Intra-abdominal abscess is an organized, contained infected fluid collection within the abdomen, holding significant importance among surgical emergencies. Most common causes include postoperative complications (50% — especially colorectal, appendectomy, bariatric surgery), gastrointestinal perforation (perforated appendicitis, diverticulitis, peptic ulcer), pancreatitis (infected necrosis, infected pseudocyst), trauma, and Crohn's disease complications. Bacteriology is generally polymicrobial with gram-negative enteric bacilli (E. coli, Klebsiella), anaerobes (Bacteroides fragilis), and enterococci being most commonly isolated. CT is the most sensitive diagnostic modality (95% sensitivity) showing rim enhancement (abscess wall/capsule), central low-density fluid collection, gas bubbles, and surrounding inflammatory fat stranding. Treatment includes antibiotics plus percutaneous drainage (for abscesses >3 cm) or surgical drainage (when percutaneous access not feasible). Untreated abscesses can lead to sepsis, multi-organ failure, and death — mortality reaches 80% in untreated cases.
Age Range
20-80
Peak Age
50
Gender
Equal
Prevalence
Common
Intra-abdominal abscess formation follows the sequence: bacterial contamination → inflammatory response → localization → maturation. Bacterial contamination usually originates from GI perforation or surgical contamination — enteric flora (E. coli, Bacteroides, enterococci) spill into the peritoneal cavity. Acute inflammatory response involves neutrophil migration, complement activation, and cytokine release (TNF-alpha, IL-1, IL-6). Peritoneal defense mechanisms attempt to localize infection: the omentum migrates to the inflammation site ('abdominal policeman') and forms fibrin network limiting the infected area. Successful localization creates the abscess capsule (fibrosis and granulation tissue) with central necrotic/purulent material accumulation. Neovascularization of the abscess wall forms the basis of rim enhancement on CT — newly formed capillaries accumulate contrast creating a bright peripheral ring. The central purulent material is avascular showing no enhancement and appears as low-density (10-30 HU) fluid. Gas bubbles result from metabolic activity of gas-forming anaerobic bacteria (especially Bacteroides, Clostridium) producing CO₂, H₂, and H₂S through fermentation. On DWI, purulent content shows marked diffusion restriction — high cellular density (neutrophils, bacteria, debris) and high viscosity restrict water diffusion; this is the most sensitive MR sequence for abscess diagnosis and differentiates from simple fluid collections (seroma, lymphocele).
Rim enhancement is enhancement of the abscess capsule/wall as a bright peripheral ring in portal venous phase. Granulation tissue and neovascularization cause contrast accumulation in the capsular area; central purulent material is avascular showing no enhancement. This density contrast (bright ring + dark center) is the cornerstone of CT abscess diagnosis. Double target sign (inner ring: granulation tissue, outer ring: surrounding reactive inflammation) may be seen in advanced cases.
Portal venous phase CT shows fluid collection with rim enhancement (bright peripheral ring). Abscess wall/capsule shows smooth or irregular enhancement; wall thickness usually 2-5 mm. Central fluid collection shows low density (10-30 HU) without enhancement — purulent content is avascular. Gas bubbles within collection appear as very low density foci (-1000 HU) — pathognomonic for gas-forming anaerobic bacteria. Surrounding fat stranding represents inflammatory exudation into peritoneal/mesenteric fat. Reactive thickening of adjacent organ walls may be seen. Abscess anatomical location provides source clue: right lower quadrant (appendicitis), left lower quadrant (diverticulitis), subphrenic (postoperative), pelvic (gynecologic), perihepatic (biliary).
Report Sentence
Fluid collection with rim enhancement and gas bubbles in [location] consistent with intra-abdominal abscess; percutaneous drainage should be considered.
