Brain MRI Protocol for New Right-Sided Weakness with Normal Exam
Order a brain MRI with diffusion-weighted imaging (DWI), FLAIR, T2-weighted sequences, and add susceptibility-weighted imaging (SWI) and arterial spin labeling (ASL) perfusion to maximize detection of acute ischemic stroke that may not be visible on standard sequences.
Core Sequences Required
The essential baseline protocol should include 1:
- Sagittal T1-weighted imaging
- Axial T2-weighted FLAIR
- Axial diffusion-weighted imaging (DWI) with ADC mapping
- Axial susceptibility-weighted imaging (SWI)
This 4-sequence protocol identified all Level I findings (requiring immediate management changes) in a study of 996 patients with new neurological complaints, including 56 acute infarcts, 16 hemorrhages, and 15 mass lesions 1.
Critical Additional Sequences
Arterial Spin Labeling (ASL) Perfusion
Add ASL perfusion imaging to your protocol - this is particularly crucial when the clinical presentation suggests stroke but the exam appears normal. ASL provides non-contrast perfusion assessment and has proven diagnostic value in exactly this scenario 2, 3.
The evidence strongly supports ASL inclusion:
- Provides additional findings in 24.5% of patients with acute infarcts 4
- Can detect hypoperfusion when DWI is falsely negative 3
- A recent case report demonstrated ASL revealing reduced perfusion in a patient with stroke symptoms whose initial DWI was completely normal 3
Vascular Imaging
Include time-of-flight (TOF) MR angiography to evaluate for:
- Large vessel occlusion
- Arterial stenosis
- Vascular abnormalities that explain the weakness 2
This is essential because the 2024 ACR guidelines emphasize rapid detection of large vessel occlusion, which dramatically changes management even in the extended time window up to 24 hours 5.
Why This Matters for Your Patient
A normal neurological exam does not exclude acute stroke. The combination of new focal weakness with a normal exam creates a diagnostic dilemma where:
- DWI can be falsely negative in hyperacute stroke (first few hours) or in small vessel/posterior circulation strokes 3
- ASL perfusion may be the only positive finding initially, showing hypoperfusion before cytotoxic edema develops 6, 3
- SWI detects microhemorrhages and susceptibility vessel signs that indicate thrombus location 6, 2
Sequences to Skip
Do not order gadolinium contrast unless the above sequences reveal findings requiring further characterization (e.g., mass lesion, enhancement pattern needed). The 3-sequence protocol missed primarily enhancing lesions 1, but in acute stroke evaluation, contrast is not part of initial imaging 5.
Clinical Pitfall
The most dangerous error here is assuming a normal exam means no stroke and ordering inadequate imaging. The ASL sequence specifically addresses this gap - it detected stroke in a patient whose DWI was negative 3. Given that treatment windows now extend to 24 hours for large vessel occlusion 5, missing the diagnosis has significant mortality and morbidity implications.