What are the specific imaging criteria for determining eligibility for reperfusion therapies in patients with acute ischemic stroke?

Specific Imaging Criteria for Reperfusion Therapy Eligibility in Acute Ischemic Stroke

The primary imaging criterion for determining eligibility for reperfusion therapies in acute ischemic stroke is the absence of intracranial hemorrhage on non-contrast CT or MRI, which should be completed within 25 minutes of emergency department arrival and interpreted within 45 minutes. 1, 2

Initial Imaging Requirements

• Non-contrast CT or MRI must be performed immediately to exclude intracranial hemorrhage and determine the extent of ischemic changes 2
• CT scan should not show evidence of multilobar infarction (hypodensity >1/3 cerebral hemisphere), which is a contraindication for IV thrombolysis 1
• For patients with suspected large vessel occlusion (LVO), non-invasive angiography (CTA) should be performed to determine eligibility for mechanical thrombectomy 1
• CTA should ideally include "aortic arch-to-vertex" imaging to assess both extracranial and intracranial circulation 3

Time-Based Imaging Criteria

For 0-4.5 Hour Window (IV Thrombolysis)

• Primary goal is to exclude hemorrhage and assess early ischemic changes 2
• Either non-contrast CT or MRI is appropriate 1, 2
• Advanced imaging should not delay initiation of IV thrombolysis if the patient meets clinical criteria 1, 2

For 6-24 Hour Window (Mechanical Thrombectomy)

• Patients with LVO in the anterior circulation require advanced imaging (CTP or DW-MRI, with or without MRI perfusion) 1
• Imaging must demonstrate a small infarct core but large perfusion lesion (penumbra) or severe clinical deficit 4
• The presence of salvageable tissue (penumbra) is critical for determining eligibility in extended time windows 5, 6

Specific Imaging Findings That Determine Eligibility

• Contraindications on imaging:

  • Presence of intracranial hemorrhage on CT or MRI 1
  • CT showing multilobar infarction (hypodensity >1/3 cerebral hemisphere) 1
  • Large established infarct core on DWI-MRI 4

• Favorable imaging findings:

  • Small ischemic core with good leptomeningeal collaterals 5
  • Evidence of penumbral tissue (salvageable brain tissue) 4, 6
  • DWI-FLAIR mismatch on MRI for patients with unknown onset time 4, 6

Imaging Protocols for Different Clinical Scenarios

For IV Thrombolysis Candidates (0-4.5 hours)

• Non-contrast CT or MRI to exclude hemorrhage 1, 2
• Vascular imaging (CTA or MRA) to identify potential LVO 1, 3

For Mechanical Thrombectomy Candidates

• Non-contrast CT or MRI to exclude hemorrhage 1
• CTA to confirm LVO 1, 3
• Advanced imaging (CT perfusion or DWI-MRI with perfusion) for patients in extended time windows (6-24 hours) or unknown onset 1, 4

For Unknown Onset Time (Including Wake-Up Stroke)

• MRI with DWI-FLAIR mismatch protocol to identify strokes likely within 4.5 hours 4, 6
• CT perfusion or MR perfusion to identify salvageable tissue 4, 6

Common Pitfalls to Avoid

• Delaying IV thrombolysis while waiting for advanced imaging in patients who are clearly within the 4.5-hour window and have no contraindications on non-contrast CT 2
• Failing to perform vascular imaging (CTA) in patients who may be candidates for mechanical thrombectomy 1, 3
• Relying solely on time windows rather than tissue status for treatment decisions in borderline cases 4, 6

Practical Implementation

• A standardized imaging approach should be used, with all relevant studies conducted in as few sessions as possible to avoid treatment delays 1, 2
• Initial imaging should be completed and interpreted within 45 minutes of emergency department arrival 1, 2
• For patients outside standard treatment windows, advanced imaging can help identify those who may still benefit from reperfusion therapy 4, 5, 6