There are two kinds of bone scans. One tests bone density and is used to diagnose osteoporosis, a condition in which the bones become weak and brittle. The other, known as scintigraphy, is used to discover the site of a stress fracture or the presence of arthritis, infection, or bone cancer. This article discusses the second kind. Bones are live, dynamic tissue that are constantly remodeling. Old bone tissue is dissolved, new bone tissue is created, and minerals are stored away for later use. The soft core known as the bone marrow continuously manufactures blood cells. These processes of growth and change are part of the body's metabolic process. The bone scan known as scintigraphy detects areas of increased or decreased bone metabolism and can indicate hidden bone fractures, bone infections, arthritis, cancer, or the cause of unexplained bone pain.

Scintigraphy is a nuclear medicine test that requires no preparation other than the removal of jewelry, dentures, and any other metal objects. The only pain that may be felt is when tiny amounts of radioactive materials called tracers are injected into a vein in the arm.

The tracers are radionuclides that are attached to particular biological molecules. These molecules are attracted to and accumulate in the bones, taking the tracers with them. The tracers emit waves of radiation that can be detected by a special gamma camera.

It takes about two to four hours for the tracers to circulate and collect in the bones. Several glasses of water are drunk during this time, causing frequent urination. This helps to clear the body of the unabsorbed tracer materials.

After the appropriate amount of time, the patient rests on a padded table while the gamma camera, housed in an arm-like device, passes over the body and records patterns of tracer absorption in the bones. It is important to lie very still. A scan of an individual bone can take from 10 to 30 minutes and an entire skeletal scan can run from 45 minutes to an hour.

After the scan, the radioactivity generated in the body by the tracers (less than that of a chest X-ray) generally disappears within one to three days.

The information from the gamma camera is recorded in a computer that processes the data and creates an image. Normal radiotracer uptake in the bones appears as a uniform gray in the image.

Evidence of abnormal metabolism shows up either as darker "hot spots" with greater tracer uptake or as lighter, "cold spots" with little or no tracer uptake. Hot spots represent increased bone metabolism while cold spots indicate decreased bone metabolism. With arthritis, the tracers show up on the bone surfaces of the joints.

While scintigraphy can detect subtle changes in bone before they are visible by X-ray, it does not necessarily determine the cause of an abnormality. Other tests may be performed to aid in establishing a diagnosis. The bone scan findings are then correlated with the test results, other imaging studies, and clinical information.

In extremely rare cases, a person may develop a rash, swelling, or anaphylaxis as a result of this scan.