Abdominal X-ray - Abnormal bowel gas pattern
Free gas/pneumoperitoneum

Key points

  • Bowel perforation is a surgical emergency
  • An ERECT chest X-ray should be requested if perforation is suspected
  • Be familiar with Rigler's sign

Free gas, or pneumoperitoneum, is gas or air trapped within the peritoneal cavity, but outside the lumen of the bowel. Pneumoperitoneum can be due to bowel perforation, or due to insufflation of gas (CO2 or air) during laparoscopy. Both these causes have identical X-ray appearances, but very different clinical significance.

Bowel perforation

Bowel perforation is a surgical emergency. All medical students and junior doctors must therefore be familiar with the X-ray appearances of pneumoperitoneum in the clinical context of an acute surgical abdomen.

Erect chest X-ray

Patients presenting with an acute surgical abdomen should be investigated with an ERECT chest X-ray, as well as the standard supine abdominal X-ray. The patient should be positioned sitting upright for 10-20 minutes prior to acquiring the erect chest X-ray image. This allows any free intra-abdominal gas to rise up, forming a crescent beneath the diaphragm. It is said that as little as 1ml of gas can be detected in this way.

Bowel perforation is a favourite of the finals radiology OSCE. If you are shown an abdominal X-ray you should request to see the erect chest X-ray.

Air/gas under the diaphragm - erect chest X-ray

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Air/gas under the diaphragm - erect chest X-ray

  • This patient has a large volume of free gas under the diaphragm. Dark crescents have formed separating the thin diaphragm from the liver on the right, and bowel on the left.
  • This patient had a perforated duodenal ulcer.

Air/gas under the diaphragm - close up

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Air/gas under the diaphragm - close up

  • If perforation is suspected you must look very closely. In this patient, only a very thin crescent has formed under only the right hemidiaphragm.
  • Pneumoperitoneum due to insufflation of gas at laparoscopy has identical appearances.

Abdominal X-ray

Although the erect chest X-ray is a much more sensitive investigation for pneumoperitoneum, there are several signs that may be useful in detecting free gas on an abdominal X-ray.

Rigler's/double wall sign

Rigler's sign (also known as the double wall sign) is the appearance of lucency (gas) on both sides of the bowel wall.

Rigler's/double wall sign - diagram

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Rigler's/double wall sign - diagram

  • Normally only the inner wall of the bowel is visible
  • If there is pneumoperitoneum both sides of the bowel wall may be visible

Rigler's/double wall sign - example

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Rigler's/double wall sign - example

  • The double wall sign (Rigler's sign) is visible
  • Gas separates bowel segments and forms sharp angles and triangles (asterisks)

Football sign - example

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Football sign - example

  • 2 radiographs were required to completely cover the abdomen in this large patient
  • A large volume of free gas has risen to the front of the peritoneal cavity resulting in a large round black area - 'football sign'
  • The double wall sign (Rigler's) is also visible (arrowhead)

Liver edge - example (close up)

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Liver edge - example (close up)

  • Gas may be seen outlining soft tissues structures such as the falciform ligament, or the liver edge