal-inherent photon statistics and insufficient photodetector gain of the APDs. However, in our new follow-up test program employing the same electronics and readout scheme, silicon photomultipliers (SiPMs) replacing the previously used APDs showed excellent timing behavior of 420 and 235 ps FWHM for 1 and 3 photoelectrons, respectively. Timing resolution with 511 keV gammas on LSO reached 400 ps FWHM without time walk corrections.