A Healthy Volunteer FDG-PET Study on Annihilation Radiation Non-collinearity

We evaluated the non-collinearity of annihilation radiation photons from the human body for the first time. This is a fundamental but little investigated matter in positron emission tomography (PET). In this paper, the physical cause of the small angular deviation from 180deg is described, and a new methodology is proposed to evaluate it under in vivo conditions of a spatially distributed radiation source and a limited acquisition time. This new method is a physical conversion from the photopeak spectrum shape into the angular distribution based on the conservation laws of momentum and energy. A healthy volunteer study using ¹⁸FDG and a Ge semiconductor detector reveals that the angular distribution is approximately a single Gaussian function with the FWHM of 0.54 plusmn 0.02 deg. This is about 10% larger than the value widely used in PET calculations. Finally, the theoretical limits of PET resolution assuming the ideal radiation detectors are estimated by the convolution of a line spread function of the angular deviation with that of the positron range for ¹⁸F in water equivalent tissues.