What is the appropriate imaging workup for a patient with suspected intestinal obstruction?

Imaging for Intestinal Obstruction

CT abdomen and pelvis with intravenous contrast is the definitive imaging study for suspected intestinal obstruction and should be obtained immediately, bypassing plain radiographs in most cases. 1

Initial Imaging Strategy

Proceed directly to CT with IV contrast when clinical suspicion for intestinal obstruction is high. CT achieves diagnostic accuracy exceeding 90-96% for detecting the presence, location, and cause of obstruction, vastly outperforming plain radiographs which have only 30-77% accuracy and are misleading in 20-40% of patients. 1, 2

Why CT is Superior

CT provides critical information that plain films cannot reveal: 1

• Confirms presence and grade of obstruction with 93-96% sensitivity 1, 2
• Identifies the exact transition point using multiplanar reconstructions 1
• Determines the underlying cause in 87-95% of cases (adhesions, hernias, malignancy, volvulus) 2, 3
• Detects life-threatening complications including bowel ischemia, strangulation, closed-loop obstruction, and perforation 1

CT Protocol Specifications

• Use IV contrast to assess bowel wall perfusion and detect ischemia 1
• Do not use oral contrast for suspected obstruction—the fluid-filled dilated bowel provides natural contrast, and oral contrast adds no diagnostic value while potentially causing aspiration 1
• Obtain multiplanar reconstructions to improve accuracy in localizing the transition zone 1

When Plain Radiographs May Be Considered

Plain abdominal radiographs should only be used as an initial screening tool when CT is not immediately available or in resource-limited settings. 1 However, recognize their severe limitations:

• Sensitivity for small bowel obstruction is only 50-77%, with specificity of 50-72% 1, 2
• Diagnostic in only 50-60% of cases, inconclusive in 20-30%, and misleading in 10-20% 1
• Cannot identify the cause of obstruction in 93% of cases 2
• Cannot detect bowel ischemia, strangulation, or other complications 1

Critical pitfall: Do not waste time obtaining plain radiographs when CT is available—this delays definitive diagnosis and appropriate treatment. 1

CT Findings Requiring Emergency Surgery

Immediately consult surgery when CT demonstrates: 1

• Reduced or absent bowel wall enhancement (indicates ischemia)
• Closed-loop obstruction (C-shaped or U-shaped dilated loop)
• Pneumatosis intestinalis or mesenteric venous gas (advanced ischemia)
• Pneumoperitoneum (free intraperitoneal air indicating perforation)
• Mesenteric edema with ascites and absence of small-bowel feces sign (high-risk for ischemia)

Alternative Imaging Modalities

Ultrasound

Bedside ultrasound has 88-91% sensitivity and 76-96% specificity for diagnosing intestinal obstruction and can serve as an alternative when CT is unavailable. 1, 2 Diagnostic criteria include dilated loops >2.5-3 cm and decreased/absent peristalsis. 1 However, ultrasound cannot reliably identify the cause or detect ischemia. 2

MRI

MRI should be reserved for special populations where radiation exposure is a concern: 1

• Pregnant patients (use noncontrast sequences only)
• Children
• Young patients with repetitive episodes requiring multiple imaging studies

MRI achieves 95% sensitivity and 100% specificity for bowel obstruction but requires 20-40 minutes acquisition time versus <1 minute for CT, limiting its use in acutely ill patients. 1

Special Consideration: Low-Grade or Intermittent Obstruction

Standard CT has lower sensitivity (48-50%) for low-grade or intermittent partial obstruction. 1 When initial CT is equivocal but clinical suspicion remains high:

• Administer 50-150 mL water-soluble contrast (Gastrografin) via nasogastric tube after adequate gastric decompression 1
• Obtain abdominal X-ray at 24 hours to assess contrast progression 1
• Failure of contrast to reach colon at 24 hours predicts need for surgery with 96% sensitivity and 98% specificity 1

Safety Precautions for Water-Soluble Contrast

Do not administer water-soluble contrast until: 1

• Stomach is adequately decompressed via NG tube (prevents aspiration pneumonia)
• Patient is adequately rehydrated with IV fluids (prevents hypovolemic shock from osmotic fluid shifts)
• At least 48 hours of conservative management has elapsed (safer timing)

Large Bowel Obstruction

For suspected large bowel obstruction, CT scan achieves 93-96% sensitivity and 93-100% specificity, far superior to plain radiographs (84% sensitivity, 72% specificity). 1 Water-soluble contrast enema has 96% sensitivity and 98% specificity for diagnosing large bowel obstruction but cannot distinguish the underlying cause. 1

Common Pitfalls to Avoid

• Do not order plain radiographs when CT is available—this wastes critical time without adding diagnostic information 1
• Do not order follow-up plain radiographs after CT—they add no value and can be misleading 1
• Do not delay CT imaging in patients with peritoneal signs, fever, hypotension, tachycardia, or elevated lactate 1
• Do not use oral contrast for suspected obstruction—it provides no benefit and may cause aspiration 1
• Do not extend conservative management beyond 48-72 hours without repeat CT if no clinical improvement occurs 1