On CT imaging of small bowel obstruction, which sign indicates the transition point between proximal dilated bowel and distal collapsed bowel, and is the most specific for mechanical obstruction?

Explanation

## Transition Zone Sign in Mechanical Small Bowel Obstruction **Key Point:** The **transition zone sign** is the abrupt change in caliber from proximal dilated small bowel to distal decompressed bowel, visible on CT as a focal narrowing point. This is the **most specific sign for mechanical obstruction**. **High-Yield:** The transition zone localizes the site of obstruction and helps differentiate mechanical from paralytic ileus (which shows diffuse dilatation without a focal transition). ### CT Findings in Small Bowel Obstruction | Finding | Definition | Specificity for Mechanical Obstruction | |---------|-----------|----------------------------------------| | **Transition Zone** | Abrupt change from dilated to collapsed bowel | **Highest** — nearly pathognomonic | | **Valvulae Conniventes** | Mucosal folds extending across entire lumen | Non-specific; normal anatomy | | **Plicae Circulares** | Circular mucosal folds in small bowel | Non-specific; normal anatomy | | **Haustra** | Incomplete mucosal folds in colon | Not relevant to small bowel obstruction | **Clinical Pearl:** A **clear transition zone** on CT is diagnostic of mechanical obstruction and helps the surgeon plan the approach (proximal vs. distal obstruction). **Mnemonic:** **TRANS** = **TR**ansition **A**brupt **N**arrowing **S**ite = mechanical obstruction. ### Additional CT Signs of Obstruction - **Small bowel fecalization** (debris in small bowel) - **Whirl sign** (volvulus) - **Closed-loop obstruction** (U-shaped or C-shaped loop) - **Mesenteric ischemia signs** (target sign, pneumatosis, portal venous gas) [cite:Robbins & Cotran 10e Ch 17]