A bone scan (skeletal scintigraphy) is a common nuclear medicine procedure used to assess the health of the entire skeletal system. It provides a functional image of the bones, detecting changes in metabolism or turnover throughout the body. Physicians often use this highly sensitive test when a patient has unexplained bone pain or when a condition affecting bone activity is suspected. Because it can detect abnormalities earlier than traditional X-rays, it is a valuable diagnostic tool.
How Bone Scans Identify Problem Areas
The procedure involves injecting a small amount of a radioactive substance, or radiotracer, into a vein. This tracer, typically Technetium-99m complexed with a phosphate compound, travels through the bloodstream and is absorbed by the bones. The tracer binds to crystals in the bone matrix in proportion to local blood flow and osteoblastic activity. Areas undergoing rapid bone turnover, such as those affected by injury or disease, accumulate a higher concentration of the radiotracer. These appear as darker or brighter spots, known as “hot spots,” on the final image. Conversely, “cold spots” show very little tracer accumulation, indicating a lack of blood supply or a destructive lesion. Mapping the radiotracer distribution helps physicians identify abnormal bone activity.
Medical Conditions That Cause Abnormal Results
An abnormal bone scan signals a disturbance in the bone’s normal process of remodeling and repair. Interpreting these findings requires correlation with a patient’s medical history and physical examination, as many conditions can cause similar patterns of activity. Because the test is highly sensitive, it detects any process that stimulates osteoblastic activity, ranging from severe systemic diseases to minor injuries.
Malignancy
Bone scans are frequently used to screen for metastatic disease, which is the most common cause of multiple, widespread hot spots in adults. Primary cancers originating in the prostate, breast, lung, kidney, or thyroid frequently metastasize to the bone. This spread causes an aggressive local reaction resulting in intense tracer uptake. Primary bone tumors, though less common, also show increased radiotracer activity due to rapid growth and associated bone destruction and repair. However, some cancers, such as multiple myeloma, may present as cold spots because they destroy bone without stimulating the repair process that draws in the tracer.
Trauma
Trauma is a common cause of focal hot spots, appearing as increased activity due to the body’s attempt to repair damage. While this includes obvious broken bones, the scan is particularly useful for identifying occult or hairline fractures not visible on standard X-rays. Stress fractures, resulting from repetitive microtrauma, also show high tracer uptake. These often appear as linear areas of increased activity in weight-bearing bones like the tibia or foot.
Infection
Infection of the bone (osteomyelitis) causes a localized inflammatory response that significantly increases both blood flow and bone turnover at the site. This intense activity results in a pronounced hot spot on the bone scan, often preceding changes visible on plain radiographs. A three-phase bone scan captures images immediately after injection, minutes later, and then hours later. This technique helps differentiate an active bone infection from simple soft tissue inflammation, as the high uptake is a direct result of the body’s attempt to fight the bacteria.
Degenerative and Metabolic Conditions
Non-cancerous diseases affecting bone structure and metabolism also produce abnormal bone scan results. Degenerative joint disease (severe arthritis) commonly results in increased tracer uptake around the joints, reflecting chronic wear and subsequent bone remodeling. Paget’s disease of bone, a chronic disorder of abnormal destruction and regrowth, typically causes characteristic patterns of uptake in large areas of the skull, pelvis, or long bones. Other conditions, such as fibrous dysplasia or avascular necrosis (where bone tissue dies due to poor blood supply), can cause abnormal tracer distribution, sometimes appearing as a cold spot surrounded by a reactive hot rim.
What Happens After an Abnormal Finding
An abnormal bone scan is rarely considered a definitive diagnosis on its own. It serves as a highly sensitive signal directing the next steps in the diagnostic journey. Because the scan indicates where the problem is located and the level of metabolic activity, but not the specific cause, a physician will almost always order correlative imaging to better characterize the abnormality.
Correlative Imaging
High-resolution imaging, such as a computed tomography (CT) scan or a magnetic resonance imaging (MRI) scan, provides detailed anatomical information about the flagged area. A CT scan is excellent for visualizing bone structure and detecting subtle destruction or new bone formation. An MRI provides superior detail of the soft tissues, bone marrow, and surrounding structures. These images help narrow the possibilities by showing the physical appearance of the lesion, determining if it is a simple fracture, a cyst, or a solid tumor.
Clinical Correlation and Biopsy
The scan findings must be carefully combined with the patient’s full medical history, symptoms, and laboratory test results, a process known as clinical correlation. For example, a single hot spot in the spine of a patient with a recent fall is likely a fracture. Conversely, multiple hot spots in a patient with a history of prostate cancer are highly suggestive of metastatic disease. If the cause remains unclear, or if malignancy or chronic infection is suspected, a bone biopsy may be necessary. The biopsy provides confirmation by allowing a pathologist to examine the cells under a microscope to determine the exact nature of the abnormal tissue.