What is the recommended contrast protocol for a CT abdomen scan to rule out Crohn's disease when CT enterography is not available?

CT Abdomen with IV Contrast is Recommended for Ruling Out Crohn's Disease When CT Enterography is Not Available

For ruling out Crohn's disease when CT enterography is not available, CT abdomen with IV contrast alone is the preferred protocol, as positive oral contrast may obscure subtle mural enhancement patterns that are key to diagnosing active inflammation. 1

Rationale for IV Contrast Without Oral Contrast

• IV contrast is essential for detecting active Crohn's disease as it allows visualization of mural enhancement, which is a key indicator of active inflammation 1
• While CT enterography (with neutral oral contrast) would be ideal, when unavailable, standard CT with IV contrast alone is preferable to CT with positive oral contrast 1
• Positive oral contrast can obscure subtle stratified mural enhancement patterns and areas of active inflammation that are critical for diagnosis 1
• Without IV contrast, inflammatory processes can only be inferred by secondary findings like wall thickening, which may not be present in mild inflammation 1

Clinical Decision Algorithm

1. First choice (when available): CT enterography with neutral oral contrast and IV contrast

   • Provides optimal bowel distention and visualization of inflammatory changes 1
   • Allows assessment of the entire small bowel 1

2. When CT enterography is unavailable:

   • Use CT abdomen with IV contrast alone if:

     • Patient presentation suggests active inflammation 1
     • Need to evaluate for complications like abscess or fistula 1

   • Consider CT with both IV and positive oral contrast if:

     • Specific concern for bowel obstruction 1
     • Need to evaluate for fistula formation (sensitivity ranges from 68% to 100%) 1

Diagnostic Performance

• CT with IV contrast demonstrates excellent sensitivity (75-90%) and specificity (>90%) for detecting Crohn's disease when compared to endoscopic standards 1
• IV contrast timing is important - studies have shown that scans can be effectively obtained at either 40 or 70 seconds after IV contrast administration 2
• CT with IV contrast provides high diagnostic performance for:
  • Stenosis/obstruction (sensitivity 85-94%) 1
  • Abscess detection (sensitivity 86-100%) 1
  • Fistula identification (sensitivity 68-100%, though variable) 1

Important Caveats and Considerations

• Patient's clinical status should guide the choice - in acutely ill patients who cannot tolerate large volumes of oral contrast, CT with IV contrast alone is appropriate 1
• For pediatric patients, the same principles apply, though radiation dose considerations become more important 1
• If the patient can tolerate oral contrast, neutral contrast (as used in enterography) is preferable to positive contrast for visualizing mucosal inflammation 1
• Iohexol (Omnipaque) is commonly used as IV contrast, with typical adult dosage of 100-150 mL of Omnipaque 300 3
• Be aware that CT without enterography technique may miss more subtle findings of Crohn's disease, particularly in patients with mild symptoms 1

Alternative Approaches When CT Enterography is Not Available

• MR enterography could be considered if available, especially for patients requiring repeated imaging (to reduce radiation exposure) 4
• Conventional fluoroscopy with small bowel follow-through is less commonly used now but remains an option in specific scenarios 1
• Ileocolonoscopy should complement cross-sectional imaging when possible, as it permits direct visualization and biopsy of the terminal ileum and colon 1