What is the imaging modality of choice for a patient presenting with symptoms of a cerebrovascular accident (CVA) and hemiplegia 12 hours ago, between Magnetic Resonance Imaging (MRI) and Computed Tomography (CT) scan?

Imaging of Choice for CVA with Hemiplegia 12 Hours After Onset

For a patient presenting 12 hours after stroke onset with hemiplegia, either non-contrast CT or MRI without contrast are both acceptable initial imaging modalities, though MRI with diffusion-weighted imaging (DWI) is superior for detecting acute ischemia and should be preferred when rapidly available. 1

Primary Imaging Approach

Non-Contrast CT Head

• Non-contrast CT remains essential and widely acceptable as the initial imaging study at 12 hours post-stroke, primarily to exclude intracranial hemorrhage before any thrombolytic therapy and to evaluate for early ischemic changes 1
• CT should be completed within 45 minutes of emergency department arrival and interpreted rapidly to guide treatment decisions 2
• At 12 hours, CT will show more obvious ischemic changes than in the hyperacute phase, though it remains less sensitive than MRI for detecting the full extent of infarction 3, 4

MRI Without Contrast (Preferred When Available)

• MRI with DWI is superior to CT for detecting acute ischemic stroke, with 83% sensitivity versus 26% for CT in diagnosing any acute stroke 3
• Within the first 3 hours, MRI detects acute ischemia in 46% of patients compared to only 7% with CT; this superiority persists well beyond 12 hours 3, 4
• A streamlined MRI protocol including DWI, FLAIR, and gradient-echo/susceptibility-weighted imaging (SWI) can be completed in approximately 10 minutes, making it competitive with CT 5, 2
• MRI is equally accurate as CT for detecting acute hemorrhage when gradient-echo or SWI sequences are included, eliminating the traditional concern about missing hemorrhage 5, 6

Critical Imaging Components at 12 Hours

Essential Sequences for MRI Protocol

• DWI: Most sensitive sequence for acute ischemia detection, showing restricted diffusion within minutes of onset with 88-100% sensitivity 5, 2
• FLAIR: Identifies subacute changes, chronic lacunar infarcts, and helps determine stroke age 5, 2
• Gradient-echo (GRE) or SWI: Excludes hemorrhage and detects microbleeds, blood products, and intravascular thrombus 5, 2
• Apparent diffusion coefficient (ADC) maps: Eliminate T2 "shine-through" effect and increase specificity 5

Vascular Imaging Considerations

• At 12 hours post-onset, add CT angiography (CTA) of head and neck or MR angiography (MRA) to the initial imaging to evaluate for large vessel occlusion (LVO), as patients may still be candidates for endovascular therapy up to 24 hours with appropriate imaging selection 1
• Recent randomized controlled trials have demonstrated benefit of endovascular therapy for LVO up to 24 hours, making urgent LVO detection critical even at 12 hours 1
• CTA provides rapid vascular assessment and is usually appropriate when performed with non-contrast CT 1
• MRA (preferably non-contrast time-of-flight for head, contrast-enhanced for neck) can be combined with brain MRI in a single session 2

Extended Window Considerations (6-24 Hours)

Perfusion Imaging

• Beyond 6 hours, perfusion imaging (CT perfusion or MR perfusion) may be required to determine eligibility for endovascular therapy by identifying salvageable penumbra versus irreversibly infarcted core 1, 5
• Major trials demonstrating EVT benefit at 6-24 hours used either DWI-MRI or CT perfusion for patient selection 1
• The rapidity of CT perfusion acquisition is a strongly relevant clinical consideration in most settings compared to MR perfusion 1

Algorithmic Approach at 12 Hours

1. Immediate imaging goal: Exclude hemorrhage, confirm ischemic stroke, detect LVO 1

2. Choose initial modality based on availability:

   • If MRI available within 10-15 minutes: Perform MRI without contrast (DWI, FLAIR, GRE/SWI) + MRA head and neck 2
   • If MRI not rapidly available: Perform non-contrast CT + CTA head and neck 1

3. Add perfusion imaging if:

   • LVO is confirmed on CTA/MRA AND
   • Patient may be candidate for endovascular therapy AND
   • Need to assess salvageable tissue 1

4. Proceed directly to catheter angiography only if:

   • Highly suspected LVO with clear etiology (e.g., hyperdense MCA sign with new atrial fibrillation) AND
   • Immediate conversion to endovascular therapy is planned 1

Common Pitfalls to Avoid

• Do not delay treatment waiting for MRI if CT is immediately available and shows no contraindications to therapy 2
• Do not assume CT is sufficient for detecting all acute ischemia—CT misses significant ischemic changes that MRI would detect, particularly in posterior circulation and small cortical infarcts 5, 3
• Do not skip vascular imaging at 12 hours—patients may still be candidates for mechanical thrombectomy up to 24 hours with appropriate selection 1
• Do not order contrast-enhanced CT or MRI of the brain—there is no relevant literature supporting contrast-enhanced parenchymal imaging for acute ischemic stroke evaluation 1, 2

Quality of Life and Mortality Considerations

• Rapid LVO detection at 12 hours is critical because stroke due to LVO is a significant source of death and disability, and endovascular therapy up to 24 hours has demonstrated benefit in reducing morbidity and mortality 1
• MRI's superior sensitivity for detecting acute ischemia provides more accurate prognostic information and helps identify patients at risk for complications 5, 3
• Early identification of hemorrhagic transformation on MRI (detected in more patients than CT) impacts anticoagulation decisions and secondary prevention strategies 3, 6