What is the window for CT (Computed Tomography) and MRI (Magnetic Resonance Imaging) in acute ischemic stroke?

Imaging Time Windows for CT and MRI in Acute Ischemic Stroke

For acute ischemic stroke patients who are candidates for IV thrombolysis (0-4.5 hour time window), either non-contrast CT (NCCT) or MRI should be performed immediately to exclude intracranial hemorrhage and determine the extent of ischemic changes, with initial imaging completed within 45 minutes of emergency department arrival. 1

CT Imaging Window

• NCCT is the most widely available and fastest initial imaging modality for acute stroke evaluation and should be interpreted within 45 minutes of patient arrival in the emergency department 1
• NCCT primarily serves to exclude intracranial hemorrhage (absolute contraindication to IV tPA) and assess for early ischemic changes 1
• Early ischemic changes on NCCT include hyperdense MCA/basilar artery sign, sulcal effacement, basal ganglia/subcortical hypodensity, and loss of cortical gray-white differentiation 2
• Frank hypodensity on NCCT involving more than one-third of MCA territory is a relative contraindication for IV tPA due to increased risk of hemorrhagic transformation 1
• Using specialized "Stroke Windows" settings significantly improves detection of early ischemic changes compared to standard window settings (70% vs 18%) 2

MRI Imaging Window

• MRI with diffusion-weighted imaging (DWI) is superior to NCCT for detecting acute ischemia with very high sensitivity and specificity 1
• A standardized multimodal MRI protocol (DWI, FLAIR, GRE/SWI, PWI, MRA) can be performed in approximately 10 minutes, making it competitive with CT for acute stroke evaluation 1
• MRI is as reliable as CT for excluding hyperacute intracerebral hemorrhage, with gradient-echo sequences particularly sensitive for detecting hemorrhage 3, 1
• The presence of a small number of microbleeds (<5) on gradient-echo MRI is not a contraindication to IV tPA within the 3-hour window 1

Time-Based Imaging Recommendations

0-4.5 Hours (IV tPA Window)

• Primary goal: Exclude hemorrhage and assess extent of ischemic changes 1
• Either NCCT or MRI is appropriate, but should not delay IV tPA administration 1
• Initial imaging should be completed and interpreted within 45 minutes of ED arrival 1
• If endovascular therapy is being considered, vascular imaging (CTA, MRA) should be added but should not delay IV tPA if indicated 1, 4

Beyond 4.5 Hours

• For patients outside the standard IV tPA window (>4.5 hours), more advanced imaging is recommended 1
• CT perfusion and MRI perfusion/diffusion imaging can identify salvageable tissue (penumbra) and may help select patients who could benefit from late reperfusion therapy 1
• For patients being considered for endovascular therapy (up to 8 hours or beyond), vascular imaging is strongly recommended during initial evaluation 1, 4

Imaging Protocol Selection

• Standardized imaging approach should be used with all relevant studies conducted in as few sessions as possible to avoid treatment delays 1
• Three main imaging strategies for acute stroke evaluation:
  1. NCCT followed by digital subtraction angiography (if endovascular therapy is considered) 1
  2. NCCT plus CTA with or without perfusion CT 1, 4
  3. MRI plus MRA with or without perfusion imaging 1, 4

Common Pitfalls and Caveats

• Delaying IV tPA while waiting for advanced imaging is a critical error - if the patient is within the 4.5-hour window and has no contraindications on NCCT, IV tPA should be initiated without waiting for additional imaging 1
• Early ischemic changes on NCCT are often subtle and frequently overlooked with standard window settings - specialized stroke window settings significantly improve detection 2, 5
• For patients with wake-up stroke or unknown onset time, NCCT-based treatment decisions can be safe with comparable rates of symptomatic intracerebral hemorrhage to standard window patients 6
• MRI access may be limited in some facilities, particularly during off-hours, which may impact the choice of imaging protocol 1