Characterization of scintillation pulses in time determination in TOF-PET

Scintillation pulses generated from positron emission tomography (PET) detectors contain the information of annihilation events including timing and energy. Conventionally, the peak of the pulse is used to pick the time information. However, little investigation has been reported about the beginning of a pulse which may keep the essential time information. The first optical photon of a scintillation pulse usually submerges in the noise and hard to detect. By characterizing the pulses, we set up a model composed of the leading edge, decay and noise to pick the first photon. The investigation of pulse model may provide an efficient way to improve the timing resolution for time-of-flight (TOF) PET. The initial model of scintillation pulse is obtained by averaging an amount of pulses. It gives a rough expression of pulse shape and shows an interesting detail of leading edge which is not a complete straight line but with curves in both ends. To validate the previous scintillation pulse model, we set up the optical transport simulation on Geant4 Application for Tomography Emission (GATE) which could complete the process from the interaction of gamma photon and scintillator to the optical transportation in crystal until being detected by PMT modules. From the simulation, we obtain a pulse library of optical photon pulses. Meanwhile, we get the mean pulse by averaging and the coincidence timing resolution of 58.3 ps FWHM which is determined by the first optical photon. In the further experiments, special detectors are introduced to ensure accordant scintillation pulses which deposit whole energy (511 keV) by photoemission. By averaging the uniformed pulse library, it will show an accurate mean-pulse and noise model. Based on this model, we will optimize the characterization for scintillation pulses and apply it to the time determination by leading edge fitting which is efficient in digital pulse processing for TOF PET.