Reading an abdominal X-ray: the gas pattern
- Reading
An abdominal X-ray shows where the gas in the gut is and how wide the bowel is, which helps spot a blockage, a stalled bowel or a hole.
What it is
The plain abdominal X-ray is usually a single supine film taken from the front, from the diaphragm to the pubic bones. Its main value is the bowel gas: where it is, how wide the loops are and whether it reaches the rectum. Small bowel lies centrally and shows folds (valvulae conniventes) that cross the whole width of the loop; colon lies around the edges and shows haustra that cross only part of it. The 3-6-9 rule gives the upper limits of normal width: about 3 cm for small bowel, 6 cm for colon and 9 cm for the caecum. The film also shows calcifications, bones, soft-tissue outlines and tubes. It is quick, cheap and low in radiation, but it is often normal or nonspecific; CT has replaced it for most acute abdomens, and free air is best sought on an erect chest X-ray.
What to check
- Gas pattern: is the bowel dilated, and is it small bowel, colon or both?
- Small bowel: central, folds cross the full width; colon: peripheral, haustra cross part of the width.
- 3-6-9 rule: small bowel over 3 cm, colon over 6 cm or caecum over 9 cm is dilated.
- Gas in the rectum and colon argues against a complete obstruction.
- Free air (Rigler sign), abnormal gas in the bowel wall, portal veins or bile ducts.
- Calcifications, bones (spine, pelvis), soft-tissue outlines and any lines or tubes.
Reading it step by step (and why)
- Check the patient, date and position (supine or erect) — air-fluid levels need an erect or decubitus film.
- Check coverage from diaphragm to pubic symphysis — hernial orifices and lung bases are easy to cut off.
- Find the stomach, small bowel, colon and rectal gas — the distribution tells obstruction from ileus.
- Measure the widest loop of each kind with the 3-6-9 rule — width, not number of loops, defines dilatation.
- Look for gas on both sides of the bowel wall (Rigler sign) — it means free intraperitoneal air.
- Look for gas in the bowel wall, portal veins or biliary tree — they point to ischaemia or a fistula.
- Trace the psoas, liver, spleen and kidney outlines — loss or displacement suggests fluid or a mass.
- Scan for calcifications along the kidneys, ureters, gallbladder, pancreas and aorta — stones and aneurysms show here.
- Review the lumbar spine, pelvis, hips and lung bases — fractures, bone lesions and basal pneumonia mimic abdominal pain.
- Check the tip of any tube or drain — a misplaced nasogastric tube is a common, important finding.
Common mistakes
- Calling a loop small bowel or colon by position alone; check the folds and haustra.
- Calling a normal film reassuring: early obstruction, ischaemia and appendicitis often look normal.
- Looking for free air on a supine film only; an erect chest X-ray or a left lateral decubitus film is far more sensitive.
- Forgetting the edges: a groin hernia, a lung-base pneumonia or a hip fracture can be the answer.
- Mistaking pelvic phleboliths for ureteric stones.
Next step
- Suspected perforation: erect chest X-ray now; if the patient cannot sit up, a left lateral decubitus film or CT.
- Dilated small bowel or colon with pain and vomiting: CT abdomen and pelvis with IV contrast to find the cause and level.
- Suspected mesenteric ischaemia: do not wait for the X-ray; ask for CT angiography and call surgery.
- Free air, pneumatosis with portal venous gas, or a closed-loop pattern: call the surgical team and the radiologist at once.
- Unexplained finding or a film you are unsure of: ask the radiologist to review it before acting.
Test yourself
Think of the answer, then tap the question to see it.
How do you tell small bowel from colon on an abdominal X-ray?
Small bowel lies centrally and its folds cross the whole width of the loop; colon lies peripherally and its haustra cross only part of the width.
What does the 3-6-9 rule state?
The upper limits of normal width: about 3 cm for small bowel, 6 cm for colon and 9 cm for the caecum.
What is the Rigler sign?
Both the inner and the outer surface of the bowel wall are visible because there is gas on both sides of it: free intraperitoneal air on a supine film.
Which film is best for small amounts of free air?
An erect chest X-ray, with the patient upright for several minutes; a left lateral decubitus film if they cannot sit up. CT is the most sensitive.
Elsewhere in DurgiPedia
Connections
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: Ileus (StatPearls (NCBI Bookshelf), 2023); Small Bowel Obstruction (StatPearls (NCBI Bookshelf), 2025); Bowel Perforation (StatPearls (NCBI Bookshelf), 2025).
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Representative model, for educational purposes only.