Inclusion Criteria:  Symptomatic paroxysmal atrial fibrillation or persistent atrial fibrillation diagnosed - within 24 months preceding the consent date. Planned radiofrequency ablation for atrial fibrillation consisting of pulmonary vein isolation. Able and willing to comply with study follow-up requirements Able and willing to provide written informed consent  Exclusion Criteria:  Any clinical contra-indication to ablation Any disease limiting life expectancy to < 1year Contra-indication to MRI including renal dysfunction (eGFR<30ml/min) Potential participant currently pregnant or breast feeding Prior ablation, cardiac surgery or presence of any prosthetic valves Myocardial infarction or percutaneous coronary intervention within 3 months preceding consent date Hypertrophic cardiomyopathy or other inherited cardiac condition Presence / likely implant of any pacemaker, cardiac resynchronization therapy device or implantable defibrillator within 1 year Enrolment in an existing interventional atrial fibrillation study which alters treatment or follow-up. Use of amiodarone for any indication within 6 months prior to the planned ablation procedure Use of any anti-arrhythmic drug for an arrhythmia indication other than atrial fibrillation Prior documented cavotricuspid isthmus dependent right atrial flutter and planned cavotricuspid isthmus ablation to be performed at the time of atrial fibrillation ablation Unable to understand verbal or written explanations given in English or German as appropriate.