PET scan (positron emission tomography) of the brain of a person suffering from Alzheimer's disease, a progressive form of dementia. This subject, aged 75 years, has had the disease for 4- 5 years. PET scans utilise radioactive chemicals, analogues of substances participating in the brain's metabolism, to give 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. Here, activity is confined to the top & bottom of the brain; dark blue/black areas represent virtually no activity. Compare with P332/124, same style of scan but of normal subject.

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