evaluate the impact of various features in CMR on the mid-term/long-term success rate of rhythm control for paroxysmal atrial fibrillation/flutter .

• Atrial fibrillation/flutter are widely prevalent.
• Reported antiarrhythmic drug efficacy to restore & maintain sinus rhythm is 52% which is far from optimal.
• On the other hand, 5-year arrhythmia-free survival after catheter ablation was previously reported to be 63%.
• Therefore, there is a need for better characterization of the underlying atrial substrate, to improve treatment outcomes in terms of efficacy and safety

Inclusion Criteria:

• Age>18 years.
• Patients undergoing successful cardioversion (pharmacologic or electric) for atrial fibrillation/flutter, OR symptomatic patients with paroxysmal atrial fibrillation/flutter scheduled for trans-catheter ablation.
• Index cardioversion OR Index ablation procedure.

Exclusion Criteria:

• LA thrombus on TEE or CMR.
• Contraindications to CMR.
• Contraindications to Gd-based contrast.

Based on the previously reported antiarrhythmic drug efficacy of 52% in maintaining sinus rhythm:

• For Wilcoxon-Mann-Whitney test for difference in 2 group means: assuming an alpha error probability of 0.05, a study power of 60-70%, and a 1:1 ratio of successful rhythm control vs atrial fibrillation/flutter recurrence, a total sample size of 86-106 would be needed (43-53 for each arm). [calculated by G*Power software v.3.1.9.7]
• To be able to build multivariable logistic regression models including, e.g., 4-5 predictor variables, a reasonable sample size would be 80-100 patients.
• Patient enrollment will continue until the projected sample size is satisfied or until 24 months have lapsed, whichever comes first.