Investigation of the block effect in LSO detectors

The block effect is the degradation in spatial resolution that exists when there are more scintillation crystals than photomultiplier tubes in a PET detector. In this study the block effect was measured for the Siemens-CTI Hi-Rez PET detector, a LSO block detector. The effect of changing gamma-ray interaction depth in the scintillation crystals in the Hi-Rez detector was also examined. These results are compared to results obtained with two BGO PET detectors; the Siemens-CTI HR+ and the GE Advance. The magnitude of the block effect in the central three crystal columns of the Hi-Rez detector was found to be negligible, while the magnitude of the block effect varied from 0.6 mm to 2.9 mm in the remaining crystal columns. Events that occurred at different crystal depths were decoded consistently in the central three crystal columns, and inconsistently in the remaining crystal columns. Previously, we found that the block effect was 0.9 mm in the GE Advance block detector, negligible for the edge crystals in the CTI HR+ block detector, and averaged 1.2 mm in the center crystals of the CTI HR+ block detector. In the GE Advance detector, events that occurred at different crystal depths were decoded consistently. In the CTI HR+ block detector, events that occurred deeper in the crystal had a greater probability of being positioned inaccurately. The results from the three detectors examined suggest that the inconsistent positioning of events that occur at different crystal depths contribute to the block effect, in both LSO and BGO block detectors.