Bone Three Phase Scan

A Three-Phase Bone Scan, also known as Triphasic Bone Scintigraphy, is a highly sensitive nuclear medicine imaging technique used for evaluating bone metabolism and detecting various musculoskeletal disorders. This diagnostic scan utilizes Technetium-99m labeled methylene diphosphonate (Tc-99m MDP), a radiotracer that concentrates in bone tissues depending on blood flow, metabolic activity, and calcium content.

The scan is performed in three distinct phases flow phase, blood pool phase, and delayed phase each offering crucial diagnostic insights into different aspects of bone pathology. This test is especially effective in identifying osteomyelitis, stress fractures, bone infarctions, painful prosthesis, reflex sympathetic dystrophy, and bone graft viability.

What is a Three-Phase Bone Scan?

• A Three-Phase Bone Scan is a nuclear medicine imaging study used to assess bone turnover and vascularity in the body.
• It involves injecting a small amount of radioactive tracer (Tc-99m MDP) into the bloodstream.
• The scan is captured in three time-dependent phases to evaluate blood flow, soft tissue uptake, and bone mineralization.
• It is non-invasive and can image the entire skeletal system in one session.

Phases of the Three-Phase Bone Scan

The test includes three main stages:

1.  Flow Phase (Dynamic Phase)

• Immediately after injection of the radiotracer, dynamic images are taken for about 60 seconds.
• This phase evaluates blood perfusion to the area of interest.
• High activity may indicate hyperemia (increased blood flow), as seen in infections, inflammation, or trauma.

2. Blood Pool Phase (Soft Tissue Phase)

• Conducted 5 to 10 minutes after tracer injection.
• Focuses on tracer activity in the extracellular fluid and soft tissues.
• Helps differentiate soft tissue involvement from pure bone pathology.
• Useful in assessing conditions like cellulitis versus osteomyelitis.

3.  Delayed Phase (Bone Phase)

• Performed approximately 2.5 to 3 hours post-injection.
• Reflects bone turnover and calcium content in the skeletal tissues.
• Tracer uptake in bones is proportional to osteoblastic activity and bone remodeling.
• Abnormal uptake can indicate bone infection, fracture, metastasis, or infarction.

What Conditions Can Be Diagnosed Using a Three-Phase Bone Scan?

The Three-Phase Bone Scan is useful in the assessment of a wide range of bone and joint-related conditions:

• Osteomyelitis – Differentiates between bone infection and soft tissue infection.
• Stress Fractures / Stress Reactions – Especially in athletes or military personnel with vague bone pain.
• Painful Prosthetic Joint – Helps distinguish between prosthetic loosening and infection.
• Sacroiliitis – Identifies inflammation in the sacroiliac joints, often seen in spondyloarthropathies.
• Reflex Sympathetic Dystrophy (RSD) / Complex Regional Pain Syndrome (CRPS) – Early detection in painful, swollen limbs.
• Bone Infarcts / Avascular Necrosis – Detects impaired blood supply to bone.
• Bone Graft Viability – Assesses successful uptake and integration of the graft.
• Trauma Evaluation – Identifies subtle or hidden fractures that may not be visible on X-ray.
• Bone Tumors / Metastasis – Evaluates skeletal spread of cancers like breast, prostate, or lung cancer.

How Does the Three-Phase Bone Scan Work?

Tc-99m MDP, the radiopharmaceutical agent, binds to hydroxyapatite crystals in bone.

• The uptake of the tracer is proportional to osteoblastic (bone-forming) activity and regional blood flow.
• Early phase shows vascular supply; delayed phase shows bone turnover.
• Normal bone shows uniform uptake; abnormal areas show either increased or decreased uptake.
• Tracer is excreted through kidneys, so hydration and urination help eliminate it from the body post-scan.

Procedure for a Three-Phase Bone Scan

• Preparation: No fasting needed; patient advised to drink water before and after scan.
• Injection: Tc-99m MDP is injected intravenously.
• Imaging Phases:
  • Flow phase: Begins immediately for 1 minute
  • Blood pool phase: After 5–10 minutes
  • Delayed phase: After 3 hours

Whole Body Imaging: Performed in the delayed phase to assess for metastatic or systemic bone disease.

• Duration: Total time may be around 3–4 hours due to delay between phases.
• Post-scan Instructions: Patient can resume normal activities; advised to stay hydrated to flush out the tracer.

Advantages of Three-Phase Bone Scan

• Highly sensitive for early detection of bone abnormalities.
• Useful in differentiating soft tissue from bone pathologies.
• Whole body coverage helps in identifying multiple sites of involvement.
• Safe and non-invasive with minimal radiation exposure.
• Can identify disease processes even before structural changes are visible on X-rays or MRI.

Limitations

• Less specific – may require correlation with other imaging (MRI/CT) or biopsy.
• Cannot always determine the exact cause of abnormal uptake.
• False positives possible in post-surgical or healing bones.

Conclusion

The Three-Phase Bone Scan is a powerful diagnostic imaging tool in nuclear medicine, especially for conditions involving bone pain, infection, trauma, and prosthesis evaluation. Its ability to capture blood flow, soft tissue activity, and bone metabolism in a single test makes it highly valuable in clinical decision-making.