The semi-erect frontal chest radiograph shows ‘Rigler Sign’ or Double wall sign indicating the presence of pneumoperitoneum.

Normally gas only outlines the inner aspect of the bowel loop. In the presence of free intraperitoneal air, the outer aspect of the bowel wall is visualised clearly due to the presence of air inside the bowel loop (physiological) and outside the bowel loop (pathological).This constitutes Rigler sign.

The sign is named after an American Radiologist Dr. Leo Rigler who first described it.

The above patient underwent a CT scan which confirmed pneumoperitoneum related to a perforated peptic ulcer.

Reference: https://pubs.rsna.org/doi/10.1148/radiol.2283020302