Gastrointestinal and Abdominal Radiology
Core concepts
Build a systematic approach to the acute abdomen: bowel obstruction (dilated loops, air-fluid levels, transition point), perforation (free intraperitoneal air, best seen on upright or decubitus views), ischemia (pneumatosis, portal venous gas, thumbprinting), and inflammation. Know the CT features of appendicitis (dilated appendix >6 mm, periappendiceal fat stranding, appendicolith) and diverticulitis (left-sided, pericolic stranding, abscess). Understand the liver: segmental anatomy by the portal veins and hepatic veins, and the imaging hallmarks of hemangioma (peripheral nodular enhancement with centripetal fill-in), HCC (arterial enhancement with washout), and metastases. Recognize the double-duct sign, the target sign of intussusception, and the coffee-bean sign of sigmoid volvulus.
Traps
Do not call every dilated bowel an obstruction: ileus shows diffuse dilation without a transition point. Free air on supine radiographs may be subtle (Rigler sign, falciform ligament sign). Beware attributing right lower quadrant pain to appendicitis when the appendix is retrocecal or when a Meckel diverticulum or mesenteric adenitis is present. Distinguish a true diverticulum from a pseudodiverticulum. On CT, do not mistake a collapsed stomach or bowel for a mass.
Rule of thumb
For any acute abdominal series, look for gas outside the bowel first, then a transition point, then the caliber and distribution of loops.
Worked example
A patient with left lower quadrant pain and fever: CT shows a thickened sigmoid colon with pericolic fat stranding and a small fluid collection, consistent with complicated diverticulitis; the collection dictates percutaneous drainage rather than antibiotics alone.