Depth of interaction for PET using segmented crystals

The authors investigate the use of segmented scintillation crystals to obtain depth of interaction (DOI) information in positron emission tomography (PET) detectors. Coupling segmented crystals to a photodetector at one end yields distinct pulse heights from the different segments and the depth of interaction is determined to within the segment length. The method is simple to implement because no additional instrumentation is required. Since the technique uses light attenuation, a scintillator with sufficient light yield is required. The authors found that 4×4×10 mm³ segments of lutetium oxyorthosilicate (LSO) provide enough light for this application. The energy spectra of segmented crystals show clearly distinct photopeaks for each segment and the photopeak separation can be adjusted by altering the optical couple between segments. An ambiguity exists between photopeak events from an attenuated crystal and scattered events from the crystal without attenuation. This leads to the same error as a system with no depth information. Using 10 mm segments the authors find that the fraction of the events in the lower energy window due to scatter contamination in 2-segment (3-segment) crystals increases from 8% (13%) to 32% (60%) for a source in air vs. behind 16 cm of water. In imaging situations with minimal scatter medium (e.g. rodents) DOI is accurately provided by the segmented crystals, while in the presence of scatter (e.g. WB, cardiac) the method is only partially effective, and improvements in sensitivity, image quality and reduced scanner cost must be carefully evaluated