Healthy and cancerous brain. Coloured positron emission tomography (PET) scan of an axial section through two human brains, one healthy (at left) and one affected by a tumour (at right). These colour-coded scans show brain activity from blue (lowest blood supply) through green and yellow to red (highest blood supply). At left the cerebral hemispheres of the healthy brain show a regular yellow band. At right, the tumour appears blue within the yellow band. It shows low blood supply because of tissue damage or tissue death at the tumour. PET scans detect emissions caused by a radioactive tracer injected into the bloodstream.