INVESTIGATION OF LSO CRYSTALS FOR HIGH-SPATIAL-RESOLUTION POSITRON EMISSION TOMOGRAPHY

In order to achieve high sensitivity and maintain good uniform spatial resolution over the field of view in high resolution PET systems, ade quate depth of interaction information must be extracted from the crys tal. A phoswich detector can supply one solution to the depth of inter action problem. In this approach,two or more scintillators exhibiting different light decay times are positioned on top of each other and se parated by pulse shape discrimination. Initially, our experiments focu sed on separating different types of scintillators such as LSO and GSO or LSO and YSO. These combinations were all well separated as expecte d. During the investigation, a shift in the time distribution of diffe rent samples of LSO was noticed. Further investigation showed two grou ps of LSO. The shift in the zero cross time was more than twice the FW HM of the time distribution. A single photon experiment revealed that the decay time of the 'fast' crystal was 33.4 nanoseconds while the de cay of the 'slow' crystal was 42.2 nanoseconds. A spectral plot reveal ed that the spectral output of the 'slow' crystal was skewed to the lo nger wavelengths as compared to the 'fast' crystal. Further investigat ion on other crystal samples revealed decay times between the two extr emes, suggesting a continuum in the light decay.