18O is a natural, stable isotope of oxygen, and it is supplied in the form of O-18 water (H218O). 18O is an important precursor for the production of fluorodeoxyglucose (FDG) used in positron emission tomography (PET). PET is a nuclear medicine imaging technique which produces a three-dimensional image or map of functional processes in the body. The system detects pairs of gamma rays emitted indirectly by a positron-emitting radionuclide (tracer), which is introduced into the body on a biologically active molecule. Images of tracer concentration in three-dimensional space within the body are then reconstructed by computer analysis. In modern scanners, this reconstruction is often accomplished with the aid of a CT X-ray scan performed on the patient during the same session, in the same machine.
Generally, in the radiopharmaceutical industry, enriched water (H218O) is bombarded with hydrogen ions in either a cyclotron or a linear accelerator, creating fluorine-18.This is then synthesized into FDG and injected into a patient.