Diagnostic Ultrasound - Abdomen and Pelvis

Peritoneal Space Abscess
TERMINOLOGY
Definitions
• Localized abdominal collection of pus
IMAGING
General Features
• Best diagnostic clue
○ Fluid collection with mass effect ± gas bubbles or air-fluid
level
• Location
○ Anywhere within abdominal cavity
○ Typical intraperitoneal spaces are cul-de-sac, Morison
pouch, and subphrenic spaces
• Size
○ Highly variable; 2-15 cm in diameter or diffuse peritoneal
collection
• Morphology
○ Hypoechoic or anechoic fluid collection ± septations and
debris
Ultrasonographic Findings
• Grayscale ultrasound
○ Complex fluid collection with internal low-level echoes,
membranes, or septations
○ Dependent echoes representing debris may produce
fluid-fluid level
○ Bright linear echoes with reverberation artifacts
representing gas bubbles; highly suggestive of infection
○ Inflamed fat adjacent to abscess presents as echogenic
mass
– Usually seen with abscesses due to appendicitis,
diverticulitis, complicated acute cholecystitis,
inflammatory bowel disease, and pancreatitis
○ Bacterial peritonitis: Primary or secondary to other
intraabdominal infection or perforated viscus
– Ascites with internal echoes from particulate debris or
pus, loculations, internal septations, gas
– Diffuse thickening of peritoneum (parietal and
visceral), mesentery, and omentum
○ Postoperative peritoneal abscess
– Close to site of surgery, around the tip of drainage
catheter (if blocked), dependent parts of peritoneal
cavity (supine patients)
○ Tuberculous peritonitis: Matted bowel loops with
heterogeneous interbowel exudate
– ± necrotic lymphadenopathy (mesenteric and
retroperitoneal), may progress to liquefaction and
abscess formation
○ Sclerosing peritonitis: Major complication of continuous
ambulatory peritoneal dialysis (CAPD) with secondary
infection
– Hyperperistaltic bowel loops with both free and
loculated ascites (earlier sign)
– Later: Matted, clumped bowel loops tethered to
posterior abdominal wall by uniformly echogenic
enveloping membrane (1-4 mm thick)
• Color Doppler
○ Hypervascular periphery, avascular center of abscess;
adjacent inflamed fat may be hyperemic
Radiographic Findings
• Radiography
○ Insensitive
○ Abnormal gas collection with air-fluid level
○ Soft tissue "mass" or focal ileus
○ Subphrenic abscess: Pleural effusion and lower lobe
atelectasis
Fluoroscopic Findings
• Abscess sinogram
○ Useful after percutaneous drainage, defines residual
cavity
○ Detection of fistulas to bowel, pancreas, or biliary duct
CT Findings
• Low-attenuation fluid collection with peripheral rim
enhancement
• ± gas/mass effect/fat stranding
MR Findings
• T2-bright fluid collection with enhancing rim post contrast
• High T1 signal suggests hemorrhagic, proteinaceous, or
mucinous content
Nonvascular Interventions
• US-guided: Diagnostic or therapeutic aspiration and
percutaneous abscess drainage (PAD)
Nuclear Medicine Findings
• In-111 WBC scan preferred
○ Persistent nonphysiologic activity, focal or diffuse
• Tc-99m-labeled HMPAO WBC: Alternative for pediatric
patients or patients with inflammatory bowel disease
Imaging Recommendations
• Best imaging tool
○ CECT: More sensitive for deeper, larger, or gascontaining
collections and as screening test
○ Bedside US: For critically ill or postoperative patients, can
be effective screening tool to localize intraperitoneal
abscess or collections
– US may be suboptimal due to limited patient mobility,
open wounds, dressings, drainage tubes, paralytic
ileus
○ MR: Alternative to CT for patients with contrast allergy
or impaired renal function
• Protocol advice
○ CECT: Oral and IV contrast for best accuracy
○ US: Evaluation of dependent portion of peritoneal cavity
or area surrounding operative site with transabdominal
and transvaginal probes
○ Scanning over sites of tenderness improves detection,
abscesses tender with transducer pressure
DIFFERENTIAL DIAGNOSIS
Loculated Ascites
• Evidence for cirrhosis or chronic liver disease
• Passively conforms to peritoneal space
• May contain septations and thinner, smoother wall
Diagnoses: Abdominal Wall/Peritoneal Cavity
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