Radiology · Erect chest X-ray + CT abdomen

Pneumoperitoneum (Air Under the Diaphragm)

A crescent of air under the diaphragm on an erect chest X-ray — pneumoperitoneum, most often from a perforated hollow viscus. The CT confirms free intraperitoneal gas.

Erect chest X-ray with a labelled lucent crescent of air under the left hemidiaphragm, next to an axial CT showing free air around the liver and stomach
Free air under the left hemidiaphragm on chest X-ray; CT shows extensive pneumoperitoneum.Image: Cerevisae, via Wikimedia Commons — CC BY-SA 4.0.

Written & reviewed by: Kinase Medical Team · Last reviewed: 30 September 2026 · Editorial policy

Key features

  • Lucent crescent of free gas under the hemidiaphragm on an erect chest X-ray.
  • The erect film is very sensitive — Radiopaedia notes that about 1 mL of free gas can be detected.
  • Most common cause: disruption of a hollow viscus wall (e.g. perforated peptic ulcer); free gas is also expected after abdominal surgery.
  • Other plain-film signs: Rigler (double-wall) sign, football sign.
  • A left lateral decubitus view helps if the patient cannot sit up; CT is the most sensitive test.

Differential diagnosis

Differential diagnosis of Pneumoperitoneum (Air Under the Diaphragm)
ConditionHow to tell it apart
Chilaiditi sign (pseudopneumoperitoneum)Colon interposed between the liver and diaphragm mimics free gas; a left lateral decubitus view or CT separates the two.

How it appears in NEET PG

A classic surgery/radiology image: an erect chest film in a patient with sudden severe abdominal pain, asking for the sign or the most likely cause (perforated peptic ulcer).

Frequently asked questions

Why is an erect chest X-ray used to detect pneumoperitoneum?

Free gas rises to the highest point of the abdomen, under the diaphragm, where it is easy to see against the liver. Even a small volume can be detected this way.

What is the Rigler sign?

Both the inner and outer walls of a bowel loop are visible because gas lies on both sides of the wall — a sign of pneumoperitoneum on an abdominal film.

What mimics air under the diaphragm?

The Chilaiditi sign — colon interposed between the liver and diaphragm — is the classic pseudopneumoperitoneum.

Sources

  1. Radiopaedia — Pneumoperitoneum
  2. Harrison's Principles of Internal Medicine (textbook)

Keep practising

For exam preparation and medical education only — not a substitute for clinical judgement, diagnosis or treatment of any patient.