Positron Emission Tomography (often called PET) is a medical imaging technique. A specially constructed device can show the path and distribution of a weakly radioactive substance. This substance is called tracer. The tracer dissolves in the blood, which will carry it the brain. There, it concentrates around brain structures that are particularly active. A PET scan detects the radioactivity and converts it into a digital image of the brain, highlighting the active areas. PET scans are often used to detect tumors. Different tracers are used for different kinds of scans. Very often, the tracers are similar to sugar, or are sugars which have been specially modified for the purpose.