Abdominal Abscess

Localized abdominal collection of pus or infected fluid

CT is imaging test of choice : Low density, loculated, encapsulated fluid collection with peripheral rim enhancement

• Simple fluid density (0-10 HU) or slightly hyperdense

• Internal gas in absence of intervention/drainage highly suspicious for infected collection

• "Abscess" suggests discrete, drainable fluid collection: Differentiate from ill-defined inflammation and fluid that is not drainable (i.e., phlegmon)

• Adjacent fat stranding, edema, and fascial thickening due to inflammation

• Intraparenchymal abscess (liver, kidney, etc.) often surrounded by low-density parenchymal edema

US : Complex fluid collection with internal low-level echoes, membranes, or septations

• Increasing complexity within abscess fluid suggests thicker, more viscous contents

• Greater complexity on US often implies greater difficulty in drainage (especially with small-caliber catheters)

• Center of abscess avascular on color Doppler imaging with peripheral hyperemia

Many different causes, including postoperative setting, enteric perforation, generalized bacteremia, and trauma

Increased incidence in diabetics, immunocompromised patients, and postoperative patients

Differentiating abscess from noninfected collections after surgery may be difficult and requires correlation with clinical symptoms of infection or fluid aspiration

Localized abdominal collection of pus or infected fluid

Best diagnostic clue

• Loculated, encapsulated fluid collection with peripheral rim enhancement ± gas bubbles or air-fluid level on CECT

Location

• Can occur anywhere within abdominal cavity, including intraperitoneal space, extraperitoneal spaces, or intraparenchymal

Size

• Highly variable

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    2-15 cm in diameter, microabscesses < 2 cm

Morphology

• Low-density round or oval collection of fluid with a peripheral enhancing rim

Low density, loculated, encapsulated fluid collection with peripheral rim enhancement

• May be simple fluid density (0-10 HU) or slightly hyperdense

• Often adjacent fat stranding, edema, and fascial thickening due to inflammation

• Intraparenchymal abscess (liver, kidney, spleen, etc.) often shows surrounding low-density parenchymal edema

Presence of internal gas (~ 50% of cases) in absence of intervention highly suspicious for infected collection

Term "abscess" suggests discrete, drainable fluid collection: Differentiate from ill-defined inflammation and fluid that is not drainable (i.e., phlegmon)

Can be difficult to distinguish infected from noninfected (e.g., seroma, lymphocele, hematoma) collections

Typically central core of abscess demonstrates fluid signal (low-signal T1WI, high-signal T2WI)

• Internal complexity may slightly alter signal characteristics (e.g., hemorrhage, proteinaceous content)

Enhancing peripheral rim on T1WI C+ images

Abscesses anywhere in abdomen tend to show restricted diffusion (high signal on DWI with low ADC values)

• Lower ADC values than noninfected fluid collections

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    However, lack of restricted diffusion cannot exclude possibility of abscess (overlap in ADC values with necrotic tumors and noninfected collections)

Usually evidence of adjacent soft tissue edema around abscess (high T2 signal)