What is the underlying principle of PET imaging?

A positron-emitting atom, typically a part of a biomolecule of interest, is injected intravenously. After equilibration, the unique decay properties of the positron undergoes annihilation with an electron, emitting apair of high-energy photons (511 keV) that exit the body and are absorbed (thereby detected) by crystals in a ring of detectors. Computer algorithms reconstruct the data set in multiple planes, localizing the radiopharmaceutical. Metabolic and molecular events are thus localized tomographically within the body.