Pneumoperitoneum
- Erect CXR (~10 min upright) = best projection (can detect ~1–2 mL under the diaphragm); left lateral decubitus AXR if the patient cannot sit (air over the liver); CT = most sensitive (IOC), with lung window.
- Causes: perforated peptic ulcer (commonest), perforated diverticulitis, malignancy, bowel ischaemia, iatrogenic (post‑operative free gas is normal up to ~3–7 days; laparoscopy, peritoneal dialysis), pneumatosis cystoides (benign, asymptomatic), NEC (neonates).