Background and Purpose-At 1.5 T, diffusion-weighted imaging-fluid-attenuated inversion recovery (DWI-FLAIR) mismatch helps identify strokes within 4.5 hours of onset. However, at 3T, studies have found divergent results. The goal of this study was to determine whether DWI-FLAIR mismatch at 3T would also be helpful for identifying patients within 4.5 hours of symptom onset. Methods-All patients presenting with an ischemic stroke in the middle cerebral artery territory and explored with 3T MRI within 12 hours between November 2007 and April 2012 were included in this retrospective study. Two readers analyzed the DWI and FLAIR images. Logistic regression was performed to determine independent predictors of FLAIR visibility. Also, the predictive values of a mismatch for identifying patients with stroke onset ≤4.5 hours were estimated. Results-The study included 194 patients. The only predictive factor of FLAIR visibility was delayed MRI acquisition. The DWI-FLAIR mismatch was able to identify patients within 4.5 hours of stroke onset with relatively low sensitivity (0.55; 95% confidence interval, 0.48-0.63), low specificity (0.60; 95% confidence interval, 0.42-0.77), high positive predictive value (0.88; 95% confidence interval, 0.82-0.94), and very low negative predictive value (0.19; 95% confidence interval, 0.11-0.28). In addition, 44.5% of patients had a positive FLAIR sequence within 4.5 hours. Conclusions-This study improves our understanding of DWI-FLAIR mismatch as an imaging biomarker for wake-up management of patients with stroke. At 3T, the presence of a DWI-FLAIR mismatch was able to identify stroke onset of <4.5 hours. from symptom onset within the first 4.5 hours because of low sensitivity. 10The goal of the present study was to determine whether the DWI-FLAIR mismatch at 3T might be a good marker for identifying those patients who are within 4.5 hours of ischemic stroke symptom onset. Materials and Methods PatientsPatients with ischemic strokes in the middle cerebral artery territory who underwent 3T MRI at our institution during the first 12 hours of symptom onset were retrospectively studied during the period from November 2007 to April 2012. They were identified from the registry of the stroke unit. Patients were included if they had a precise time of stroke symptom onset and if they had undergone DWI and FLAIR imaging. The study had the approval of the local institutional review board. According to the study design, written informed consent was waived.Initially, 234 patients were included in the study. Patients with FLAIR images showing major artifacts (n=20; 8.9%) were excluded, as were those with multiple lesions of different ages (n=20; 8.9%), because of the difficulty of attributing lesions to the stroke symptoms used to date it.11 Ultimately, data from 194 patients were analyzed. For each patient, demographic data (sex, age), National Institutes of Health Stroke Scale (NIHSS) score at admission, and stroke symptom onset time to determine the stroke onset-to-MRI delay were collected. MRI Proto...