Brain tumour. Coloured Positron Emission Tomography (PET) scan of an axial section through a human brain containing a tumour (blue, at right). The colour-coded scan shows brain activity from blue (lowest) through green and yellow to red (highest). The high activity areas show increased blood supply. The tumour, which is possibly caused by cancer, appears less active. This may be because of tissue damage or death at the tumour that has reduced the local blood supply. PET scans detect the emissions caused by the presence of a radioactive tracer injected into the patient's bloodstream.