Normal patterns and variants in single-photon emission computed tomography and positron emission tomography brain imaging.

One of the most important issues in evaluating functional brain scans for research or clinical purposes is to be able to identify normal variants. Determining the baseline "normal" state of the brain is not easy to characterize since many normal brain functions and mental processes affect brain activity. This article reviews issues pertaining to the technical and neurophysiological aspects of functional brain imaging that might alter "normal" activity and will also consider how normal brain activity changes throughout the lifespan.