What type of computed tomography (CT) scan should be ordered for a patient suspected of having a stroke or transient ischemic attack (TIA)?

CT Imaging for Stroke Assessment

For patients suspected of having a stroke or TIA, order a non-contrast CT head immediately as the essential first imaging study to exclude intracranial hemorrhage, which is critical for determining eligibility for thrombolytic therapy. 1

Initial Imaging: Non-Contrast CT Head

Non-contrast CT of the head is the recommended initial imaging modality because it rapidly excludes intracranial hemorrhage—an absolute contraindication to thrombolytic agents, anticoagulants, and antiplatelet therapy used to treat stroke. 1 This study should be completed and interpreted within 45 minutes of emergency department arrival. 2, 3

What Non-Contrast CT Accomplishes:

• Excludes hemorrhage with excellent sensitivity 2
• Identifies stroke mimics including infection, masses, and other lesions that can present with stroke-like symptoms 1
• Detects early ischemic changes that correlate with subsequent stroke risk 1
• Assesses infarct extent: Frank hypodensity involving more than one-third of the MCA territory is a relative contraindication to IV tPA due to hemorrhagic transformation risk 3

Essential Add-On: Vascular Imaging

After the initial non-contrast CT, immediately add CT angiography (CTA) of the head and neck with IV contrast for comprehensive stroke evaluation. 1

Why CTA Head and Neck Are Critical:

CTA head with IV contrast rapidly identifies:

• Large vessel occlusions requiring endovascular therapy 2
• Intracranial atherosclerosis and steno-occlusive disease 1
• Site of arterial occlusion with 95% sensitivity and 100% specificity 4

CTA neck with IV contrast evaluates:

• Carotid stenosis in anterior circulation TIA/stroke patients who are revascularization candidates 1
• Vertebrobasilar arterial disease 1
• Arterial dissection 1

The American Heart Association guidelines specifically recommend noninvasive cervical carotid imaging within 48 hours for TIA or minor stroke patients who are candidates for carotid endarterectomy or stenting. 1

Optimal "One-Stop-Shop" CT Protocol

Order: Non-contrast CT head + CTA head and neck + CT perfusion (if beyond standard treatment window)

This combined approach can be performed efficiently on modern CT scanners, reducing imaging time by approximately 43% and contrast dose by 40% compared to separate acquisitions. 5 The combination of CT perfusion and CTA provides the most accurate assessment of:

• Infarct core (via decreased cerebral blood volume on perfusion) with 80% sensitivity and 97% specificity 4
• Salvageable tissue at risk (via increased mean transit time) 4
• Collateral circulation status 4

When to Add CT Perfusion:

• Patients presenting beyond 6 hours from symptom onset who may still be endovascular therapy candidates 2, 3
• Unknown time of symptom onset (wake-up strokes) 2
• CT perfusion differentiates salvageable penumbra from irreversibly infarcted core, essential for treatment selection in extended time windows 2, 3

Alternative: MRI-Based Protocol

If MRI is immediately available without delaying treatment, MRI brain without contrast (including DWI, FLAIR, and GRE/SWI sequences) combined with MRA head and neck is superior to CT for detecting acute ischemia, particularly in the posterior circulation. 6, 2, 3

MRI Advantages:

• DWI detects acute ischemia with 77% sensitivity in the first 3 hours versus only 16% for CT 6, 3
• Superior for posterior circulation strokes where CT sensitivity is particularly poor 6
• Can be completed in approximately 10 minutes with standardized protocols 2, 3
• MRA head (non-contrast) and MRA neck (contrast-enhanced) provide comprehensive vascular assessment in a single session 6, 3

Critical Caveat:

Never delay IV thrombolytic therapy to obtain MRI. If the patient is within the 4.5-hour window and non-contrast CT excludes hemorrhage, initiate IV tPA immediately without waiting for additional imaging. 2, 3

Common Pitfalls to Avoid

• Ordering only parenchymal imaging without vascular imaging misses large vessel occlusions requiring endovascular therapy and critical carotid stenosis requiring urgent revascularization 1, 2
• Delaying treatment for advanced imaging: Time is brain—if within the standard treatment window with no contraindications on non-contrast CT, start IV tPA first 2, 3
• Not completing diagnostic evaluation within 48 hours: The 90-day stroke risk after TIA is 11.6%, with 8.8% occurring within the first 7 days, making rapid complete evaluation essential 1
• Assuming normal CT excludes TIA: Non-contrast CT is often normal in TIA; follow-up MRI is reasonable to confirm diagnosis and predict early stroke risk 1

Imaging That Should NOT Be Ordered Initially

• CT head with IV contrast alone: No evidence supports this for stroke evaluation 1
• CT head without and with IV contrast: No evidence supports this approach 1
• CT perfusion as initial study: Not typically used for initial TIA assessment, though helpful for risk stratification in specific cases 1
• Catheter angiography: Too invasive for initial evaluation; reserve for secondary workup when endovascular intervention is being considered 1