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Positron Emission Tomography (PET) 

Definition

Positron emission tomography (PET) is a scanning technique used in conjunction with small amounts of radiolabeled compounds to visualize brain anatomy and function.

Purpose

PET was the first scanning method to provide information on brain function as well as anatomy. This information includes data on blood flow, oxygen consumption, glucose metabolism, and concentrations of various molecules in 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 drug addiction, 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 of tests.

Description

A very small amount of a radiolabeled compound is inhaled by or injected into the 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 positron collides 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 computer uses this information to generate three-dimensional, cross-sectional images that represent the biological activity where the radiolabeled compound has accumulated.

A related technique is called single photon emission computed tomography (CT) scan (SPECT). SPECT is similar to PET, but the compounds used contain heavier, longer-lived radioactive atoms that emit high-energy photons, called gamma 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.

Risks

Some of radioactive compounds used for PET or SPECT scanning can persist for a 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.

Resources

Periodicals

"Studies Argue for Wider Use of PET for Cancer Patients." Cancer Weekly Plus December 15, 1997: 9.

Key terms

Electron — One of the small particles that make up an atom. An electron has the same mass and amount of charge as a positron, but the electron has a negative charge.

Gamma ray — A high-energy photon, emitted by radioactive substances.

Half-life — The time required for half of the atoms in a radioactive substance to disintegrate.

Photon — A light particle.

Positron — One of the small particles that make up an atom. A positron has the same mass and amount of charge as an electron, but the positron has a positive charge.

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PET positron emission tomography.

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PET

abbr.

positron emission tomography

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PET

positron emission tomography, a nuclear medicine technique that combines computed tomography and radioisotope brain scanning.

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pet

a nonfood animal included in a human household as a companion and on a status almost equivalent to that of a human being. They are used as instructional media for children about biological matters, as companions for lonely people of all sorts, as a guarding and watchdog presence, as psychological support for disturbed people and as a means of entrance into a different social group. Called also companion animals. Includes dogs, cats, cage birds, aquarium fish, and exotic species such as monkeys, alligators, tortoises, monkeys, big cats, axolotls, newts, yabbies, seahorses, snakes, gerbils, hamsters, mice, rats and spiders.

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pet-assisted therapy, pet facilitated therapy

the use of animals in a specific medical program as an adjunct to conventional therapy, especially for seriously ill and recuperating persons and persons with psychological problems. See also animal facilitated therapy.

children's p's

those pets which can be properly cared for by children; usually taken to include the common companion pets, dogs, cats and cage birds, and the unusual species such as lizards, tortoises, terrapins, hamsters, guinea pigs, mice, rats, gerbils, rabbits, and even snakes and spiders.

exotic p's

those other than the conventional dogs, cats, aquarium fish and cage birds.