Gastrointestinal Imaging
Core framework
GI imaging questions test pattern recognition in the acute abdomen, chronic liver disease, and pancreaticobiliary disease. For the acute abdomen, know CT signs of appendicitis (dilated appendix >6 mm, periappendiceal fat stranding, appendicolith, arrowhead sign), diverticulitis (left-sided, pericolic fat stranding, with Hinchey staging), and small bowel obstruction (transition point, bowel >3 cm, valvulae conniventes). Closed-loop obstruction and pneumatosis/portal venous gas are surgical emergencies.
Liver
HCC in a cirrhotic liver is diagnosed noninvasively using LI-RADS: arterial phase hyperenhancement plus washout and/or capsule, with LR-5 indicating definite HCC. Cholangiocarcinoma shows peripheral rim enhancement with delayed central enhancement and capsular retraction - a classic trap when distinguished from HCC. Hemangioma shows peripheral nodular enhancement with centripetal fill-in. Focal nodular hyperplasia has a central scar and retains hepatocyte-specific contrast on hepatobiliary phase, unlike adenoma (except the β-catenin subtype).
Pancreas and bowel
Pancreatic adenocarcinoma: hypoenhancing mass with ductal cutoff and upstream atrophy. Acute pancreatitis: interstitial edematous versus necrotizing, with revised Atlanta severity. Inflammatory bowel disease: mural stratification, skip lesions (Crohn) versus continuous involvement (ulcerative colitis).
Traps and rule of thumb
Traps: calling a hypoenhancing mass with delayed enhancement an HCC; missing a second transition point; forgetting that a normal-appearing appendix does not exclude early appendicitis if the cecum is inflamed (cecal barotrauma).
Rule of thumb: in cirrhosis, if the lesion shows arterial hyperenhancement with washout, think LR-5/HCC; if it shows progressive delayed enhancement with capsular retraction, think cholangiocarcinoma.