Spatial resolution in PET and the effect of gamma-ray interaction depth in block detectors

The spatial resolution in PET is poorer than that of CT or MRI. All modern PET scanners use block detectors, i.e. clusters of scintillation crystals coupled to four photomultiplier tubes (PMTs). Some of the loss of spatial resolution in PET is attributed to the use of block detectors, because a photon that interacts with one crystal in the cluster may be incorrectly positioned, resulting in blurring of the reconstructed image, called the “block effect”. We examined the effect of changing gamma‐ray interaction depth in the scintillation crystals in detectors from the CTI HR+ and GE Advance PET scanners. We postulated that the depth at which the gamma‐ray interacts may contribute to the “block effect” blurring. The “block effect” was measured for both detectors, and it was found to be 1.2 mm for the central crystals and negligible for the edge crystals in the CTI HR+ block. However, it was 0.9 mm in all crystals of the GE Advance detector. In the CTI HR+ detector, a depth dependence on the positioning of the event was observed, as was a dependence on the crystal location (edge vs. centre). In the GE Advance detector, no such dependence was observed. These results suggest that the depth of interaction of an annihilation photon may contribute to the block effect in detectors that use crystals cut from a single scintillation crystal (pseudo‐discrete crystals). In detectors that use discrete crystals no additional blurring as a function of gamma‐ray interaction depth in the detectors was observed. © 2005, American Association of Physicists in Medicine. All rights reserved.