Fluorine-18 is a fluorine radioisotope which is an important source of positrons. It has a mass of 18.0009380(6) u and its half-life is 109.771(20) minutes.
Fluorine-18 is an important isotope in the radiopharmaceutical industry, and is primarily synthesized into fluorodeoxyglucose (FDG) for use in positron emission tomography (PET scans). It is substituted for hydroxyl and used as a tracer in the scan. Its significance is due to both its short half-life and the emission of positrons when decaying. In the radiopharmaceutical industry, it is made using either a cyclotron or linear particle accelerator to bombard a target, usually of pure or enriched oxygen-18-water with high energy protons (typically ~18 MeV). Fluorine-18 is often substituted for hydroxyl due to similar steric and electrostatic properties. This may however be problematic in certain applications due to possible changes in molecule polarity.