Simulation and measurement of spatial resolution in detection of annihilation radiation with BGO crystals

Spatial resolution performance measurements of PET scanners are sensitive to the long tails of the angular distribution of the annihilation radiation, to a degree not previously appreciated. The non-Gaussian nature of the positron range, angular distribution of annihilation radiation, and scatter-induced spatial distribution of scintillator energy deposition prevents simple (e.g. quadrature-based arguments) methods of performance estimation, and requires full numerical convolution of the 3D spatial profiles. The authors have developed a Monte Carlo simulation of 511 keV gamma ray interactions with scintillator crystals to model the spatial distributions obtained in projection measurements of annihilation radiation from point sources. The authors find good agreement between the simulations and experimental results obtained with BGO-based crystals, both for optically-isolated crystals embedded in a crystal matrix, and for a conventional, Anger-based BGO block detector. There is approximately a 0.3 mm difference in the coincidence response function FWHM between 4 mm isolated and Anger crystals which is reproduced by the Monte Carlo calculations