The effects of detector material and structure on PET spatial resolution and efficiency

Many annihilation photons undergo one or more Compton interactions in PET (positron emission tomography) scintillation crystals before being absorbed photoelectrically or escaping from the crystal. Only the site of the first interaction is on the original trajectory and expresses the exact localization of the annihilation. In larger crystals or block detectors the light from all scintillations appears to originate from the centroid of all interactions. The number of interactions and tangential component of the mean square distance between the centroid and the point of first interaction have been simulated using Monte Carlo techniques. This blurring component is 0.73 mm for 511-keV gamma rays in BGO and 1.4 mm for NaI crystals which are 30 mm deep and 50 mm wide. This error is small for BGO crystals compared with the quantization implied by allocating each detection to one 5-6-mm-wide crystal of an encoded block.<>