How to read an abdominal X-ray

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

The abdominal X-ray (AXR) is requested far less than it used to be, because CT and ultrasound answer most abdominal questions better. It still has a clear role: suspected bowel obstruction, suspected toxic megacolon in known inflammatory bowel disease, some foreign bodies, and follow-up of known urinary stones. When you do meet one, the commonest error is to look only at the bowel and miss the free gas, the stone or the bone lesion. This page gives you a fixed order: gas pattern → bowel calibre → small versus large bowel → extraluminal gas → organs and soft tissue → calcification → bones → devices.

1. When is an AXR the right test?

The Royal College of Radiologists' iRefer guidance lists the situations where an AXR may help. In summary:

• Indicated: suspected bowel obstruction; suspected toxic megacolon in known IBD; a swallowed or inserted foreign body that is sharp or potentially poisonous; renal colic follow-up in a patient with known radiopaque stones.
• Not indicated: non-specific abdominal pain, constipation as a routine investigation, or as the first-line test for suspected perforation.
• Suspected perforation: request an erect chest X-ray (and in most cases CT, which is more sensitive). See pneumoperitoneum.
• Suspected first renal colic: non-contrast CT (CT KUB) is the reference test (NICE NG118).
• Suspected gallstones: ultrasound. Only about 10–15% of gallstones (some sources say up to 20%) are visible on plain film.

2. Pre-checks and adequacy

• Patient, date and time, and previous imaging for comparison.
• Projection: in the UK the standard AXR is supine AP. An erect abdominal film is no longer routine. Without a horizontal beam you will not see fluid levels.
• Coverage: ideally from the hemidiaphragms to the hernial orifices (the inguinal and femoral regions). Larger patients may need two films. Say if an area is missing.
• Side marker checked against the anatomy (liver on the right, gastric bubble on the left).

3. Bowel gas pattern and calibre

Where is the gas?

Gas is normally seen in the stomach, throughout the colon and rectum, and in a few small-bowel loops. Two or three small-bowel loops of normal calibre are acceptable. Ask whether gas reaches the rectum: rectal gas suggests there is no complete distal obstruction, but it does not exclude one, especially after a rectal examination or enema.

How wide is the bowel? The 3-6-9 rule

Dilatation then needs a pattern:

Pattern	Typical film	Read more
Small bowel obstruction	Multiple central dilated loops over 3 cm; colon collapsed or gasless beyond the block	SBO
Large bowel obstruction	Dilated peripheral colon with haustra, cut off at a transition point; small bowel may also dilate if the ileocaecal valve is incompetent	LBO
Volvulus	A massively dilated loop: sigmoid "coffee bean" pointing to the right upper quadrant, or caecum displaced to the mid or left upper abdomen	Volvulus
Paralytic ileus	Both small and large bowel dilated, gas reaching the rectum, no transition point	SBO
Toxic megacolon	Dilated, featureless transverse colon with mucosal islands or thumbprinting	Toxic megacolon

4. Small versus large bowel

Feature	Small bowel	Large bowel
Position	Central	Peripheral, framing the abdomen
Folds	Valvulae conniventes: thin, closely spaced folds that cross the whole width of the loop ("stack of coins")	Haustra: thicker, more widely spaced folds that cross only part of the width
Normal maximum diameter	About 3 cm	About 6 cm (caecum about 9 cm)
Contents	Gas and fluid	Gas, fluid and often faeces (mottled appearance)

The distinction is not always clean. Very distended loops lose their folds, the descending and sigmoid colon may have few haustra, and the transverse and sigmoid colon can hang down into the centre of the abdomen. Use several features together.

5. Gas outside the bowel

Gas in the wrong place is often more important than dilated bowel.

• Pneumoperitoneum (free intraperitoneal gas). On a supine film the key sign is Rigler's sign (the double-wall sign): both the inner and outer walls of a bowel loop are visible because gas lies on both sides. Others include the football sign (a large oval lucency, mainly in children), the falciform ligament sign, the triangle sign (a triangular pocket of gas between loops) and gas in the right upper quadrant over the liver. Rigler's sign usually needs a moderate volume of gas [VARIES]; overlapping loops can mimic it. Next step: erect chest film or CT.
• Retroperitoneal gas: streaky gas outlining the kidney or psoas, for example after duodenal perforation.
• Gas in the bowel wall (pneumatosis): linear or bubbly lucencies parallel to the wall. In adults this can mean bowel ischaemia; in neonates it is a sign of necrotising enterocolitis.
• Portal venous gas: branching lucencies reaching the periphery of the liver. Often ominous in adults (ischaemia).
• Biliary gas (pneumobilia): branching lucencies near the centre of the liver. Common after sphincterotomy or a biliary stent; also part of Rigler's triad in gallstone ileus (with small bowel obstruction and an ectopic gallstone).

