Positron emission tomography (PET) scanning.

Positron emission tomography (PET) scanning permits the noninvasive determination of biological function, metabolism, and pathology following the administration of short-lived positron-emitting radiopharmaceuticals (radioactive pharmaceuticals). Positron-emitting radiopharmaceuticals are drugs that contain a radioactive atom that is transformed into a more stable atom by emitting a positron. A positron is a subatomic particle that has the same mass and charge as an electron, but the charge is positive rather than negative. When an energetic positron is emitted by a radioactive atom, it quickly loses its energy in the surrounding medium and comes to rest. The positron then combines with a free electron in the medium, the two particles are annihilated, and their energy is converted into two 511,000-volt-potential (511 keV) photons that travel in exactly opposite directions. The detection of these two photons in coincidence (simultaneously) by placing a ring of small radiation detectors around the patient is the basic principle used in PET scanning. The radiation detectors convert these light photons into electrical signals that are fed to a computer, which reconstructs the distribution of radioactivity in the desired organ and presents the information as an image on a video screen.