A 3-phase bone scan is a specialized nuclear medicine test designed to evaluate the metabolic activity and blood flow within the skeletal system. This procedure involves injecting a small, safe amount of a radioactive material, known as a radiotracer, into a vein. The designation “3-phase” refers to the specific timing of image acquisition, which tracks the radiotracer’s movement from the bloodstream into the soft tissues and eventually into the bone itself. This dynamic imaging provides insight into the physiological processes occurring in the body, going beyond a standard anatomical image.
Specific Conditions Diagnosed by the Scan
Physicians often order a 3-phase bone scan to distinguish between active bone disease and issues confined to the surrounding soft tissues. The ability to assess vascularity and blood flow makes this scan effective for evaluating acute processes where timing is significant. A primary indication is differentiating between osteomyelitis (a bone infection) and cellulitis (an infection of the soft tissue). The scan clarifies the exact location of the pathology, as osteomyelitis is active within the bone, while cellulitis is confined to non-bony structures.
The scan is also used to assess complex regional pain syndrome (CRPS), a chronic pain condition often presenting with abnormal blood flow and inflammation. It evaluates acute trauma, such as stress fractures or occult fractures not visible on standard X-rays. For patients with joint replacements, the test helps determine if a painful prosthesis is due to mechanical loosening or an underlying infection.
The Procedure: Detailing the Three Imaging Phases
The 3-phase bone scan begins with the intravenous injection of a bone-seeking radiotracer, most commonly Technetium-99m Methylene Diphosphonate (Tc-99m MDP). The patient is positioned under a gamma camera immediately before the injection to ensure the area of interest is captured from the start. The procedure is structured into three distinct phases, each designed to capture a specific biological process.
Flow or Perfusion Phase
This first phase begins instantly upon injection of the tracer. For approximately 60 seconds, the camera rapidly acquires images to monitor the arterial blood supply to the area of interest. This dynamic imaging provides a detailed picture of the blood flow, indicating whether delivery to the tissue is prompt and symmetrical.
Blood Pool or Soft Tissue Phase
This second phase is captured immediately following the flow images, typically within 5 to 15 minutes post-injection. This set of static images shows the accumulation of the radiotracer in the capillaries and the surrounding soft tissues. Increased activity in this phase, without significant uptake later, suggests soft tissue inflammation or infection, such as cellulitis.
Delayed or Bone Phase
This final phase occurs approximately two to four hours after the initial injection. During this waiting period, the tracer is carried by the bloodstream and chemically adsorbed onto the hydroxyapatite crystals in the bone matrix, reflecting osteoblastic activity. The images acquired now represent the long-term metabolic activity of the bone. Areas of high bone turnover, such as fractures, tumors, or chronic infection, show increased radiotracer uptake.
Patient Preparation and Post-Scan Instructions
Preparation for a 3-phase bone scan is straightforward and helps ensure clear, high-quality images. Patients are advised that no fasting is required, and they can continue to take most medications as usual. They must remove all metal objects, including jewelry and belts, as these can interfere with the gamma camera’s detection of the tracer.
The total procedure is lengthy, often taking up to five hours due to the two to three-hour delay between the second and third phases. This waiting period allows the radiotracer to circulate and properly accumulate in the bone tissue. Patients are usually allowed to leave the facility during this delay but must return punctually for the final set of images.
The radiotracer emits a very low level of radiation. To expedite the clearance of any unabsorbed tracer, patients are encouraged to drink extra fluids, particularly water, following the injection and before the final scan. This hydration aids in flushing the material from the system through urination, which also helps clear the bladder for clearer pelvic imaging.