An erect chest radiograph of a patient with acute abdomen shows a crescent of gas beneath the right hemidiaphragm. Which diagnosis does this represent?

• A Right-sided pneumothorax
• B Right lower lobe atelectasis
• C Subphrenic abscess
• D Pneumoperitoneum ✓

Explanation

Free intraperitoneal gas (pneumoperitoneum) accumulates under the diaphragm in the erect position and appears as a radiolucent crescent beneath the right hemidiaphragm on an erect chest radiograph. The right side is more commonly seen because the left may be masked by the gastric bubble. The most common cause is a perforated hollow viscus (peptic ulcer perforation). Pneumothorax appears as a white pleural line with absent lung markings peripherally. Subphrenic abscess shows an air-fluid level below the diaphragm.

Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.

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