Plain abdominal X-ray

What is a plain abdominal X-ray?

The plain abdominal X-ray is readily available. It is often used for urgent investigation - for example, of acute abdominal pain.

Investigations are normally undertaken after history and examination. The merits of any investigation should always be balanced against cost and risk.

Employing a consistent routine when examining abdominal X-rays will improve detection of abnormal findings.

Other imaging techniques should be considered, including ultrasound, CT scans and MRI scans.

Value and limitations

Diagnosis or management is not often changed by the X-ray and this raises questions about the value of such investigation. A 2020 systematic review and meta-analysis concluded, based on abundant available evidence, major advances in diagnostic imaging and changes in the management of certain diseases, that there was no place in current practice for routine plain abdominal radiography in the work-up of adult patients with acute abdominal pain.¹ There is also evidence that many doctors would benefit from further training in reading X-rays.²

Erect abdominal X-rays are used to look for fluid levels in obstruction or ileus. Air may be seen under the diaphragm in an erect film if the bowel has been perforated, although a CXR is more usual to look for that sign. Abdominal X-ray is of no value in haematemesis. Avoiding erect pictures where unnecessary and avoiding plain films for haematemesis will reduce the level of unnecessary films.³

Indications

Renal colic

• A 'KUB' picture is requested. This is a large film that is designed to take in the kidneys, ureters and bladder.

• About 90% of renal stones are radio-opaque. Uric acid stones may be missed.

• False positives may occur from phleboliths that are most common in the pelvic veins. False negatives may arise, especially if stones are small.

• Calcification may represent gallstones but only a minority of gallstones are radio-opaque. Gallstones become more frequent with age and are often asymptomatic.⁴

• Doctors in A&E tend to be poor at identifying stones on plain films but, if urinalysis is negative, the diagnosis is unlikely to be renal colic.⁵

Intestinal obstruction

• Erect and supine films are used to confirm the diagnosis.

• Obstruction of the small bowel shows a ladder-like series of small bowel loops but this also occurs with an obstruction of the proximal colon. Fluid levels in the bowel can be seen in upright views.

• Distended loops may be absent if obstruction is at the upper jejunum.

• Obstruction of the large bowel is more gradual in onset than small bowel obstruction. The colon is in the more peripheral part of the film and distension may be very marked.

• Fluid levels will also be seen in paralytic ileus when bowel sounds will be reduced or absent rather than loud and tinkling as in obstruction.

• In an erect film, a fluid level in the stomach is normal as may be a level in the caecum. Multiple fluid levels and distension of the bowel are abnormal.

Perforation of the intestine

• If the bowel has been perforated and a significant amount of gas has been released it will show as a translucency under the diaphragm on an erect film.

• Gas will also be found under the diaphragm for some time after laparotomy or laparoscopy.

Appendicitis

• An appendicolith may be apparent in an inflamed appendix in 15% of cases but as a diagnostic point in the management of appendicitis, the plain X-ray is of very limited value.⁶

• It may be of value in infants, although CT scanning has significant advantages over X-rays in such cases.⁷

Intussusception

• Intussusception occurs in adults and children.

• A plain abdominal X-ray may show some characteristic gas patterns.⁸

• A sensitivity and specificity of 90% adds to this rather difficult diagnosis but ultrasound is vastly superior.⁹

Detection of swallowed foreign bodies

• Plain X-ray will detect the presence of radiopaque foreign bodies.

• A plain abdominal X-ray will show 90% of cases of 'body packing' (internal concealment of drugs to avoid detection) but there can be occasional false positives.¹⁰

Suggested approach to viewing films

• Identify the name and date on the film. If there are previous films, use them for comparison.

• Identify the projection of the film - most are anteroposterior (AP).

• Identify the view taken - 'supine', 'erect' or 'lateral decubitus'.

• Confirm that an adequate area has been covered, especially for a KUB. An abdominal film should include the lower anterior ribs.

• Check exposure. If the spine is visible, most structures to be seen will be visible. Under-penetration is not usually a problem. Overexposure (dark areas) should be viewed with a bright light.

• Artefacts may be immediately obvious. Piercing of the umbilicus is very popular, especially in young women but genital piercing is not infrequent. Metallic objects are obvious. There may be clips or materials from previous surgery. Occasionally a retained surgical instrument is seen. Swabs contain a radio-opaque band.

Below is a diagrammatic representation of the radiological anatomy of the abdomen:

Abdominal anatomy

Solid organs, hollow organs and bones can be classified as:

• Visible or not visible.

• Normal in size, enlarged, or too small.

• Distorted or displaced.

• Abnormally calcified.

