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

As small animal positron emission tomography (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 the intrinsic detector spatial resolution and whether the photoelectric interaction, and therefore high Z materials, outperform dense medium Z scintillators. In particular, with the EGSnrc 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 that 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 the intrinsic detector 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 effects of multiple scattering, electron range, and fluorescence emission to the spatial resolution are smallest for medium Z, high-density materials like YAP:Ce. The fraction of misplaced events, which are defined here as F=N_Wrong/N_Tot, is F_0.5mm = 41 % 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 misplaced events are, respectively, F_0.5mm = 32% and F_0.1mm = 47%. We conclude that for ultra-high-resolution PET detectors, dense medium Z scintillators, such as YAP:Ce, may outperform high Z materials.