The injection is a phosphate-based compound that has an attached radioactive label. This phosphate-based substance is injected into the blood stream, and is then attracted to areas where bone metabolic activity is high. The attached radiolabel can be detected, and the areas of bone activity are seen as "hot spots" on the bone scan image.
Over several hours, the radiolabeled substance is excreted by the kidneys into urine. Bone scans are used to detect fractures, infections, tumors, and other conditions that may increase the rate of bone turnover.