Positron Emission Tomography (PET)

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Related to Positron Emission Tomography (PET): PET scan, positron emission tomography (PET) scan

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 and function.


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.


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.


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.



"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.


any method that produces images of single tissue planes. In conventional radiology, tomographic images (body-section radiographs) are produced by motion of the x-ray tube and film or by motion of the patient that blurs the image except in a single plane. In reconstruction tomography (CT and PET) the image is produced by a computer program. Called also laminagraphy, planigraphy, body-section technique.

computerized axial tomography
see computed tomography (below).
computed tomography (CT)
a revolutionary radiological imaging modality that uses computer processing to generate an image (CT scan) of the tissue density in a 'slice' about 0.5 inch thick through the patient's body. Called also computerized axial tomography (CAT) and computerized transaxial tomography (CTAT).
Because CT is noninvasive and has high contrast resolution, it has replaced some radiographic procedures using contrast media. However, in some areas the injection of contrast further enhances the image. CT also has a better spatial resolution than scintillation imaging (about 1 mm for CT compared with 15 mm for a scintillation camera).
positron emission tomography (PET)
a combination of computed tomography and scintillation scanning. Natural biochemical substances or drugs tagged with a positron-emitting radioisotope are administered to the subject. After injection, the tagged substance (tracer) is localized in specific tissues like its natural analog. When the isotope decays, it emits a positron, which then annihilates with an electron of a nearby atom, producing two 511 keV gamma rays traveling in opposite directions 180° apart. When the gamma rays trigger a ring of detectors around the subject, the line between the detectors on which the decay occurred is stored in the computer. A computer program (reconstruction algorithm), like those used in computed tomography, produces an image of the distribution of the tracer in the plane of the detector ring.
Most of the isotopes used in PET scanning have a half-life of only 2 to 10 minutes. Therefore, they must be produced by an on-site cyclotron and attached chemically to the tracer and used within minutes. Because of the expense of the scanner and cyclotron, PET is used only in research centers.
ultrasonic tomography
the ultrasonographic visualization of a cross-section of a predetermined plane of the body; see B-mode ultrasonography.
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