Neurologic Imaging

Which Imaging Study Is Best for an Acute Focal Neurologic Deficit Within 6 Hours?

May 26, 2026 · Pouyan Golshani, MD, Interventional Radiologist

It’s 2 a.m. in the emergency department, and you’re evaluating a 68-year-old patient who presented with sudden-onset aphasia and right-sided weakness. Her family confirms she was last known to be at her neurologic baseline just three hours ago. The clinical suspicion for an acute ischemic stroke is high, and every minute counts. Your immediate decision—which imaging study to order—will dictate the entire course of her care, from thrombolysis eligibility to potential mechanical thrombectomy. This article provides a focused, evidence-based workflow for this precise clinical scenario: a new, fixed or worsening focal neurologic defect with an onset of less than six hours. For this time-sensitive presentation, the American College of Radiology (ACR) rates MRA head and neck without and with IV contrast as Usually Appropriate, providing the comprehensive data needed to make critical therapeutic decisions.

Who Fits This Clinical Scenario for Suspected Acute Stroke?

This guidance is specifically for patients presenting with a new focal neurologic deficit that is either fixed or actively worsening, where the time from symptom onset (or last known well) is less than six hours. This “hyperacute” window is critical for interventions like intravenous thrombolysis and endovascular thrombectomy.

Inclusion criteria for this workflow:

New Focal Deficit: The patient has a clearly defined, localized neurologic symptom such as hemiparesis, hemisensory loss, aphasia, neglect, or visual field loss.
Fixed or Worsening: The symptoms are persistent and not improving.
Time Window: The patient was last known to be well less than six hours ago.

It is crucial to distinguish this presentation from similar but distinct clinical scenarios that follow different diagnostic pathways:

If symptoms have completely resolved: The patient has likely experienced a Transient Ischemic Attack (TIA). This requires an urgent but different workup, detailed in the ACR variant for TIA.
If symptom onset was more than 6 hours ago: The patient falls into a different ACR variant. While imaging is still critical, the therapeutic options and imaging goals (e.g., assessing for large ischemic core) change.
If a non-contrast CT has already confirmed hemorrhage: The workup immediately pivots to the “Proven parenchymal hemorrhage” scenario, focusing on cause and management rather than ischemia.
If the primary suspicion is venous thrombosis: Presentations like headache, seizures, or deficits not conforming to an arterial territory may point toward dural venous sinus thrombosis, which has its own dedicated imaging protocol.

What Diagnoses Are You Working Up in This Scenario?

When a patient presents with an acute focal deficit, the imaging workup is designed to rapidly differentiate between several critical possibilities. The choice of study is driven by the need to confirm or exclude these diagnoses to guide immediate, time-sensitive treatment.

Acute Ischemic Stroke (AIS)
This is the primary and most common diagnosis of concern. An AIS occurs when a blood clot obstructs an artery supplying the brain, leading to cell death in the affected territory. Imaging must rapidly identify the presence of ischemia, determine its extent (core vs. penumbra), and, crucially, locate the responsible vascular occlusion. Identifying a Large Vessel Occlusion (LVO) in the internal carotid, middle cerebral, or basilar arteries is paramount, as these patients may be candidates for mechanical thrombectomy.

Intracranial Hemorrhage (ICH)
Accounting for a significant minority of stroke presentations, ICH is a “can’t-miss” diagnosis. A ruptured blood vessel causes bleeding into the brain parenchyma. The clinical presentation can be indistinguishable from an ischemic stroke, but the treatment is diametrically opposed—thrombolytics are contraindicated and potentially fatal. The first step in any acute stroke imaging is to definitively rule out hemorrhage.

Stroke Mimics
Several non-vascular conditions can present with acute focal neurologic deficits, masquerading as a stroke. These include seizure with postictal paralysis (Todd’s paralysis), complex migraine with aura, hypoglycemia, and functional neurologic disorders. While the clinical history can raise suspicion for a mimic, imaging is essential to exclude a true vascular event before pursuing alternative diagnoses.

