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Introduction: Arterial spin labeling (ASL) MRI can non-invasively measure quantitative CBF. Evaluation of hemodynamics in the ipsilateral hemisphere is a common practice for selecting patients for therapy. In this study, we hypothesized that the contralateral CBF (cCBF) may identify patients with high collateral capacity and better outcome.Hypothesis: In acute stroke, higher CBF in the unaffected hemisphere is associated with better neurological outcome.Methods: Patients were part of the prospective ‘iCAS’ (imaging the Collaterals in Acute Stroke) study. Inclusion criteria were: ischemic hemispheric stroke (< 16 hrs onset to imaging time [OIT]), age>=18, informed consent, and technically adequate imaging including GRE, DWI, and 3D pseudocontinuous ASL. Outcomes were assessed by NIHSS at baseline, day 1, and day 5; and mRS at day 30 and day 90. After image registration to an MNI template, mean cCBF was calculated at standard ASPECTS levels in the contralateral hemisphere. Patients were dichotomized by median cCBF into low and high cCBF groups. Results are reported as medians with interquartile ranges [IQR]. Outcome differences were assessed with Wilcoxon (NIHSS) and Fisher’s exact test (mRS).Results: 61 patients met inclusion criteria: 32 F, age 66 yrs [54-77], OIT 4.8 hrs [3.4-7.2], baseline NIHSS 13 [8-19], 36 underwent thrombectomy [28 with final TICI >= 2b], cCBF 38.8 [31-46] ml/100 g/min. There was no difference between groups in age, gender, OIT, or reperfusion. Median NIHSS at baseline/day1/day5 for low and high cCBF groups was 13/14/11 and 12/6/4, respectively, which was significantly different on day 1 (p=.009) and day 5 (p=.031). Patients with higher cCBF had lower contralateral arterial transit time (p=.029) and better day 90 mRS (p=.029).Conclusion: Higher ASL cCBF predicts better outcome in acute stroke independent of baseline NIHSS and reperfusion status. This may reflect a better underlying capacity for collateral flow to the ischemic hemisphere.