Peritoneum, Retroperitoneum and Abdominal Wall

Calcified LN from granulomatous disease (TB/histoplasmosis) mottled 10-15mm, most common in mesentery; esp from drinking unpasteurised milk. Phleboliths are round/oval, up to 5mm, commonly with central lucency. Nodes are enlarged if >15mm maximum short axis diamter if solitary, >10mm multiple.


Morison pouch (R hepatorenal fossa) is the most dependent portion in supine patient, collecting fluid and metastases. Pouch of Douglas (females) most dependent when erect. Ascites seen on XR if >500mL with veiling density (grey abdomen), indistinct liver/spleen/psoas margins, medially displaced organs, bulging flank stripe, increased SB separation, “dog’s ears” (fluid either side of bladder). Ascites is serous (-10 to 10 HU from cirrhosis, hypoproteinemia, CHF), exudative (>15HU from abscess, pancreatitis, peritonitis, perforation), haemoperitoneum (acute ~ 45HU from trauma, surgery or spontaneous) or neoplastic; may also include urine, bile, chyle. More complicated ascities may contain debris, septations associated with inflammatory or malignant process. High density fluid also seen in TB.


From bowel perforation (most commonly duodenum or gastric ulcer; caecum tends to perforate when >9cm, in toxic megacolon >5.5cm), trauma, surgery (usually resolves 3-4 days, progressively decreases), gas-producing organisms, orogenital insufflation, COPD, alveolar rupture. Signs on supine XR include Rigler’s sign (both sides bowel wall can be seen due to free gas), outlining of falciform ligament and liver edges, football sign (gas under anterior abdo wall), triangular/linear gas in RUQ, cupola sign – air in the anterior central leaf of diaphragm.

Pneumatosis cystoides intestinalis/coli – spherical or oval subserosal gas, benign.


Irritants causing peritonitis include:

  • Perforated biliary tract with bile leak – Induces severe peritonitis, usually with bacterial superinfection.
  • Acute pancreatitis – Leakage of pancreatic enzymes induces fat necrosis with globules of fat in fluid, damage to bowel causing leak of bacteria.
  • Foreign body – Granuloma type reaction with fibrous scarring.
  • Endometriosis – Haemorrhage into the peritoneum, blood acting as an irritant.
  • Ruptured dermoid cyst – Keratins invoke intense granulomatous reaction.
  • Perforated hollow viscus – E.coli, streotococci, S.aureus, enterococci, C.perfringens.
  • Spontaneous bacteral peritonitis – Uncommon, most in patients with cirrhosis and ascites (see in 10%), children with nephrotic syndrome. Unknown route of entry for bacteria.

Sentinal loop – Single dilated loop adjacent to inflammation.

Abscesses are loculated fluid collections, thick irregular walls, may have internal debris, fluid-fluid level, gas is highly specific, adjacent fascial thickening, increased density of adjacent fat with stranding.


Serosa-lined pouch of peritoneum (hernia sac). Acquired hernias are most commonly anteriorly. Venous stasis and oedema leads to incarceration (permanent entrapment) then arterial and venous compromise (strangulation) leading to infarction. Types include:

  • Inguinal – Direct is medial to the inferior epigastric vessels; indirect is lateral.
  • Femoral
  • Obturator
  • Umbilical
  • Surgical
  • Spigelian (lateral ventral) – Through the spigelian fascia, between rectus abdominus and semilunar line.


Retroperitoneal tumours – 80% are malignant, large, heterogeneous and cystic, displacing and compressing organs.

  • Lipoma – Well-defined, homogeneous fat.
  • Liposarcomas – Heterogeneous with areas of fat.
  • Teratoma
  • Lymphangioma
  • Neurogenic tumours – Schwannoma, neurofibroma, ganglioneuroma.
  • Lymphoma
  • Desmoid tumour
  • Malignant mesenchymoma
  • Fibrosarcoma
  • Leiomyosarcoma – Most are extravascular, some intravascular (1/3 involve IVC). Large soft tisue mass isodense to muscle, necrosis, cystic degeneration.


Normal nodes are oval, homogeneous, low T1 high T2. Lymphadenopathy includes single enlarged nodes (maximum short axis >10mm except retrocrural 6mm, porta 6, gastrohepatic ligament 8, pelvic 15), multiple separate lobulated nodes, bulky conglomerate masses. Cacification in inflammation, mucinous adenocarcinomas, carcinomas, sarcomas, treated lymphoma. Most pathological nodes have low HU < muscle, lower density nodes include nonseminomatous testicular carcinoma, TB, lymphoma. Sonographic silhouette sign – conglomerate retroperitoneal nodes obscuring echognic aortic wall. Sandwich sign – entraped mesenteric vessels.

Peritoneal metastases from ovaries, colon, stomach, or pancreatic carcinoma. Implants in pelvic cul-de-sac, right paracolic gutter, greater omentum. Peritoneal surface nodules, omental caking, displaced bowel from anterior abdominal wall, thickened nodular bowel wall (serosal implants), ascites which may be loculated with debris.

Carcinoid and adenocarcinoma metastases cause prominent mesenteric desmoplastic reaction (cf none in melanoma).

