Radioactive Tracers

Radioactive tracers in medicine (also called radio-pharmaceuticals) use the fact that specific tissues accumulate specific substances. Labelling one of these leads to information on the specific tissue. For example, the thyroid gland removes iodine from the blood. When iodine-123 is injected into the blood, it collects in the thyroid like any isotope of iodine. However, it emits gamma radiation that reveals if the gland is working at the normal rate. Many types of compounds can be radiolabelled, including salts, small organic compounds, and proteins, antibodies, or red blood cells.

Think about how aircraft have bright flashing lights on their undersides. These do not effect the aircraft's ability to fly, but make it visible in the dark night sky. The radionuclide is like a flashing light on a compound. Although we cannot see its radioactive beam with our eyes, a suitable instrument will detect it clearly against a dark nonradioactive background.