Positron Emission Tomography (PET) is a medical imaging modality measuring the in vivo processes of cells and molecules using radiotracers. Compared to other medical imaging techniques, PET still suffers from limited spatial resolution and signal-to-noise ratio. One key factor to resolution degradation is the distance the emitted positron travels before annihilating with an electron. This distance is called positron range. Radionuclides emitting high-energy positrons are subject to more spatial