Treatment allocation: Patients will be randomized to receive either Aspirin (ASA) or anticoagulants (ratio 1:1). ASA means Aspirin™ 300 mg q.d. given orally. In patients who cannot swallow safely, ASA will be given intravenously (e.g. Aspegic™ 250 mg q.d.) until swallowing function has recovered. Anticoagulants mean the use of vitamin K antagonists (i.e., phenprocoumon [Marcoumar™] or acenocoumarol [Sintrom™]) with a target international normalized ratio (INR) of 2.0 - 3.0. Until the target INR has been reached, patients will receive anticoagulation with i.v. heparin or low molecular weight heparin). The choice of the distinct agent (e.g. phenprocoumon or acenocoumarol) can be chosen by the treating physician and the patient, taking into account the experience with these agents (phenprocoumon is more commonly used in the German speaking part, acenocoumarol is usually used in the French speaking part of Switzerland). Treatment should start as soon as diagnosis has been established, informed consent by patients or next-to-kin was obtained and baseline magnetic resonance (MR)-images have been performed (maximum time window maximum 24 hours). Treatment duration will be 90 days +/-14 days (i.e., until the follow-visit 2).