What is the most appropriate initial imaging modality for a fluctuating heel wound?

Imaging for Fluctuant Heel Wound

For a fluctuant heel wound concerning for abscess or soft tissue infection, ultrasound is the most appropriate initial imaging modality after plain radiographs, as it rapidly identifies fluid collections, guides immediate aspiration, and evaluates soft tissue involvement without radiation exposure. 1, 2

Initial Imaging Approach

Step 1: Plain Radiographs First

• Obtain standard three-view foot radiographs (AP, lateral, and oblique) as the initial study to exclude underlying osteomyelitis, gas in soft tissues, or foreign bodies before proceeding to soft tissue evaluation. 1, 3, 4
• Add an axial Harris-Beath view specifically for calcaneal (heel bone) assessment, as this optimally visualizes the posterior foot anatomy that standard views may miss. 3, 4
• Weight-bearing views are not necessary in this acute infectious scenario and may be contraindicated with a fluctuant wound. 4

Step 2: Ultrasound for Soft Tissue Evaluation

• After radiographs, ultrasound is the preferred next study (rated 8/9 "usually appropriate") for evaluating soft tissue infections and fluid collections in the heel region. 1
• Ultrasound excels at identifying abscesses, defining their extent, and differentiating cellulitis from drainable fluid collections—critical for determining if surgical drainage is needed. 2
• The real-time capability allows immediate correlation with the symptomatic area and can guide needle aspiration for culture and drainage in the same session. 2, 5
• Ultrasound detects early inflammatory changes in the plantar fascia, Achilles tendon, and surrounding soft tissues that may be involved in the infectious process. 5, 6

When to Escalate to MRI

If ultrasound shows deep extension, concern for osteomyelitis persists despite negative radiographs, or surgical planning requires detailed anatomic mapping, MRI with IV contrast is the next appropriate study (rated 9/9 "usually appropriate"). 1

MRI Advantages in This Context:

• Superior delineation of fluid collections, areas of necrosis, and extent of soft tissue involvement with contrast enhancement. 1
• Detects early osteomyelitis with 90%+ sensitivity before radiographic changes appear (which take 10-14 days). 1
• Evaluates deep structures including bone marrow, joints, and tendon involvement that ultrasound cannot adequately assess. 1

MRI Without Contrast Alternative:

• If IV contrast is contraindicated (renal failure, allergy), MRI without contrast is rated 7/9 "usually appropriate" as an alternative, though contrast significantly improves soft tissue characterization. 1

CT Has Limited Role

CT with IV contrast is rated only 6/9 "may be appropriate" and should be reserved for situations where MRI is contraindicated or unavailable. 1

• CT is inferior to both ultrasound and MRI for soft tissue infection evaluation. 1
• Consider CT primarily if there is concern for radiopaque foreign body (glass, metal, gravel) that radiographs did not clearly demonstrate. 1

Critical Pitfalls to Avoid

• Never skip plain radiographs—they are essential to exclude osteomyelitis, foreign bodies, and gas-forming infections before focusing on soft tissue imaging. 1, 3
• Do not order MRI as the first study—it is expensive, time-consuming, and ultrasound provides faster, equally effective soft tissue assessment for initial management decisions. 1, 2
• Do not rely on clinical examination alone to distinguish cellulitis from abscess—ultrasound changes management in 30-40% of cases by identifying drainable collections. 2
• Ensure aspiration/culture is performed if a fluid collection is identified—imaging guides therapy but microbiologic diagnosis is essential for antibiotic selection. 1, 2

Algorithm Summary

1. Plain radiographs (3 views + axial calcaneal) → exclude bone involvement, foreign body, gas 1, 3, 4
2. Ultrasound → identify abscess, guide drainage, assess soft tissue extent 1, 2, 5
3. MRI with contrast (if needed) → deep extension, suspected osteomyelitis, surgical planning 1

This approach prioritizes rapid diagnosis and intervention to prevent progression to deeper infection, limb-threatening complications, and the need for more extensive surgical debridement—directly impacting morbidity and quality of life outcomes. 1, 2