A doctor may order a bone scan when they need to look beyond the physical structure of the skeleton and assess its metabolic activity. This diagnostic imaging tool uses a small, safe amount of radioactive material, called a radiotracer, to highlight areas of abnormal bone turnover. Unlike a standard X-ray, which primarily shows bone density and shape, a bone scan reveals how actively the bone tissue is breaking down and rebuilding itself. Increased or decreased uptake of the tracer suggests a problem, prompting the physician to investigate various conditions that cause this irregular activity.
Evaluating Cancer Spread
The bone scan is highly valued in oncology for its sensitivity in detecting metastatic disease. This test can often identify these secondary bone lesions much sooner than conventional X-rays, making it a powerful tool for cancer staging and monitoring. Many common cancers, including those of the breast, prostate, and lung, frequently spread to the skeleton, where they trigger a localized, high-speed bone repair process.
The radiotracer, typically Technetium-99m methylene diphosphonate (Tc-99m MDP), is absorbed by hydroxyapatite crystals in the bone, accumulating in areas of increased osteoblastic activity or blood flow. These sites of rapid bone remodeling, characteristic of a metastatic lesion, appear as “hot spots” on the final image. The scan provides a full-body view of the skeleton to determine the number and extent of metastatic sites and evaluate a patient’s response to cancer treatment. While highly sensitive, the scan’s results must be interpreted alongside other imaging studies and clinical findings because other conditions can also cause these hot spots.
Investigating Unexplained Pain and Trauma
Another frequent reason for a bone scan is to investigate localized bone pain or suspected injuries that are not clearly visible on initial X-rays. Standard X-rays may not show signs of damage until weeks after an injury, especially for small or fine fractures. The bone scan can identify these hidden injuries because the body’s immediate response is to initiate a repair process that increases bone turnover and blood flow to the site.
The scan is particularly effective for diagnosing stress fractures, often sustained by athletes through repetitive motion. These stress injuries, along with occult fractures—those not easily seen on X-rays—show up as distinct areas of increased tracer uptake within a few days of onset. Detecting this metabolic healing response aids prompt diagnosis, allowing doctors to quickly implement treatment plans for fractures in areas like the hip, wrist, or foot.
Identifying Infections and Systemic Disorders
The bone scan is also used to diagnose and monitor various infectious and metabolic diseases that affect bone activity. One such condition is osteomyelitis, a bone infection where the body’s inflammatory response causes a localized increase in blood flow and bone formation. A specialized three-phase bone scan, which involves taking images at different time intervals, is often used when a bone infection is suspected to help distinguish it from soft tissue inflammation.
Systemic bone disorders also rely on the bone scan for diagnosis. Paget’s disease, a chronic condition where normal bone remodeling is disrupted by excessive breakdown and regrowth, causes intense, widespread tracer uptake that is easily visible on the scan. Additionally, the test can help assess certain types of arthritis by showing increased activity in the joints, and it is also used in cases of avascular necrosis, where a lack of blood supply can lead to bone tissue death.
What to Expect During the Procedure
The bone scan procedure typically begins with a technologist injecting the radiotracer, often Tc-99m MDP, into a vein in the arm or hand. This radioactive material is not a dye and generally causes no side effects or sensations. A waiting period of approximately two to four hours follows the injection, allowing the tracer to circulate through the bloodstream and bind to the areas of active bone. Patients are usually asked to drink several glasses of water during this time to help flush any unbound tracer out of the body through the kidneys.
Before the imaging phase, the patient is asked to empty their bladder, as residual tracer in the urine can interfere with the image quality, especially in the pelvic region. The patient then lies still on a table while a gamma camera slowly scans the body, detecting the low-level gamma rays emitted by the tracer in the bones. This scanning process is painless and typically takes between 30 minutes and one hour. The radiation dose involved is quite low, and the radioactive substance naturally decays and leaves the body within a day.