Investigation of depth-of-interaction by pulse shape discrimination in multicrystal detectors read out by avalanche photodiodes

The measurement of depth of interaction (DOI) within detectors is necessary to improve resolution uniformity across the FOV of small diameter PET scan ners. DOI encoding by pulse shape discrimination (PSD) has definite advanta ges as it requires only one readout per pixel and it allows DOI measurement of photoelectric and Compton events. The PSD time characteristics of vario us scintillators were studied with avalanche photodiodes (APD) and the iden tification capability was tested in multi-crystal assemblies with up to fou r scintillators. In the PSD time spectrum of an APD-GSO/LSO/BGO/CsI(TI) ass embly, four distinct time peaks at 45, 26, 88 and 150 ns relative to a fast test pulse, having resolution of 10.6, 5.2, 20 and 27 ns, can be easily se parated. Whereas the number and position of scintillators in the multi-crys tal assemblies affect detector performance, the ability to identify crystal s is not compromised. Compton events have a significant effect on PSD accur acy, suggesting that photopeak energy gating should be used for better crys tal identification. However, more sophisticated PSD techniques using parame tric time-energy histograms can also improve crystal identification in case s where PSD time or energy discrimination alone is inadequate. These result s confirm the feasibility of PSD DOI encoding with APD-based detectors for PET.