A bone scan is a specialized diagnostic imaging procedure used to assess the health and activity within the skeletal system. It provides detailed visual information about the bones, helping medical professionals identify areas of unusual physical or chemical changes. This test serves as a tool to gain insights into various bone conditions, aiding in the detection of underlying issues.
How Bone Scans Work
A bone scan begins with the intravenous injection of a small amount of a radioactive tracer, or radiopharmaceutical, into a vein. This tracer, often technetium-99m labeled with methylene diphosphonate (Tc-99m MDP), travels through the bloodstream and accumulates in bone areas with increased metabolic activity or blood flow, often indicating bone remodeling or repair.
After a waiting period, usually two to four hours, allowing the tracer to concentrate in the bones and unabsorbed tracer to flush from the body, imaging begins. A gamma camera detects the radiation emitted by the tracer, capturing signals that a computer processes to create detailed images of the skeletal system. These images reflect the tracer’s distribution and uptake, providing a functional map of bone activity.
Understanding Bone Scan Images
A normal bone scan typically displays a uniform and symmetrical distribution of the radioactive tracer throughout the skeleton. This indicates balanced bone metabolism and healthy bone structure. Some areas, like the kidneys and bladder, may appear brighter due to the excretion of unabsorbed tracer, and certain joints might show slightly more uptake due to normal physiological processes.
Deviations from this even uptake highlight areas of concern, manifesting as “hot spots” or “cold spots.” Hot spots are regions with increased radiotracer accumulation, appearing darker or brighter on the image compared to surrounding bone. These areas suggest heightened bone metabolic activity, such as rapid bone breakdown and repair, increased blood flow, or increased mineralization.
Conversely, cold spots show decreased or absent tracer uptake, appearing lighter or fainter on the scan. This reduced uptake indicates diminished metabolic activity, a lack of blood supply, or areas where bone tissue has been destroyed or is not actively remodeling. The presence of hot or cold spots signals an alteration in bone function or structure, prompting further investigation.
What Bone Scans Detect
Bone scans are highly sensitive diagnostic tools for detecting a wide range of bone conditions by identifying areas of abnormal bone metabolism. Hot spots are frequently observed in bone trauma, including fractures not readily visible on conventional X-rays, such as stress fractures or difficult-to-locate breaks.
Bone infections, known as osteomyelitis, typically present as hot spots due to the body’s increased metabolic response. Various forms of arthritis, involving inflammation and changes in joint structure, can also lead to increased tracer uptake and appear as hot spots.
Bone scans are widely used to detect benign and cancerous tumors, and to identify if cancer from other parts of the body, such as breast or prostate, has spread to the bones, appearing as hot spots. Conditions like Paget’s disease, characterized by abnormal bone remodeling, also manifest as hot spots.
While less common, cold spots can indicate conditions such as avascular necrosis, where bone tissue dies due to a lack of blood supply, or certain bone cysts or tumors like multiple myeloma. While a bone scan pinpoints areas of abnormal activity, additional tests are often needed to determine the precise cause.