What is the most helpful diagnostic study to determine eligibility for mechanical thrombectomy in ischemic stroke?

Perfusion Computed Tomography

For this patient presenting 8 hours after last known well with an NIHSS of 10, perfusion computed tomography (CTP) is the most helpful diagnostic study to determine eligibility for mechanical thrombectomy, as it is required to assess core-perfusion mismatch in the extended time window (6-24 hours) when selecting patients according to DAWN or DEFUSE-3 criteria. 1

Algorithmic Approach to Imaging Selection

Initial Imaging (All Patients)

• Noncontrast CT head must be performed immediately to exclude hemorrhage and assess ASPECTS score 1
• CT angiography (CTA) should be obtained simultaneously to identify large vessel occlusion 1
• These studies should not delay IV alteplase if the patient is within 4.5 hours 1

Time-Based Advanced Imaging Requirements

Early Window (0-6 hours from last known well):

• CTP is not necessary for thrombectomy decision-making 1
• Noncontrast CT plus CTA is sufficient if ASPECTS ≥6, NIHSS ≥6, and large vessel occlusion is confirmed 2
• Obtaining CTP in this window may cause harmful delays 1

Extended Window (6-24 hours from last known well):

• CTP or DW-MRI with perfusion is required to determine eligibility 1
• Must demonstrate sizable mismatch between ischemic core and hypoperfusion area 1
• DAWN and DEFUSE-3 trials, which established benefit in this window, both used perfusion imaging as eligibility criteria 1
• Strict adherence to DAWN or DEFUSE-3 criteria is mandatory when selecting patients beyond 6 hours 1

Why Other Options Are Incorrect for This Patient

Noncontrast CT alone:

• Essential first step but insufficient for extended window eligibility determination 1
• Cannot assess perfusion mismatch required for 6-24 hour window 1

Noncontrast MRI:

• DW-MRI can show core infarct but requires perfusion sequences (not just noncontrast) for extended window selection 1
• Takes longer than CT-based protocols without improving outcomes 3

aPTT and INR:

• Should be obtained but must not delay reperfusion therapy 1
• Only blood glucose measurement must precede IV alteplase 1
• Do not determine thrombectomy eligibility 1

Critical Caveats for CTP Interpretation

Recanalization artifact:

• Spontaneous recanalization before CTP can cause gross underestimation of core infarct volume by automated software 4
• The entire composite of imaging (noncontrast CT, CTA, and CTP) must be examined together 4
• If noncontrast CT shows established infarct but CTP shows minimal core, suspect recanalization 4

Technical considerations:

• CTP adds approximately 4 minutes to scan time compared to CT alone 5
• Improves decision-maker confidence and creates trend toward appropriate thrombectomy recommendations 5
• The combination of noncontrast CT, arterial-phase CTA, and CTP achieves highest inter-rater agreement 5

Practical Implementation

For this specific patient:

• Last known well was 8 hours ago (extended window)
• NIHSS 10 suggests potential large vessel occlusion
• Immediate sequence: Noncontrast CT → CTA → CTP 1
• If large vessel occlusion confirmed on CTA with favorable perfusion mismatch on CTP, proceed to thrombectomy 1
• ASPECTS must be ≥6 on noncontrast CT 2
• Core-perfusion mismatch criteria from DAWN or DEFUSE-3 must be met 1