Radioisotope brain scans, composite image. At left are patients being scanned in radiology departments. At top centre are brain scans produced using radioactive tracers. A symbolic representation of an atom is just left of centre, with a table of commonly used radioactive tracers behind it. At right is a human upper body and skeleton. Radioactive tracers are injected into the blood to reveal areas of disease or high metabolic activity in the brain. After injection the body is scanned by a special camera that can detect the tracers.