Positron emission tomography (PET)

Positron emission tomography (PET) is a scanning technique used in conjunction with small amounts of radiolabeled compounds to visualize brain anatomy andfunction. PET was the first scanning method to provide information on brainfunction as well as anatomy. This information includes data on blood flow, oxygen consumption, glucose metabolism, and concentrations of various moleculesin brain tissue.

PET has been used to study brain activity in various neurological diseases and disorders, including stroke, epilepsy; Alzheimer's disease, Parkinson's disease, and Huntington's disease; and in some psychiatric disorders, such as schizophrenia, depression, obsessive-compulsive disorder, attention-deficit hyperactivity disorder, and Tourette syndrome.

PET studies have helped to identify the brain mechanisms that operate in drugaddiction, and to shed light on the mechanisms by which individual drugs work. PET is also proving to be more accurate than other methods in the diagnosis of many types of cancer. In the treatment of cancer, PET can be used to determine more quickly than conventional tests whether a given therapy is working. PET scans also give accurate and detailed information on heart disease, particularly in women, in whom breast tissue can interfere with other types oftests.

A very small amount of a radiolabeled compound is inhaled by or injected intothe patient. The injected or inhaled compound accumulates in the tissue to be studied. As the radioactive atoms in the compound decay, they release smaller particles called positrons, which are positively charged. When a positroncollides with an electron (negatively charged), they are both annihilated, and two photons (light particles) are emitted. The photons move in opposite directions and are picked up by the detector ring of the PET scanner. A computeruses this information to generate three-dimensional, cross-sectional imagesthat represent the biological activity where the radiolabeled compound has accumulated.

A related technique is called single photon emission computed tomography scan(CT scan) (SPECT). SPECT is similar to PET, but the compounds used contain heavier, longer-lived radioactive atoms that emit high-energy photons, calledgamma rays, instead of positrons. SPECT is used for many of the same applications as PET, and is less expensive than PET, but the resulting picture is usually less sharp than a PET image and reveals less information about the brain.

Some of radioactive compounds used for PET or SPECT scanning can persist fora long time in the body. Even though only a small amount is injected each time, the long half-lives of these compounds can limit the number of times a patient can be scanned.

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