Healthy brain. Coloured Single Photon Emission Computed Tomography (SPECT) scan of the brain of a normal human subject. This is a horizontal section through the brain. Colour-coding is: high brain activity (red, yellow); low activity (grey, blue). Active/inactive regions are roughly symmetrical in both left and right cerebral hemispheres in this healthy patient. In SPECT scanning a radioactive tracer is injected into the bloodstream to reveal metabolic activity in the brain. SPECT scans can also show brain areas affected by disease or brain dysfunction. (See photo's: M108/340 for Alzheimer's disease; M210/102 for migraine).