A 52-year-old man with a history of peptic ulcer disease presents with acute severe epigastric pain and abdominal rigidity. CT imaging shows free air in the peritoneal cavity. Which imaging finding best distinguishes a perforated peptic ulcer from a perforated diverticulitis?

## Distinguishing Perforated Peptic Ulcer from Perforated Diverticulitis ### Key Anatomical Difference **Key Point:** The location and distribution of free air on CT is the most reliable imaging discriminator between these two perforations. ### Free Air Distribution Patterns | Feature | Perforated Peptic Ulcer | Perforated Diverticulitis | |---------|------------------------|-------------------------| | **Primary location of free air** | Anterior peritoneal cavity, lesser sac | Pelvis, paracolic gutters | | **Typical trajectory** | Tracks superiorly along anterior peritoneum | Tracks inferiorly to pelvis | | **Associated findings** | Visible ulcer crater, minimal bowel wall thickening | Inflamed diverticulum, bowel wall thickening, pericolic fat stranding | | **Abscess formation** | Rare (acute rupture) | Common (contained perforation) | ### Why Location Matters 1. **Peptic ulcer perforation**: Occurs in the anterior wall of the duodenum or gastric antrum. Free air immediately tracks **superiorly and anteriorly** into the anterior peritoneal cavity and lesser sac due to gravity and anatomical planes. 2. **Diverticular perforation**: Occurs in the sigmoid colon (or right colon). Free air tracks **inferiorly and laterally** into the pelvis and paracolic gutters, often becoming loculated and forming abscesses. **High-Yield:** Free air in the **lesser sac and anterior peritoneal cavity** is pathognomonic for perforated peptic ulcer; free air in the **pelvis and paracolic gutters** suggests colonic perforation (diverticulitis). ### Clinical Pearl In perforated peptic ulcer, patients often present with **acute onset** and minimal abscess formation because the perforation is typically acute and catastrophic. In contrast, perforated diverticulitis often has a **subacute course** with loculated collections and abscess formation due to the inflammatory nature of the disease. **Tip:** Always assess the **anatomical location of free air** first—it is more specific than the amount of free air or presence of abscess.