Free gas says a viscus has perforated; the site is read from where the gas and fluid concentrate — gas at the falciform ligament and around the duodenum, a wall defect in the anterior duodenal bulb or gastric antrum — and it decides the operation.
Orient first
- Upper GI perforation (stomach, duodenum) gives larger volumes of free gas, gathering anterior to the liver and in the fissure for the ligamentum teres.
- Lower GI perforation gives small gas bubbles in the pelvis and faecal matter; gas confined to the mesentery points to the colon.
- The ulcer is usually on the anterior wall of the duodenal bulb; a posterior ulcer penetrates the pancreas and bleeds rather than perforates.
Acquire the study
- Portal venous phase CT abdomen-pelvis; review on lung window (W 1500, L −600) and soft-tissue window with coronal reformats.
The manoeuvre
- Lung window: free gas anterior to the liver, in the fissure for the ligamentum teres and in the lesser sac.
- Concentration of gas bubbles and fluid next to the duodenal bulb or antrum in the axial and coronal plane points to the site.
- Look for a focal wall defect or wall thickening with surrounding fat stranding at the site.
- Report free fluid volume and a collection that needs drainage; note if gas extends into the retroperitoneum (duodenal D2–D3).
What confirms it
- Free intraperitoneal gas with a focal duodenal or gastric wall defect or peri-duodenal gas/fluid concentration.
What licenses you to exclude it
- A CT with no free gas on lung window makes perforation unlikely but a sealed perforation can leave only fluid and fat stranding at the site — describe it.
The classic misread
- Overlooking a few bubbles because the scan was read only on soft-tissue window.