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Imaging the acute abdomen: free air, obstruction, ischaemia and inflammation

Contrast CT is the main test for the acute abdomen in adults; free air, a transition point in obstruction, signs of strangulation and bowel ischaemia are the findings that require surgery — and abdominal radiographs and US are supporting tools.

לקריאה בעברית

Why it matters in the exam

Free air (pneumoperitoneum): on an erect chest film — air under the diaphragm; on a supine film (for example in a ventilated patient) — Rigler's sign (both sides of the bowel wall are seen), the "football sign" and the falciform ligament. CT is the most sensitive. Free air without perforation: after surgery (usually absorbed within a few days), in peritoneal dialysis and in pneumatosis cystoides. Portal venous gas reaches the liver periphery, while biliary gas (pneumobilia) is concentrated centrally near the hilum — portal gas with pneumatosis raises suspicion of bowel ischaemia.

Small bowel obstruction: loops above 3 cm, a transition point, "small bowel faeces" and air–fluid levels; the commonest cause — adhesions. Closed-loop obstruction: a C- or U-shaped loop with two transition points and a mesenteric "whirl" sign. Signs of strangulation: reduced wall enhancement, mesenteric oedema, free fluid and pneumatosis — urgent surgery. In adhesive obstruction without signs of strangulation: oral water-soluble contrast; arrival in the colon within 24 hours predicts resolution without surgery. Large bowel obstruction: diameter above 6 cm, and a caecum above 9 cm at risk of perforation. Sigmoid volvulus: a "coffee bean" whose apex points to the right upper quadrant, usually in an elderly patient; caecal volvulus: a dilated caecum that migrates to the left upper quadrant.

Acute mesenteric ischaemia: CTA in arterial and portal phases. An SMA embolus usually lodges beyond the origin of the middle colic artery; thrombosis starts at the arterial origin on atherosclerosis; venous thrombosis — thick oedematous bowel; and NOMI in a patient in shock. Late signs: pneumatosis and portal venous gas. Ischaemic colitis affects the watershed areas — the splenic flexure and the rectosigmoid junction — in the elderly. Appendicitis: diameter above 6 mm, non-compressible, wall enhancement and fat stranding; US first in children and pregnancy, and MRI in pregnancy when US is equivocal; CT in adults. Diverticulitis — CT; a large abscess is drained percutaneously. Epiploic appendagitis — a fatty oval with a dense ring and a central dot; omental infarction — larger fat, usually on the right, without a ring. Both are treated conservatively.

Key points

• CT is the main test for the adult acute abdomen.
• Small bowel above 3 cm, colon above 6, caecum above 9.
• Closed loop and strangulation signs — surgery.
• Mesenteric ischaemia — CTA; pneumatosis and portal gas are late.
• Appendix above 6 mm; US in children and pregnancy.

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Sources

• ACR Appropriateness Criteria: Suspected Small-Bowel Obstruction, 2019 — CT as initial imaging
• Bologna Guidelines for Diagnosis and Management of Adhesive Small Bowel Obstruction (WSES), 2017 update — Water-soluble contrast challenge
• ACR Appropriateness Criteria: Imaging of Mesenteric Ischemia, 2018 — CTA

Sources are given for further reading; the text is written afresh and not copied from them.

Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.