What is the diagnostic approach for a patient presenting with symptoms suggestive of posterior circulation stroke or transient ischemic attack (TIA), such as vertigo, dysarthria, or ataxia, using Magnetic Resonance Angiography (MRA) of the posterior circulation?

MRA Posterior Circulation for Suspected Stroke/TIA

In patients presenting with symptoms suggestive of posterior circulation stroke or TIA (vertigo, dysarthria, ataxia), MRA of the head and neck should be performed as part of the initial vascular imaging evaluation, ideally in conjunction with brain MRI with diffusion-weighted sequences, which is the preferred imaging modality for detecting posterior fossa ischemia. 1, 2

Initial Imaging Strategy

Brain Parenchymal Imaging First

• Non-contrast CT is typically performed first in the emergency setting to rapidly exclude hemorrhage and determine eligibility for thrombolysis, though it has extremely low sensitivity (~10%) for detecting acute posterior circulation infarcts 2, 1
• MRI brain without contrast with diffusion-weighted imaging (DWI) is the imaging modality of choice for posterior circulation stroke, as it can detect ischemic changes within minutes of onset and has far superior sensitivity compared to CT 1, 2, 3
• DWI sequences do not require contrast administration for acute stroke evaluation 2

Vascular Imaging with MRA

MRA head and neck should be performed in patients with suspected posterior circulation ischemia, particularly those being considered for mechanical thrombectomy 1, 2

Specific MRA Protocol Recommendations:

• MRA of both head AND neck is recommended rather than either alone, as vertebrobasilar pathology can occur at multiple levels—from vertebral artery origins (most common atherosclerotic site) to the basilar apex 1
• MRA is preferred over ultrasound for vertebral artery evaluation because ultrasound has difficulty visualizing vertebral artery origins 1
• CTA of head and neck is an acceptable alternative to MRA and may be preferred when MRA is contraindicated or unavailable 1, 2

Clinical Context That Guides Imaging Decisions

High-Risk Features Requiring Urgent Vascular Imaging:

• Focal neurologic deficits suggesting brainstem or cerebellar localization (crossed sensory/motor findings, Horner syndrome, dysphagia, cranial nerve palsies) 4
• Symptoms suggesting specific vascular territories: dysphagia, nausea/vomiting, and Horner syndrome correlate with proximal territory; unilateral weakness and facial palsy with middle territory; visual field loss and lethargy with distal territory 4
• Recurrent or fluctuating symptoms suggesting hemodynamic insufficiency 1

Diagnostic Pitfalls to Avoid:

• Do not rely solely on NIHSS score to determine imaging urgency—posterior circulation strokes can present with NIHSS of 0 (isolated truncal ataxia, headache, vertigo) yet still represent large vessel occlusion 1
• Do not assume isolated dizziness is benign—while isolated vertigo is rare in posterior circulation stroke (1.4%), dizziness was present in 47% of posterior circulation stroke patients in the New England Medical Center registry 5, 4
• Do not delay transfer for extensive imaging at primary care facilities—arrange immediate transfer to comprehensive stroke centers for patients with suspected posterior circulation stroke 2
• Be aware that MRA can have diagnostic pitfalls: basilar artery fenestration can mimic thrombus on MRA, and complementary imaging (CTA or transcranial Doppler) may be needed to clarify the diagnosis 6

Advanced Vascular Imaging Considerations

When Standard MRA/CTA May Be Insufficient:

• Conventional catheter angiography remains the reference standard for confirming vertebrobasilar insufficiency and vertebral artery dissection, and allows dynamic evaluation of vessel patency with head positioning 1
• 4D flow MRI can add hemodynamic information when used in conjunction with CTA, particularly for identifying major hemodynamic disturbances in patients with proximal/middle territory infarcts and vascular pathology on CTA 7
• Transcranial Doppler can demonstrate flow abnormalities distinguishing vertebrobasilar insufficiency from other causes of vertigo 1

Evidence Quality Note

The 2024 ACR Appropriateness Criteria provides the most current guideline-level evidence for imaging approach to dizziness and ataxia 1, while the 2011 multisociety guidelines on extracranial vertebral artery disease establish the Class I recommendation for noninvasive vascular imaging (CTA or MRA) as part of the evaluation 1. The 2019 neurointerventional guidelines emphasize that CTA has become the primary method for identifying large vessel occlusion in most stroke patients, with similar logic applying to posterior circulation 1.