Imaging technique which measures the cerebral concentration of positron-emitting isotopes which are incorporated into biologically active compounds. The concentration of the tracers is determined by detectors positioned outside the head, and tomographic images are constructed by a computer in a way similar to CT or MRI. PET scanning can be used to measure cerebral blood flow, oxygen uptake, or glucose utilization. The technology is available at a few large medical centers where it is used in both research and specialized clinical applications; however it is not utilized routinely in diagnosis.