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Use of Fluorine 18 in Nuclear Medicine
Fluorine 18 is a positron emitter with a half life of 109.77 minutes. It is used in PET scans.
Decay mode: 9F18 ? 8O18 + +1e0 (1.656 MeV). Oxygen 18 is stable. The positron travels only a
few millimetres in the human body before it meets an electron, when both are annihilated
producing two gamma ray photons. The energy of each gamma ray photon is 511 MeV, the rest
mass of the electron, so presumably the positron looses some energy prior to annihilation. The
typical exposure from a PET scan is about 7 mSv.
Production: Irradiation of the stable oxygen 18 isotope with high energy protons (18 MeV) from
a cyclotron. 8O18 + +1p1 ? 9F18 + 0n1. The target is either normal or enriched water.
Half life: 109.77 minutes, which means that the radioisotope must be transported post haste.
Ideally a nuclear medicine facility at a hospital would have its own cyclotron.
Fluorine is used as a radioactive tracer in PET Scans mostly as Fluorodeoxyglucose (more
precisely 2-Deoxy-2-fluoro-D-glucose or FDG for short), which is a glucose analogue and is
taken up by body cells that are high users of glucose such as brain and cancer cells. The presence
of Fluorine in the molecule inhibits the body metabolising it as a normal glucose molecule. Once
the Fluorine has decayed to Oxygen 18 the normal metabolic process proceeds. A typical dose is
5 - 10 millicuries or 200 to 400 MBq administered by saline drip. The patient is scanned an hour
later during which the patient should avoid activity so that the FDG does not go to active
Flumazenil (C15H14FN3O3 ) inhibits the action of benzodiazepines such as Librium and Valium,
which have a slowing down effect on the central nervous system, they are used in the treatment of
insomnia, seizures and alcohol withdrawal. Flumazenil is mostly used with patients who become
excessively drowsy with benzodiazepines, and sometimes in the treatment of overdoses.
When one of the carbon atoms is replaced with a Carbon 11 atom, flumazenil becomes a
radiopharmaceutical as Carbon 11 is a positron emitter. The PET scan reveals the distribution of
the receptor sites for both the benzodiazepines and for the flumazenil. However the half life of
Carbon 11 is 20.38 min with a positron with energy of 0.45 MeV
The Cooperative Research Centre for Biomedical Imaging Development (CRCBID) based at
ANSTO has produced flumazenil with a Fluorine 18 atom (18FMZ). The longer half life of
Fluorine 18 will enhance this diagnostic tool. The press release and the article in The Age
discuss the benefits to epilepsy sufferers.
http://www.ansto.gov.au/ The Education Resources section of this website has physics notes,
worksheets and booklets.