WAKE-UP is a clinical trial of MRI based thrombolysis in acute stroke patients with unknown time of symptom onset, e.g. due to recognition of stroke symptoms on awakening. Intravenous thrombolysis with Alteplase is available as effective and safe treatment of acute stroke within 4.5 hours of symptom onset. However, in about 20% of acute stroke patients time of symptom onset is unknown. This large group of patients is currently excluded from treatment with Alteplase. The objective of the research proposed in the WAKE-UP project is to provide effective treatment options for this large group of acute stroke patients.