Is CT with or without contrast recommended for diagnosing an abscess in the foot?

CT with IV Contrast is Recommended for Diagnosing Foot Abscess

CT with IV contrast is the preferred imaging modality for diagnosing an abscess in the foot, as it provides superior visualization of abscess walls and surrounding structures.

Rationale for CT with IV Contrast

• When CT is obtained for suspected abscess, IV contrast is preferable and usually appropriate as it improves visualization of the abscess wall 1
• For suspected soft tissue infections including abscesses, CT with IV contrast is rated higher than CT without contrast 1
• IV contrast helps characterize and detect subtle tissue abnormalities and complications 1
• CT with contrast provides better delineation of rim enhancement of fluid collections, which aids in the diagnosis of abscess 1

Imaging Algorithm for Suspected Foot Abscess

1. Initial Assessment:

   • Plain radiographs may be useful as a first step to rule out gas-forming infections or foreign bodies
   • For suspected soft tissue gas, radiographs are rated highest (9/9) as first study 1

2. Definitive Imaging:

   • CT with IV contrast: Primary recommended modality
     • Provides excellent visualization of abscess walls
     • Less operator-dependent than ultrasound 1
     • Maintains accuracy in the presence of dressings, drains, or significant tissue edema 1
     • Superior for detecting soft tissue gas, deep tissue structures, and adjacent bone involvement 2

3. Alternative Options (if CT with contrast is contraindicated):

   • CT without IV contrast: Acceptable alternative if contrast is contraindicated
   • MRI with and without contrast: High sensitivity but less readily available in emergency settings
   • Ultrasound: Better for very superficial abscesses but operator-dependent and limited for deep structures

Special Considerations

• Timing matters: Early abscesses (less than 5 days from symptom onset) may be missed on imaging, with sensitivity of CT and MRI increasing to 100% only after 6 days 3
• Drainage guidance: CT is particularly useful not only for diagnosis but also for guiding percutaneous drainage procedures 4
• Limitations: Small abscesses may be missed on CT, and early abscesses without significant fluid collection may be difficult to detect 2

Pitfalls to Avoid

• Relying solely on ultrasound for deep foot abscesses, as it has lower sensitivity (75%) compared to CT (88%) for deep abscesses 2
• Performing CT without contrast when contrast is not contraindicated, as this reduces the ability to visualize abscess walls
• Delaying imaging in cases with high clinical suspicion, as early abscesses may have false negative results 3
• Overlooking the need for follow-up imaging if initial results are negative but clinical suspicion remains high

By following this evidence-based approach with CT with IV contrast as the primary imaging modality, clinicians can optimize the detection of foot abscesses, leading to earlier intervention and improved patient outcomes.