Positron emission tomography (PET) scan of the brain (basal ganglia level) of a normal human subject. The colour-coded scan through this cerebral layer shows brain activity: from low (blue) to high (yellow). PET scanning relies on a radioactive tracer, injected into the bloodstream, to reveal metabolic activity in the brain. As seen here, normal brain metabolic activity produces a roughly symmetrical pattern in the yellow areas of left and right cerebral hemispheres. In many brain dysfunctions (such as Alzheimer's disease, epi- lepsy and schizophrenia), PET scans are useful in pinpointing specific brain areas which are meta- bolically affected by disease.