Abdominal X-Ray (Supine, Erect)

The abdominal radiograph images gas in the gastrointestinal tract, soft tissue outlines, calcification, foreign bodies and bones.

The supine anteroposterior film (centred at the iliac crests, including the diaphragm to the symphysis pubis) is the basic view; an erect or left lateral decubitus film (horizontal beam) is added to show air fluid levels and free intraperitoneal gas, although an erect chest radiograph is more sensitive for free gas.

Supine films show the bowel diameter, distribution of gas and mucosal pattern: small bowel loops lie centrally with valvulae conniventes crossing the whole lumen, whereas the large bowel lies peripherally with haustra that do not cross the lumen. Normal diameters are under 3 cm for small bowel, under 6 cm for colon and under 9 cm for the caecum.

The effective dose is about 0.7 mSv for a supine film, similar to 3 months of natural background radiation.

Since CT became widely available, the plain abdominal film has a limited role in acute abdominal pain in adults, where it is rarely diagnostic (diagnostic yield about 10%) and often misleading. It remains useful for suspected small bowel or large bowel obstruction in settings without CT, for volvulus, toxic megacolon in colitis, ingested or inserted foreign bodies and drug packets, tube and stent positions, constipation and faecal loading in children, and as a baseline scout film.

Indications. Suspected bowel obstruction (vomiting, distension, absolute constipation, colicky pain). Suspected perforation (together with an erect chest film). Toxic megacolon or severe colitis for daily monitoring. Ingested foreign body, button battery, magnets or packets of drugs. Abdominal pain in an unwell patient where CT is not available.

Position of nasogastric or feeding tubes, stents and catheters. Constipation assessment in children. Suspected renal or ureteric stone when KUB is requested. Not for routine abdominal pain without red flags, and not for vague symptoms, because the dose is not justified by the yield.

Contraindications and precautions. Pregnancy: avoid unless the benefit is clear, as the foetal dose is higher than for chest films (about 1 to 3 mGy); ultrasound or MRI are preferred. Patients who are too unwell to stand: use the left lateral decubitus view held for 5 minutes. Remove clothing with metal and previous contrast residues from the field. Document the last menstrual period in women of childbearing age.

Preparation and technique. No fasting is needed. Exposure at end expiration, with the patient lying flat. For the erect view the patient stands for at least 5 minutes beforehand to allow air to collect. Include the hernial orifices (inguinal region) when obstruction is suspected.

Normal appearances. Scattered gas in the stomach, caecum, and a few loops of colon and small bowel; faeces mottled in the right colon and rectum. Psoas outlines and flank stripes visible. Renal outlines in some patients. Normal calcification only in costal cartilage, phleboliths in the pelvis and calcified mesenteric nodes. No free gas, no dilated loops and no air fluid levels (a few small levels in the normal small bowel are acceptable).

Interpretation. Read systematically: gas pattern, soft tissues, bones, calcifications, and tubes, lines and foreign bodies.

• Small bowel obstruction: dilated loops over 3 cm, central distribution, valvulae conniventes, multiple air fluid levels on an erect film (stepladder pattern), little or no gas in the colon. Causes are adhesions (commonest), hernia, tumour, Crohn disease and volvulus; closed loop obstruction shows a gasless or fluid filled loop and risks strangulation.
• Large bowel obstruction: dilated colon over 6 cm (caecum over 9 cm), haustral markings, peripheral distribution, gas stopping at the point of obstruction, with a competent ileocaecal valve (distended colon, normal small bowel) or an incompetent valve (small bowel dilatation). Causes include colorectal cancer, volvulus and diverticular stricture.
• Sigmoid volvulus: a massively dilated, inverted U (coffee bean) loop arising from the pelvis and rising toward the right upper quadrant. Caecal volvulus: a dilated loop arising from the right iliac fossa pointing to the left upper quadrant, with collapsed distal colon.
• Toxic megacolon: transverse colon over 6 cm with loss of haustra, thumbprinting and mucosal islands in a patient with severe colitis.
• Paralytic ileus: generalised gaseous dilatation of small and large bowel with gas in the rectum.
• Pneumoperitoneum: on a supine film, the Rigler sign (gas on both sides of the bowel wall), the football sign (large central gas collection), the falciform ligament sign, and the double wall sign; on erect films, a crescent of gas under the diaphragm.
• Pneumatosis intestinalis and portal venous gas: branching gas lucency in the bowel wall or liver periphery, suggesting bowel ischaemia, necrotising enterocolitis in neonates or severe infection.
• Calcification and stones: renal and ureteric calculi, gallstones (only 10 to 15% are opaque), pancreatic calcification (chronic pancreatitis), aortic and splenic artery calcification (aneurysm over 3 cm), phleboliths (round with a lucent centre in the pelvis), calcified lymph nodes, uterine fibroids, adrenal calcification, appendicolith.
• Foreign bodies: coins, batteries (double ring or halo), magnets, dense packets of drugs ("body packer") and retained surgical swabs with radio opaque markers.
• Faecal loading: mottled faecal material throughout the colon, often with overflow in children.
• Soft tissue: hepatosplenomegaly (displaced bowel), ascites (centralised bowel loops, ground glass appearance), enlarged bladder, pelvic masses, and loss of the psoas shadow in retroperitoneal disease.
• Bones: sclerotic or lytic metastases, degenerative changes, fractures, and sacroiliitis.