6. Organs, calcification, bones and devices

Solid organs and soft tissues

Look at the liver, spleen, kidneys and bladder outlines, and for any soft-tissue mass displacing bowel. The psoas outlines are often described but are unreliable: they are frequently not seen in normal people. Ascites gives a diffuse grey appearance with bowel floating centrally; ultrasound is the test.

Calcification

• Urinary tract: about 80–90% of renal stones are radiopaque. Trace the ureter along the tips of the lumbar transverse processes, across the sacroiliac joints, to the ischial spines and bladder. Pelvic phleboliths (small round calcified veins, often with a lucent centre) mimic distal ureteric stones. Uric acid stones are radiolucent. See abdominal calcification and stones.
• Gallstones: a minority are visible. A porcelain gallbladder has a calcified wall.
• Pancreas: speckled calcification across the upper abdomen at about L1–L2 suggests chronic pancreatitis.
• Vessels: curvilinear calcification of the aortic wall; a width of more than 3 cm suggests an abdominal aortic aneurysm (ultrasound or CT confirms). Splenic artery calcification is common in older people.
• Others: appendicolith, calcified fibroids, calcified mesenteric nodes, costal cartilage, and teeth in an ovarian dermoid.

Bones

Review the lower ribs, lumbar spine, pelvis, sacroiliac joints and hips. Look for fractures, sclerotic or lytic metastases, Paget disease and sacroiliitis. A hip fracture or bone metastasis can be the most important finding on an AXR requested for something else.

Devices and foreign bodies

Name every line, drain, stent, clip, intrauterine device, NG tube and feeding tube. Check that NG tubes follow the expected course below the diaphragm, under local policy (see NG tube position). Look for ingested or inserted foreign bodies.

7. Presenting the AXR

See how to present an X-ray for general technique.

8. Common pitfalls

• The "gasless" abdomen. Fluid-filled obstructed loops may show almost no gas. A near-normal AXR does not exclude obstruction; CT does better.
• Forgetting the hernial orifices. Gas-filled bowel below the inguinal ligament points to an obstructed hernia.
• Pseudo-Rigler sign. Two overlapping loops can make a wall look outlined on both sides.
• Phleboliths mistaken for ureteric stones, and the reverse.
• Pseudo-obstruction (Ogilvie syndrome) cannot be reliably told from mechanical large bowel obstruction on the AXR. CT is needed.
• Recent surgery. Free gas can be normal for days after abdominal surgery; correlate clinically.

Red flags: act now

What to focus on, by role

Role	Priorities
Medical student	3-6-9, small versus large bowel, and the signs of free gas. Practise saying the system aloud.
Nurse	Recognise when a film is being used to check an NG tube or a foreign body, and escalate obvious free gas or a grossly distended abdomen.
Junior doctor	Request an AXR only when it will help, recognise obstruction and volvulus, and know when to go straight to CT. Check the hernial orifices on the film and the patient.
Radiographer	Coverage from diaphragm to hernial orifices, the need for a second exposure in larger patients, and pregnancy checks before abdominal exposure.
Advanced / FRCR	Closed-loop obstruction, pneumatosis versus pseudo-pneumatosis, and calcification patterns.

Practise on real abdominal X-rays →

References

1. Radiology Masterclass. Abdominal X-ray introduction. radiologymasterclass.co.uk
2. Royal College of Radiologists. iRefer: making the best use of clinical radiology. irefer.org.uk
3. Radiopaedia. 3-6-9 rule. radiopaedia.org
4. Radiopaedia. Pneumoperitoneum. radiopaedia.org
5. Radiology Masterclass. Small bowel obstruction. radiologymasterclass.co.uk
6. NICE. Renal and ureteric stones: assessment and management (NG118). nice.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.