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