CATHERINE POUEDRAS / SCIENCE PHOTO LIBRARY CATHERINE POUEDRAS / SCIENCE PHOTO LIBRARY
Positron emission tomography (PET) scan of the brain of a patient with Alzheimer's disease (senile dementia). The colour-coded scan shows metabolic activity throughout this axial section, from low (blue) to high (red). PET scanning utilises an injected radioactive tracer (in this case an analogue of glucose) to reveal variations in metabolic activity in the brain. A normal scan would reveal a more symmetrical pattern of high activity across the cerebral hemispheres (the outer left & right areas of the image). Here, the blue-green area at bottom left reveals a particularly low level of metabolic activity. Alzheimer's is the most common form of dementia.
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