On the efficiency of NaI/LSO and YSO/LSO phoswich block detectors

In order to obtain a high and uniform spatial resolution in PET, the depth-of-interaction information (DOI) has to be established. This information can be obtained by using a phoswich detector, such as GSO and LSO or other scintillator combinations. The authors are looking into efficiency questions concerning phoswich detectors. As a part of this investigation, NaI(Tl)/LSO and YSO/LSO phoswich block pairs have been investigated in both singles mode and in coincidence counting mode in order to measure the counting efficiency for 511 keV annihilation photons. The reason for having a soft scintillator in front is to be able to switch modality, combing both PET and SPECT in the same detector. When operated in PET mode, however, both scintillator layers are used. The results were compared to LSO only data, the LSO detector being the same as was used in the phoswich block. The two phoswich blocks were investigated both experimentally and theoretically. A satisfactory agreement was found between the experiments and theory, with the latter based on simulations with GEANT. The phoswich combination is, as expected, more effective than the LSO only detector due to intra-detector events. For a soft scintillator like NaI or YSO, however, the phoswich gain is small for 511 keV photons with an energy threshold above 350 keV in both scintillators.