Participation in exercise improves cardiovascular health. However, long-term endurance exercise may increase the risk of an irregular heart rhythm called atrial fibrillation (AF). In AF, blood flow is altered, increasing the risk of clot formation in the heart which may enter the circulation and cause a stroke. The risk of stroke can be reduced with the use of blood thinning medication. Athletes with atrial fibrillation, due to their healthy lifestyle, are generally felt to be at low risk of stroke and many would not be offered blood thinning treatment using risk scores used in clinical practice. In a recent survey of almost one thousand athletes, the investigators found that there was an increased risk of stroke in those with atrial fibrillation, even in those without other risk factors for stroke. To further investigate these findings, this study will use MRI scanning to look at the hearts and brains of athletes aged between 40-64 years old. The researchers will assess athletes with and without atrial fibrillation, as well as some athletes with atrial fibrillation who have had a stroke previously. The MRI scans will measure heart size and function as well as blood flow patterns in the heart. The study will determine whether athletes with atrial fibrillation have evidence of stroke on brain MRI and whether these are related to abnormal flow patterns. The results will help us decide whether a larger study should be performed.