Can an ultrasound detect an abscess not visible on Computed Tomography (CT) scan?

Ultrasound Can Detect Abscesses Not Visible on CT Scan

Yes, ultrasound can detect abscesses that are not visible on CT scan, with research showing ultrasound has higher sensitivity (96.7%) compared to CT (76.7%) for diagnosing superficial abscesses. 1

Comparative Diagnostic Performance

Ultrasound Advantages

• Higher sensitivity for superficial abscesses (96.7% vs 76.7% for CT) 1
• Can detect fluid collections with greater accuracy in certain locations
• Allows real-time evaluation with the ability to:
  • Assess internal characteristics of abscess cavities
  • Identify necrosis or debris within the abscess
  • Detect associated pathology (foreign bodies, fistulas, vascular injury) 1
• Useful for detecting fluid collections including:
  • Joint effusions
  • Abscesses
  • Infected tendon sheaths 1

CT Advantages

• Higher specificity for abscesses (91.4% vs 85.7% for ultrasound) 1
• Superior for detecting:
  • Soft tissue gas (most sensitive modality) 1
  • Deep tissue structures
  • Adjacent bone involvement 1
  • Periosteal reaction and bone erosions

Evidence from Clinical Studies

A key study by Gaspari et al. demonstrated that ultrasound correctly diagnosed abscesses in 29 out of 30 patients, while CT only identified 23 out of those same 30 patients with abscesses 1. This translates to ultrasound detecting approximately 20% more abscesses than CT in this population.

For specific types of abscesses:

• Splenic abscesses: CT and MRI appear to be the best tests, with both sensitivities and specificities ranging from 90% to 95% 1
• Liver abscesses: CT has shown 97% sensitivity compared to 79% for ultrasound 2
• Perianal abscesses: Ultrasound demonstrates high accuracy in identification 1

Location-Specific Considerations

Superficial Abscesses

• Ultrasound is superior for superficial abscesses, particularly those near the skin surface
• Color Doppler can assess vascularity and hyperemia without IV contrast 1

Deep Abscesses

• CT or MRI generally preferred for deep abscesses
• A retrospective review showed CT sensitivity of 88% vs 75% for ultrasound in detecting intra-abdominal abscesses 1

Perineal/Perianal Region

• MRI has the highest detection rates for anorectal abscesses but limited availability in emergency settings 1, 3
• Endoanal ultrasound is good for fistula detection but requires special skills and may be poorly tolerated by patients with acute pain 1, 3

Clinical Pitfalls and Considerations

1. Operator Dependency: Ultrasound results are highly operator-dependent, especially in complex anatomical regions 1

2. Patient Factors:

   • Body habitus can limit ultrasound visualization
   • Pain and tenderness may limit adequate examination

3. Abscess Characteristics:

   • Small abscesses may be missed on CT 1
   • Early abscesses without significant fluid collection may be missed on ultrasound 2
   • Diffuse inflammatory patterns can mask abscesses on both modalities 2

4. Anatomical Challenges:

   • Dome of liver abscesses are frequently missed by ultrasound 2
   • Deep pelvic abscesses may require specialized techniques for visualization

Practical Approach

When abscess is suspected but CT is negative:

1. Consider ultrasound evaluation, especially for superficial locations
2. Use MRI when available for complex cases or when both CT and ultrasound are equivocal
3. Remember that different imaging modalities may complement each other rather than being redundant 4

Ultrasound-guided drainage has been shown to decrease clinical failure rates compared to drainage without ultrasound, highlighting the value of ultrasound not just for diagnosis but also for treatment 5.