In this research study the investigators want to learn more about how well the investigators can visualize scar tissue in the heart by MRI. In patients with congenital heart disease who need a procedure in the electrophysiology laboratory, how the MRI findings match the findings in the electrophysiology laboratory is not known. This study works to answer these questions. Participants will undergo a cardiac MRI as part of the routine clinical care that was ordered by their doctors and additional imaging by cardiac MRI will be performed.

Atrial arrhythmias including intra-atrial reentrant tachycardia (IART) and atrial fibrillation (AF) routinely develop after surgical repair of congenital heart disease (CHD), contributing to heart failure exacerbation, increased hospital resource use, and reduced health-related quality of life. The combination of atriotomy scars, intra-cardiac suture lines, and chronic pressure or volume overload from residual lesions creates the necessary milieu of heterogeneous atrial fibrosis capable of supporting wavefront reentry. While catheter ablation has become a primary tool in the management of IART and AF, long-term ablation outcomes have stagnated over the preceding decade despite advances in mapping and ablative technologies. Left atrial (LA) fibrosis analysis using 3-dimensional (3D) late gadolinium enhancement (LGE) cardiac magnetic resonance imaging (CMR) has shown utility in the management of adults with structurally normal hearts and atrial fibrillation (AF), having associations with endocardial bipolar voltage amplitude, likelihood of maintenance of sinus rhythm after ablation, and thromboembolic risk. Excellent reproducibility of LA fibrosis quantification has been demonstrated in adults with structurally normal hearts and AF. To date, the use of 3D LGE in CHD has been limited to the ventricles. Prior studies have described altered LA function in adolescent and young adult patients with rTOF. Additionally, right atrial (RA) functional abnormalities have also been described in patients with rTOF. No prior studies have attempted to validate this technology in the atrium of patients with congenital heart disease. Our studies aims to investigate the reproducibility of atrial fibrosis quantification by cardiac MRI and may provide insights to correlations with voltage mapping in the electrophysiology laboratory.

Inclusion Criteria:

• Patients >13 years old with congenital heart disease referred for cardiac MRI and receiving gadolinium as part of routine clinical care or for pre-ablation planning will be included.

Exclusion Criteria:

• Those with self-reported anxiety or claustrophobia and/or the presence of a permanent pacemaker or implantable cardioverter defibrillator will be excluded.

The hospital at which the investigators are based is a referral center for patients with congenital heart disease. These patients sometimes require a cardiac MRI. Patients >13 years old with congenital heart disease referred for CMR and meeting eligibility criteria will be approached for this study.