A triple phase bone scan is a specialized medical imaging test that provides detailed information about bone health and surrounding soft tissues. It helps identify various conditions affecting the skeletal system by tracking the absorption of a small amount of radioactive material. This allows for assessment of blood flow, soft tissue changes, and bone metabolism in specific areas.
How the Scan Works
A triple phase bone scan utilizes a radioactive tracer, typically technetium-99m methylene diphosphonate (Tc99m-MDP), which is injected into a vein. This tracer travels through the bloodstream and is absorbed by bone cells, particularly in areas where bone metabolism is higher due to injury, infection, or disease. A gamma camera then detects the radiation emitted by the tracer, creating images that reveal areas of increased or decreased uptake.
The “triple phase” refers to three distinct sets of images taken at different time points, each providing unique insights. The first, or “vascular/blood flow” phase, involves immediate imaging (within seconds) after the tracer injection to assess blood perfusion to the area of interest. This helps identify conditions with altered blood flow, such as acute inflammation or infection.
The second phase, known as the “soft tissue/blood pool” phase, is captured a few minutes after the injection. These images show how the tracer pools in the soft tissues surrounding the bone, distinguishing between bone and soft tissue abnormalities. For example, cellulitis, a soft tissue infection, would show increased activity in this phase without significant bone uptake in later images.
The final and most informative phase is the “bone/delayed” phase, taken typically two to four hours after the injection. During this time, the tracer has accumulated in the bone itself, highlighting areas of increased bone turnover or remodeling. Analyzing all three phases together allows clinicians to differentiate between various conditions, such as early infections versus soft tissue inflammation, or acute fractures versus older injuries.
Conditions Diagnosed by the Scan
A triple phase bone scan is frequently used to diagnose and monitor a range of bone and soft tissue conditions. It is particularly effective in detecting bone infections, known as osteomyelitis, often identifying changes days or weeks before they become visible on standard X-rays. The scan can help distinguish osteomyelitis from cellulitis, as bone infections typically show increased tracer activity across all three phases, localizing to the bone in the delayed images.
The scan also aids in evaluating stress fractures, which may not be apparent on conventional X-rays, by revealing areas of increased metabolic activity within the bone. It is useful for identifying occult (hidden) fractures, avascular necrosis (bone tissue death due to lack of blood supply), and bone tumors, both primary and metastatic. For tumors, the scan can help determine their location, extent, and activity, assisting in staging and treatment planning.
It can also diagnose Complex Regional Pain Syndrome (CRPS), a chronic pain condition, by showing characteristic changes in blood flow and bone metabolism. It is also employed to assess the integration and healing of bone grafts after surgery, providing valuable information on graft viability.
What to Expect During the Scan
Undergoing a triple phase bone scan involves several steps over a period of about five hours, though much of this time is a waiting period. Generally, there is no special preparation required before the test, such as fasting. Upon arrival at the nuclear medicine department, a technologist will review your medical history, including any allergies and current medications.
A small amount of the radioactive tracer will be injected intravenously, typically into a vein in your arm or hand. Immediately following this injection, the first two phases of imaging will begin. You will be positioned on an imaging table, and a gamma camera will capture images of the area of interest for a few minutes to assess blood flow and soft tissue pooling.
After these initial images, there will be a waiting period, usually two to three hours, to allow the tracer to accumulate in your bones. During this time, you may be asked to drink plenty of fluids and empty your bladder, as this helps to clear background radiation and improve image clarity. You can typically leave the department during this waiting period, but it’s important to return at the scheduled time for the final imaging.
For the third and final phase, you will again lie on the imaging table while the gamma camera takes delayed images of your bones. This session can last anywhere from 30 minutes to an hour, depending on the specific areas being scanned. After all images are acquired, you can usually resume your normal activities and diet without any specific restrictions.
Understanding Your Results and Safety
Once the triple phase bone scan is complete, a radiologist, a doctor specializing in medical imaging, will carefully interpret the images. They will look for areas of abnormal tracer uptake, which appear as “hot spots” (increased uptake) or “cold spots” (decreased uptake), indicating potential issues. The radiologist considers the patterns of uptake across all three phases, along with your clinical history, to provide a comprehensive report.
Your referring physician will then discuss the results with you, explaining what the findings mean for your specific condition and guiding further treatment or management. The interpretation is highly specific, differentiating, for example, between soft tissue inflammation and bone infection based on the tracer’s behavior in each phase.
Regarding safety, the radioactive tracer used in a triple phase bone scan delivers a very low dose of radiation, comparable to that received from other common imaging tests like X-rays or CT scans. The amount of radiation is well within safe limits and is generally considered not harmful. The tracer is not a dye and typically does not cause allergic reactions, though mild discomfort at the injection site is possible. Your body naturally eliminates the tracer over a few hours to a day through urine, so staying well-hydrated after the scan can help expedite this process.