In a 68‑year‑old woman with atrial fibrillation who presents 10 hours after acute right facial droop, right upper‑extremity weakness, and aphasia, NIH Stroke Scale 11, non‑contrast head CT without hemorrhage and CT angiography showing left middle cerebral artery M1 occlusion, which additional imaging study is most useful to determine candidacy for mechanical thrombectomy?

Perfusion CT is the Most Useful Additional Test

For this 68-year-old woman presenting 10 hours after stroke onset with confirmed M1 occlusion, perfusion CT is the most useful additional test to determine mechanical thrombectomy candidacy, as it identifies salvageable penumbra versus irreversibly infarcted core in the extended time window. 1, 2, 3

Why Perfusion Imaging is Essential Beyond 6 Hours

• CT perfusion becomes mandatory for patients presenting 6-24 hours after symptom onset to select candidates for endovascular therapy by demonstrating viable tissue that can be saved 2, 3
• The major late-window thrombectomy trials (DAWN and DEFUSE-3) specifically required perfusion imaging (either CT perfusion or MRI diffusion/perfusion) to identify patients with small ischemic cores who would benefit from intervention 4
• Perfusion CT differentiates between infarcted core and salvageable penumbra, which is the critical determination for treatment eligibility in the extended window 3
• At 10 hours post-onset, this patient falls squarely into the extended window where tissue viability—not just vessel patency—determines treatment benefit 1, 2

Why Other Options Are Less Useful

Coagulation Studies (INR, PT, aPTT)

• While coagulation parameters are important baseline labs, they should never delay imaging or treatment decisions 2
• These tests are primarily relevant for IV thrombolysis contraindications, but this patient is already beyond the 4.5-hour tPA window 4
• For mechanical thrombectomy, coagulation abnormalities are not absolute contraindications—the procedure can often proceed with appropriate precautions 4

Platelet Count

• Platelet count is a routine safety parameter but does not determine thrombectomy candidacy 4
• Thrombocytopenia below 100,000/μL would be a relative contraindication, but this does not help identify which patients will benefit from the procedure 4

FLAIR MRI or T1/T2-Weighted MRI

• While MRI with diffusion-weighted imaging is superior to CT for detecting acute ischemia, it does not provide the critical perfusion mismatch information needed for extended-window decision-making 1, 3
• FLAIR can identify stroke onset timing in wake-up strokes (FLAIR-negative/DWI-positive suggests <4.5 hours), but this patient has a known 10-hour onset 1
• Standard anatomic MRI sequences show the infarct but not the salvageable penumbra 3

Practical Implementation

The recommended imaging protocol for this patient is:

• Non-contrast CT head (already completed—shows no hemorrhage or obvious large infarct) 2
• CT angiography (already completed—confirms M1 occlusion) 2
• CT perfusion immediately to quantify ischemic core volume and penumbra 2, 3

Interpretation criteria for thrombectomy eligibility:

• Small ischemic core volume (typically <70 mL on automated software) 4
• Substantial penumbra (mismatch ratio typically >1.8) 4
• Target mismatch profile indicating salvageable tissue despite the 10-hour delay 4, 3

Critical Pitfall to Avoid

• Do not obtain perfusion imaging in obvious thrombectomy candidates within the first 6 hours, as this causes harmful treatment delays 2
• However, at 10 hours post-onset, perfusion imaging is not optional—it is the evidence-based standard for patient selection 4, 2, 3