Ultrasonographic Features of Abscess on Endoscopic Ultrasound
On endoscopic ultrasound (EUS), an abscess appears as a hypoechoic or anechoic fluid collection with a defined but often irregular wall, demonstrating peripheral Doppler signal in the wall rather than central flow. 1
Key Sonographic Characteristics
Fluid Content and Echogenicity
- The abscess cavity contains hypoechoic fluid that appears as a confined fluid collection on EUS imaging. 1
- The fluid component is anechoic or hypoechoic, distinguishing it from solid inflammatory masses. 1
- Unlike free peritoneal fluid, an abscess does not conform to normal peritoneal reflections. 1
Wall Characteristics
- The abscess demonstrates a defined wall, though this wall is often irregular rather than smooth. 1
- The wall consists of a rim of inflammatory tissue that appears as poorly demarcated lesions on EUS. 1
- Peripheral Doppler signal is detectable in the abscess wall, not centrally within the fluid collection. 1
Distinguishing Features from Inflammatory Mass
- An inflammatory mass (phlegmon) appears as an ill-defined hypoechoic mass without a definable wall and shows detectable internal color Doppler signal throughout. 1, 2
- In contrast, an abscess has a defined wall with peripheral rather than central vascularity. 1
- Contrast-enhanced ultrasound can aid differentiation, as inflammatory masses demonstrate central enhancement while abscesses show only rim enhancement. 1
Clinical Context and Location
Perianal Abscesses
- In perianal Crohn's disease, abscesses are defined radiologically as confined fluid collections with rim enhancement. 1
- The location should be classified as perianal, intersphincteric, ischiorectal, or suprasphincteric. 1
Intra-abdominal Abscesses
- Mesenteric and intra-abdominal abscesses should be documented with precise dimensions, enteric location, and relationship to adjacent organs. 1
- The estimated volume of fluid content and technical feasibility of image-guided drainage should be assessed. 1
Critical Reporting Elements
When an abscess is identified on EUS, the report must include:
- Precise dimensions of the fluid collection. 1
- Location relative to bowel and adjacent structures. 1
- Estimated volume of drainable fluid content. 1
- Assessment of technical feasibility for image-guided drainage. 1
Common Pitfalls
- Do not mistake an abscess for simple free fluid—abscesses have defined walls and do not follow peritoneal reflections. 1
- Avoid confusing inflammatory masses with abscesses; the presence of a defined wall and peripheral rather than central Doppler signal distinguishes abscesses. 1, 2
- Small fluid components within inflammatory masses may eventually form abscesses and require close monitoring. 1