Abdominal Abscess

Published on 19/07/2015 by admin

Filed under Radiology

Last modified 22/04/2025

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

image Internal gas in absence of intervention/drainage highly suspicious for infected collection
image “Abscess” suggests a discrete, drainable fluid collection: Differentiate from ill-defined inflammation and fluid that is not drainable (i.e. phlegmon)
image Adjacent fat stranding, edema, and fascial thickening due to inflammation
image Intraparenchymal abscess (liver, kidney, etc.) often surrounded by low-density parenchymal edema
• US: Complex fluid collection with internal low-level echoes, membranes, or septations

image Increasing complexity within abscess fluid suggests thicker, more viscous contents
image Greater complexity on US often implies greater difficulty in drainage (especially with small-caliber catheters)
image 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
image
(Left) Axial CECT in an elderly postoperative patient demonstrates a rounded complex fluid collection image with gas bubbles image and an enhancing capsule image, findings diagnostic for an abdominal abscess.

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(Right) Axial CECT in a elderly postoperative patient demonstrates multiple loculated fluid collections image with prominently enhancing capsules image and mass effect on adjacent structures, representing abdominal abscesses. Note the air-fluid level image within one of the abscesses.
image
(Left) Axial CECT shows a large pelvic abscess image following hysterectomy. Note the presence of a discrete enhancing rim and mass effect on adjacent loops of bowel and the bladder.

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(Right) Axial CECT shows placement of a percutaneous drainage catheter image 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

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

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

image Highly variable

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

image 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

image May be simple fluid density (0-10 HU) or slightly hyperdense
image Often adjacent fat stranding, edema, and fascial thickening due to inflammation
image 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)

image 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) 

image 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)

Ultrasonographic Findings

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

image Dependent echoes represent debris within abscess

– Increasing complexity within abscess fluid suggests thicker, more viscous contents
– Greater complexity on US often implies more difficult drainage (especially with small-caliber catheter)
image Posterior acoustic through transmission may vary depending on composition of fluid in abscess

– Abscesses with thick, viscous, proteinaceous fluid may have relatively little through transmission
image Center of abscess is typically avascular on color Doppler imaging, with peripheral hyperemia
image Fat surrounding abscess may appear markedly echogenic due to inflammation

– Inflamed fat hyperemic on color Doppler
image Internal echogenic foci with ring-down artifact and posterior “dirty” acoustic shadowing suggest presence of gas

Radiographic Findings

• Radiography

image Soft tissue “mass” or density ± internal ectopic gas (about 50% of cases) or air-fluid levels

– May be associated with loss of soft tissue-fat interface
image Dilated bowel loops due to focal ileus
image Subphrenic abscess often results in adjacent pleural effusion and lower lobe atelectasis

Fluoroscopic Findings

• Abscess sinogram

image Useful after percutaneous drainage to assess presence of residual abscess cavity
image Defines catheter position and communication with abscess
image Identifies fistulization of abscess with adjacent bowel, pancreas, or biliary tree

Nuclear Medicine Findings

• Ga-67 scan

image Most often utilized for chronic infections and fever of unknown origin
image Nonspecific, as Ga-97 may demonstrate uptake with tumors, such as lymphoma, as well as chronic granulomatous processes (i.e., sarcoidosis)
• In-111 or Tc-99m-labelled white blood cell (WBC) scan

image Most often utilized for acute infections or inflammatory bowel disease
image 73-83% sensitivity
image False-positives with bowel infarct or hematoma
• Newer agents

image Indium-labeled polyclonal IgG
image Tc-99m-labeled monoclonal antibody

Imaging Recommendations

• Best imaging tool

image CECT
• Protocol advice

image  

DIFFERENTIAL DIAGNOSIS

Postoperative Lymphocele

• History of lymph node dissection with collection adjacent to surgical clips along lymphatic drainage pathways
• Loculated collection of simple fluid attenuation without peripheral enhancement or internal gas

Biloma

• Fluid collection adjacent to biliary tree in patient with history of biliary or hepatic surgery
• Usually simple fluid attenuation without peripheral enhancement or internal gas (unless superinfected)