Cervical Artery Dissection
Though less common, a tear in the wall of a carotid or vertebral artery is an important cause of stroke, particularly in younger patients. A dissection can lead to thromboembolism or vessel occlusion. A comprehensive vascular study that includes the neck is necessary to identify this underlying cause.

Why Is MRA of the Head and Neck the Recommended Study for Suspected Acute Stroke?

For a patient with a new focal deficit within the 6-hour window, the ACR identifies several imaging options as Usually Appropriate, including non-contrast CT, CTA, and various MRI/MRA protocols. However, MRA head and neck without and with IV contrast offers the most comprehensive single examination to answer the key clinical questions.

The power of an MRI-based approach lies in its multi-sequence capability. A “stroke protocol” MRI provides several distinct pieces of information:

Diffusion-Weighted Imaging (DWI): This sequence is exceptionally sensitive for detecting cytotoxic edema within minutes of ischemic onset, making it far superior to CT for confirming early ischemia.
Gradient Echo (GRE) or Susceptibility-Weighted Imaging (SWI): These sequences are highly sensitive for detecting acute or chronic blood products, reliably ruling out the intracranial hemorrhage that would preclude thrombolysis.
Magnetic Resonance Angiography (MRA): This component visualizes the arteries from the aortic arch through the circle of Willis. It can identify the site of a large vessel occlusion, assess for underlying stenosis, and detect abnormalities like a cervical artery dissection. The addition of intravenous contrast can improve vessel characterization and diagnostic confidence.

This combination directly addresses the primary differential diagnoses: confirming ischemia, ruling out hemorrhage, and identifying the vascular lesion. Furthermore, MRI and MRA achieve this with no ionizing radiation (adult_rrl=O 0 mSv).

How Do Alternative Studies Compare?

CT head without IV contrast: Also rated Usually Appropriate, this is often the first imaging test performed in many emergency departments due to its speed and wide availability. Its primary role is to rapidly exclude hemorrhage. However, it is insensitive to early ischemic changes (the “early ischemic signs” are subtle and often absent in the first few hours). It provides no information about the blood vessels.
CTA head and neck with IV contrast: Also Usually Appropriate, this is an excellent and fast alternative for identifying LVO. It is often performed immediately after a non-contrast head CT. The combination of non-contrast CT and CTA provides the two most critical pieces of data for thrombectomy decisions: no hemorrhage and a target vessel occlusion. The main trade-off is the use of ionizing radiation (adult_rrl=☢☢☢ 1-10 mSv) and iodinated contrast.
US duplex Doppler carotid artery: This is rated Usually not appropriate in the acute setting. While useful for evaluating chronic carotid stenosis, it cannot assess the intracranial vessels where most LVOs occur and provides no information about the brain parenchyma. Its use is deferred for secondary stroke workup.

The choice between a CT/CTA pathway and an MRI/MRA pathway often depends on institutional protocols, scanner availability, and patient-specific factors like contraindications to MRI or renal insufficiency.

Once you’ve decided on MRA head and neck without and with IV contrast, our protocol guide covers the technique, contrast, and reading principles: MRA Neck With and Without Contrast.

What’s Next After MRA? Downstream Workflow

The results of the initial imaging study will immediately branch the patient’s management into one of several pathways. The goal is to move from diagnosis to definitive therapy as quickly as possible.

If the MRA is positive for LVO and a small ischemic core: This is a neuro-interventional emergency. The patient should be evaluated immediately for mechanical thrombectomy. The MRA provides the roadmap for the interventionalist, showing the exact location of the clot. The patient may also be a candidate for intravenous thrombolysis if within the appropriate time window and without contraindications.
If the MRA shows acute ischemia but no LVO: The patient has a small-vessel or distal embolic stroke. They should be evaluated for intravenous thrombolysis. Subsequent workup will focus on secondary prevention, including a search for the embolic source (e.g., atrial fibrillation, carotid plaque).
If the MRA is negative for both ischemia and hemorrhage: The diagnosis of stroke is less likely. Attention should turn to stroke mimics. Further workup may include an electroencephalogram (EEG) to investigate for seizure, a review of medications, and metabolic testing. A negative DWI in the setting of a persistent deficit is highly reassuring against an ischemic cause.
If the MRA shows an unexpected finding (e.g., tumor, abscess): The clinical workflow pivots entirely to address the identified pathology. This will involve consultation with the appropriate service (e.g., neurosurgery, infectious disease) and a different set of diagnostic and therapeutic steps.

