Dr. Henry Wagner, director of nuclear medicine at John Hopkins Hospital, Baltimore, demonstrates Positron Emission Tomography (PET), to produce an axial image of his brain. A high-cost technique, PET scanning involves injecting a positron- emitting radioisotope (produced in a cyclotron) into the body: the detector is sensitive to photons produced when positrons emitted collide with electrons from the molecules in the brain to which the radioisotope has been dedicated.One example is glucose, where PET scans may detect schizophrenia & Alzheimer's disease, or Parkinsonism by mapping dopamine receptor sites.