Imaging Recommendations for Acute Stroke

Initial Imaging (≤ 3 hours from symptom onset)

• Non‑contrast CT (NCCT) of the head should be performed immediately for every patient with suspected acute stroke to exclude intracranial hemorrhage (an absolute contraindication to intravenous thrombolysis) and to evaluate early ischemic changes. Class I recommendation. 1
• If MRI is readily available without delaying intravenous tPA, diffusion‑weighted imaging (DWI) should be used because it is more sensitive than CT for detecting acute ischemia and for ruling out stroke mimics. [2][3]
• CT angiography (CTA) of the head and neck may be added during the initial work‑up provided it does not postpone IV tPA administration and endovascular therapy is accessible, as it identifies large‑vessel occlusions amenable to mechanical thrombectomy. 2

Imaging After 3 Hours from Symptom Onset

• For patients considered for mechanical thrombectomy or intra‑arterial therapy, MRI‑DWI or CTA with source imaging together with vascular and perfusion studies is a Class I recommendation. [2][3]
• Vascular imaging becomes essential to locate the occlusion; proximal large‑vessel occlusions respond better to endovascular treatment than to IV thrombolysis alone. [2][3]

Role of Specific Imaging Modalities

• NCCT – reliably excludes hemorrhage and detects early ischemic changes; a hypodensity involving more than one‑third of the middle cerebral artery territory is a strong contraindication to IV tPA. 2
• MRI DWI – the most sensitive sequence for acute ischemia, outperforming NCCT and other MRI sequences. [2][3]
• FLAIR – can reveal subarachnoid hemorrhage and intravascular thrombus, aiding differentiation of stroke mimics. 3
• Gradient‑echo (GRE) MRI – identifies cerebral microbleeds; a small burden (fewer than 5 microbleeds) does not preclude IV tPA, whereas the risk associated with a larger burden (more than 5) remains uncertain. [2][3]

Timing Relative to Thrombolytic Therapy

• Advanced imaging should never delay IV tPA when the patient is within the treatment window and NCCT has ruled out hemorrhage. [2][1]
• Each 15‑minute delay in administering IV tPA reduces the probability of a favorable functional outcome. 2

Cost‑Effectiveness Considerations

• NCCT remains the most cost‑effective initial modality because it prevents inappropriate antithrombotic therapy by reliably detecting hemorrhage. 1
• Routine MRI for all acute stroke patients is not cost‑effective; selective use in cases with ambiguous clinical localization or atypical presentations may improve outcomes. 1

Integration with Clinical Stroke Severity Assessment

• The National Institutes of Health Stroke Scale (NIHSS) should be used alongside imaging findings to quantify deficit severity, guide therapeutic decisions, predict hemorrhage risk, and standardize communication among care teams. 1

Contrast Administration Is Not Required for Acute Stroke Diagnosis

Initial Imaging Recommendations

• The American College of Radiology (ACR) states that non‑contrast head CT is the essential first‑line test for all patients with suspected acute stroke to exclude intracranial hemorrhage (an absolute contraindication to thrombolysis) and to assess early ischemic changes. 4
• According to the ACR, non‑contrast CT or MRI (including diffusion‑weighted imaging) alone is sufficient to rule out acute ischemic stroke; intravenous contrast is not part of the standard diagnostic pathway for primary stroke evaluation. 4
• The ACR confirms that neither non‑contrast CT nor non‑contrast MRI requires intravenous contrast for the primary diagnosis of acute stroke. 4

Role of Contrast in Secondary Work‑up

• The ACR notes that MRI performed with and without contrast may be useful in the secondary work‑up when brain tumors or other non‑vascular lesions mimic ischemic stroke, but this is not part of routine stroke exclusion protocols. 4
• The ACR indicates that contrast‑enhanced MR angiography of the neck can more accurately quantify carotid stenosis in delayed presentations, addressing stroke etiology rather than the acute diagnosis. 4

Advanced Imaging and Perfusion Assessment

• The ACR recommends that, for patients considered for extended‑window reperfusion therapy (6–24 hours), perfusion studies such as CT perfusion or non‑contrast arterial spin labeling (ASL) MRI may be employed for patient selection. 4

Guideline Position on Contrast‑Enhanced MRA

• The ACR Appropriateness Criteria state that contrast‑enhanced MR angiography of the head has no established role in routine evaluation of acute ischemic stroke. 5

Imaging‑Based Vascular Territory Identification in Stroke

CT Identification of Vascular Territory

• The American College of Radiology states that non‑contrast CT can delineate the vascular territory of an acute stroke by showing hypodensity patterns that correspond to the anterior cerebral, middle cerebral, posterior cerebral, or vertebrobasilar arterial distributions. 6

Determination of Anterior vs. Posterior Circulation

• According to the American College of Radiology, imaging interpretation must specify whether the infarct involves the anterior circulation (carotid, anterior cerebral, and middle cerebral arteries) or the posterior circulation (vertebral, basilar, and posterior cerebral arteries) to guide subsequent therapeutic decisions. 6