A computer-enhanced x-ray technique in which patients are injected with a drug which emits a small amount of radiation. The chemical settles in tissues in direct proportion to the amount of blood flow in the area. Scanning which is coupled with a computer produces images which are useful in diagnosing various conditions. It has been most used to date in the brain, where it is helpful in detecting Alzheimer’s disease and certain psychiatric disorders. The technique is invasive, in that an injection is required. The cost of an examination is said to be less than half that of a computed tomography (CT) scan or magnetic resonance imaging (MRI).