Abdominal Abscess

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

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

“Abscess” suggests a 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

PATHOLOGY

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

CLINICAL ISSUES

• Increased incidence in diabetics, immunocompromised patients, and postoperative patients

DIAGNOSTIC CHECKLIST

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

(Left) Axial CECT in an elderly postoperative patient demonstrates a rounded complex fluid collection with gas bubbles and an enhancing capsule , findings diagnostic for an abdominal abscess.

(Right) Axial CECT in a elderly postoperative patient demonstrates multiple loculated fluid collections with prominently enhancing capsules and mass effect on adjacent structures, representing abdominal abscesses. Note the air-fluid level within one of the abscesses.

(Left) Axial CECT shows a large pelvic abscess following hysterectomy. Note the presence of a discrete enhancing rim and mass effect on adjacent loops of bowel and the bladder.

(Right) Axial CECT shows placement of a percutaneous drainage catheter using a transgluteal approach. The abscess has almost completely resolved following drainage.

TERMINOLOGY

Definitions

• Localized abdominal collection of pus or infected fluid

IMAGING

General Features

• 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

– 2-15 cm in diameter, microabscesses < 2 cm

• Morphology

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

CT Findings

• 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 a 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

MR Findings

• 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

– 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)