Introduction
Abdominal X-ray (AXR) interpretation is a high-yield OSCE station. Unlike a chest X-ray, the AXR is used for specific clinical questions — most commonly bowel obstruction, free air, and foreign bodies. A systematic approach prevents missed findings.
💎 Clinical Pearl
State the view and adequacy first. An AXR is a supine anteroposterior film — there is no lateral view in the standard abdominal series. If free air is suspected, a chest X-ray (erect) is the gold standard.
Systematic AXR Approach
Use the ABCDE approach for AXR:
🧠 Mnemonic
ABCDE for AXR:
Air (bowel gas pattern, free air)
Bowel (identify small vs large bowel; diameter)
Calcification (gallstones, renal calculi, aortic aneurysm, vascular, pancreatic)
Densities (organs: liver, spleen, kidneys; bones; foreign bodies)
Extras (drains, catheters, stents, surgical clips, pacemakers)
1. Air — Bowel Gas Pattern
Normal Bowel Gas
| Bowel | Location | Diameter | Mucosal pattern |
|---|---|---|---|
| Small bowel | Central | Under 3 cm | Valvulae conniventes (complete rings across full width) |
| Large bowel | Peripheral (frame of abdomen) | Under 6 cm (caecum under 9 cm) | Haustra (incomplete semicircular folds) |
🧠 Mnemonic
3-6-9 Rule:
Small bowel normal: under 3 cm
Large bowel normal: under 6 cm
Caecum normal: under 9 cm
Beyond these diameters = significant dilatation
Small Bowel Obstruction (SBO)
- Central dilated loops of small bowel (valvulae conniventes visible, crossing full width)
- Diameter over 3 cm
- Paucity of gas in the large bowel (if complete obstruction)
- Causes: adhesions (most common), hernias, gallstone ileus, intussusception, tumour
Large Bowel Obstruction (LBO)
- Peripheral dilated loops (haustra visible, incomplete folds)
- Diameter over 6 cm
- Small bowel may also be dilated if ileocaecal valve is incompetent
- Causes: colorectal carcinoma (most common), sigmoid volvulus, diverticular stricture, faecal impaction
2. Specific Patterns to Recognise
Sigmoid Volvulus
🧠 Mnemonic
Coffee bean sign — a massively dilated loop of sigmoid colon folded on itself, pointing toward the right upper quadrant. Looks like a coffee bean or bent inner tube.
The sigmoid colon twists on its mesentery. Often in elderly constipated patients or psychiatric patients on antipsychotics. Treatment: flatus tube decompression or sigmoidoscopy; surgery if ischaemia.
Caecal Volvulus
Dilated caecum displaced to the left upper quadrant. Associated small bowel dilatation. Less common than sigmoid volvulus. Requires surgical management.
Toxic Megacolon
⚠️ Red Flag
Transverse colon diameter over 6 cm with loss of haustral pattern, mucosal thickening or thumbprinting.
Associated with: ulcerative colitis, Crohn's disease, Clostridioides difficile colitis.
Emergency: risk of perforation. IV steroids, nil by mouth, surgical review, daily AXR monitoring.
3. Free Air (Pneumoperitoneum)
On a supine AXR, free air is difficult to see. Signs include:
- Rigler's sign: both sides of the bowel wall are visible (air inside and outside the lumen)
- Football sign: large collection of free air outlines the peritoneal cavity
- Air under the diaphragm: best seen on an erect chest X-ray
⚠️ Red Flag
Free air (pneumoperitoneum) = perforation until proven otherwise. Common causes: perforated peptic ulcer, perforated diverticular disease, perforated colon (tumour or toxic megacolon). Requires urgent surgical review.
4. Calcification
| Calcification | Location | Clinical relevance |
|---|---|---|
| Gallstones | Right hypochondrium | Only 10-15% are radio-opaque on AXR; USS is gold standard |
| Renal calculi | Along the course of the ureter (L2, pelvic brim, vesicoureteric junction) | 90% radio-opaque; CT-KUB is gold standard |
| Aortic aneurysm | Curvilinear calcification in the aortic wall, left of midline | Measure width; if over 3 cm = aneurysm |
| Pancreatic calcification | Mid-epigastric | Chronic pancreatitis (alcoholic) |
| Phleboliths | Pelvis (bilateral, small, round, lucent centre) | Benign calcification in pelvic veins; common normal variant |
5. Solid Organs and Bones
- Liver: right upper quadrant; hepatomegaly if lower edge below right costal margin
- Spleen: left upper quadrant; splenomegaly if extends beyond left iliac crest
- Psoas lines: bilateral symmetrical shadows alongside lumbar spine; absent = retroperitoneal pathology (haemorrhage, abscess)
- Spine: look for vertebral fractures, scoliosis, bony metastases
How to Present an AXR
"This is a supine abdominal X-ray of [patient name], dated [date]. The patient is adequately exposed. Looking at the bowel gas pattern: there are centrally placed dilated loops of small bowel measuring up to 4.5 cm, with valvulae conniventes visible crossing the full lumen width. There is paucity of gas in the large bowel. These findings are consistent with small bowel obstruction. I would perform a clinical assessment, insert an NGT, IV access, fluid resuscitation, and obtain a CT abdomen-pelvis to identify the cause and level of obstruction."
"What is the 3-6-9 rule for bowel diameter on AXR?"
Normal small bowel diameter is less than 3 cm. Normal large bowel diameter is less than 6 cm. Normal caecal diameter is less than 9 cm. Dilatation beyond these thresholds is clinically significant and suggests obstruction, ileus, or inflammatory bowel disease depending on the clinical context.
"How do you differentiate small bowel from large bowel on an AXR?"
Small bowel occupies a central position, has a diameter normally under 3 cm, and shows valvulae conniventes — mucosal folds that cross the full width of the lumen. Large bowel is peripherally located (framing the abdomen), has haustra (incomplete semicircular folds), a larger calibre normally under 6 cm, and contains faecal shadows. In obstruction, the dilated bowel proximal to the block determines which type is obstructed.
"What is the coffee bean sign and what does it indicate?"
The coffee bean sign (or bent inner tube sign) on AXR describes the appearance of a massively dilated loop of sigmoid colon that has twisted on its mesentery (sigmoid volvulus). The two limbs of the loop are folded together and the apex typically points toward the right upper quadrant. It is a surgical emergency as it can lead to ischaemia and perforation.
"What are the signs of pneumoperitoneum on an AXR?"
Free air is difficult to detect on a supine AXR. Rigler's sign (both sides of the bowel wall visible, indicating air inside and outside the lumen) and the football sign (large air collection outlining the peritoneal cavity) suggest pneumoperitoneum on a supine film. An erect chest X-ray is the standard investigation for free air under the diaphragm. Pneumoperitoneum indicates perforation until proven otherwise.
Related guides: Chest X-Ray Interpretation OSCE | A&E Assessment OSCE | Abdominal Examination OSCE