By using a source of positrons, for instance from the isomeric isotope tecnetium-99m (which emits positrons of an energy sufficient to penetrate tissue but not energetic enough to cause too much damage) surgeons can scan human body tissues for anomalies. The isotope, shielded in a can except for a small opening, produces a narrow beam of positrons, which are injected into the patient. When a positron encounters an electron within an atom inside the patient, the two annihilate each other producing two energetic gamma ray photon of approx 500keV in energy that emerge from the body and are detected by the scanner surrounding the patient. Differing tissue types absorb positrons with differing efficiencies, by which they can be differentiated. See Transmutation.

By scanning the positron beam around the body and observing gamma ray photons emitted in opposite directions, computers can build up a three dimensional picture of the whole body, organs and all, using a mathematical process called tomography.