What is the best imaging study to assess for a leg abscess?

Best Imaging Study for Leg Abscess Assessment

Ultrasound (US) is the best initial imaging study for assessing a leg abscess, with a sensitivity of 96.7% and specificity of 85.7% for diagnosing superficial abscesses. 1

Rationale for Ultrasound as First-Line Imaging

Ultrasound offers several advantages for leg abscess evaluation:

• High diagnostic accuracy: US correctly diagnosed abscess in 29 out of 30 patients in a comparative study 1
• Real-time visualization: Allows immediate assessment of fluid collections and internal characteristics
• Procedural guidance: Facilitates image-guided aspiration or drainage
• No radiation exposure: Safe for repeated examinations
• Cost-effective and accessible: Widely available in emergency departments and clinics
• Bedside capability: Can be performed at point-of-care without patient transport

Ultrasound Features of Abscesses

Ultrasound can identify key abscess characteristics:

• Fluid collection with internal debris
• Surrounding hyperemia (using color Doppler)
• Internal characteristics including necrosis and debris
• Associated pathology (foreign bodies, fistulas)
• Depth and extent of the collection

Algorithm for Imaging Selection

1. Initial imaging: Ultrasound for suspected superficial abscess

   • If abscess identified → proceed to ultrasound-guided drainage
   • If negative but high clinical suspicion → consider deeper imaging

2. For deeper soft tissue structures or if ultrasound is inconclusive:

   • MRI with contrast: Excellent for detecting and evaluating both superficial and deep soft tissue infections 1
   • CT with IV contrast: Alternative if MRI contraindicated; particularly useful for detecting soft tissue gas and guiding drainage procedures 1

Clinical Implications

Ultrasound significantly improves clinical outcomes:

• Reduces clinical failure rates when used to guide abscess management 2
• Changes management decisions in approximately 16% of skin and soft tissue infection cases 3
• Enables accurate needle placement for aspiration or drainage 1
• Allows differentiation between abscess (requiring drainage) and cellulitis (requiring antibiotics only)

Pitfalls and Limitations

• Depth limitations: US has limited visualization of deeper structures compared to MRI and CT 1
• Operator dependency: Accuracy depends on user experience and technique
• Bone interference: Evaluation of adjacent bone involvement may be limited 1
• Gas interference: Air within abscesses can create artifacts

For superficial leg abscesses, ultrasound provides the optimal balance of diagnostic accuracy, accessibility, and clinical utility while avoiding radiation exposure. For deeper infections or when bone involvement is suspected, MRI or CT may be necessary as second-line imaging options.