The primary aim of this prospective observational study is to investigate whether an enhanced diagnostic MRI work-up (including cardiac MRI, angiography of the aortic arch and the brain-supplying arteries) combined with an in-hospital Holter-ECG of up to 5 days duration leads to a significant increase in relevant pathologic findings with respect to stroke aetiology as compared to the findings obtained by a routine diagnostic work-up (including stroke unit monitoring, 24h-Holter-ECG, echocardiography, Doppler-ultrasound of the brain-supplying arteries) in patients with acute ischemic stroke and no atrial fibrillation according to past medical history or baseline ECG. A better understanding of the stroke aetiology may improve secondary stroke prevention and long term outcome.