MRI Stroke Protocol Guidelines

Core MRI Sequences Required

• The American College of Cardiology recommends that diffusion-weighted imaging (DWI) is the single most sensitive and specific imaging technique for detecting acute ischemia, with sensitivity/specificity of 91%/95% versus 61%/65% for CT within 6 hours 2
• DWI detects ischemic changes within minutes of symptom onset and is considered the gold standard for ischemic core assessment, according to the Journal of the American College of Cardiology 3
• The American Heart Association suggests that DWI allows differentiation of acute from chronic stroke based on temporal evolution of diffusion characteristics 5
• Hyperintensity on DWI indicates cytotoxic edema in severely ischemic brain parenchyma, as stated by the Journal of the American College of Cardiology 6
• Perfusion-weighted imaging (PWI) demonstrates the total area of acute ischemia and identifies tissue at risk (penumbra), with a perfusion/diffusion mismatch pattern identifying salvageable tissue 5
• The American College of Cardiology recommends that gradient-echo (GRE) sequence detects acute, subacute, and chronic hemorrhage with superior sensitivity compared to CT 2
• GRE identifies microhemorrhages indicating amyloid angiopathy, hypertension, or other vascular diseases, according to the American Heart Association 1, 2
• Small numbers of microbleeds (<5) do not contraindicate intravenous thrombolysis, though risk with multiple microbleeds (>5) remains uncertain, as stated by the American Heart Association 1
• FLAIR is effective for detecting subarachnoid hemorrhage, though artifacts may occur at the skull base, according to the American Heart Association 1
• FLAIR shows large hyperintensities in acute stroke and helps exclude stroke mimics, as recommended by the American Heart Association 7
• MR Angiography (MRA) provides noninvasive screening of extracranial and intracranial circulation with sensitivity/specificity of 70-100% for detecting stenoses, according to the American Heart Association 7

Timing Considerations and Treatment Windows

• The American Heart Association recommends that MRI can be used if it does not unduly delay timely administration of intravenous tPA, with the complete multimodal MRI examination (DWI, FLAIR, GRE, PWI, MRA) taking approximately 10 minutes 1, 2
• Vascular imaging is indicated even within 3 hours if an endovascular team is available and it doesn't delay tPA, as stated by the American Heart Association 1

Critical Advantages of MRI Over CT

• MRI is superior for detecting small cortical, small deep, and posterior fossa infarcts, according to the American Heart Association 7
• MRI better distinguishes acute from chronic ischemia, as recommended by the American Heart Association 7
• MRI identifies subclinical satellite ischemic lesions that provide information on stroke mechanism, according to the American Heart Association 7
• MRI is more effective than CT for excluding stroke mimics, as stated by the American Heart Association 1
• MRI avoids ionizing radiation and iodinated contrast exposure, as recommended by the American Heart Association 7

Important Caveats and Pitfalls

• The major limitation of MRI is institutional access to emergency MRI rather than acquisition time itself, according to the American Heart Association 2
• Patient contraindications include claustrophobia, cardiac pacemakers, and metal implants, as stated by the American Heart Association 7
• MRI is more susceptible to motion artifact than CT, according to the Journal of the American College of Cardiology 3
• Never delay thrombolytic therapy to obtain MRI if CT is immediately available, as recommended by the American Heart Association 2, 4