Atrial fibrillation ablation (AF) is a standard interventional treatment for patients with symptomatic AF refractory to medical treatment. The known predictive factors for the success of the procedure remain insufficient to predict the probabilities of success and to appropriately select the patients who could benefit the most from this procedure. Left atrium imaging by MRI or CT may be able to identify AF substrate. However data are lacking about the practical impact of these techniques in routine practice to predict AF ablation outcome. The CT-AF study is a prospective, interventional, multicenter cohort study. The main objective of this study is to evaluate the prognostic value of a new automated measurement technique for intra-myocardial atrial fat measurement in cardiac CT and the measurement of global left atrial strain in MRI in patients who are candidates for first AF ablation. The main outcome will be the relationship between the relative volume of left atrial fat measured with CT and total left atrial strain in MRI and recurrence of AF at 1 year after the ablation procedure (blanking period of 3 months post ablation excluded).