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The effect of recoil electron range on efficiency and spatial resolution of very high resolution animal PET instrument using the Compton PET concept was investigated with Monte Carlo simulations using Electron-Gamma Shower code (EGS4). Planar silicon detectors (4.48 cm × 1.12 cm, and 0.3, 1.0, and 1.5 mm thickness) separated by 4 cm were examined for Compton scattering detectors. Intrinsic efficiencies of very high resolution events (single scatterings in two opposed planar detectors) are 0.004, 0.046, and 0.094%, respectively. The results show that the escape of recoil electron significantly reduces the efficiency of coincidence events in 0.3 mm thick detectors. Spatial resolutions from silicon ring detectors (41 cm diameter, 4.48 cm length, and 0.3, 1.0, and 1.5 mm thickness, segmented in 0.3 × 0.3 mm 2 pads) are 279, 295, and 298 μm FWHM, respectively, at the center of the field view. The result demonstrates that degradation of spatial resolution from low energy recoil electron was small.