Use of Imaging to Select Patients for Late Window Endovascular Therapy

Gregory W. Albers, MD

Disclosures

Stroke. 2018;49(9):2256-2260. 

In This Article

Estimating the Ischemic Core With CTP

Both DAWN and DEFUSE 3 had restrictions on the size of the estimated ischemic core volume that was eligible for enrollment. For patients screened with CTP, both studies used a relative CBF threshold of <30% of normally perfused tissue to identify ischemic core with the same perfusion analysis software installed at each site. It is important to appreciate that CTP maps do not identify infarcted tissue, they identify regions with blood flow abnormities that can predict tissue fate. For example, in patients with an acute arterial occlusion, CTP can identify tissue that is likely to be already irreversibly injured before this tissue can be identified as hypodense on a noncontrast CT. Several studies have shown that relative CBF maps can provide a reasonably accurate estimate of tissue that is likely to be irreversibly injured in acute stroke patients.[4–6]

An important issue is which CBF threshold is most accurate for estimating the ischemic core in acute stroke patients because the choice of threshold can have a substantial impact on how much tissue is considered to be potentially irreversibly injured. There have been several studies that have addressed the question of which CBF threshold is the most appropriate, and in general, most studies have suggested that thresholds of around <30% to 35% are optimal. For example, in one study, 103 acute stroke patients immediately were taken to MRI after a CTP scan.[5] The DWI lesion was used as the gold standard for identifying the ischemic core. In this study, a relative CBF threshold of <38% of normal best predicted the DWI volume. However, in a few patients (<5%), the 38% threshold significantly overestimated the size of the DWI lesion. For these few patients, the CTP results could give the impression that there was less salvageable tissue than may actually be present. The investigators determined that 30% provided the most accurate threshold that did not overcall the DWI lesion. The median absolute difference in the CBF-based core with the 30% threshold was only 9 mL smaller than the DWI lesion, and there were no significant overcalls.

Prospective validation of the accuracy of the CBF <30% threshold was obtained in the SWIFT PRIME trial (Solitaire With the Intention for Thrombectomy as Primary Endovascular Treatment for Acute Ischemic Stroke).[4] In SWIFT PRIME, which used RAPID with the <30% relative CBF threshold, the ischemic core volume at baseline strongly correlated with infarct volume imaged at 27 hours in patients who achieved reperfusion; the median absolute difference between the observed and predicted volume was 10 mL. In this study, there was virtually no evidence of core overestimation; among patients where the 27-hour infarct volume was smaller than the baseline ischemic core, the median difference was only 4 mL.

Variability between CBF core volumes and tissue that is irreversibly injured can occur for a variety of reasons. One of the most important is how long the CBF abnormality has been present. If reperfusion is not promptly achieved, the final infarct volume may be larger than baseline CTP-based core estimate. In contrast, if the occlusion has been present for only a few minutes, even regions with low CBF will not yet be irreversibly injured, and early reperfusion can result in a transient ischemic attack. Therefore, stricter relative CBF thresholds may be required for patients who are imaged early after symptom onset, and this is the topic of ongoing research efforts.[7]

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