A. Saoudi et al., Investigation of depth-of-interaction by pulse shape discrimination in multicrystal detectors read out by avalanche photodiodes, IEEE NUCL S, 46(3), 1999, pp. 462-467
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.