Postoperative Seroma

• Simple fluid attenuation without peripheral enhancement
• May be loculated or contain internal gas due to recent surgery
• Differentiating loculated seroma from abscess not always possible based on imaging alone

Loculated Ascites

• Often in patients with cirrhosis, chronic liver disease, chronic renal failure, or other underlying cause for ascites
• Simple fluid attenuation with minimal mass effect, no peripheral enhancement, and no internal gas
• May demonstrate complexity (e.g., septations) on US or MR but appear simple on CT

Pancreatic Pseudocyst

• Clinical history or imaging stigmata of prior pancreatitis
• Location highly variable, but most often within pancreatic parenchyma, lesser sac, anterior pararenal space, or transverse mesocolon
• Pseudocyst usually requires several weeks to develop peripheral pseudocapsule

Abdominal Hematoma

• Attenuation variable depending on age of blood products, but clot is typically high attenuation (> 45 HU) in the acute setting and gradually decreases in attenuation over time
• May demonstrate weak peripheral enhancement as it evolves (without necessarily being infected)

Retained Oxidized Cellulose (Surgicel)

• Placed at surgery to induce hemostasis and appears as collection of gas bubbles without much fluid
• May mimic abscess, but no discrete fluid collection

PATHOLOGY

General Features

• Etiology

image Many different causes including enteric perforation (e.g., perforated appendicitis, diverticulitis), postoperative setting, generalized bacteremia, and trauma

– Postoperative abscess may be variably located depending on site of surgery, but most often occurs in intraperitoneal spaces, such as cul-de-sac, Morison pouch, subphrenic spaces
• Genetics

image Risk increased if genetically altered immune response
image Diabetics have ↑ incidence of gas-forming abscesses
• Associated abnormalities

image Pus collection, peripheral fibrocapillary “capsule,” often polymicrobial from enteric organisms

Staging, Grading, & Classification

• Organism: Bacterial, fungal, amebic
• Related to organ of origin (e.g., liver abscess)
• Intraperitoneal
• Extraperitoneal
• Communicating

image Underlying fistula to gastrointestinal tract
image Connection to biliary tract or pancreatic duct

Gross Pathologic & Surgical Features

• Often adherent omentum or bowel loops; pus collection
• May or may not have “capsule”

Microscopic Features

• White-cell debris
• Bacteria, fungi

CLINICAL ISSUES

Presentation

• Most common signs/symptoms

image Fever, chills, abdominal pain
image Tachycardia and hypotension in setting of sepsis
• Clinical profile

image Leukocytosis, positive blood cultures, elevated ESR
image Elderly and immunocompromised patients may not have fever or ↑ WBC

Demographics

• Age

image Any
• Gender

image M = F
• Epidemiology

image Most commonly in postoperative setting
image Increased incidence in diabetic and immunocompromised patients
image Microabscesses due to fungal infections in immunocompromised patients

Natural History & Prognosis

• Variable depending on extent of abscess, patient’s immune system status, and other comorbidities
• Overall excellent prognosis if treated appropriately

Treatment

• Options, risks, complications

image Percutaneous abscess drainage (PAD)

– 80% success rate, with patient selection critical for success
– Best candidates have well-defined, encapsulated, fluid-filled abscesses > 3 cm with safe catheter access route
– Drainage can be performed under CT or US guidance, with multiple approaches possible (transcutaneous, transgluteal, transrectal, transvaginal)
– Complex abscess (i.e., multiseptated) or abscess with enteric fistula may take weeks or months to drain, but most abscesses can be drained in 10-14 days
– Catheter removed when drainage < 10 cc per shift or when abscess cavity resolves on imaging
image Contraindications for PAD related to patient coagulopathy (elevated INR or low platelet count)

– Coagulopathy with prothrombin time > 3 sec
– International normalized ratio > 1.5
– Platelets < 50,000/μL
image Contraindications for PAD related to abscess

– Poorly defined collection (i.e., phlegmon) rather than discrete drainable abscess
– No safe access route for catheter insertion due to intervening bowel, adjacent vital organs, or pleura

image Crossing colon is greater risk than traversing small bowel or stomach
image Crossing sterile collections (such as hematoma) or sterile pleural effusion should be avoided due to risk of superinfection
– Gas-forming infection, e.g., emphysematous pancreatitis
– Echinococcal cyst (due to risk of leakage and resultant anaphylaxis)
– Infected necrosis
– Multiseptated abscess
– Superinfected necrotic tumor
image Surgery indications

