A 45-year-old man with a history of osteomyelitis of the left tibia 6 months ago presents with persistent localized pain and swelling at the site. Plain radiographs show no acute changes. A technetium-99m MDP bone scan is performed. Which of the following findings would best differentiate active osteomyelitis from chronic post-infectious changes?

Explanation

## Bone Scan Imaging Phases and Interpretation ### Three-Phase Technetium-99m MDP Bone Scan The technetium-99m MDP bone scan consists of three distinct phases: 1. **Blood pool phase (0–5 minutes)** — reflects regional blood flow and soft tissue perfusion 2. **Soft tissue phase (2–4 hours)** — captures blood pool clearance and soft tissue uptake 3. **Delayed phase (3–4 hours)** — represents osteogenic activity and bone metabolism **Key Point:** Active osteomyelitis produces a characteristic **three-phase pattern** with increased uptake in all phases, reflecting both increased vascularity and active bone turnover. ### Differentiation: Active vs. Chronic Osteomyelitis | Feature | Active Osteomyelitis | Chronic Post-Infectious Changes | |---------|---------------------|--------------------------------| | **Blood pool phase** | Increased (hyperemia) | Normal or mildly increased | | **Delayed phase** | Markedly increased | Mildly increased or normal | | **Pattern** | Diffuse uptake at site | Focal, less intense | | **Clinical significance** | Ongoing infection | Residual scarring, remodeling | **Clinical Pearl:** The presence of increased uptake on **both blood pool AND delayed phases** is the hallmark of active infection. Blood pool phase uptake indicates acute inflammation and hyperemia, while delayed phase uptake reflects active osteoblastic response. ### Why This Matters **High-Yield:** In a patient with a history of osteomyelitis presenting with persistent symptoms, the **three-phase pattern** (increased in all phases) confirms active recurrent or persistent infection. Chronic post-infectious changes show uptake only on delayed phase (residual bone remodeling) without significant blood pool activity. **Mnemonic:** **ABD** — **A**ctive infection shows **B**lood pool + **D**elayed phase uptake. ### Clinical Application This patient's persistent pain and swelling 6 months post-infection warrant imaging to rule out relapse. The three-phase pattern would indicate: - Active osteomyelitis requiring antimicrobial therapy - Possible need for surgical intervention if localized abscess is present - Follow-up imaging to monitor treatment response [cite:Harrison 21e Ch 155]