DWI shows marked diffusion restriction in intra-abdominal abscess — hyperintensity at high b-values (b=800-1000) and low ADC values on ADC map (usually <1.0 × 10⁻³ mm²/s, often <0.7 × 10⁻³ mm²/s). High viscosity of purulent content (thick, pus-consistency fluid) and dense cellular components (neutrophils, bacteria, fibrin, debris) restrict Brownian motion of water molecules. This is the most sensitive MR sequence for abscess diagnosis (95% sensitivity) and critical for differentiating from simple serous collections (seroma, lymphocele, biloma — high ADC, no diffusion restriction). Abscess wall usually shows intermediate DWI signal; central purulent content shows the most marked restriction.
Report Sentence
Collection shows marked diffusion restriction on DWI (ADC: [value] × 10⁻³ mm²/s) consistent with purulent content (abscess).
B-mode ultrasonography shows abscess as complex fluid collection — internal echoes (debris, fibrin), septations, thick wall, and irregular contour. Unlike simple anechoic collections, internal echogenic components are present. Gas bubbles appear as hyperechoic foci with 'dirty shadowing'. Doppler shows increased peripheral vascularity (hyperemic ring) in abscess wall; no vascularity in central fluid. US plays primary role in percutaneous drainage planning and guidance — safe access route, distance, and adjacent structures evaluated. US is less sensitive than CT for deep abscesses (retroperitoneal, interloop).
Report Sentence
Complex fluid collection with internal echoes and septations in [location] consistent with abscess on ultrasonography.
Non-contrast CT shows gas bubbles (-1000 HU) within the collection as a pathognomonic finding. Gas bubbles appear as small, round, very low-density foci accumulating at the superior aspect of the fluid collection (antigravitational). Gas-fluid level (air-fluid level) may be visible. Gas presence strongly supports abscess diagnosis but may be from bowel perforation-related free gas or iatrogenic (postoperative residual gas) — clinical correlation required. Fluid collection density ranges 10-30 HU in purulent content; hemorrhagic abscess may show higher density (30-50 HU).
Report Sentence
Gas bubbles within the collection suggest gas-forming bacterial infection (abscess).
Arterial phase evaluates inflammatory changes surrounding the abscess. Fat stranding — density increase in peritoneal and mesenteric fat (normal fat -80 to -100 HU rising to -20 to +20 HU) indicates inflammatory exudation. Reactive thickening and increased enhancement of adjacent bowel walls may be seen. Peritoneal thickening and enhancement reflect local peritonitis. Abscess wall may not show prominent enhancement in arterial phase — rim enhancement is more prominent in portal venous phase. Phlegmon (unorganized inflammatory change) may precede abscess and appears as ill-defined fat stranding area without formed collection/capsule.
Report Sentence
Extensive fat stranding surrounding the abscess with reactive thickening of adjacent bowel walls supporting active inflammatory process.
Labeled leukocyte (Tc-99m or In-111 labeled WBC) scintigraphy shows focal increased radiopharmaceutical accumulation at abscess site. This imaging method is used when CT is indeterminate or in chronic/atypical abscess suspicion. In-111 labeled leukocytes show 24-48 hour uptake with high specificity. Tc-99m HMPAO labeled leukocytes provide earlier imaging (4-6 hours). SPECT/CT fusion improves anatomical localization accuracy. Normal GI elimination limits accuracy for abdominal abscesses but value increases in chronic and atypical presentations.
Report Sentence
Focal increased uptake on labeled leukocyte scintigraphy in [location] consistent with active infectious/inflammatory focus (abscess).
Criteria
Most common abscess type developing after surgery (50%). Colorectal, appendectomy, bariatric, and hepatobiliary surgery are most common causes. Usually presents on postoperative days 5-14. Anastomotic leak, contaminated field, hematoma superinfection are etiological factors.
Distinct Features
Localized adjacent to surgical field — anastomotic line vicinity is most common location. Oral contrast CT may detect leak in anastomotic failure. Collection near surgical clips/stapler line strongly suggests postoperative abscess. Early CT (postoperative day 3-5) important for baseline assessment.
