A 52-year-old man presents with acute onset abdominal pain, abdominal distension, and vomiting. On examination, there is visible peristalsis and a palpable abdominal mass in the right lower quadrant. Plain abdominal X-ray shows dilated small bowel loops with air-fluid levels. Which is the investigation of choice to confirm the diagnosis and determine the level of obstruction?

## Investigation of Choice for Small Bowel Obstruction **Key Point:** CT abdomen and pelvis with IV contrast is the gold standard for evaluating acute small bowel obstruction — it localizes the obstruction, identifies the cause, and assesses for complications. ### Why CT is Superior | Feature | CT with IV Contrast | Barium Follow-Through | Ultrasound | MR Enterography | |---------|-------------------|----------------------|-----------|----------------| | **Sensitivity for obstruction** | 90–95% | 60–70% | 70–80% | 85–90% | | **Identifies cause** | Excellent (mass, volvulus, adhesions) | Poor | Moderate | Good | | **Speed** | Rapid (< 5 min) | Slow (hours) | Rapid | Slow (20–30 min) | | **Detects complications** | Perforation, ischemia, closed loop | Limited | Limited | Limited | | **Radiation dose** | Moderate | High (barium + fluoroscopy) | None | None | | **Use in acute setting** | Yes | No (contraindicated in complete obstruction) | Limited role | No (time-consuming) | **High-Yield:** In this case, the palpable right lower quadrant mass suggests a mechanical cause (likely intussusception, malignancy, or Crohn's disease). CT will identify the mass, show transition zone, and assess for ischemia — all critical for surgical planning. ### Clinical Pearl **Barium follow-through is contraindicated** in suspected complete small bowel obstruction because barium may impact at the obstruction site and worsen the condition or cause perforation. **MR enterography** is excellent for chronic obstruction and Crohn's disease but is too slow for acute presentations requiring urgent diagnosis and intervention. **Ultrasound** can show dilated loops and free fluid but has poor sensitivity for identifying the exact cause and level of obstruction, especially in obese patients. ### Imaging Findings in Small Bowel Obstruction on CT 1. **Dilated small bowel** (> 3 cm diameter) 2. **Transition zone** — abrupt change from dilated to collapsed bowel 3. **Cause identification** — mass, volvulus, internal hernia, adhesions 4. **Ischemic changes** — bowel wall thinning, lack of enhancement, free fluid 5. **Closed-loop obstruction** — high-risk for perforation **Mnemonic: ABCDE of CT findings in obstruction** - **A**brupt transition zone - **B**owel dilatation (> 3 cm) - **C**ause identified (mass, volvulus) - **D**etail of complications (ischemia, perforation) - **E**nhancement pattern (normal vs. compromised) [cite:Harrison 21e Ch 297]