Advanced Time-of-Flight (ToF) PET Photon Detectors

For the last three decades, a significant amount research has focused on improving the scintillation crystal of the detector to go beyond simple coincidence detection capability to time-of-flight (ToF) capability. A scintillation crystal creates a tiny cascade of thousands of visible light photons in response to absorbing a 511 keV annihilation photon interaction. The process is characterized by the crystal’s light output (number of light photons created per keV of absorbed energy) and decay constant t (ns).In a PET detector, the scintillation crystal’s properties determine how precisely one can say whether two photons are detected simultaneously. The resulting uncertainty in the arrival time difference for the two photons over many events is known as the coincidence time resolution. The scintillation crystal properties are the dominant factor that determines the time resolution: the brighter and faster the light pulse, the better the results. In ToF-PET, in addition to being able to determine to which LOR the event belongs, the detector coincidence time resolution is so good that by measuring the arrival time difference between the two photons one can constrain the two-photon emission point to a particular segment along that line. The coincidence time resolution directly maps into ToF position uncertainty along the LOR, Δx, by the formula Δx= c* Δt/2, where c is the speed of light, and Δt is the full-width-at-half-maximum (FWHM) time resolution of two detectors in coincidence. We are working on scintillation crystal, photodetector, and electronics features to enable <300 ps coincidence time resolution to advance ToF performance.