High Z and medium Z scintillators in ultra high resolution small animal PET

As small animal PET scanners are continuously improving in their performances, one is lead to the question of how far can spatial resolution go. In this paper we address the limiting effects to spatial resolution and whether the photoelectric interaction, and therefore high Z materials, outperform medium Z scintillators. In particular, with a Monte Carlo simulation, we compare the ultimate performances, in spatial resolution, of three scintillators: BGO, NaI(Tl) and YAP:Ce. BGO is the PET scintillator which has the highest photofraction whereas YAP:Ce has the lowest. NaI(Tl), instead is a relatively high Z but low density scintillator. There are three principle contributions to the degradation of spatial resolution: multiple Compton scattering electron range after a gamma interaction and K-shell fluorescence emission. We present the results of simulations of crystals with different thicknesses, with and without K-shell fluorescence emission and electron transport. We conclude that the effect of multiple scattering, electron range and fluorescence emission to the spatial resolution are smaller for low Z, high density materials like YAP:Ce. The fraction of misplaced events, defined here as F = N_Wrong/N_Tot, is F_0.5mm = 52% for BGO in the case of 0.5 mm binning, increasing to F_0.1mm = 80% for the 0.1 mm binning. In the case of YAP:Ce, the scatter fractions are respectively F_0.5mm = 27% and F_0.1mm = 44%. We conclude that for ultra high resolution PET detectors, medium Z scintillators, such as YAP:Ce, may outperform high Z materials.