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, adequate depth of interaction information must be extracted from the crystal. A phoswich detector can supply one solution to the depth of interaction problem. In this approach, two or more scintillators exhibiting different light decay times are positioned on top of each other and separated by pulse shape discrimination. Initially, our experiments focused on separating different types of scintillators such as LSO and GSO or LSO and YSO. These combinations were all well separated as expected. During the investigation, a shift in the time distribution of different samples of LSO was noticed. Further investigation showed two groups of LSO. The shift in the zero cross time was more than twice the FWHM of the time distribution. A single photon experiment revealed that the decay time of the 'fast' crystal was 33.4 nanoseconds while the decay of the 'slow' crystal was 42.2 nanoseconds. A spectral plot revealed that the spectral output of the 'slow' crystal was skewed to the longer wavelengths as compared to the 'fast' crystal. Further investigation on other crystal samples revealed decay times between the two extremes, suggesting a continuum in the light decay.