Criteria
Develops after GI perforation (appendicitis, diverticulitis, peptic ulcer, Crohn's). Located adjacent to perforation site. Free intraperitoneal air (pneumoperitoneum) may accompany. Identifying perforation source (appendicolith, diverticulum, ulcer crater) is critical for treatment planning.
Distinct Features
Perforated appendicitis: right lower quadrant/pelvic location with appendicolith. Perforated diverticulitis: left lower quadrant with diverticular changes. Perforated peptic ulcer: subphrenic/perihepatic gas with ulcer crater. Crohn's disease: ileocecal location with fistula formation.
Criteria
Infected necrosis or infected pseudocyst developing after acute pancreatitis. Usually develops 2-4 weeks after pancreatitis onset. Gas bubbles in necrotic collection are the most reliable CT finding of infection. Fine-needle aspiration (FNA) with Gram stain confirms infection.
Distinct Features
Peripancreatic location with pancreatitis history. Infected necrosis contains necrotic tissue (heterogeneous, non-enhancing solid debris) with infected fluid — walled-off necrosis (WON) terminology used. Step-up approach preferred: antibiotics → percutaneous drainage → minimally invasive necrosectomy → surgical necrosectomy. Mortality 15-30%.
Distinguishing Feature
Mesenteric hematoma shows high density (50-70 HU) on non-contrast CT; abscess shows central low density (10-30 HU) with rim enhancement. No gas in hematoma (unless superinfected); gas is pathognomonic in abscess. Hemoglobin drop and anticoagulant history in hematoma; fever, leukocytosis, and infection source expected in abscess. DWI shows marked diffusion restriction in abscess; minimal or no restriction in acute hematoma.
Distinguishing Feature
Mesenteric cyst is thin-walled (<3 mm), well-defined, water-density (0-20 HU) cystic mass; abscess is thick-walled (>3 mm), irregular with rim enhancement and gas. No surrounding fat stranding with cyst; prominent fat stranding with abscess. Cyst usually asymptomatic; abscess presents with fever, pain, leukocytosis. No DWI restriction in cyst; marked restriction in abscess.
Distinguishing Feature
PMP shows low-density mucinous ascites with peritoneal implants; abscess is localized rim-enhancing collection. Scalloping is pathognomonic in PMP — absent in abscess. Gas not expected in PMP; pathognomonic in abscess. PMP is chronic/slow; abscess presents acutely. PMP mucinous fluid usually shows no diffusion restriction on DWI; abscess shows marked restriction.
Distinguishing Feature
Bacterial peritonitis is diffuse peritoneal inflammation showing widespread peritoneal thickening, ascites, and fat stranding rather than a localized collection (abscess). Abscess is localized and contained. Free ascites in peritonitis; contained collection in abscess. Diffuse bowel wall thickening in peritonitis; local reactive changes adjacent to abscess. Peritonitis requires emergency surgical exploration; abscess may be treated with percutaneous drainage.
Urgency
emergentManagement
interventionalBiopsy
Not NeededFollow-up
3-monthIntra-abdominal abscess requires urgent diagnosis and treatment. Treatment algorithm: (1) Broad-spectrum antibiotics — gram-negative, anaerobic, and enterococcal coverage (typically piperacillin-tazobactam or meropenem + metronidazole). (2) Percutaneous drainage — CT or US-guided catheter placement for abscesses >3 cm; success rate 80-90%. Gram stain and culture from drainage fluid. (3) Surgical drainage — when percutaneous access not feasible. (4) Source control — perforation source surgically repaired or resected. Follow-up: clinical improvement and collection resolution on CT. Antibiotic duration: 4-7 days after source control. Mortality: 10-20% with treatment; 80% without.
Intraabdominal abscess can lead to sepsis and multi-organ failure if untreated. Percutaneous drainage (CT or US-guided) is the primary treatment. Antibiotic therapy accompanies drainage. Surgery may be required if drainage is insufficient. Treatment of the underlying cause is important.