Imaging of Peritoneal Cavity and Retroperitoneum

Peritoneal & Retroperitoneal Anatomy - Mapping the Maze

• Peritoneal Cavity: Serous membrane-lined space, divided into Greater and Lesser Sacs.
  • Greater Sac: Main, larger portion of the peritoneal cavity.
  • Lesser Sac (Omental Bursa): Smaller sac posterior to the stomach. Communicates with Greater Sac via Foramen of Winslow (epiploic foramen).
  • Key Fluid Collection Sites (Potential Spaces):
    • Morison's Pouch (hepatorenal space): Most dependent part of upper abdomen when supine.
    • Pouch of Douglas (rectovesical or rectouterine pouch): Most dependent part of peritoneal cavity in pelvis.
    • Paracolic Gutters (right & left): Channels for fluid and spread of infection/malignancy.
• Retroperitoneum: Space posterior to the parietal peritoneum.
  • Anterior Pararenal Space: Contains pancreas, duodenum (2nd-4th parts), ascending & descending colon.
  • Perirenal Space: Encases kidneys, adrenal glands, ureters, aorta, IVC, and perirenal fat.
  • Posterior Pararenal Space: Contains fat and nerves.

⭐ The right paracolic gutter is typically wider and deeper than the left, providing a major pathway for fluid movement from the pelvis to the upper abdomen, particularly Morison's pouch.

Imaging Modalities - Picking Your Probe

• X-ray (KUB): Initial: gas, calcifications, foreign bodies. Limited soft tissue.
• Ultrasound (USG): First-line: free fluid, collections, biliary/renal. Real-time, no radiation. Guides interventions. Doppler.
• Computed Tomography (CT): Workhorse: trauma, acute abdomen, staging.
  • NCCT: Calculi, hemorrhage.
  • Arterial Phase (20-30s): Hypervascular lesions, CTA.
  • Portal Venous Phase (60-70s): Standard, parenchymal organs.
  • Delayed Phase (3-15 min): GU tract, cholangiocarcinoma, washout.
  • 📌 Phases: No Apple Pie Daily (Non-contrast, Arterial, Portal, Delayed).
• Magnetic Resonance Imaging (MRI): Problem-solving, superior soft tissue (e.g., rectal Ca staging). No radiation.

⭐ CT is the primary modality for evaluating acute abdomen and is crucial for detecting and staging peritoneal carcinomatosis.

Peritoneal Pathologies - Cavity Conundrums

• Ascites: Fluid accumulation.
  • Types: Transudative (cirrhosis), Exudative (infection, malignancy).
  • Imaging: USG (anechoic/complex), CT (fluid density, displaced bowel loops).
  • Sites: Morison's pouch, paracolic gutters, Pouch of Douglas.
• Peritonitis: Peritoneal inflammation.
  • Causes: Infection (bacterial, TB), sterile.
  • CT: Peritoneal thickening/enhancement, mesenteric stranding, abscess.
• TB Peritonitis:
  • Types: Wet (ascites), Dry (adhesions), Fibrotic-fixed (masses).
  • CT: Smooth peritoneal thickening, omental caking, low-density nodes.
• Peritoneal Carcinomatosis: Peritoneal metastases.
  • Primaries: Ovarian, gastric, colorectal.
  • CT: Nodular peritoneal/omental thickening (caking), malignant ascites.
• Hernias: Organ protrusion via defect.
  • Types: Inguinal, femoral, umbilical, incisional, internal.
  • CT: Defines hernia, contents, complications (strangulation).

⭐ Sister Mary Joseph Nodule: Umbilical metastasis, often from GI/gynecological cancers, indicating peritoneal spread.

Retroperitoneal Pathologies - Behind-the-Lining Lesions

• Key sign: Anterior displacement of bowel/pancreas. Spaces: Anterior pararenal, perirenal, posterior pararenal.
• Retroperitoneal Collections:
  • Hematoma: Trauma, anticoagulation, AAA rupture. CT: Acute hyperdense, chronic hypodense. Sentinel clot sign.
  • Abscess: Pancreatitis, pyelonephritis, IBD. CT: Rim-enhancing fluid, gas bubbles, inflammatory stranding.
• Retroperitoneal Tumors:
  • Primary: Liposarcoma (most common, CT: fat attenuation), leiomyosarcoma.
  • Secondary: Lymphoma (commonest), metastases (renal, adrenal, GI).
• Retroperitoneal Fibrosis (Ormond's Disease):
  • Idiopathic (70%) or secondary (malignancy, drugs, infection).
  • CT/MRI: Periaortic/peri-iliac enhancing soft tissue, encasing structures (e.g., ureters leading to hydronephrosis).

  ⭐ In idiopathic Retroperitoneal Fibrosis, ureters are typically drawn medially; lateral displacement suggests malignancy.

High‑Yield Points - ⚡ Biggest Takeaways

• CT is the primary modality for evaluating peritoneal and retroperitoneal pathology.
• Pneumoperitoneum: Upright CXR for initial detection; CT confirms and localizes source.
• Ascites: CT attenuation helps differentiate types (transudate, exudate, blood).
• Retroperitoneal spaces (anterior pararenal, perirenal, posterior pararenal) are key for localization.
• Mesenteric ischemia: CTA is crucial; look for vascular signs, bowel changes, pneumatosis.
• Peritoneal carcinomatosis: CT reveals omental caking, nodules, and malignant ascites.
• Retroperitoneal fibrosis: CT/MRI shows peri-aortic/ureteric soft tissue encasement.