What is the optimal timing and protocol for accessing a perfusion scan in a patient undergoing extended thrombectomy (mechanical removal of a blood clot) for acute ischemic stroke?

Accessing Perfusion Imaging for Extended Window Thrombectomy

For patients presenting 6-24 hours after last known well with suspected large vessel occlusion, CT perfusion (CTP) or diffusion-weighted MRI with perfusion imaging is mandatory before proceeding to thrombectomy to identify salvageable brain tissue. 1

Timing and Protocol for Perfusion Imaging

Within 0-6 Hours

• Perfusion imaging is NOT required for patients presenting within 6 hours who meet standard thrombectomy criteria (NIHSS ≥6, ASPECTS ≥6, confirmed large vessel occlusion on CTA). 1
• Proceed directly to thrombectomy based on non-contrast CT and CTA alone—perfusion imaging should not delay treatment in this early window. 1

Within 6-24 Hours (Extended Window)

• Perfusion imaging is mandatory to demonstrate salvageable tissue using either: 1

  • CT perfusion (CTP), OR
  • Diffusion-weighted MRI with perfusion imaging

• Apply strict eligibility criteria from landmark trials: 1

  • DAWN criteria: Clinical-imaging mismatch (specific age/NIHSS/core volume thresholds)
  • DEFUSE-3 criteria: Ischemic core <70 mL, mismatch ratio ≥1.8, mismatch volume ≥15 mL

• These trials demonstrated dramatic benefit: 49% vs 13% (DAWN) and 44.6% vs 16.7% (DEFUSE-3) achieved functional independence with thrombectomy versus medical therapy alone. 1, 2

Critical Workflow Considerations

Do not delay thrombectomy for unnecessary testing—only blood glucose measurement is required before treatment initiation. 1

Obtain CTA without waiting for serum creatinine in patients without known renal impairment to avoid delays. 1

Do not wait to assess IV thrombolysis response before proceeding to angiography if thrombectomy is being considered. 3

Multimodal CT and MRI, including perfusion imaging, should not delay IV alteplase administration if the patient is within the thrombolysis window. 1

Special Populations

Perioperative Stroke Patients

• For patients developing stroke after cardiac surgery (particularly CABG, valve replacement, TAVR), urgent CTA and perfusion imaging is indicated given delayed neurological assessment due to anesthesia emergence. 1
• These patients should be strongly considered for extended window thrombectomy based on individualized perfusion imaging data. 1

Posterior Circulation (Basilar Artery Occlusion)

• Non-contrast CT with PC-ASPECTS ≥6 and CTA confirming basilar occlusion is sufficient for treatment decisions. 1
• MRI is helpful but not required, given difficulty assessing posterior fossa on CT. 1
• CT perfusion shows promise for patient selection but remains investigational for posterior circulation. 1
• Thrombectomy is indicated 0-12 hours (Class I) and reasonable 12-24 hours (Class IIa) based on clinical criteria alone. 1, 3

Common Pitfalls to Avoid

Never proceed to thrombectomy beyond 6 hours without perfusion imaging in anterior circulation strokes—tissue selection is mandatory for benefit in this window. 1, 3

Do not use perfusion imaging as a screening tool within 6 hours—salvageable tissue is present in >80% of patients at these early time points, making perfusion imaging of minimal value and potentially harmful due to delays. 1

Avoid treating patients who fail to meet DAWN or DEFUSE-3 criteria in the extended window—these are the only validated selection criteria with proven benefit. 1

Evidence Quality Considerations

While recent research suggests non-contrast CT alone may be sufficient even in extended windows 4, current AHA/ASA guidelines (Class I, Level A) mandate perfusion imaging for 6-24 hour patient selection. 1 The guideline recommendation takes precedence over individual research studies until guidelines are formally updated.

The DEFUSE-3 and DAWN trials provide the highest quality evidence (Level A) supporting perfusion-based selection, with number needed to treat below 3 for positive outcome shift. 2, 5