Small bowel obstruction on X‑ray

Written by the X-Ray Atlas editorial team from the sources listed below · Medical review pending · Updated 24 October 2026 · Level: Beginner

Small bowel obstruction (SBO) is a blockage that stops the normal passage of contents through the small intestine. It is one of the most common reasons for emergency general surgical admission. Patients typically present with colicky central abdominal pain, vomiting, distension and absolute constipation, although vomiting comes early and constipation late in proximal obstruction. The abdominal X-ray (AXR) can suggest SBO, but in UK practice CT is the main test: it confirms the diagnosis, finds the cause and the site, and detects strangulation, which the plain film cannot. This page covers how to recognise small bowel on the film, the signs of obstruction, the causes, gallstone ileus, and how SBO differs from ileus.

1. How to look

The standard film is a supine AP abdomen. Erect abdominal films are no longer routine in most UK departments. An erect chest X-ray is often requested at the same time to look for free gas. Use the systematic abdominal approach, and for obstruction ask:

1. Is there dilated bowel? Measure it.
2. Is it small or large bowel?
3. Is there gas in the colon and rectum beyond it?
4. Is there a visible cause: a hernia, a gallstone, gas in the biliary tree, previous surgery (clips, staples)?
5. Is there free gas or gas in the bowel wall?

2. Small bowel or large bowel?

Feature	Small bowel	Large bowel
Position	Central, many loops	Peripheral, framing the abdomen
Folds	Valvulae conniventes: thin folds that cross the whole width of the loop, closely spaced	Haustra: thicker folds that cross only part of the width, more widely spaced
Diameter	Dilated above 3 cm	Dilated above 6 cm (caecum above 9 cm)
Contents	Gas and fluid	Often contains faeces, which look mottled

The closely packed valvulae conniventes of a dilated jejunum give a "stack of coins" appearance. The ileum is relatively featureless, so distal small bowel loops can be harder to identify by their folds alone.

3. Signs of small bowel obstruction

• Multiple dilated (>3 cm) central loops of small bowel.
• Valvulae conniventes crossing the full width of the loops.
• A collapsed or gasless colon beyond the obstruction. Some gas may remain in the colon early on, or after a rectal examination or enema.
• On an erect or decubitus film, multiple air–fluid levels at different heights, giving a "step-ladder" appearance.
• The string of beads sign: small bubbles of gas trapped between the valvulae in loops that are mostly fluid-filled. It is a useful sign because it suggests obstruction even when there is little gas.

4. Causes

• Adhesions from previous surgery: the most common cause in the developed world. Look for surgical clips, staples or a scar.
• Hernias: inguinal, femoral, umbilical, incisional and internal. Always check the groins on the film for gas-filled bowel below the inguinal ligament, and examine the patient's groins clinically. A small femoral hernia is easy to miss on both.
• Malignancy: small bowel tumours, or extrinsic compression and peritoneal metastases.
• Crohn's disease strictures.
• Gallstone ileus (below).
• Intussusception, which in adults usually has a lead point such as a tumour. In children, see the paediatric abdominal X-ray.
• Volvulus of the small bowel. See volvulus.
• Bezoars and swallowed objects. See foreign bodies.

5. Gallstone ileus and Rigler's triad

Gallstone ileus is mechanical obstruction caused by a large gallstone that has passed through a fistula between the gallbladder and the bowel, usually the duodenum, and then lodged in the narrowest part of the small bowel, typically the terminal ileum. It mainly affects older people, often women. The classic plain film combination is Rigler's triad:

1. small bowel obstruction;
2. pneumobilia (gas in the biliary tree, through the fistula);
3. an ectopic gallstone, usually in the right iliac fossa, if it is calcified.

Rigler's triad is not the same as Rigler's sign, which is the double-wall sign of pneumoperitoneum. Because most gallstones are not visible on plain film, all three features are seen together in only a minority of cases. See abdominal calcification and stones.

6. Ileus versus obstruction

Paralytic (adynamic) ileus is failure of bowel movement without a mechanical blockage. It is common after abdominal surgery, and with electrolyte disturbance (especially low potassium), opiates, sepsis and other acute illness.

• In ileus, both small and large bowel tend to be dilated, gas reaches the rectum, and there is no transition point.
• In mechanical SBO, dilated small bowel lies proximal to a transition point, with collapsed bowel beyond.
• A sentinel loop is a single dilated loop next to a local inflammatory process, such as pancreatitis or appendicitis.

The distinction can be impossible on a plain film, especially after surgery. CT shows whether a transition point exists.

7. Pitfalls

• A fluid-filled, gasless obstruction can look normal.
• Large bowel obstruction with an incompetent ileocaecal valve also dilates small bowel. Always look at the colon. See large bowel obstruction.
• Missing the hernia: the groins are at the edge of the film and are often cut off.
• Calling obstruction on the number of fluid levels alone: fluid levels are also seen in ileus and gastroenteritis.
• The film cannot exclude strangulation. Ischaemia is a clinical and CT diagnosis: pain out of proportion, peritonism, fever, raised lactate.

8. Next steps

• CT abdomen and pelvis with intravenous contrast is the test of choice for suspected SBO. It identifies the transition point, the cause and signs of ischaemia, closed-loop obstruction or perforation. Many UK centres go straight to CT without an AXR when SBO is clinically likely.
• Initial management typically includes nil by mouth, intravenous fluids, analgesia, correction of electrolytes and decompression with a nasogastric tube according to local practice, with early surgical review.
• In adhesive SBO without signs of ischaemia, the World Society of Emergency Surgery (Bologna) guidelines support a trial of non-operative management, including a water-soluble contrast challenge, in which follow-up abdominal films show whether contrast reaches the colon.

Red flags: act now

What to focus on, by role

Role	Priorities
Medical student	The 3-6-9 rule, small versus large bowel features, the causes (adhesions and hernias), and Rigler's triad versus Rigler's sign.
Nurse	Vomiting, distension, fluid balance and NG tube care. Escalate worsening pain, tachycardia or fever.
Junior doctor	Examine the hernial orifices, request CT rather than relying on the AXR, and involve the surgical team early.
Radiographer	Include the hernial orifices and the diaphragm where possible; avoid unnecessary erect abdominal films.
Advanced / FRCR	Gasless obstruction, gallstone ileus, ileus versus obstruction, and correlation with CT.

Practise on abdominal X-rays →

References

1. Radiology Masterclass. Small bowel obstruction. radiologymasterclass.co.uk
2. Radiopaedia. 3-6-9 rule. radiopaedia.org
3. Ten Broek RPG, et al. Bologna guidelines for diagnosis and management of adhesive small bowel obstruction (ASBO): 2017 update. World J Emerg Surg 2018;13:24.
4. Royal College of Radiologists. iRefer: making the best use of clinical radiology. irefer.org.uk

For education only. Not a substitute for a formal report, senior review or your local policy. Never use this page to make decisions about a real patient.