PET scan (positron emission tomography) of the brain of a normal subject, aged 72 years. PET scans utilise radioactive chemicals, analogues of substances participating in the brain's metabolism, to provide an image that represents its level of function. Gamma rays emitted during the annihilation of positrons from the tracer are recorded & colour-coded according to intensity. PET is used to study regional blood flow & oxygen consumption in the normal effects of ageing. In addition, it may be applied to show forms of dementia (Alzheimer's disease), cerebral vascular disorders & Parkinson's disease. Compare with M108/136, PET of Alzheimer's disease, similar scan type.