On CT imaging, which sign indicates perforation of a hollow viscus and represents the site of defect in the bowel wall?

Explanation

## CT Imaging of Bowel Perforation — The Perforation Site Sign **Key Point:** Focal discontinuity in the bowel wall with adjacent free air is the most direct CT sign of bowel perforation and represents the actual site of the defect. ### Direct Signs of Perforation on CT | Sign | Description | Sensitivity | Specificity | |------|-------------|-------------|-------------| | **Focal bowel wall discontinuity + free air** | Visible defect in bowel wall with adjacent pneumoperitoneum | High | Very high (>95%) | | **Free air (pneumoperitoneum)** | Air in peritoneal cavity | Sensitive (~95%) | Low (can be post-op) | | **Localized free air near bowel** | Air collection adjacent to specific bowel segment | Moderate | Moderate | ### Indirect Signs of Perforation (Non-Specific) - **Peritoneal fat stranding** — Inflammation but not specific to perforation - **Ascites** — Fluid accumulation; non-specific - **Thickened peritoneum** — Peritonitis, but can occur without perforation - **Dilated bowel with transition point** — Obstruction, not perforation **High-Yield:** CT is the **gold standard** for detecting bowel perforation with sensitivity >95% and specificity >90%. It can identify: 1. The exact site of perforation (focal wall discontinuity) 2. Free air in the peritoneal cavity 3. The underlying cause (tumor, diverticulitis, ulcer, trauma) 4. Associated complications (abscess, sepsis) **Clinical Pearl:** Even small amounts of free air (as little as 1–2 mL) can be detected on modern multidetector CT, especially on lung windows. This is why CT is superior to plain radiography. **Mnemonic:** **PERFORATION on CT** = **P**neumoperitoneum + **E**xact site (wall discontinuity) + **R**egional inflammation + **F**ree air localization ### Why Focal Wall Discontinuity Is Diagnostic The presence of a **focal gap or defect** in the normally continuous bowel wall, especially when **adjacent to free air**, is virtually pathognomonic for perforation because: - It represents the actual anatomical breach - It localizes the site of perforation (aids surgical planning) - It excludes mimics like peritonitis without perforation **Warning:** Absence of visible wall defect does NOT exclude perforation. Small perforations (especially from peptic ulcer disease) may not show a visible defect on CT, but free air will still be present.