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Introduction: Radiofrequency (RFA) and cryoballon ablation are standard approaches for rhythm control of atrial fibrillation. Both strategies involve scar formation of the left atrial tissue. To date there have been no studies using post ablation cardiac magnetic resonance imaging (MRI) to assess residual fibrosis and scar created by standard radiofrequency and cryoballoon ablation. Methods: The current study is a sub analysis of the control group of the Delayed-Enhancement MRI Determinant of Successful Catheter Ablation of Atrial Fibrillation trial (DECAAF II). This was a multicenter, randomized, controlled, single blindedtrial that evaluated atrial arrhythmia recurrence (AAR) between PVI alone and PVI plus fibrosis guided ablation. Baseline MRI and 3-month post ablation MRI were obtained to assess lesion and scar formation. Results: Of the 843 patients randomized in the trial, 428 patients were in the control arm that received standard PVI alone. Of those 428 patients, 367 patients (85.7%) underwent RFA and 61 (14.3%) underwent cryoballoon ablation. Average procedure duration was 147 minutes for RFA and 107 minutes for cryoballoon (p = 0.001). The rate of AAR at 451 days was 42.0% in the RFA group and 45.9% in the cryoballoon group (p = 0.66). At the 3-month post MRI the RFA arm had significantly more left atrial scar formation compared to cryoballoon arm (8.8% vs. 6.4%, p = 0.001). Also, the RFA arm had more covered fibrosis (3.6% vs. 2.9%, p = 0.02) and less non-covered fibrosis (14.9% vs. 17.2%, p = 0.05) compared to the cryoballoon arm. The amount of scar formation did not predict AAR (p = 0.07). Conclusions: In this subanalysis of the DECAAF II trial, there was no significant difference in the rate of AAR between RFA and cryoballoon ablation. However, RFA caused significantly more left atrial scar compared to cryoballoon ablation on 3-month post MRI.

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