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Positron emission tomography—or PET for short—is an imaging tool increasingly used in medicine to diagnose and guide the treatment of a wide range of diseases and disorders, especially cancer.
PET is an example of nuclear medicine, which means it depends on the use of radioactive isotopes made in cyclotrons or nuclear reactors. A radioactive isotope is a chemical element with an unstable nucleus that, over time, decays into more stable isotopes, emitting subatomic particles in the process.
When a radioactive atom such as fluorine-18 or carbon-11 decays, a proton in the atom's nucleus turns into a neutron and emits a positron. The positron is a subatomic particle, identical to the electron except that it has the opposite electrical charge. When the positron collides with an electron in another atom, both particles are annihilated; that is, they are turned into energy, which radiates away as highly energetic photons or light, usually gamma rays. A PET machine can detect the photons emitted and calculate the approximate location of the positron before it was annihilated.
Medicine can exploit this phenomenon by using radioactive tracers (or radiotracers for short). These molecules, which contain radioisotopes, concentrate in certain cells, for example, cancer cells. Such cells are usually more active than normal cells, so they 'take up' more of the tracer. A widely-used radiotracer is fluorodeoxyglucose, a molecule similar to glucose, and hence it is used to trace cells that consume large amounts of glucose, such as brain and kidney cells and cells that have become cancerous. The fluorodeoxyglucose molecule contains a radioactive fluorine atom that decays (usually within several hours) into an oxygen atom, emitting a pair of positrons.
The radiotracer is injected into a patient's blood and its progress in the body is traced using a PET detector. When the radioactive atoms in the tracer decay, they emit positrons that can be detected electrically, making it possible to determine the position of the tracer molecule. Combining the results of millions of these detections, it is possible to build a detailed, three-dimensional map or image of the tissue or organ where the tracer has become concentrated in the body. The technique of combining all the detection data into a map or image is known as tomography.
PET is a powerful tool able to produce high-resolution images that help doctors diagnose disease early and aid their decisions about how best to treat the disease. It is often the best way to clearly identify the location and extent of an abnormality such as a tumour. This is especially so with tumours that are hard to see using techniques such as X-rays, although in practice a PET scan is often combined with an X-ray computed tomography (CT) scan.
PET can also reveal abnormalities in the body's functions over a period of time, which makes it possible to investigate disorders such as epilepsy and heart disease.
Clinicians also use an imaging tool known as single photon emission computed tomography, or SPECT for short . SPECT is a similar technique to PET but generally it is considered to be less powerful than PET. SPECT scanners usually use radiotracers with longer half lives than those used with PET (e.g. technetium-99), and can therefore investigate longer-lasting functions in the body than those typically examined with a PET scanner.
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