Abdominal and Gastrointestinal Imaging
Core concepts
Master the acute abdomen: free air (pneumoperitoneum), bowel obstruction (transition point, small vs large bowel), ischaemia (pneumatosis, portal venous gas), and appendicitis (enlarged appendix, fat stranding). Know liver lesions: haemangioma (peripheral nodular enhancement), HCC (arterial enhancement with washout in cirrhosis), metastasis (hypovascular). Understand biliary disease (gallstones, choledocholithiasis, cholangiocarcinoma) and pancreatitis (interstitial vs necrotising, complications). Learn bowel wall patterns: target sign, thumbprinting, and the CT features of Crohn disease.
Traps
- Portal venous gas and pneumatosis suggest ischaemia-do not dismiss.
- A hypovascular liver lesion in cirrhosis may still be HCC-use LI-RADS criteria.
- Gallstone ileus (Rigler triad: pneumobilia, obstruction, ectopic stone) is a classic trap.
- Appendicitis can be missed if the appendix is retrocaecal-look for secondary signs.
- Do not confuse small bowel (valvulae conniventes) with large bowel (haustra).
Worked example / rule of thumb
A cirrhotic patient with a 2 cm arterial-enhancing lesion showing washout: this is HCC by LI-RADS 5-no biopsy needed. Rule of thumb: haemangioma fills in from the periphery; HCC washes out. For obstruction, find the transition point to determine the level and cause.
Key points
- Acute abdomen: free air, obstruction, ischaemia, appendicitis.
- Liver lesion enhancement patterns distinguish haemangioma, HCC, metastasis.
- LI-RADS guides HCC diagnosis in cirrhosis.
- Rigler triad indicates gallstone ileus.
- Bowel wall signs (target, thumbprinting) suggest ischaemia or inflammation.