Should a CT scan with or without contrast be used to rule out diverticulitis?

CT Scan for Suspected Diverticulitis: With or Without Contrast

CT scan with IV contrast is the preferred imaging modality for diagnosing diverticulitis due to its superior diagnostic accuracy of 98%, though unenhanced CT is acceptable for patients with contraindications to contrast material. 1

Primary Imaging Recommendation

CT abdomen and pelvis is the most useful examination for patients with suspected diverticulitis because of:

• Superior diagnostic accuracy (98%)
• Ability to identify alternative diagnoses with similar presentation
• Capacity to risk-stratify patients for operative versus non-operative treatment
• Ability to triage between inpatient and outpatient management 1

Role of IV Contrast

• IV contrast material improves characterization and detection of subtle bowel wall abnormalities and complications of diverticulitis (e.g., abscess) 1, 2
• Contrast enhancement appears greatest immediately after bolus administration (15-120 seconds) 2
• IV contrast helps distinguish abscesses from adjacent bowel 1

When Unenhanced CT is Appropriate

• For patients with contraindications to IV contrast (e.g., renal impairment, contrast allergy)
• Unenhanced CT is still more accurate than clinical evaluation alone 1
• Unenhanced CT can be used to diagnose uncomplicated diverticulitis 1

Clinical Impact of CT Imaging

1. Early diagnosis and management:

   • CT diagnosis of uncomplicated acute diverticulitis in the emergency department can prevent unnecessary hospital admission 1
   • Early CT for acute diverticulitis can reduce hospital admission by more than 50% and shorten hospital length of stay 1

2. Complication detection:

   • CT can identify complications requiring surgical or interventional radiology management:
     • Perforation
     • Abscess formation
     • Fistula development
     • Bowel obstruction 1

3. Prognostic value:

   • CT may help predict which patients are likely to experience recurrent diverticulitis 1
   • Colonic wall thickness at the site of diverticulitis and extent of complications may predict recurrence risk 1

Alternative Imaging Modalities

• MRI: Diagnostic accuracy likely less than CT but insufficiently studied; not routinely recommended for suspected diverticulitis 1
• Ultrasound: Less accurate than CT, especially in obese patients and for distal sigmoid diverticulitis; US misdiagnosed 17% of patients with uncomplicated diverticulitis and 79% with complicated diverticulitis 3
• Plain radiography: Limited value unless complications such as free perforation or obstruction are suspected 1

Special Considerations

• Low-dose CT techniques can achieve 50-90% radiation dose reduction while maintaining similar sensitivity and specificity to standard-dose CT 1
• Oral contrast is not always necessary but may improve diagnosis of abscess by distinguishing it from adjacent bowel 1
• Combining unenhanced CT with contrast-enhanced CT (biphasic imaging) is unnecessary and not recommended 1

Common Pitfalls

1. Misdiagnosis risk: Clinical assessment alone is often insufficient and may lead to misdiagnosis 1
2. Cancer mimicry: Perforated colon cancer can mimic both clinical and radiographic findings of diverticulitis 1
   • CT findings suggesting cancer rather than diverticulitis include pericolonic lymphadenopathy (>1 cm in short axis)
3. Contrast-related risks: The decision to employ contrast enhancement should be based upon careful evaluation of clinical findings, other radiological data, and unenhanced CT findings 2

In conclusion, while both contrast-enhanced and unenhanced CT scans can diagnose diverticulitis, IV contrast is preferred when not contraindicated as it improves detection of complications and characterization of bowel wall abnormalities.