Positron Emission Tomography (PET) maps metabolic function in oncology and cardiology using beta-plus radiotracers. An emitted positron travels a short distance in tissue before encountering an orbital electron. The pair undergoes matter-antimatter annihilation, converting rest mass into two opposed 511 keV gamma ray photons emitted at 180°. Coincident detection of these pairs defines lines of response for tomographic reconstruction.