Memory Loss & the Brain

Like computed tomography (CT) and magnetic resonance imaging (MRI), PET is a method for providing images of the brain via sophisticated computer analysis. However, PET images detail brain function rather than structure (see also fast MRI). For this reason, PET images are often shown superimposed over a traditional MRI or CT image, allowing precise localization of the brain structures which are active.

PET involves radioactive variants of biologically-important molecules such as oxygen, glucose, hormones and neurotransmitters. These molecules are injected into the bloodstream and carried throughout the body. The level of radiation involved is about equivalent to two chest X-rays, and is not believed to cause any harm. The radioactive molecules emit radiation that can be detected by a PET scanner to allow study of their distribution and uptake in brain tissue. This information is relayed to a computer which reconstructs an image displaying colored patches where accumulated radiation is highest (usually colored red) and lowest (usually colored blue). Such an image is called a PET scan.

It is assumed that brain areas which are especially active during a given task will use more oxygen and glucose; thus, by watching where radioactively-tagged oxygen or glucose accumulate on the PET scan, it is possible to "see" which brain regions are active. Thus, if the frontal lobes appear in bright red on a PET scan, this implies that frontal lobes are especially active during the current task.

PET is also useful in the study of brain abnormalities, by comparing the activity patterns in healthy individuals against the patterns obtained in patients. For example, the focus seizure in an epileptic patient is often associated with decreased activity, which can be observed through PET. Various psychiatric disorders such as schizophrenia and manic-depression can result in altered PET activity, as can transient ischemic attacks, Parkinson's disease, multiple sclerosis, epilepsy, cerebrovascular disorder and Alzheimer's disease.

Further reading: Illustrated Guide to Diagnostic Tests, 2nd edition. Springhouse Corporation, Springhouse PA, 1998.