What is the best initial imaging study for a small bowel obstruction (SBO)?

Best Initial Imaging for Small Bowel Obstruction

CT abdomen and pelvis with IV contrast is the best initial imaging study for suspected small bowel obstruction (SBO) due to its superior diagnostic accuracy of >90% and ability to identify the cause, location, and complications of obstruction. 1

Diagnostic Accuracy of Imaging Modalities

Imaging Modality	Diagnostic Accuracy	Key Benefits
CT with IV contrast	>90%	Identifies cause, location, complications, ischemia
Plain Radiographs	60-70%	Limited utility, cannot reliably identify cause
Ultrasound	High sensitivity (91%)	Limited in adults, useful in pediatrics
MRI	95% sensitivity	Alternative for pregnant patients, children

Why CT is Superior as Initial Test

CT abdomen and pelvis with IV contrast provides critical information that impacts management:

• Confirms presence of obstruction with high accuracy
• Identifies the transition point and etiology
• Detects complications requiring urgent surgery:
  • Closed-loop obstruction
  • Strangulation
  • Ischemia
  • Perforation
  • Volvulus

CT findings that suggest ischemia include abnormal bowel wall enhancement, intramural hyperdensity, bowel wall thickening, mesenteric edema, ascites, and pneumatosis 1. The presence of ischemia warrants immediate surgical intervention.

Important CT Protocol Considerations

• No oral contrast needed: The fluid in obstructed bowel provides adequate intrinsic contrast
• IV contrast preferred: Essential for assessing bowel wall perfusion and detecting ischemia
• Multiplanar reconstructions: Increases accuracy in locating the transition zone 1

Role of Plain Radiographs

The ACR Appropriateness Criteria panel did not reach consensus on recommending radiographs of the abdomen and pelvis in patients with acute presentation of suspected SBO 1. Plain films have limited sensitivity (60-70%) and cannot reliably identify the cause or location of obstruction 1.

While some institutions still use radiographs as an initial screening tool, proceeding directly to CT is more efficient and provides definitive information that impacts management decisions.

Special Populations

• Pregnant patients and children: MRI is the preferred alternative to avoid radiation exposure (sensitivity 95%, specificity 100%) 1
• Pediatric patients: Ultrasound has shown good diagnostic accuracy and should be considered 1

Water-Soluble Contrast Studies

Water-soluble contrast agents have both diagnostic and potential therapeutic value in adhesive SBO:

• Can predict resolution of SBO with conservative therapy (96% sensitivity, 98% specificity)
• Appearance of contrast in the colon within 24 hours suggests successful non-operative management 1
• However, these studies are typically performed after initial CT diagnosis in cases of suspected adhesive SBO

Common Pitfalls to Avoid

1. Delaying CT for plain films: When SBO is clinically suspected, proceeding directly to CT provides more definitive information
2. Using oral contrast: Can delay diagnosis, increase patient discomfort, and risk aspiration
3. Misinterpreting adynamic ileus as mechanical SBO: CT helps distinguish between these entities
4. Missing closed-loop obstruction: Requires careful evaluation of CT images in multiple planes

CT abdomen and pelvis with IV contrast remains the most efficient and accurate initial imaging study for suspected SBO, providing crucial information that guides appropriate management and improves patient outcomes.