A bone scan is a nuclear imaging test that helps diagnose and track several types of bone disease. Your doctor may order a bone scan if you have unexplained skeletal pain, a bone infection or a bone injury that can't be seen on a standard X-ray.

A bone scan can also be an important tool for detecting cancer that has spread (metastasized) to the bone from the tumor's original location, such as the breast or prostate.

If you have unexplained bone pain, a bone scan might help determine the cause. The test is very sensitive to any difference in bone metabolism. The ability to scan the entire skeleton makes a bone scan very helpful in diagnosing a wide range of bone disorders, including:

• Fractures
• Arthritis
• Paget's disease of bone
• Cancer originating in bone
• Cancer that has metastasized to bone from a different site
• Infection of the joints, joint replacements or bones (osteomyelitis)
• Impaired blood supply to bones or death of bone tissue (avascular necrosis)

Although the test relies on radioactive tracers to produce the images, these tracers produce very little radiation exposure — less than a CT scan.

You don't need to restrict your diet or avoid particular activities in preparation for a bone scan. Let your doctor know if you've taken a medicine containing bismuth, such as Pepto-Bismol, or if you've had an X-ray test using barium contrast material within the past four days. Barium and bismuth can interfere with bone scan results.

Immediately before the test, you may be asked to remove jewelry or other metal objects.

Bone scans aren't usually performed on pregnant women or nursing mothers because of concerns about radiation exposure to the baby. Tell your doctor if you're pregnant — or think you might be pregnant — or if you're nursing.

A bone scan is a nuclear imaging procedure. In nuclear imaging, tiny amounts of radioactive materials (tracers) are injected into a vein and taken up in varying amounts at different sites in the body.

Areas of the body where cells and tissues are repairing themselves most actively take up the largest amounts of tracer. Nuclear images highlight these areas, suggesting the presence of abnormalities associated with disease or injury.

A bone scan procedure includes both an injection and the actual scan.

The injection

Tracers will be injected into a vein in your arm. The amount of time between the injection and scan varies, depending on the reason your doctor has ordered the scan.

Some images may be taken immediately after the injection. But the main images are taken two to four hours later to allow the tracer to circulate and be absorbed by your bones. Your doctor may recommend that you drink several glasses of water while you wait.

The scan

You'll be asked to lie still on a table while an arm-like device supporting a tracer-sensitive camera passes back and forth over your body. The scan itself can take up to an hour. The procedure is painless.

Your doctor might order a three-phase bone scan, which includes a series of images taken at different times. A number of images are taken as the tracer is injected, then shortly after the injection, and again three to five hours after the injection.

To better see some bones in your body, your doctor might order additional imaging called single-photon emission computerized tomography (SPECT). This imaging can help with conditions that are especially deep in your bone or in places that are difficult to see. During a SPECT scan, the camera rotates around your body, taking images as it rotates.

After the test

A bone scan generally has no side effects, and no follow-up care is needed. The radioactivity from the tracers is usually completely eliminated two days after the scan.

A doctor who specializes in reading images (radiologist) will look for evidence of abnormal bone metabolism on the scans. These areas appear as darker "hot spots" and lighter "cold spots" where the tracers have or haven't accumulated.

Although a bone scan is very sensitive to abnormalities in bone metabolism, it's less helpful in determining the exact cause of the abnormality. If you have a bone scan that shows hot spots, more testing may be needed to determine the cause.