When to use contrast versus non-contrast head computed tomography (CT) scans?

Contrast versus Non-Contrast Head CT: When to Use Each

Non-contrast head CT should be the first-line imaging choice for most acute neurological presentations, with contrast-enhanced CT reserved only for specific clinical scenarios where intracranial infection, tumor, or inflammatory pathologies are suspected. 1

General Principles for Head CT Selection

Non-Contrast Head CT (NCHCT)

• First-line imaging test for most acute neurological presentations including:
  • Altered mental status
  • Trauma
  • Suspected stroke
  • Headache with concerning features
  • Seizures
  • Acute neurological deficits

Contrast-Enhanced Head CT

• Second-line imaging after initial NCHCT when specifically looking for:
  • Intracranial infection (abscess, meningitis, encephalitis)
  • Primary or metastatic tumors
  • Inflammatory conditions
  • Vascular abnormalities requiring better visualization

Clinical Decision Algorithm

When to Order Non-Contrast Head CT:

1. Acute presentations requiring rapid assessment:

   • Altered mental status or delirium
   • Suspected stroke (within first 24 hours)
   • Acute headache with concerning features
   • New focal neurological deficits
   • Seizures
   • Head trauma
   • Suspected intracranial hemorrhage

2. High-risk patients with:

   • History of trauma or falls
   • Anticoagulant use
   • Hypertension
   • Focal neurological deficits
   • Signs of elevated intracranial pressure
   • Significant deterioration of consciousness 1

When to Add Contrast (After Initial NCHCT):

1. Abnormal findings on NCHCT requiring further characterization:

   • Mass lesions
   • Areas of edema with unclear etiology
   • Suspected abscess

2. Specific clinical scenarios even with normal NCHCT:

   • Known or suspected cancer with neurological symptoms
   • Signs of infection with fever and neurological changes
   • Immunocompromised patients with neurological symptoms
   • Suspected inflammatory conditions 1, 2

Evidence-Based Considerations

Diagnostic Yield

• Non-contrast head CT is sufficient for initial evaluation in most emergency settings
• When NCHCT is normal, adding contrast rarely changes management in the acute setting 2, 3
• In one study, only 1 out of 379 patients had an abnormal contrast CT following a normal non-contrast CT 3
• Repeat NCHCT with the same clinical indication as a prior negative study has a significantly lower yield (1.8%) compared to first-time examinations (4.3%) 4

Specific Clinical Scenarios

• Stroke evaluation: NCHCT is essential to rule out hemorrhage before thrombolytic therapy; contrast may obscure early hemorrhagic complications 1
• Altered mental status: NCHCT is the first-line test; contrast adds little value unless specific pathologies are suspected 1
• New focal neurologic deficits: NCHCT first to assess for hemorrhage or large infarct, followed by MRI which is more sensitive than CT for acute infarct 1

Common Pitfalls to Avoid

1. Ordering contrast CT as initial test: This adds unnecessary risk, cost, and time without improving diagnostic yield in most acute settings 1, 2, 3

2. Failing to consider radiation exposure: The significant increase in imaging volume has led to a sixfold increase in annual per capita radiation exposure, with predictions that 2% of future cancers will be caused by CT radiation exposure 5

3. Not considering contrast risks: Contrast media carries risks of allergic reactions and nephrotoxicity, especially in patients with renal impairment 6

4. Automatic "with and without contrast" ordering: This practice is not supported by evidence for initial imaging in most acute settings 1

Key Takeaways

• NCHCT should be the default first imaging test for acute neurological presentations
• Adding contrast should be a deliberate decision based on specific clinical suspicions or abnormal findings on NCHCT
• Clear communication between ordering physicians and radiologists is essential for appropriate study selection 6
• Following evidence-based guidelines can reduce unnecessary radiation exposure, contrast risks, and healthcare costs