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Positron emission tomography

Positron emission tomography (PET) is a nuclear medicine medical imaging technique which produces a three dimensional image or map of functional processes in the body.

A short-lived radioactive tracer isotope which decays by emitting a positron, chemically combined with a metabolically active molecule, is injected into the living subject (usually into blood circulation). There is a waiting period while the metabolically active molecule (usually a sugar) becomes concentrated in tissues of interest, then the subject is placed in the imaging scanner. The short-lived isotope decays, emitting a positron. After travelling up to a few millimeters the positron annihilates with an electron, producing a pair of gamma ray photons moving in opposite directions. These are detected when they reach a scintillator material in the scanning device, creating a burst of light which is detected by photomultiplier tubes. The technique depends on simultaneous or coincident detection of the pair of photons: photons which do not arrive in pairs (i.e., within a few nanoseconds) are ignored. By measuring where the gamma rays end up, their origin in the body can be plotted, allowing the chemical uptake or activity of certain parts of the body to be determined. The scanner uses the pair-detection events to map the density of the isotope in the body, in the form of slice images separated by about 5mm. The resulting map shows the tissues in which the molecular probe has become concentrated, and is read by a nuclear medicine physician or radiologist, to interpret the result in terms of the patient's diagnosis and treatment. PET scans are increasingly read alongside CT scans, the combination giving both anatomic and metabolic information (what the structure is, and what it is doing). PET is used heavily in clinical oncology (medical imaging of tumours and the search for metastases) and in human brain and heart research.

Alternative methods of scanning include computed tomography (CT), magnetic resonance imaging (MRI) and functional magnetic resonance imaging (fMRI) and single photon emission computed tomography (SPECT).

However, while other imaging scans such as CT and MRI, isolate organic anatomic changes in the body, PET scanners are capable of detecting areas of molecular biology detail (even prior to anatomic change) via the use of radiolabelled molecular probes that have different rates of uptake depending on the type of tissue involved. The changing of regional blood flow in various anatomic structures (as a measure of the injected positron emitter) can be visualized and relatively quantified with a PET scan.

In order to properly interpret a PET scan result, the resolution of the scanner must be known. Typically, determining the resolution of a PET scanner is done with tiny wires that have been irradiated in a nuclear reactor or a particle accelerator. Another process involves using tiny beads of zeolite that have been dipped into a saline solution containing technetium-99m. (Bailey, et. al., 2004)

Radionuclides used in PET scanning are typically isotopes with short half lives such as carbon-11, nitrogen-13, oxygen-15, and fluorine-18 (half-lives of 20 min, 10 min, 2 min, and 110 min respectively). Due to their short half lives, the isotopes must be produced in a cyclotron at or near the site of the PET scanner. Currently, 18-F is the only isotope approved by the FDA for distribution in the US. Rubidium-82 is allowed limited use for myocardial perfusion experiments. These isotopes are incorporated into compounds normally used by the body such as glucose, water or ammonia and then injected into the body to trace where they become distributed.

PET as a technique for scientific investigation is limited by the need for clearance by ethics committees to inject radioactive material into participants, and also by the fact that it is not advisable to subject any one participant to too many scans. Furthermore, due to the high costs of cyclotrons needed to produce the short-lived radioisotopes for PET scanning (for example 18-F), few hospitals and universities are capable of performing PET scans.

However, with the recent decision of Medicare to cover PET scans for specific patients, there has been a recent trend of increase in clinical use of PET scans throughout the United States.