Pseudomyxoma Peritonei

Gelatinous ascites from intraperitoneal spread of mucin producing cells, ie ruptured appendiceal mucocele, mucinous adenocarcinoma (ovary, colon, rectum). Punctate/ringlike calcifications, mottled densities, septations, mass effect with scalloping on liver and bowel.

Peritoneal Mesothelioma

Primary tumour of the peritoneal lining. 20-30% of mesotheliomas, almost always associated with asbestos (?swallowed asbestos fibres penetrate through intestinal wall). Nodular irregular peritoneal and omental thickening/masses forming large plaques and omental caking. Adjacent bowel may be involved and fixed. Rarely multilocular cystic form. Poor prognosis. DDx metastatic adenocarcinoma.


Bulky adenopathy, confluent around mesenteric vessels with sandwich sign (preserved fat around vessels).

Mesenteric Desmoid Tumour

Benign but locally aggressive solid fibrous tumour. Most multiple in mesentery, muscles of anterior abdominal wall and psoas. Most associated with Gardner syndrome. Homogeneous mass, well-defined or infiltrative borders.

Mesenteric Teratoma

Heterogneous with cystic and solid components, may contain calcium or fat.

Desmoplastic Tumour

Primary malignant tumour of the peritoneum and retroperitoneium in children and young adults. Small round cell tumour similar to Ewing sarcoma etc.

TB peritonitis

GI Duplication Cyst

Congenital partial or complete replica of SB lined by intestinal epithelium, most from distal SB. May communicate with normal lumen. Thick-walled cyst containing layers of bowel wall, serous fluid. May have adenocarcinomas within.

Sclerosing Peritonitis

Unknown aetiology. Uncommon, variable inflammation, fat necrosis and fibrosis in root of mesentery. Soft tissue infiltration (‘misty mesentery’).


(Mesenteric cyst). Benign cystic lymphatic vascular channels. Most thin-walled, septations and mutiple loculations with chylous, serous, haemorrhagic or mixed fluid. Fluid-debris or fluid-fat levels. In omentum, root of SB mesentery, mesocolon or retroperitoneum. May be complicated by infection or haemorrhage.

AIDS in the Abdomen

Multiple opportunistic infections and malignancies at multiple sites. 90% have abdominal involvement. With advent of better drug therapy, patients live longer with increasing malignancy.

  • Extrapulmonary P jiroveci infection in liver, spleen, kidney, pancreas, lymph nodes. Diffuse or punctate calcifications.
  • Oesophagitis, gastric antritis, duodenitis from Candida albicans and CMV.
  • Cryptosporidium, Isospora belli protazoan severe diarrhoea.
  • Cholangitis (resembling sclerosing cholangitis), acalculous cholecystitis from Cryptosporidium, CMV.
  • Other infections include TB, MAI, herpesvirus, toxoplasma gondii, enteamoeba histolytica, giardia lamblia, cryptococcus neoformans.
  • Kaposi sarcoma (KS) – Multicentric arising from lymphatic epithelium in all tissues. ?From herpes-type virus transmitted primarily by anal intercourse (KS is uncommon in women and heterosexual men). Typically vascular nodule on skin or mucous membranes (eg GIT). Classic KS is limited, confined to face, extremities, oral mucosa, may convert to epidemic KS at any time. Epidemic KS is disseminated with lesions in LN (brightly enhancing), visceral organs (liver uniform or ring-enhancing nodules), GIT (in 40-50% with nodules, plaques, polyps, thickened folds), bone marrow.
  • AIDS-related lymphoma (ARL) – Extremely aggressive, responds poorly to therapy, commonly extranodal in CNS, bone marrow, GIT (focal or difuse wall thickening with eccentric homogeneous mass, rectal/perianal common), liver (multiple/single hypodense lesions), spleen (but minimal/absent hepatosplenomegaly unless focal lesiosn present), kidney (hypodense nodules).
  • Adenopathy – Persistent generalised <10mm is reactive hyperplasia preceding onset of AIDS. >15mm suggests ARL/KS/TB/MAI.

Pelvic Lipomatosis

Excessive perirectal and perivesical fat. Bladder elongated and lifted up out of pelvis.

Retroperitoneal Fibrosis

(Sclerosing retroperitonitis). Rare fibrous plaque in lower retroperitoneum (usually L4-5) encasing and compressing aorta, IVC, medial deviation of ureters causing narrowing and hydronephrosis. Early stage ceullar and oedematous; later dense hyalinised collagen. May extend to involve the mesentery. Smooth extrinsic narrowing of ureter with proximal hydronephrosis, may cause teardrop configuration to bladder, narrowing of sigmoid colon. Doesn’t tend to extend behind vessels (cf lymphoma).

  • Primary/idiopathic (70%, Ormond disease) – ?Autoimmune. 15% also have other process eg mediastinal fibrosis, Riedel thyroiditis, sclerosing cholangitis, orbital pseudotumour.
  • Secondary – From drugs (methysergide, ergotamine, beta-blockers), adjacent inflammatory conditions (vasculitis, diverticulitis, Crohn diseaes, inflammatory aneurysm), malignant diseae (lymphoma, urinary tract, small metastatic foci eliciting fibrotic reaction), radiation, TB, syphilis, actinomycosis, fungi.