Pitfalls to Avoid (and When to Get Help)

In the high-stakes environment of acute stroke care, several common pitfalls can delay or misdirect care.

Delaying the Scan: Time is brain. Any delay in getting the patient to the scanner reduces the efficacy of potential treatments. Activate your institution’s stroke alert system immediately upon suspicion.
Ordering an Incomplete Study: Ordering only a “brain MRI” without specifying a stroke protocol or vascular imaging (MRA) can miss the crucial information about vessel occlusion needed for thrombectomy decisions.
Ignoring Patient Contraindications: Failing to screen for MRI contraindications (e.g., incompatible pacemakers, metallic foreign bodies) before sending the patient to the scanner can cause dangerous delays.
Misinterpreting Early CT Signs: Relying solely on a non-contrast CT to rule out ischemia can be misleading. A “normal” CT in the first few hours does not exclude a devastating ischemic stroke.

If the imaging findings are complex, equivocal, or do not match the clinical picture, immediate consultation with a neuroradiologist and the stroke neurology team is essential.

Frequently Asked Questions

Why not just get a non-contrast head CT on every patient with a focal deficit?

A non-contrast head CT is an excellent and often necessary first step, rated as ‘Usually Appropriate’ by the ACR. Its primary purpose is to rapidly and reliably rule out hemorrhage, which is critical before administering thrombolytics. However, it is insensitive for detecting early ischemia and provides no information about the blood vessels. For a patient who may be a candidate for thrombectomy, a vascular study like CTA or MRA is required to identify a large vessel occlusion. Many centers perform a non-contrast CT followed immediately by a CTA for this reason.

Is there a role for CT Perfusion (CTP) in this <6 hour window?

CT Perfusion is rated as ‘May be appropriate’ by the ACR for this scenario. Its main role is to help identify the ischemic penumbra (brain tissue at risk but still salvageable) versus the infarct core (irreversibly damaged tissue). While it can be very useful, especially in patients with an unclear time of onset or in an extended time window (6-24 hours), it is not always necessary in the <6 hour window if the non-contrast CT and CTA provide sufficient information to proceed with thrombolysis or thrombectomy.

What if my patient has a contraindication to MRI, like a pacemaker?

If a patient has an absolute contraindication to MRI, the recommended pathway is a non-contrast head CT followed by a CTA of the head and neck. This combination effectively rules out hemorrhage and identifies any large vessel occlusion, providing the essential information needed to make decisions about intravenous thrombolysis and mechanical thrombectomy. This CT-based pathway is also rated as ‘Usually Appropriate’ and is a common protocol in many stroke centers.

Does the ‘with and without IV contrast’ part of the MRA recommendation matter?

The ‘without and with’ contrast recommendation for MRA provides the most complete dataset. The ‘without contrast’ portion includes the essential DWI and GRE/SWI sequences for ischemia and hemorrhage detection. The ‘with contrast’ MRA can improve visualization of the arteries, help characterize plaque, and increase sensitivity for detecting subtle abnormalities like vessel dissection or vasculitis. While a non-contrast MRA is still highly valuable, the addition of contrast provides incremental diagnostic information.

If the MRA is negative but the patient’s deficit persists, what should I do?

A negative stroke-protocol MRI, particularly a negative DWI sequence, makes a significant ischemic stroke highly unlikely. In this case, you should broaden your differential diagnosis to include stroke mimics. This includes investigating for postictal (Todd’s) paralysis with an EEG, checking for severe hypoglycemia, considering a complex migraine, or evaluating for a functional neurologic disorder. A neurology consultation is critical in this situation to guide the subsequent workup.

Reviewed by Pouyan Golshani, MD, Interventional Radiologist — May 26, 2026