Use of a central positron emitting reference source to improve the timing alignment of a singles list-mode small animal PET scanner

To minimize the system-wide timing resolution of a PET scanner, and hence the applied coincidence window, a delay must be added to the signals from each crystal to compensate for inherent time differences between detectors. Without an absolute time reference, this must be been done iteratively. We propose using a new timing probe, consisting of a Ge-68 source imbedded in a plastic scintillator coupled to a fast PMT, to time-calibrate the MADPET-II small animal PET scanner. The probe provides a precise trigger signal with each positron decay and emission of 511 keV photons and, when placed in the center of the scanner, allows all crystals to be aligned to a common reference. MADPET-II is based on the one-to-one coupling of LSO crystals to APD detectors, each of which is read out by a dedicated electronic channel. Time stamps for each singles event are stored in list-mode format, allowing time offsets to be assigned directly to each detector. Signals from the probe were fed into an Ortec 934 CFD, and the trigger signals were digitised using the MADPET-II data acquisition system. Histograms generated by the time difference between the detection of 511 keV photons in the detectors and the corresponding positron decay in the probe give the time offsets for each crystal. After applying the offsets to the singles data, the system-wide timing resolution of the 256 channel MADPET-II prototype improved from 16.5 ns FWHM to 10.2 ns FWHM (450 keV threshold, F-18 flood source).