🔬 Abdominal X-ray

The Abdominal X-ray (AXR) is a plain radiograph of the abdomen used to evaluate the bowel gas pattern, detect free air, and identify radiopaque stones or foreign bodies. While its clinical utility has diminished with the widespread availability of CT, it remains a useful initial screen in specific presentations such as bowel obstruction or monitoring inflammatory bowel disease. It is usually performed in the supine position.

Suspected acute bowel obstruction or volvulus. Clinical suspicion of a perforated viscus (though erect CXR is preferred for free air). Assessment of toxic megacolon in known inflammatory bowel disease. Localisation of radiopaque foreign bodies. Investigation of severe constipation or faecal impaction. Monitoring of known renal calculi (KUB view). Not indicated for routine investigation of non-specific abdominal pain.

The patient typically lies supine (on their back) for a plain film of the abdomen. The X-ray beam is directed vertically through the abdomen to the detector plate beneath. An 'erect' abdominal film is rarely performed now, as it provides little additional information over a supine film and an erect CXR. For renal stones, a 'KUB' film (Kidneys, Ureters, Bladder) is requested, which ensures the entire urinary tract is captured from the T12 level to the pubic symphysis.

A normal bowel gas pattern with small amounts of air in the stomach, small bowel, and colon. No evidence of significant bowel dilatation (maximum 3cm small, 6cm large, 9cm caecum). No free intraperitoneal air. No abnormal calcifications (e.g., stones). Visualisation of the psoas muscle shadows and solid organ outlines (liver and kidneys) if visible. Intact bony structures of the spine and pelvis. No radiopaque foreign bodies.

Follow a systematic approach: 'BBC'—Bowel gas pattern (small vs large bowel, diameters, distribution); Bones (lumbar spine, pelvis, ribs); Calcification and Organs (liver, spleen, kidneys, bladder, stones). Differentiate small bowel (central, valvulae conniventes crossing the whole width) from large bowel (peripheral, haustra appearing as incomplete clips). Look for 'Rigler's sign' or 'Football sign' indicating perforation. Assessment of the psoas shadows and pelvic area is also important.

Dilated small bowel loops (>3 cm) or large bowel loops (>6 cm, or >9 cm for the caecum) suggest intestinal obstruction. Volvulus may show the 'coffee bean' sign (sigmoid) or 'embryo' sign (caecal). Pneumoperitoneum (free air) is best seen on an erect CXR but may appear as Rigler's sign (air on both sides of the bowel wall) on an AXR. Faecal loading or impaction is seen as 'mottled' shadows. Radiopaque gallstones or renal stones may be visible. Thumbprinting indicates bowel wall oedema, often due to ischaemia or inflammatory bowel disease.

The AXR has a more limited role than previously, as CT abdomen is significantly more sensitive and specific for most acute abdominal pathologies. However, it remains useful in the rapid assessment of suspected bowel obstruction, especially in patients with 'drip and suck' management. It is also the first-line investigation for identifying and monitoring the passage of radiopaque ingested foreign bodies or assessing the severity of toxic megacolon in ulcerative colitis.

Over-interpreting normal gas patterns or 'pseudo-obstruction' as mechanical obstruction. Failing to recognise signs of perforation (Rigler’s sign) on a supine film. Assuming a normal AXR rules out significant pathology like a leaking AAA or mesenteric ischaemia. Misidentifying faecal material as a mass or vice versa. Relying on AXR when a CT would be more appropriate and definitive for the clinical question.

AXR has low sensitivity for many common pathologies, including appendicitis, cholecystitis, and pancreatitis. It involves a relatively high radiation dose compared to a CXR (equivalent to several months of background radiation). It often fails to pinpoint the exact site or cause of an obstruction. Many gallstones and renal stones are radiolucent and thus invisible on X-ray. It should not delay more definitive imaging like CT if the patient is acutely unwell.

Candidates should know the '3-6-9' rule for bowel diameters. Differentiating between small and large bowel obstruction on a plain film is a high-yield exam skill. Be aware that most gallstones (85%) are radiolucent and will not be seen. Understand that AXR is contraindicated in pregnancy unless there is a life-threatening reason and it is the only viable option. Always check for 'Rigler's sign' which implies a surgical emergency.