The gray outer surface is the surface of the brain from MRI and the inner colored structure is cingulate gyrus, part of the brain's emotional system visualized with PET.
Photo by Monte S. Buchsbaum, M.D.
Also developed in the 1970s, PET scans allow one to observe blood flow or metabolism in any part of the brain. In a PET scan, the subject is injected with a very small quantity of radioactive glucose. The PET then scans the absorption of the radioactivity from outside the scalp. Brain cells use glucose as fuel, and PET works on the theory that if brain cells are more active, they will consume more of the radioactive glucose, and if less active, they will consume less of it.
A computer uses the absorption data to show the levels of activity as a color-coded brain map, with one color (usually red) indicating more active brain areas, and another color (usually blue) indicating the less active areas.
PET imaging software allows researchers to look at cross-sectional "slices" of the brain, and therefore observe deep brain structures, which earlier techniques like EEGs could not. PET is one of the most popular scanning techniques in current neuroscience research.