Timing measurements from a TOF-PET scanner using local PMT triggering

The development of high light output scintillation crystals with fast decay times has made clinical whole body TOF-PET scanners possible. Current clinical systems based on LYSO have timing resolutions near 600 ps (FWHM), but considerably better resolution is possible with faster scintillators. Bench top measurements have indicated that the LaBr3 modules used in a proto-type TOF PET scanner should allow for a resolution of 315-330 ps. To achieve the best system timing resolution a new "local" triggering scheme has been developed, in which the seven PMTs used for energy and positioning determination are also used to produce a trigger. The electronics use fast components, and have been found to introduce a timing jitter of less than 50 ps. The local triggering design reduces the effects of pulse pileup and dark current noise, greatly reducing the degradation of the timing resolution at higher rates. With these new electronics we have achieved a system time resolution of 420 ps with little degradation at the rates expected for clinical FDG studies. The combination of the continuous design of our scanner and the local triggering scheme makes this design concept scalable to any configuration of PMTs in a full PET scanner.