Brain MRI Protocol for Acute Stroke Evaluation
For acute stroke evaluation, the recommended MRI protocol includes diffusion-weighted imaging (DWI), fluid-attenuated inversion recovery (FLAIR), gradient-recalled echo (GRE) or susceptibility-weighted imaging (SWI), with MR angiography (MRA) of the head and neck, which can be completed in 6-10 minutes. 1, 2
Core MRI Sequences Required
The essential sequences for acute stroke MRI include:
- Diffusion-weighted imaging (DWI): Most sensitive and specific for detecting acute ischemia within minutes of onset, far superior to CT and other MRI sequences 2
- FLAIR (Fluid-Attenuated Inversion Recovery): Detects subacute ischemia, excludes stroke mimics, and can identify subarachnoid hemorrhage 1, 2
- GRE (Gradient-Recalled Echo) or SWI (Susceptibility-Weighted Imaging): Excludes intracranial hemorrhage with sensitivity equal to or better than CT, detects microbleeds and intravascular thrombus 1, 2
- T1-weighted and T2-weighted sequences: Assess chronic structural changes and atrophy 3
Vascular Imaging Component
MR angiography (MRA) of both intracranial and cervical arteries should be performed during the initial evaluation to identify large vessel occlusion, stenosis, and determine stroke mechanism 1. This is particularly critical if endovascular therapy is being considered 1, 2.
Advanced Imaging for Extended Time Windows
For patients presenting 6-24 hours from last known well or with wake-up stroke:
- Perfusion imaging (MRP or ASL) should be added to identify salvageable tissue using core/perfusion mismatch 4
- DWI-FLAIR mismatch can identify patients who may benefit from IV thrombolysis in extended windows (>4.5 hours from last known well) 4
Protocol Timing and Efficiency
Modern stroke MRI protocols can be completed rapidly:
- Standard multimodal protocol: 10 minutes for DWI, FLAIR, GRE, MRA, and perfusion 2
- Ultrafast protocols: 3-6 minutes using echo-planar imaging and parallel acquisition techniques 5, 6
- Acquisition should not unduly delay IV thrombolysis if the patient is within the treatment window 2
Clinical Decision Algorithm
For IV Thrombolysis Candidates (0-4.5 hours):
- Minimum: DWI, FLAIR, GRE to exclude hemorrhage and assess ischemic extent 1
- Add MRA if endovascular therapy is available and does not delay treatment 2
For Endovascular Therapy Candidates:
- Complete protocol with vascular imaging (MRA head and neck) is strongly recommended during initial evaluation 1
- Perfusion imaging if beyond 6 hours from onset 4
For TIA or Non-Thrombolysis Candidates:
- Multimodal MRI is preferred over CT 1
- Focus on DWI (positive in ~40% of TIAs), vascular imaging (MRA or CTA), and stroke mechanism determination 1
Key Advantages Over CT
MRI provides superior detection of:
- Acute ischemia (DWI sensitivity approaches 100% vs. <60% for early CT) 2
- Small infarcts and posterior fossa lesions 2
- Stroke mimics 2, 3
- Microhemorrhages and chronic vascular disease 2
- Hemorrhage detection (GRE/SWI equal to or better than CT) 2, 7
Important Caveats
- MRI should only be used if it does not delay treatment and if the center can achieve speed comparable to CT-based protocols 2, 8
- The presence of <5 microbleeds on GRE is not a contraindication to IV thrombolysis; risk with >5 microbleeds is uncertain 2
- 24/7 availability of MRI with rapid acquisition protocols is essential for routine use in acute stroke 2, 9
- All imaging must be completed within 30 minutes of hospital arrival 4, 9