Diffusion-weighted imaging-fluid attenuated inversion recovery mismatch in nocturnal stroke patients with unknown time of onset

Background: More than a quarter of patients with ischemic stroke (IS) are excluded from thrombolysis because of an unknown time of symptom onset. Recent evidence suggests that a mismatch between diffusion-weighted imaging (DWI) and fluid attenuated inversion recovery (FLAIR) imaging could be used as a surrogate for the time of stroke onset. We compared used the DWI-FLAIR mismatch and the FLAIR/DWI ratio to estimate the time of onset in a group of patients with nocturnal strokes and unknown time of onset. Methods: We used a prospectively collected acute IS patient database with MRI as the initial imaging modality. Nineteen selected nocturnal stroke patients with unknown time of onset were compared with 22 patients who had an MRI scan within 6 hours from stroke onset (control A) and 19 patients who had an MRI scan between 6 and 12 hours (control B). DWI and FLAIR signal was rated as normal or abnormal. FLAIR/DWI ratio was calculated from independent DWI and FLAIR ischemic lesion volumes using semiautomatic software. Results: The DWI-FLAIR mismatch was different among groups (unknown 43.7%; control A 63.6%; control B 10.5%; Fisher-Freeman-Halton test; P =.001). There were significant differences in FLAIR/DWI ratio among the 3 groups (unknown 0.05 ± 0.12; control A 0.17 ± 0.15; control B 0.04 ± 0.06; Kruskal-Wallis test; P <.0001). Post-hoc pairwise comparisons revealed that FLAIR/DWI ratio from the unknown group was significantly different from the control B group (P =.0045) but not different from the control A group. DWI volumes were not different among the 3 groups. Conclusions: A large proportion of patients with nocturnal IS and an unknown time of stroke initiation have a DWI-FLAIR mismatch, suggesting a recent onset of stroke.