What is the recommended approach for an MRI stroke protocol?

MRI Stroke Protocol

For acute stroke evaluation, a multimodal MRI protocol should include diffusion-weighted imaging (DWI), perfusion-weighted imaging (PWI), gradient-echo (GRE) sequences, FLAIR, and MR angiography (MRA), which can be completed in approximately 10 minutes and provides superior diagnostic accuracy compared to CT alone. 1

Core MRI Sequences Required

Diffusion-Weighted Imaging (DWI)

• DWI is the single most sensitive and specific imaging technique for detecting acute ischemia, far surpassing non-contrast CT and all other MRI sequences with sensitivity/specificity of 91%/95% versus 61%/65% for CT within 6 hours 1
• DWI detects ischemic changes within minutes of symptom onset and is considered the gold standard for ischemic core assessment 1, 2
• DWI allows differentiation of acute from chronic stroke based on temporal evolution of diffusion characteristics 1
• Hyperintensity on DWI indicates cytotoxic edema in severely ischemic brain parenchyma 1

Perfusion-Weighted Imaging (PWI)

• PWI demonstrates the total area of acute ischemia and identifies tissue at risk (penumbra) 2, 3
• The perfusion/diffusion mismatch pattern (larger perfusion defect surrounding smaller DWI lesion) identifies salvageable tissue 1
• PWI more accurately reflects the extent of neurological dysfunction than DWI alone 3

Gradient-Echo (GRE) Sequence

• GRE detects acute, subacute, and chronic hemorrhage with superior sensitivity compared to CT 1
• GRE identifies microhemorrhages indicating amyloid angiopathy, hypertension, or other vascular diseases 1
• **Small numbers of microbleeds (<5) do not contraindicate intravenous thrombolysis**, though risk with multiple microbleeds (>5) remains uncertain 1
• GRE can detect intravascular thrombus without requiring vascular imaging techniques 1

FLAIR (Fluid-Attenuated Inversion Recovery)

• FLAIR is effective for detecting subarachnoid hemorrhage, though artifacts may occur at the skull base 1
• FLAIR shows large hyperintensities in acute stroke and helps exclude stroke mimics 1
• FLAIR combined with GRE exceeds CT sensitivity for detecting thrombus within vasculature 1

MR Angiography (MRA)

• MRA provides noninvasive screening of extracranial and intracranial circulation with sensitivity/specificity of 70-100% for detecting stenoses 1
• MRA identifies acute proximal large-vessel occlusions but cannot reliably identify distal or branch occlusions 1
• Intracranial MRA (3D time-of-flight or contrast-enhanced) should be included to localize vessel occlusion 1, 2

Timing Considerations and Treatment Windows

Within 3-6 Hours of Symptom Onset

• MRI can be used if it does not unduly delay timely administration of intravenous tPA 1
• The complete multimodal MRI examination (DWI, FLAIR, GRE, PWI, MRA) can be performed in 10 minutes, making it competitive with CT 1
• Vascular imaging is indicated even within 3 hours if an endovascular team is available and it doesn't delay tPA 1

Beyond 6 Hours or Unknown Onset

• Multimodal imaging with perfusion assessment becomes essential for treatment selection 2, 4
• DWI or perfusion imaging is required to determine ischemic core in late-window patients 1
• Advanced imaging guides decisions for mechanical thrombectomy or intra-arterial therapy 1

Critical Advantages of MRI Over CT

• MRI is superior for detecting small cortical, small deep, and posterior fossa infarcts 1, 4
• MRI better distinguishes acute from chronic ischemia 1
• MRI identifies subclinical satellite ischemic lesions that provide information on stroke mechanism 1
• MRI is more effective than CT for excluding stroke mimics 1
• MRI avoids ionizing radiation and iodinated contrast exposure 1

Important Caveats and Pitfalls

Access and Contraindications

• The major limitation is institutional access to emergency MRI rather than acquisition time itself 1
• Patient contraindications include claustrophobia, cardiac pacemakers, and metal implants 1
• MRI is more susceptible to motion artifact than CT 1

Interpretation Considerations

• Not every hyperintensity on DWI represents ischemic stroke—specificity is lower than sensitivity, requiring correlation with clinical presentation 5
• DWI can show subtle abnormalities in venous stroke that differ from arterial ischemia patterns 6
• DWI provides better diagnostic contribution in the first week of stroke and in patients with small lesions or preexisting ischemic damage 7

Treatment Delay Warning

• Never delay thrombolytic therapy to obtain MRI if CT is immediately available 1, 2
• FDA approval for tPA only requires exclusion of intracranial hemorrhage within 45 minutes, not specifically requiring CT 1
• Time-sensitive nature of reperfusion therapies must be balanced against comprehensive imaging 2, 4