What is the recommended imaging test for suspected bowel obstruction?

Imaging for Bowel Obstruction

CT abdomen and pelvis with IV contrast is the imaging modality of choice for suspected bowel obstruction, with diagnostic accuracy exceeding 90% for both small and large bowel obstruction. 1, 2

Primary Imaging Recommendation

CT should be performed with IV contrast (when not contraindicated) and without oral contrast for suspected high-grade obstruction. 1, 2 The American College of Radiology establishes CT as the gold standard because it:

• Confirms or excludes obstruction with >90% accuracy 1, 2
• Distinguishes mechanical obstruction from adynamic ileus with high accuracy 1, 2
• Identifies the transition point and cause of obstruction 1, 2
• Detects life-threatening complications including ischemia, strangulation, closed-loop obstruction, and volvulus 1, 2, 3

CT Protocol Specifications

IV contrast is strongly preferred to evaluate bowel perfusion and identify ischemia, which carries up to 25% mortality if missed. 1, 2 Key protocol elements include:

• Oral contrast is NOT required and should be avoided in suspected high-grade obstruction - the nonopacified fluid in dilated bowel provides adequate intrinsic contrast 1, 2
• Oral contrast delays diagnosis, increases patient discomfort, raises aspiration risk, and can obscure abnormal bowel wall enhancement patterns that indicate ischemia 1, 4
• Multiplanar reformations significantly improve accuracy for locating transition points and planning surgical intervention 1, 4
• Thin sections should be used to evaluate regions of interest 5

Role of Plain Radiography

Plain abdominal radiographs have limited diagnostic value with sensitivity and specificity of only 60-70% and are frequently inconclusive, requiring additional imaging. 2 However, the American College of Radiology notes they may be appropriate as an initial examination to direct further workup, particularly in resource-limited settings. 2, 4 Radiographs can:

• Confirm or exclude small bowel obstruction in straightforward cases 1
• Detect free air suggesting perforation 1
• Show air-fluid levels and dilated bowel loops 4

The traditional clinical-radiographic evaluation fails to establish the diagnosis in 20-52% of cases, missing complete obstruction in over half of patients. 6

Critical CT Findings Requiring Urgent Surgery

Specific CT signs mandate immediate surgical intervention because they indicate complications with high morbidity and mortality: 1, 3

• Abnormally decreased or increased bowel wall enhancement (ischemia) 1, 4
• Intramural hyperdensity on noncontrast CT 1, 4
• Pneumatosis or mesenteric venous gas 1, 4
• Closed-loop obstruction with "beak sign" at transition point 1, 4
• Internal hernias 1

Unfortunately, CT sensitivity for ischemia is only 14.8% prospectively and 29.6-51.9% retrospectively, despite high specificity when signs are present. 1 This means clinical correlation remains essential - if clinical suspicion for ischemia is high despite negative CT, surgical exploration is still warranted. 1

Adjunct Imaging Modalities

Water-soluble contrast challenge can predict success of conservative management - if contrast reaches the colon within 24 hours, non-operative management is likely to succeed. 2, 4 This is particularly useful for:

• Partial obstruction 4
• Differentiating partial from complete obstruction 4
• Functional grading of obstruction severity 6

CT enterography is equally appropriate for intermittent or low-grade subacute bowel obstruction with indolent presentation. 2

Large Bowel Obstruction Considerations

For suspected large bowel obstruction, CT with IV contrast (without oral contrast) has 91% sensitivity, specificity, positive predictive value, and negative predictive value. 7 Additional prone and/or decubitus scans should be obtained when clarification of a possible transition point is needed. 7 CT effectively identifies obstructing masses (typically carcinoma) in the majority of cases. 7

Common Pitfalls to Avoid

• Never delay CT by administering oral contrast in suspected high-grade obstruction - this increases complications without improving diagnostic accuracy 1, 4
• Do not rely on plain films alone - they miss the diagnosis in up to half of cases and provide no information about cause or complications 2, 6
• Do not assume absence of CT ischemia signs excludes ischemia - sensitivity is poor, so maintain high clinical suspicion 1
• Recognize that CT can miss incomplete/low-grade obstruction - consider adjunct enteroclysis or water-soluble contrast studies in these cases 5