What are the CT perfusion criteria for mechanical thrombectomy (MT) in patients with acute stroke, including National Institutes of Health Stroke Scale (NIHSS) score and blood pressure requirements?

CT Perfusion Criteria for Mechanical Thrombectomy in Acute Stroke

For patients presenting within 0-6 hours of symptom onset, CT perfusion is not required for thrombectomy decision-making—noncontrast CT with ASPECTS ≥6 plus CTA confirming large vessel occlusion is sufficient, whereas patients presenting 6-24 hours from last known well must have CT perfusion or DW-MRI demonstrating salvageable tissue with strict adherence to DAWN or DEFUSE-3 mismatch criteria. 1, 2

Early Window (0-6 Hours): Basic Imaging Criteria

Within 6 hours, proceed with thrombectomy based on noncontrast CT and CTA alone without perfusion imaging. 2

Required criteria for thrombectomy in the early window include:

• Age ≥18 years 3
• Pre-stroke mRS 0-1 (functionally independent) 1, 3
• NIHSS ≥6 (moderate to severe deficit) 1, 3
• ASPECTS ≥6 on noncontrast CT (limited established infarction) 1, 3
• Causative occlusion of ICA or MCA-M1 confirmed on CTA 1, 3
• Treatment initiation (groin puncture) within 6 hours of symptom onset or last known well 3

The American College of Radiology explicitly states that CTP is unnecessary in this time window when these criteria are met. 2

Extended Window (6-24 Hours): Mandatory Perfusion Imaging

Beyond 6 hours, advanced perfusion imaging becomes mandatory to demonstrate salvageable brain tissue. 1, 2

DEFUSE-3 Criteria (6-16 Hours)

For patients 6-16 hours from last known well, the American Heart Association gives a Class I, Level A recommendation for thrombectomy when meeting DEFUSE-3 criteria: 1

• Core infarct volume <70 mL (measured as CBF <30% on CTP or DWI lesion on MRI) 1
• Mismatch ratio >1.8 (ratio of hypoperfusion volume to core volume) 1
• Absolute mismatch volume >15 mL 1
• Anterior circulation LVO (ICA or MCA-M1) 1

The DEFUSE-3 trial demonstrated 44.6% versus 16.7% achieved good functional outcome (mRS 0-2) with thrombectomy versus control (RR 2.67,95% CI 1.60-4.48, P<0.0001). 1

DAWN Criteria (6-24 Hours)

For patients 6-24 hours from last known well, the American Heart Association provides Class I (6-16h) or Class IIa (16-24h) recommendations using DAWN clinical-imaging mismatch criteria: 1

Age 80 years or older:

• NIHSS ≥10
• Core volume <21 mL 1

Age <80 years:

• NIHSS ≥10 and core <31 mL, OR
• NIHSS ≥20 and core <51 mL 1

The DAWN trial showed 49% versus 13% achieved good functional outcome with thrombectomy versus control (adjusted difference 33%, 95% CI 21-44). 1

Critical guideline statement: Only DAWN or DEFUSE-3 eligibility criteria should be used for patient selection beyond 6 hours—these are the only RCTs demonstrating benefit in this window, and strict adherence is mandatory in clinical practice. 1

Blood Pressure Requirements

Patients eligible for IV thrombolysis who will undergo thrombectomy must have blood pressure lowered below 185/110 mmHg before alteplase is initiated. 1

Hypotension and hypovolemia should be corrected to maintain systemic perfusion necessary to support organ function. 1

Technical and Procedural Considerations

Imaging Acquisition Protocol

• Noncontrast CT head must be performed immediately to exclude hemorrhage and assess ASPECTS 2
• CTA should be obtained simultaneously to identify large vessel occlusion 2
• For extended window patients, CTP or DW-MRI with perfusion is required to determine eligibility 1, 2
• Only blood glucose measurement must precede treatment—other labs (CBC, electrolytes, INR, aPTT) should be obtained but must not delay reperfusion therapy 1, 2

Reperfusion Goal

The technical goal is achieving mTICI 2b/3 reperfusion to maximize probability of good functional outcome. 1, 3 In the HERMES pooled analysis, 71% of patients achieved mTICI 2b/3 reperfusion. 1

Relationship to IV Thrombolysis

• Eligible patients should receive IV alteplase even if thrombectomy is planned 1
• Do NOT evaluate response to IV thrombolysis before proceeding with catheter angiography—observing for clinical response is not required and not recommended 1, 3

Common Pitfalls and Caveats

Perfusion Imaging Interpretation Errors

Recanalization of LVO before or during imaging can cause CTP post-processing software to erroneously underestimate or show normal core infarct volume. 4 This occurs in approximately 5.9% of cases and represents a critical caveat not widely reported. 4 The entire composite of hyperacute CT imaging must be examined while making decisions, not relying solely on automated software output. 4

Large Core Considerations

Recent research suggests that patients with large core (>50 mL) may still benefit from thrombectomy if significant perfusion mismatch exists (mismatch ratio >1.2). 5, 6 However, current AHA/ASA guidelines maintain strict DAWN/DEFUSE-3 criteria with core volume limits. 1 The interaction between treatment effect and mismatch ratio is significant (p<0.001), with adjusted OR of 6.8 for good outcome when mismatch ratio >1.2 versus 0.7 when absent. 5

Mismatch Profile Importance

Even in the early window (<6 hours), perfusion mismatch profiles predict clinical response to thrombectomy. 7 Approximately 80% of LVO patients have evidence of penumbra regardless of infarction volume, and both target mismatch (adjusted OR 3.3) and any mismatch (adjusted OR 5.9) independently predict functional recovery. 7 Reperfusion benefits patients with mismatch but not those without it. 7

Low NIHSS Scores

Do not exclude thrombectomy based solely on low NIHSS scores in the presence of confirmed LVO and significant penumbra on perfusion imaging. 8 Case evidence demonstrates successful outcomes even with NIHSS of 0 when perfusion imaging shows large penumbra (70 mL) with minimal core. 8

Age Considerations

Advanced age >80 years alone should not be an exclusion criterion—elderly patients comprise the majority of stroke victims and may still benefit from thrombectomy. 3