Determination of the centroid of interaction of crystals in block detectors for PET

Cut blocks of bismuth germanate (BGO) are the most common detectors used in current positron emission tomography (PET) imaging systems. The distribution of light received by four photo-multiplier (PMT) cathodes is used to assign each gamma ray to one crystal. Most 511-keV gamma rays undergo Compton scattering in the crystal before being photoelectrically absorbed. This, and the effects of dense metal septa between blocks change the response of the crystals depending on their position in the block and the angle of incidence of gamma rays on the block. For each projection, the authors used Monte Carlo simulation to determine the “centroid of interaction” for each crystal in the block. They found the crystals appear to be packed slightly closer in each block than their geometrical distance. They found the location of the crystal´s centroid relative to its center changes along each projection. The authors simulated the 4×4 crystal arrays of the Scanditronix PC2048-B, determined the centroid of interaction for each crystal, then modified the reconstruction program´s interpolation parameters. The spatial resolution improved by up to 10%, and a distortion of 1.5 mm at the edge of the imaging field was eliminated