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Small and large bowel obstruction on X-ray and CT

  • Finding

When the bowel is blocked, the loops above the block swell with gas and fluid and the bowel below empties. X-ray suggests it; CT finds where and why.

Updated

What it is

In mechanical obstruction the bowel above the blockage dilates with gas and fluid while the bowel beyond it collapses. In small bowel obstruction the X-ray shows central dilated loops over about 3 cm with folds crossing the whole loop, often stacked in a step-ladder, multiple air-fluid levels on an erect film and little or no gas in the colon. Adhesions from earlier surgery are the commonest cause in adults; hernias lead in patients without prior surgery and worldwide. In large bowel obstruction the colon dilates (over 6 cm, caecum over 9 cm) up to the block; colorectal cancer is the commonest cause, then volvulus and diverticular stricture. CT with IV contrast is the key test: it confirms the obstruction, finds the transition point between dilated and collapsed bowel, shows the cause and looks for the complications that need surgery now, closed loop and strangulation. Ileus (a stalled bowel without a blockage) dilates small bowel and colon alike, with gas reaching the rectum and no transition point.

What it looks like

  • Small bowel obstruction: central loops over 3 cm, folds across the full width, little colonic gas.
  • Erect film: multiple air-fluid levels at different heights; a string of small gas bubbles (string-of-pearls sign) in fluid-filled loops.
  • Large bowel obstruction: colon over 6 cm (caecum over 9 cm) to an abrupt cut-off; small bowel dilates too if the ileocaecal valve leaks.
  • CT: a transition point from dilated to collapsed bowel; the small-bowel faeces sign often sits just before it.
  • Sigmoid volvulus: a huge inverted-U loop rising from the pelvis (coffee-bean sign); bird's-beak taper on contrast enema; whirl of mesentery on CT.
  • Caecal volvulus: a large gas-filled caecum displaced toward the left upper quadrant, with small bowel dilatation.

Why it looks that way

  • Swallowed air and gut secretions build up above the block, so loops dilate and fluid layers out under the gas.
  • Bowel beyond the block empties over hours, so gas in the rectum argues against a complete obstruction.
  • A closed loop (obstructed at both ends, as in volvulus) fills with no way out, raises wall pressure and cuts off blood flow.

Differential and pitfalls

  • Signs of strangulation on CT: poorly enhancing or thickened wall, mesenteric oedema, free fluid, pneumatosis, portal venous gas.
  • Closed loop on CT: a U- or C-shaped fluid-filled loop with two transition points close together and a twisted mesentery.
  • Ileus vs obstruction: ileus dilates small bowel and colon with gas to the rectum and no transition point; seen after surgery, with low potassium, opioids or sepsis.
  • Colonic pseudo-obstruction mimics large bowel obstruction without a lesion; CT shows no cut-off.
  • A caecum widening beyond about 9 cm, especially with a competent ileocaecal valve, risks perforation.
  • Early obstruction or fluid-filled loops can give a near-normal X-ray; do not let it reassure you.

Next step

  • Suspected obstruction: CT abdomen and pelvis with IV contrast; oral contrast is usually not needed and can delay the scan.
  • Closed loop, strangulation signs, free air or peritonitis: call the surgical team now.
  • Adhesive small bowel obstruction without danger signs: surgeons may give water-soluble oral contrast and repeat the film; contrast reaching the colon predicts resolution without surgery.
  • Suspected sigmoid volvulus without signs of ischaemia: CT to confirm, then endoscopic decompression by the surgical or GI team.
  • Large bowel obstruction: CT to find the cause; a tumour needs staging and a surgical plan, not barium.

Test yourself

Think of the answer, then tap the question to see it.

What is the commonest cause of small bowel obstruction in an adult who has had abdominal surgery?

Adhesions. In patients without previous surgery, and worldwide, hernias are a leading cause.

Which CT findings suggest strangulated bowel?

Reduced or absent wall enhancement, wall thickening, mesenteric oedema or haziness, free fluid, and gas in the bowel wall or portal veins.

What does the coffee-bean sign represent?

A massively dilated sigmoid loop in sigmoid volvulus: an inverted U rising from the pelvis, its two walls meeting in a central line like the cleft of a coffee bean.

How does ileus differ from mechanical obstruction on imaging?

Ileus dilates small bowel and colon together with gas reaching the rectum and no transition point; obstruction shows dilated bowel above a transition and collapsed bowel beyond it.

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Educational content; it does not replace a radiologist's report and is not a clinical instruction. Each study is ordered on the patient's assessment and your institution's protocols.

Sources: Small Bowel Obstruction (StatPearls (NCBI Bookshelf), 2025); Volvulus (StatPearls (NCBI Bookshelf), 2022); Sigmoid Volvulus (StatPearls (NCBI Bookshelf), 2023); Ileus (StatPearls (NCBI Bookshelf), 2023).

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