• Containing abnormal gas, fluid, or discrete calculi.

Bones

Identify:

• Lower rib cage.

• Lumbar spine.

• Sacrum.

• Pelvis.

• Hip joints.

Check for:

• Cortical outline.

• Joint and disc space.

• Trabecular pattern.

• General bone density.

• Lysis, fracture, sclerosis.

• Epiphyseal lines.

Solid organs

• Liver:
  

  • There is soft tissue density in the right upper quadrant that displaces any bowel from this area.

• Spleen:
  

  • Soft tissue mass in the left upper quadrant about the size of a fist (usually is not visible).

• Kidneys:
  

  • A shadow may be visible.

  • The left kidney is higher than the right. The upper poles tilt medially.

  • They should be about three vertebrae in size.

• Psoas muscles:
  

  • Form straight lines extending infero-laterally from the lumbar spine to the lesser trochanter of the femur.

• Bladder:
  

  • If the bladder is full, it will appear as a soft tissue density in the pelvis.

• Uterus:
  

  • Sits on top of and may indent the bladder.

  • It is often not seen on plain films.

• Prostate:
  

  • Sits deep in the pelvis.

  • Usually only seen if calcified.

Hollow organs

• Stomach:
  

  • When supine, air in the stomach will rise anteriorly and fluid will pool posteriorly.

• Small bowel:
  

  • Gas will be seen in polygonal shapes, due to peristalsis.

  • Normal small bowel is 2.5-3.0 cm in diameter.

  • Valvulae may be seen crossing the entire lumen.

  • Often, little small bowel is seen on a plain film.

• Appendix:
  

  • Occasionally, an appendicolith is seen.

  • Less commonly, barium from an old study, or ingested foreign bodies will appear in the appendix.

• Colon:
  

  • Start in the right iliac fossa with the caecum that may show fluid levels.

  • Follow it up to the hepatic flexure, over to the splenic flexure and down into the pelvis. It may be filled with air or faeces. Shape may be altered by redundant bowel. The colon is in the periphery of the abdomen.

Normal calcification

• Costal cartilage.

• Mesenteric lymph nodes.

• Pelvic vein phleboliths.

• Prostate gland.

Abnormal calcification

Calcium indicates pathology in:

• Pancreas.

• Renal parenchymal tissue.

• Blood vessels and vascular aneurysms.

• Gallbladder fibroids (leiomyoma).

Calcium can make the following pathology visible:

• Biliary calculi.

• Renal calculi.

• Appendicolith.

• Bladder calculi.

• Teratoma.

Other calcification

• Costal cartilages may be calcified, especially in the elderly. It can look dramatic but is benign.

• Mesenteric lymph nodes may calcify and be confused with ureteric calculi. They are usually oval in shape. The line of the ureter is along the transverse processes of the lumbar vertebrae. Phleboliths from calcified pelvic veins may appear like bladder stones. Calcification may appear in the ageing prostate, low down in the pelvic brim. Prostate calcification may also occur in malignancy but it is not diagnostic.

• The pancreas lies at the level of the T9 to T12 vertebrae. Calcification occurs in chronic pancreatitis and may show the whole outline of the gland.

• Between the levels of T12 and L2, nephrocalcinosis may be seen. Calcification of the renal parenchyma indicates pathology including hyperparathyroidism, renal tubular acidosis and medullary sponge kidney.

• Calcification of blood vessels usually affects the arteries and can be quite striking. The whole vessel may be outlined by calcium. Extensive calcification may indicate widespread atheroma, especially in diabetes.

• Abdominal aortic aneurysms are usually below the 2nd lumbar vertebra. Calcification may make them obvious and can give a rough indication of the internal diameter. Abdominal ultrasound is required for accurate assessment and to determine the need for surgery or follow-up.

• Uterine fibroids can become calcified.

• Gallstones are visible in only 10-20% of cases. Ultrasound is vastly superior but plain abdominal X-ray is often the initial investigation in patients with abdominal pain. The gallbladder may become calcified after repeated episodes of cholecystitis. This is called a porcelain gallbladder and may become malignant.

• Renal calculi tend to obstruct at certain sites, especially the pelvi-ureteric junction, brim of the pelvis and vesico-ureteric junctions.¹¹

• In the pelvic region, bladder calculi may occasionally be seen. Bladder stones are usually quite large and often multiple. Calcification of a bladder tumour may also occur. Schistosomiasis may produce calcification of the bladder wall. This can sometimes be seen in X-rays of mummies of ancient Egyptian pharaohs.

• Sometimes, ovarian teratoma may show a tooth. This is of passing interest, although such an ovarian tumour can undergo torsion.