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 lig ht output of LSO (similar to 5-6 times BGO) would allow the constructi on of a detector block that would have similar intrinsic resolution ch aracteristics at 140 keV as a conventional high resolution BGO block d etector at 511 keV (similar to 4 mm FWHM). However, the intrinsic radi oactivity of LSO prevents the use of this scintillator in single photo n counting mode. YSO is a scintillator with higher light output than L SO but worse absorption characteristics than LSO at higher energies. I n a phoswich detector block the two detector materials are combined, w here YSO is placed in a front layer and is used for single photon imag ing and LSO in a second layer is used for PET imaging. Since the scint illation decay times in YSO and LSO are significantly different (70 an d 40 ns, respectively), events in the two detector materials can be se parated by pulse shape discrimination, In measurements of the system r esolution using a high resolution collimator, the spatial resolution a t 10 cm collimator distance was 8.5 mm for the phoswich detector compa red to 7.6 mm for a conventional scintillation camera. However, in SPE CT simulations of the two detector systems of a 20 cm diameter hot spo t phantom imaged at different collimator distances using a high resolu tion collimator, no appreciable difference in resolution was seen in t he reconstructed images between the two camera systems, including the ideal situation of zero distance between collimator and phantom.