Pain sensation. Coloured positron emission tomography (PET) scan of the brain responding to pain (red). The left hemisphere has been cut-away and the front of the brain is at left. The PET scan is superimposed onto a blue three- dimensional magnetic resonance imaging (MRI) scan. The central white matter is coloured yellow/green, while the outer grey matter is blue. The PET scan shows cerebral blood flow in an area associated with pain sensation. There are many areas of the brain associated with pain perception. PET scans view radioactive substances injected into the blood.