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 left & right areas are asymmetrical; metabolic activity is patchy & at a lower level on the right side.
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