Gastrointestinal and Hepatobiliary Imaging

Plain films, emergencies and children

• 3‑6‑9 rule (small bowel 3, colon 6, caecum 9 cm). Valvulae conniventes (complete, central) vs haustra (incomplete, peripheral). Step‑ladder = SBO. CT = IOC (cause, closed loop, strangulation).
• SBO: adhesions are common even without prior surgery; also assess hernias and other causes; LBO: colorectal cancer, volvulus. Competent IC valve → caecal perforation risk.
• Free gas: erect CXR best plain film; CT most sensitive. Rigler (double wall), football, falciform ligament, cupola, Doge's cap (Morison's pouch). Chilaiditi = mimic.
• Pneumobilia central (ERCP, anastomosis, gallstone ileus); portal venous gas peripheral (ischaemia, NEC). Rigler's triad: pneumobilia + SBO + ectopic stone.
• Sigmoid volvulus: coffee bean, bent inner tube, Frimann‑Dahl; bird's beak on enema; decompress. Caecal: comma/embryo in LUQ.
• HPS: US ≥ 3 mm thick / ≥ 15 mm long; single bubble. Double bubble = duodenal atresia (Down). Triple = jejunal. Malrotation: upper GI study (DJ flexure low/right), whirlpool on US; bilious vomiting = emergency. Intussusception: US target/pseudokidney; claw sign; air enema reduction. NEC: pneumatosis. Hirschsprung: transition zone, RS index < 1. Meconium ileus: microcolon, soap bubble.

Contrast studies, stomach and bowel

• Leak → water‑soluble contrast. Fistula‑in‑ano IOC = MRI.
• Achalasia: bird's beak, no fundal gas, Hurst phenomenon; manometry gold standard. Carcinoma: shouldered irregular (apple‑core/rat‑tail). DES: corkscrew. Web: anterior post‑cricoid (Plummer–Vinson). Zenker: posterior, Killian's dehiscence. Boerhaave: Naclerio V.
• Benign gastric ulcer: projects outside, Hampton line, radiating folds. Malignant: within lumen, Carman meniscus, clubbed folds. Linitis plastica = leather bottle. Sliding hernia (GEJ up) vs rolling (fundus up, volvulus risk). Reverse‑3 (Frostberg) = pancreatic head mass.
• Crohn's: terminal ileum, skip, string sign of Kantor, cobblestone, comb sign, creeping fat, fistulae; CT/MR enterography. UC: rectum‑up continuous, collar‑stud ulcers, lead‑pipe, toxic megacolon > 6 cm. TB: conical caecum, Fleischner (inverted umbrella), Stierlin, necrotic nodes.
• Diverticulitis: CT. Apple‑core = colorectal cancer; rectal staging MRI. Mexican hat = pedunculated polyp.
• Appendicitis: US (children), CT (adults), MRI (pregnancy); > 6 mm non‑compressible; caecal bar, arrowhead; Alvarado. AMI: CTA (SMA embolus; venous = thick wall); pneumatosis + portal gas late.

Liver, biliary tree and pancreas

• Abscess: US first; CT double target; pyogenic cluster sign, multiple, biliary source; amoebic single right lobe posterosuperior.
• Hydatid (WHO‑IWGE): CE1 sand/double wall; CE2 daughter cysts (rosette); CE3a water‑lily; CE4 ball of wool; CE5 calcified. Lung: water‑lily, Cumbo sign.
• Liver: 75% portal / 25% arterial; HCC arterial. Haemangioma: peripheral nodular centripetal fill, light‑bulb T2. FNH: central scar T2 bright, late enhancement. Fibrolamellar: young, normal AFP, calcified T2‑dark scar. HCC: APHE + washout + capsule (LI‑RADS 5), portal vein tumour thrombus. Mets commonest malignant; hypervascular: NET, RCC, thyroid, melanoma, choriocarcinoma.
• Cirrhosis: caudate hypertrophy, nodular surface, splenomegaly, varices. Surveillance US ± AFP 6‑monthly.
• Gallstones: US, shadowing, mobile; WES sign; 10–15% radio‑opaque; Mercedes‑Benz. Cholecystitis: wall > 3 mm, fluid, sonographic Murphy; HIDA most specific (rim sign = gangrene). MRCP for CBD stones. Adenomyomatosis: comet tails. GB cancer: north India, segments IV/V. Todani I commonest; V = Caroli (central dot).
• Pancreatitis: US for stones; CECT at 72–96 h; Atlanta APFC/pseudocyst (> 4 wk) vs ANC/WON; Balthazar CTSI; sentinel loop, colon cut‑off. Chronic: calcification, chain of lakes; tropical calcific (south India). Cancer: hypoenhancing mass, double duct sign, > 180° SMA encasement unresectable. Serous cystadenoma: central stellate calcified scar.