A standard bone scan (skeletal scintigraphy) is a nuclear medicine procedure that views the metabolic activity of the skeleton. It involves injecting a radiotracer, typically Technetium-99m-methylene diphosphonate (Tc-99m MDP), which is absorbed by the bones. A specialized camera captures the radiotracer uptake, highlighting areas of high bone turnover that may indicate disease or injury. The triple phase bone scan (TPBS) is a specialized, time-sensitive variation of this test. This procedure provides detailed, sequential information about the blood supply, soft tissue inflammation, and bone activity within a localized area of concern.
The Three Imaging Phases
The triple phase bone scan is distinguished by three distinct sets of images captured at different time intervals to provide a comprehensive picture of the pathology. The first set, known as the flow or perfusion phase, is captured immediately after the radiotracer is injected. This phase lasts approximately 60 seconds, assessing the blood flow into the area of interest. Increased flow suggests an active process requiring an elevated blood supply, such as acute inflammation or infection.
The second set of images is the blood pool or soft tissue phase, acquired a few minutes after injection, typically between five and fifteen minutes. At this point, the radiotracer has perfused the soft tissues and vascular spaces surrounding the bone. Analyzing this phase helps differentiate between a process confined to the soft tissue, such as cellulitis, and a deeper issue affecting the bone itself. Increased activity indicates soft tissue inflammation or heightened vascular permeability.
The final set of images is the delayed or bone metabolism phase, performed two to four hours later. This phase represents the standard bone scan. This interval allows the radiotracer to clear from the soft tissues and concentrate in the bone mineral matrix. The resulting images depict areas of high osteoblastic activity, which is the body’s response to disease, healing, or trauma.
Specific Medical Indications
The triple phase bone scan is frequently ordered when a physician needs to distinguish between soft tissue and bone-related problems, which are often unclear on standard X-rays. A primary application is differentiating osteomyelitis (a bone infection) from cellulitis (a soft tissue infection). In cellulitis cases, the initial two phases show increased radiotracer uptake due to inflammation, but the delayed phase will be normal because the infection has not reached the bone.
For osteomyelitis, all three phases will show increased activity: high flow, high soft tissue concentration, and significantly increased uptake in the bone on the delayed images. The TPBS is also valuable for evaluating subtle bone trauma, such as occult or stress fractures, which may not be visible on conventional radiographs for days or weeks. The flow and blood pool phases detect the increased blood supply and vascular response associated with the early stages of a stress injury.
The TPBS also plays a role in evaluating Complex Regional Pain Syndrome (CRPS), a chronic pain condition involving vascular instability. The flow and blood pool images can capture characteristic vascular changes, such as asymmetric blood flow or increased soft tissue activity, which helps confirm the diagnosis. Furthermore, the TPBS is utilized to assess prosthetic joints when infection, loosening, or inflammation around the implant is suspected. The three-phase data helps isolate the cause of pain, determining if the issue is a surgical complication or a separate infection.
Preparing for the Scan and the Procedure
Preparation for the triple phase bone scan is typically minimal, though patients are advised to arrive well-hydrated. Drinking extra water helps the body clear the radiotracer more quickly from soft tissues, improving the clarity of the final bone images. Upon arrival, a technologist administers the radiotracer dose through an intravenous injection, usually into a vein in the arm.
Imaging begins immediately after the injection to capture the flow and blood pool phases, which takes about 15 minutes. Following this initial session, the patient is free to leave for two to four hours while the radiotracer circulates and is absorbed by the skeleton. Patients are encouraged to drink fluids during this waiting period and to empty their bladder immediately before the final, delayed imaging session.
The final scan involves the patient lying still on an imaging table while a gamma camera captures the delayed images of the bones, which takes approximately one hour. The amount of radiation exposure from the radiotracer is low, comparable to a routine X-ray, and it naturally leaves the body within a few days. After the final images are taken, the patient can resume all normal activities, and the interpreting radiologist sends the report to the referring physician.