LEARNING OBJECTIVES-       Sustained re-entrant tachyarrhythmias treatment has become pivotal in the grown-up congenital heart patients clinical management. -       Cardiac LGE-MRI tissue characterization integrated with high definition electroanatomic map could allow fast recognition and effective treatment of substrate of tachyarrhythmias.-       Cardiac LGE-MRI areas were suggestive of post-surgical changes both in atrium and ventricle. -       High-density electro-anatomical map localized areas of slow conduction identifying conducting isthmuses of re-entrant arrhythmias.