PET imaging requires the use of radionuclides to label the substances necessary for biological metabolism and inject them into the human body to characterize the physiological and biochemical activity as tracer metabolic intensity.
Radionuclides decay beta to produce positrons. Positrons travel very short distances in the organism, then they meet with the negative electrons in the organism and annihilate, resulting in a pair of gamma photons with equal energy in opposite directions. By detecting the information of gamma photons, the time and location of annihilation events can be inferred.