© Wolters Kluwer Health, Inc. All rights reserved. BACKGROUND Patients with blunt cerebrovascular injuries are at risk of thromboembolic stroke. Although primary prevention with antithrombotic therapy is widely used in this setting, its effectiveness is not well defined and requires further investigation. The aim of this study was to evaluate the utility of magnetic resonance imaging (MRI)-detected ischemic brain lesions as a possible future outcome for randomized clinical trials in this patient population. METHODS This prospective observational study included 20 adult blunt trauma patients admitted to a level I trauma center with a screening neck CTA showing extracranial carotid or vertebral artery injury. All subjects lacked initial evidence of an ischemic stroke and were managed with antithrombotic therapy and observation and then underwent brain MRI within 30 days of the injury to assess for ischemic lesions. The MRI scans included diffusion, susceptibility, and Fluid-attenuated Inversion Recovery (FLAIR) sequences, and were reviewed by two neuroradiologists blinded to the computed tomography angiography (CTA) findings. RESULTS Eleven CTAs were done in the emergency department upon admission. There were 12 carotid artery dissections and 11 unilateral or bilateral vertebral artery injuries. Median interval between injury and MRI scan was 4 days (range, 0.1-14; interquartile range, 3-7 days). Diffusion-weighted imaging evidence of new ischemic lesions was present in 10 (43%) of 23 of the injured artery territories. In those injuries with ischemic lesions, the median number was 8 (range, 2-25; interquartile range, 5-8). None of the lesions were symptomatic. Blunt cerebrovascular injury was associated with a higher mean ischemic lesion count (mean count of 3.17 vs. 0.14, p < 0.0001), with the association remaining after adjusting for injury severity score (p < 0.0001). CONCLUSION In asymptomatic blunt trauma patients with CTA evidence of extracranial cerebrovascular injury and treated with antithrombotic therapy, nearly half of arterial injuries are associated with ischemic lesions on MRI. LEVEL OF EVIDENCE Therapeutic/care management, level IV.