What is the recommended imaging protocol for a patient presenting with symptoms of a stroke, specifically regarding MRI brain with or without contrast (Magnetic Resonance Imaging)?

MRI Brain Imaging Protocol for Acute Stroke

For acute stroke evaluation, non-contrast CT is the preferred initial imaging modality due to speed and availability, with MRI brain without contrast reserved for specific clinical scenarios where it will not delay treatment. 1, 2

Initial Imaging Strategy (0-4.5 Hour Window)

The primary goal is rapid exclusion of hemorrhage to enable IV thrombolysis, and this must be accomplished within 45 minutes of emergency department arrival. 1, 2

Non-Contrast CT as First-Line

• Non-contrast CT should be the initial imaging study for most acute stroke patients because it rapidly excludes intracranial hemorrhage and can be completed and interpreted within the critical 45-minute window. 1, 2
• CT is sufficient to initiate IV tPA without waiting for additional imaging if the patient is within the treatment window and has no contraindications. 1, 2
• The presence of intracranial hemorrhage (excluding microbleeds) is an absolute contraindication to thrombolysis. 1

MRI Without Contrast - Limited Role in Hyperacute Setting

• MRI brain without contrast may delay endovascular therapy in anterior circulation large vessel occlusion, which causes potential harm from delayed treatment. 1
• While MRI with diffusion-weighted imaging (DWI) is superior to CT for detecting acute ischemia (sensitivity 83% vs 26%), the time required for acquisition makes it less practical in the hyperacute setting. 3
• MRI should only be performed acutely if it does not delay IV thrombolysis. 1

Specific Scenarios Where MRI Without Contrast Is Indicated

Wake-Up Strokes

• DWI-MRI and FLAIR sequences are necessary to determine eligibility for thrombolytics in wake-up strokes. 1
• Current AHA guidelines specifically recommend MRI for evaluation of wake-up strokes. 1

Extended Time Window (6-24 Hours)

• For patients presenting 6-24 hours after symptom onset, DWI-MRI can identify candidates eligible for endovascular therapy without radiation or contrast exposure. 1
• This is particularly valuable in patients with renal insufficiency or contrast allergy, where TOF MRA combined with DWI-MRI can identify arterial occlusions. 1

Posterior Circulation Strokes

• MRI is more sensitive for detecting brainstem infarcts, which may be missed on early CT. 1
• However, the rapidity of CT still makes it preferable for initial evaluation even in basilar occlusion. 1

MRI With and Without Contrast - Secondary Workup Only

MRI brain with and without IV contrast is rarely indicated in acute stroke and should only be used in secondary workup when stroke mimics are suspected. 1

Limited Indications

• Brain tumors or other conditions mimicking ischemic stroke may require contrast-enhanced MRI for diagnosis. 1
• This is not a first-line test and should only be performed after initial stroke evaluation is complete. 1
• Contrast-enhanced MRI perfusion may detect additional at-risk regions in late-presenting strokes but is not standard surveillance imaging. 1

Critical Pitfalls to Avoid

Time Is Brain

• Delaying IV tPA while waiting for MRI is a critical error - if the patient is within 4.5 hours and non-contrast CT shows no contraindications, thrombolysis should begin immediately. 2
• The 45-minute door-to-imaging interpretation time is a CMS quality measure and must be met. 1, 2

Contrast Administration Errors

• CT head with IV contrast has no role in acute ischemic stroke evaluation and may obscure early complications such as hemorrhage. 1
• Contrast should never be given for initial brain parenchymal imaging in suspected stroke. 1

MRI Limitations

• MRI may not visualize all acute strokes, particularly very early cortical or small brainstem infarcts. 4
• Despite superior sensitivity overall, MRI detected acute ischemic stroke in only 46% of patients scanned within 3 hours versus 7% for CT in one study, though final sensitivity was 83% vs 26%. 3

Practical Algorithm

For patients presenting with acute stroke symptoms:

1. Immediate non-contrast CT (within 30 minutes of arrival) for all patients 1, 2
2. If hemorrhage excluded and patient eligible for IV tPA: Begin thrombolysis without delay 1, 2
3. If considering endovascular therapy: Add CTA or proceed directly to angiography 1
4. Reserve MRI without contrast for:
   • Wake-up strokes requiring DWI/FLAIR mismatch 1
   • Extended window patients (6-24 hours) needing tissue viability assessment 1
   • Patients with contraindications to CT contrast 1
5. MRI with contrast: Only for suspected stroke mimics in secondary workup 1