Critical values and findings needing immediate action.

• Pneumoperitoneum: urgent surgical review and resuscitation.
• Closed loop obstruction, signs of strangulation, caecal diameter over 9 to 12 cm, or sigmoid volvulus: urgent surgical review, nasogastric decompression and endoscopic or operative decompression of volvulus.
• Toxic megacolon: intravenous fluids, steroids as appropriate, early surgical review.
• Portal venous gas or pneumatosis with abdominal pain: suspect ischaemia or necrotising enterocolitis, urgent surgical review and CT.
• Button battery or multiple magnets in the oesophagus or stomach: urgent endoscopic removal.
• Dense packets of drugs in an unwell patient: toxicology advice, avoid endoscopic retrieval, and monitor for rupture.
• Aortic calcification outlining an aneurysm with pain: urgent ultrasound or CT angiography.

Limitations and artefacts.

• Low sensitivity and specificity: plain films detect obstruction in about 60 to 70% and miss closed loop obstruction, early strangulation, appendicitis, pancreatitis, diverticulitis, mesenteric ischaemia and most causes of acute abdominal pain. A normal film does not exclude serious disease.
• Free gas is detected on supine films in only 50 to 60% of perforations; erect chest films (sensitivity about 70 to 80%) and CT (over 95%) are better.
• Bowel diameter varies with the patient and position, and gas may be absent in a fluid filled obstructed bowel. Fluid filled loops may be invisible.
• Small bowel and colon are confused when loops are redundant; use haustra and valvulae pattern and position.
• Overlying bones, faeces and contrast from earlier studies obscure details; faecal material may mimic masses or pneumatosis.
• Calcified lymph nodes, phleboliths and costal cartilage mimic ureteric stones; gallstones are mostly invisible.
• Radiation dose is higher than for the chest film and gonadal exposure is not negligible; repeat films should be limited.
• Patients unable to stand produce poor erect films; use decubitus views.

Special groups.

• Children: single supine film, with erect or decubitus views for suspected perforation; the gas pattern of the neonate (double bubble sign in duodenal atresia, paucity of gas in oesophageal atresia without fistula, pneumatosis in necrotising enterocolitis) is diagnostic.
• Pregnancy: avoid; ultrasound or MRI instead.
• Elderly patients: constipation, volvulus, and mesenteric ischaemia are frequent; CT is justified early.
• Patients with stomas, drains or previous surgery: baseline comparison with previous films.
• Regions: in many hospitals without CT, the plain film with ultrasound remains the first investigation for obstruction, volvulus, perforation and foreign bodies, and sigmoid volvulus and intussusception are more common.

Follow up tests. Erect chest radiograph for pneumoperitoneum, CT abdomen and pelvis with intravenous contrast for obstruction, ischaemia and perforation, water soluble contrast study for adhesive obstruction or distal colonic obstruction, ultrasound for appendicitis, intussusception, gallstones, aortic aneurysm and ascites, contrast enema for volvulus and intussusception reduction, endoscopy for foreign bodies, serum lactate, amylase and C-reactive protein, and surgical review.