A bone scan, or skeletal scintigraphy, is a nuclear medicine procedure that provides a functional assessment of the skeleton. Unlike a standard X-ray, which primarily visualizes bone structure, a bone scan assesses bone metabolism and function by detecting areas of high cellular activity. The resulting image reveals patterns of bone turnover, blood flow, and repair, helping physicians diagnose conditions like hidden fractures, infections, and the spread of cancer before structural changes are visible.
The Mechanism of Visualization
Creating a bone scan image relies on introducing a small, safe amount of a radioactive substance, known as a radiotracer, into the body. This radiotracer, typically Technetium-99m complexed with a diphosphonate, is injected intravenously. The diphosphonate mimics phosphate, allowing it to bind to the crystalline hydroxyapatite in the bone.
The tracer accumulates in the skeleton in proportion to osteoblastic activity (new bone formation and remodeling). Areas with greater blood flow or increased metabolic turnover absorb more radiotracer. After a waiting period of two to four hours, which allows the tracer to circulate and bind, a gamma camera is used.
The gamma camera detects the gamma rays emitted by the absorbed radiotracer. This process translates the radiation signals into a two-dimensional image, or scintigram, mapping the radiotracer distribution. The final image represents the bone’s biological activity, highlighting areas of increased or decreased metabolism.
Interpreting a Standard Scan
A normal bone scan presents a uniform and symmetrical distribution of the radiotracer throughout the major bones. The image is typically displayed in shades of gray or false color, where intensity corresponds to the amount of tracer uptake. A healthy scan reflects balanced and consistent bone turnover.
Certain areas naturally show higher uptake due to increased physiological activity. These include joints (shoulders, hips, knees) due to weight-bearing stress, and the growth plates of long bones in children due to active growth.
Other structures where the tracer is normally seen are the sternum, sacroiliac joints, kidneys, and bladder. Tracer that does not bind to the bone is cleared through the urinary system, causing the kidneys and bladder to appear brightly illuminated. Recognizing these areas of expected intensity prevents mistaking normal function for disease.
Identifying Abnormal Findings
Abnormal findings on a bone scan are visually characterized by distinct deviations from the expected uniform pattern, generally categorized as either “hot spots” or “cold spots.”
Hot Spots
Hot spots appear darker or brighter than the surrounding bone, indicating significantly increased radiotracer accumulation and high metabolic activity. This intense uptake signifies that the bone is actively responding to a stimulus, such as repair, inflammation, or rapid cell turnover.
The presence of a hot spot can signify a variety of conditions. These include a healing fracture that may be too small or recent to be visible on an X-ray, or an infection like osteomyelitis. Metastatic cancer, where a tumor has spread to the bone, often shows up as multiple, intense hot spots across the skeleton. Conditions involving rapid bone remodeling, such as arthritis and Paget’s disease, also manifest as increased tracer uptake.
Cold Spots
Conversely, cold spots are regions that appear blank, pale, or significantly less intense than the surrounding healthy bone, indicating a lack of radiotracer uptake. This visual absence suggests a reduction or complete lack of blood flow, or a localized area where bone metabolism has been halted or bone tissue has been destroyed.
One common cause of a cold spot is avascular necrosis, which is the death of bone tissue due to an interruption of its blood supply. Some aggressive tumors can also present as cold spots if they destroy bone faster than the body can repair it. The interpretation of both hot and cold spots relies on their location, shape, and pattern of distribution, which helps the physician link the visual anomaly to a specific underlying medical condition.
These visual findings provide sensitive evidence of bone pathology, guiding further diagnostic testing and treatment planning.