– Extensive intraperitoneal abscesses
– Debridement of necrotic infected tissue
– Failed percutaneous drainage
image Small abscesses (usually < 3 cm) may be treated conservatively with antibiotic therapy

DIAGNOSTIC CHECKLIST

Consider

• Differentiating an abscess from noninfected collections in postoperative setting (e.g., seroma, lymphocele, hematoma) may be difficult and requires correlation with clinical symptoms of infection or fluid aspiration

Image Interpretation Pearls

• Half of abscesses do not contain gas or air-fluid levels; mass effect and enhancing rim highly suggestive in appropriate clinical context

image
(Left) Axial CECT in a woman who presented with lower abdominal pain, fever, and tenderness shows extensive free intraperitoneal gas image.
image
(Right) Axial CECT in the same patient shows a loculated abscess image immediately adjacent to a segment of sigmoid colon with extensive diverticulosis image, which proved to be the source of the free air and abscesses.
image
(Left) Axial CECT in an elderly woman after ERCP and papillotomy shows a large collection of gas and fluid image dissecting through the retroperitoneal spaces, especially the anterior pararenal space and the interfascial plane image.

image
(Right) Axial CECT in the same patient demonstrates the retroperitoneal abscess. The perforation site was the 2nd portion of the duodenum image at the papillotomy site.
image
(Left) Axial CECT in a young woman with progressive fever and abdominal pain over several days reveals a large periappendiceal abscess image with mass effect displacing the small bowel, bladder, and uterus.

image
(Right) Axial CECT in a patient who underwent a cholecystectomy shows a collection of gas image, but very little fluid, in the cholecystectomy bed. Surgical clips image are also seen. This represents oxidized cellulose (Surgicel), which was placed for hemostasis at surgery, and not an abscess.
image
Axial CECT shows multiple postoperative abscesses. Note fluid collections with enhancing rims and mass effect image.

image
Axial CECT in the same patient shows ectopic gas bubbles within abscess in pelvis image.
image
Axial CECT of gas-forming retrocecal abscess from perforated diverticulum. Note ectopic gas collection from posterior to cecum image, cecal thickening, and adjacent fat stranding image.
image
Transverse transabdominal ultrasound of postoperative abscess on grayscale image. Note hypoechoic fluid collection image with fluid-fluid level image.
image
Axial CECT of intramural abscess of sigmoid from diverticulitis. Note long segment of markedly thickened sigmoid image and ill-defined adjacent fluid collection image.
image
Axial CECT in the same patient shows more discreet abscess collections on a lower plane of the section image.
image
Axial CECT shows pyogenic postoperative abscess image after bowel resection. Note multiple fluid collections with enhancing rims; gas is seen only in pelvic abscess image.
image
Axial CECT shows pyogenic postoperative abscess after bowel resection. Note multiple fluid collections with enhancing rims. Gas is noted in pelvic abscess image, but not in other collections.
image
Axial CECT demonstrates a gas-forming pyogenic liver abscess in a diabetic patient. Note air-fluid level within the abscess cavity image.
image
Axial ultrasound shows liver abscess, demonstrating linear high-amplitude echoes with “dirty” distal acoustic shadowing representing gas image.
image
Axial CECT shows amebic abscess. Note peripheral low-attenuation zone of edema image surrounding the abscess.
image
Sagittal ultrasound shows amebic abscess. Note low level echoes image within the hypoechoic mass, and the lack of distal acoustic enhancement.
image
Sagittal ultrasound shows fungal microabscesses due to systemic candidiasis. Note multiple “target” lesions image.
image
Sagittal ultrasound of spleen demonstrates abscess with low-level echoes image.
image
Axial CECT shows an encapsulated fluid collection image with peripheral rim enhancement in the right lower quadrant. In the midst of the fluid collection is a tubular structure image, representing a ruptured appendix.
image
Axial CECT in the same patient demonstrates a pigtail catheter image placed percutaneously in the collection, with near resolution of the fluid within the abscess.

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