Can colitis be diagnosed on a computed tomography (CT) scan without contrast?

Diagnosis of Colitis on CT Scan Without Contrast

Yes, colitis can be diagnosed on CT scan without contrast, although contrast-enhanced CT provides superior diagnostic accuracy for characterizing complications and subtle inflammatory changes.

Diagnostic Capabilities of Non-Contrast CT for Colitis

Non-contrast CT can detect several key findings of colitis:

• Colonic wall thickening
• Pericolic inflammation and fat stranding
• Diverticular inflammation in diverticulitis
• Extraluminal gas in cases of perforation
• Abscess formation in complicated cases
• Luminal narrowing 1, 2, 3

Limitations of Non-Contrast CT

Without IV contrast, certain important features cannot be optimally assessed:

• Mural enhancement patterns (which help characterize active inflammation)
• Subtle bowel wall abnormalities
• Adjacent liver parenchymal hyperemia (an early finding in some forms of colitis)
• Vascular complications
• Small or subtle abscesses may be harder to distinguish from adjacent bowel 1

Comparison of Contrast vs. Non-Contrast CT for Colitis

Feature	Non-Contrast CT	Contrast-Enhanced CT
Wall thickening	Visible	Visible
Pericolic inflammation	Visible	Visible
Extraluminal gas	Visible	Visible
Mural enhancement	Not visible	Visible
Subtle inflammation	Limited detection	Better detection
Abscess delineation	Limited	Superior
Diagnostic accuracy	Lower (approximately 30% less accurate)	Higher [1,4]

Clinical Recommendations

1. First-line imaging: For patients without contraindications to IV contrast, CT with IV contrast is preferred for diagnosing colitis due to its superior diagnostic accuracy 1.

2. When non-contrast CT is appropriate:

   • Patients with contraindications to IV contrast (severe renal impairment, contrast allergy)
   • When gross inflammatory changes are expected (severe colitis)
   • As an initial screening tool in acute settings when contrast administration would delay care 1, 4

3. Alternative imaging when contrast CT is contraindicated:

   • Ultrasound (US) - useful for initial evaluation but has lower sensitivity and specificity compared to CT
   • MRI - excellent alternative but limited by availability and longer acquisition times
   • Non-contrast CT - still useful despite limitations 1

Specific Colitis Types and CT Findings

Different types of colitis may be suggested by specific CT findings, even without contrast:

• Ulcerative colitis: Continuous involvement, typically starting from rectum, wall thickness averaging 7.8 mm, submucosal fat deposition in chronic cases 5
• Crohn's colitis: Skip lesions, greater wall thickness (average 11.0 mm), often involves terminal ileum 5
• Infectious colitis: Often associated with ascites, may show right-sided predominance 2, 3
• Ischemic colitis: Vascular distribution pattern, often involving watershed areas 2, 3
• Diverticulitis: Focal asymmetric process with fascial thickening and inflamed diverticula 2

Pitfalls and Caveats

• Non-contrast CT may miss subtle inflammatory changes, potentially leading to false negatives
• Radiation exposure is a concern, particularly in patients with inflammatory bowel disease who may require repeated imaging 6
• The final diagnosis of colitis type typically requires correlation with clinical, laboratory, and endoscopic findings 2
• In elderly patients, the clinical presentation of colitis may be atypical, making imaging even more important for diagnosis 1

In emergency settings where rapid diagnosis is crucial and the patient cannot tolerate oral contrast, non-contrast CT remains a valuable diagnostic tool for colitis, though with recognized limitations compared to contrast-enhanced studies.