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

The performance of a YSO/LSO phoswich detector module is evaluated for potential use as in a combined PET/SPECT imaging system. The high light output of LSO (/spl sim/5-6 times EGO) would allow the construction of a detector block that would have similar intrinsic resolution characteristics at 140 keV as a conventional high resolution EGO block detector at 511 keV (/spl sim/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 at higher energies. In a phoswich detector block the two detector materials are combined, where YSO is placed in a front layer and is used for single photon imaging and LSO in a second layer is used for PET imaging. Since the scintillation decay times in YSO and LSO are significantly different (70 and 40 ns, respectively), events in the two detector materials can be separated by pulse shape discrimination. In measurements of the system resolution using a high resolution collimator, the spatial resolution at 10 cm collimator distance was 8.5 mm for the phoswich detector compared to 7.6 mm for a conventional scintillation camera. However, in SPECT simulations of the two detector systems of a 20 cm diameter hot spot phantom imaged at different collimator distances using a high resolution collimator, 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.