Performance of a YSO/LSO detector block for use in a PET/SPECT system

In recent years, there has been an increased interest in using conventional SPECT scintillation cameras for PET imaging, however, the count rate performance is a limiting factor. The modular block detectors used in modern PET systems do not have this limitation. In this work, the performance of a detector block design which would have high resolution and high count rate capabilities in both detection modes is studied. The high light output of LSO (~5-6 times BGO) would allow the construction of a detector block that would have similar intrinsic resolution characteristics at 140 keV as a conventional high resolution BGO block detector at 511 keV (~4 mm FWHM). However, the intrinsic radioactivity of LSO prevents the use of this scintillator in single photon counting mode. YSO is a scintillator with higher light output than LSO but worse absorption characteristics than LSO. YSO and LSO could be combined in a phoswich detector block, where YSO is placed in a front layer and is used for low energy (SPECT) imaging and LSO in a second layer is used for PET imaging. Events in the two detector materials can be separated by pulse shape discrimination, since the decay times of the light in YSO and LSO are different (70 and 40 ns, respectively). Although the intrinsic resolution of the block detector with discrete elements is worse than for a NaI camera, this would not be a limiting factor. Simulations of a 20 cm diameter hot spot phantom imaged at different collimator distances using a high resolution collimator and scintillation camera system was compared to a block detector camera. No appreciable difference in resolution was seen in the reconstructed images between the two camera systems, including the ideal situation of zero distance between collimator and phantom