A central positron source to perform the timing alignment of detectors in a PET scanner

Accurate timing alignment and stability is important to maximize the true counts and minimize the random counts in positron emission tomography. It importance increases in time-of-flight scanners. We propose using a central positron emitting source enclosed in a detector which detects the excess energy of the positron before it annihilates as a timing reference. All crystals can be time-aligned with respect to this central source. We evaluated 10 μCi ²²Na and ⁶⁸Ge sources embedded in cylinders of plastic scintillator coupled to a fast PMT. Light flashes produced after the parent isotope emits positrons are detected, and the anode signals from the PMT are the reference time for each positron decay. The time delay before the gamma ray is detected by the scanner´s conventional gamma ray detectors is the time offset to be applied to that crystal. Since all detectors are almost the same distance from the central source, time-of-flight errors are minimized. Preliminary results show a mean signal amplitude of >0.5 V from ²²Na at 1000 V PMT bias, a timing FWHM of 850 psec with respect to a small LSO crystal. This suggests it could be useful to align both conventional and time-of-flight PET scanners.