Abdominal X-ray - Abnormal bowel gas pattern
Free gas mimics

Key points

  • Certain normal structures can mimic pneumoperitoneum

Stomach bubble

The normal stomach bubble should not be confused with free intra-abdominal gas.

The stomach bubble forms a round/ovoid shape under the left hemidiaphragm. As the stomach has a thick wall, there is a thick line separating gas in the stomach from air in the lungs.

In contrast to this, free intra-abdominal gas forms a crescent under the diaphragm, and is separated from the lungs only by the thin membrane of the diaphragm.

Normal stomach bubble - erect chest X-ray

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Normal stomach bubble - erect chest X-ray

  • Round/ovoid - 'bubble' shape
  • Thick upper wall
  • Fluid level or food contents

Chilaiditi's phenomenon

In patients who have small livers (cirrhosis), or flattened diaphragms due to lung hyperexpansion (emphysema), a void is created within the upper abdomen above the liver. This space may be filled by bowel. If this bowel is air filled then it may mimic free gas.

Chilaiditi's phenomenon - example

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Chilaiditi's phenomenon - example

  • Gas forms a near crescent shape under the right hemidiaphragm
  • There is however a thick hemidiaphragm (partly consisting of bowel wall)
  • Gas can be seen to lie within bowel
  • Importantly, this patient with hyperexpanded lungs, due to emphysema, did not have acute abdominal pain

False Rigler's/double wall sign

Be careful not to mistake the gas within two adjacent bowel segments for Rigler's sign.

False Rigler's/double wall sign

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False Rigler's/double wall sign

  • Gas seen on both sides of the bowel wall is contained within adjacent bowel
  • There are no black triangles or sharp angles on the outside of the bowel wall

False football sign

Don't mistake normal abdominal fat for free gas.

False football sign - example

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False football sign - example

  • 1 - Perirenal fat (retroperitoneal)
  • 2 - Peritoneal fat (next to the liver)
  • 3 - Abdominal wall fat (separating muscles of the abdominal wall)

Page author: Salisbury NHS Foundation Trust UK (Read bio)

Last reviewed: October 2019