What are the key findings on a CT image to diagnose small bowel obstruction?

Key CT Findings for Diagnosing Small Bowel Obstruction

CT abdomen and pelvis with IV contrast is the imaging modality of choice for diagnosing small bowel obstruction, with diagnostic accuracy exceeding 90% for high-grade obstruction. 1

Essential CT Diagnostic Criteria

The fundamental CT findings that establish the diagnosis of small bowel obstruction include:

• Dilated small bowel loops measuring ≥2.5 cm proximal to the obstruction site 2
• Collapsed or normal-caliber bowel distal to the point of obstruction 2
• Transition point where bowel caliber changes from dilated to decompressed, which can be identified with 64% accuracy and is enhanced by multiplanar reformations 3, 1

Critical Findings Indicating Complications

Signs of bowel ischemia on CT warrant immediate surgical intervention and include 1, 4:

• Abnormally decreased or increased bowel wall enhancement
• Intramural hyperdensity on noncontrast images
• Bowel wall thickening
• Mesenteric edema and ascites
• Pneumatosis intestinalis or mesenteric venous gas

These ischemic findings are present in only 20% of patients with surgically confirmed ischemic bowel, making CT less reliable for detecting this complication 3.

Identifying the Cause of Obstruction

CT correctly identifies the etiology in 85% of confirmed small bowel obstruction cases 5. The modality excels at demonstrating:

• Extrinsic causes: hernias, extrinsic masses, closed-loop obstruction (look for "beak sign" at transition point) 6, 4
• Intrinsic causes: adenocarcinoma, Crohn disease, intussusception, intramural hemorrhage 6
• Intraluminal causes: bezoars, "small bowel feces sign" 6, 4
• Masses: correctly identified in 69% of cases 3

Important caveat: CT cannot reliably differentiate adhesions from internal hernias or radiation enteritis, and adhesions (the most common cause) are correctly identified in only 21% of cases 3, 5.

Optimal CT Technique

Do not administer oral contrast in suspected high-grade obstruction 1, 4. The rationale includes:

• Intrinsic bowel fluid provides adequate contrast 1
• Oral contrast delays diagnosis and increases patient discomfort 1
• Positive oral contrast obscures abnormal bowel wall enhancement patterns critical for detecting ischemia 1
• Risk of vomiting and aspiration is increased 4

IV contrast is preferable to assess bowel perfusion and detect ischemia, though noncontrast CT has comparable accuracy for diagnosing obstruction presence (but reduced sensitivity for ischemia) 1.

Multiplanar Reformations

Multiplanar reconstructions significantly improve diagnostic performance by 1, 4:

• Increasing accuracy and confidence in locating transition zones
• Better evaluating the 3-D anatomy of complex obstructions
• Providing useful surgical planning information

Special Considerations for Low-Grade Obstruction

Standard CT has markedly reduced sensitivity (48-50%) for low-grade or intermittent obstruction compared to high-grade obstruction 1. In these cases:

• Bowel loops may appear unremarkable with standard technique 1
• CT enteroclysis (nasoduodenal tube with controlled contrast infusion) offers improved sensitivity and specificity 1
• CT enterography (oral contrast protocol without intubation) may be beneficial though evidence is limited 1
• Optional re-imaging at 24 hours after oral contrast administration can demonstrate passage beyond a transition point, confirming partial obstruction 1

Clinical Impact

CT findings correctly modify management in 21% of patients by either changing conservative management to operative (18% of cases) or preventing unnecessary surgery by distinguishing ileus from obstruction 5. CT changes the pre-scan diagnosis regarding presence, cause, or severity of obstruction in 21-44% of cases 5.