What is the recommended state-of-the-art imaging modality for acute stroke diagnosis?

State-of-the-Art Imaging for Acute Stroke Diagnosis

For acute stroke diagnosis, multimodal CT imaging (non-contrast CT, CT angiography, and CT perfusion) is the recommended state-of-the-art approach due to its widespread availability, rapid acquisition, and comprehensive evaluation of both brain parenchyma and vasculature. 1

Initial Imaging Assessment

• The primary goal of imaging in acute stroke is to distinguish between hemorrhagic and ischemic stroke, which is crucial for treatment decisions 2
• Non-contrast CT (NCCT) remains the most widely used initial imaging modality due to its:
  • Rapid acquisition (seconds)
  • Widespread availability
  • Excellent sensitivity for detecting hemorrhage 2, 3
• MRI with diffusion-weighted imaging (DWI) is more sensitive than NCCT for early detection of ischemic changes but should not delay treatment if not immediately available 2

Imaging Protocol Based on Treatment Window

For Patients Within 4.5-Hour Window (IV tPA Candidates)

• Either NCCT or MRI is recommended to exclude intracranial hemorrhage before IV tPA administration 2
• If MRI can be performed without delaying treatment, it offers superior sensitivity for detecting early ischemia 2
• Frank hypodensity on CT involving more than one-third of MCA territory is a contraindication to IV tPA, while early subtle signs are not 2

For Patients Who Are Endovascular Therapy Candidates

• Vascular imaging (CTA, MRA, or conventional angiography) is strongly recommended during initial evaluation 2
• Three potential imaging strategies exist:
  1. NCCT followed by immediate angiography
  2. NCCT with CTA (with or without CT perfusion)
  3. MRI/MRA with DWI and perfusion imaging 2
• The choice depends on institutional capabilities and whether advanced imaging can be performed quickly (24/7) 2

Components of State-of-the-Art Multimodal CT

1. Non-contrast CT (NCCT):

   • Excludes hemorrhage
   • Identifies early ischemic changes
   • Excludes stroke mimics 2

2. CT Angiography (CTA):

   • Identifies large vessel occlusions requiring endovascular therapy
   • Evaluates cervical and intracranial vasculature
   • CTA source images provide qualitative cerebral blood volume maps 2, 1

3. CT Perfusion (CTP):

   • Differentiates between infarcted core and salvageable penumbra
   • Essential for selecting patients beyond 6 hours from symptom onset
   • Automated software provides quantitative assessment of ischemic tissue 1, 4

Multimodal MRI Protocol

• Offers superior tissue characterization but may be less readily available than CT 2, 3
• Key sequences include:
  • Diffusion-weighted imaging (DWI): Most sensitive for early infarct detection
  • Gradient-recalled echo (GRE): Excludes hemorrhage
  • FLAIR: Shows abnormal fluid accumulation
  • MR angiography (MRA): Demonstrates vascular occlusion
  • Perfusion-weighted imaging: Identifies tissue at risk 2

Special Considerations

• For patients beyond 6 hours from symptom onset ("late window"), multimodal imaging with perfusion assessment is essential for treatment selection 1
• In TIA patients, multimodal MRI is preferred over NCCT due to higher sensitivity in detecting subtle lesions 2
• DWI positivity in TIA patients indicates higher risk for recurrent ischemic events 2

Pitfalls and Caveats

• Focusing solely on NCCT without vascular imaging may miss large vessel occlusions requiring endovascular therapy 2
• Delaying treatment for advanced imaging must be balanced against time-sensitive nature of reperfusion therapies 2
• Automated perfusion software interpretation requires quality control and clinical correlation 1
• Renal function and contrast allergies must be considered when using CTA and CTP 4

Algorithmic Approach to Acute Stroke Imaging

1. Perform NCCT immediately to exclude hemorrhage
2. If within 4.5 hours and IV tPA candidate, administer tPA after NCCT if no contraindications
3. Proceed with vascular imaging (CTA) to identify large vessel occlusion
4. Consider perfusion imaging (CTP) if:
   • Beyond 6 hours from symptom onset
   • Uncertain time of onset
   • Considering endovascular therapy 2, 1