A bone scan is a test that identifies new areas of bone growth or breakdown. It can be done to evaluate damage to the bones, detect cancer that has spread (metastasized) to the bones, and monitor conditions that can affect the bones (including infection and trauma).
During a bone scan, a small amount of radioactive substance (radionuclide) is injected into the bloodstream. The substance concentrates in areas where the bone is abnormal. A camera takes pictures of the location of the radioactive substance in the bone. Areas of abnormal bone collect a lot of the radioactive substance and show up as "hot spots" on the picture. A bone scan can often detect a problem days to months earlier than a regular X-ray test.
WebMD Medical Reference from Healthwise