NATIONAL CANCER INSTITUTE / SCIENCE PHOTO LIBRARY NATIONAL CANCER INSTITUTE / SCIENCE PHOTO LIBRARY
Brain tumour. Coloured Positron Emission Tomography (PET) scan of a human brain containing a tumour (yellow, at lower left). The colour-coded scan shows brain activity from blue (lowest) through green and yellow to red (highest). The tumour, which might be caused by cancer, appears bright because the cells within it are more active. PET scans detect the emissions caused by the presence of a radioactive tracer which has been injected into the bloodstream. More blood is delivered to areas of the brain that are active.
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