Investigation of the block effect in positron emission tomography

Spatial resolution in PET depends on a number of factors. Among these is the block effect, which has been introduced to account for the degradation in spatial resolution observed in imaging systems which use block detectors and which is not observed in systems with one-to-one coupling of scintillation crystals to photomultiplier tubes. The origin of the block effect is not well understood. We present a comparison of the block effect for two commercially available block detectors: CTI HR+ and GE Advance. We find that the block effect contributes 1.2 mm and 1.3 mm, added in quadrature to the spatial resolution, for the CTI HR+ and GE Advance detectors respectively. Both of these values are considerably lower than the value of 2 mm cited in the literature. We also perform a study of the effect of multiple interactions of gamma rays within the crystal array, which is one factor which is thought to contribute to the block effect. We find only a marginal change in the spatial resolution when multiple photon interactions are minimized. Finally we study the effect of contributions to the summed image from events at different crystal depths and show that the event location depends on the depth of interaction.