 Substudy 1: Use of cardiac MRI to evaluate ablation lesions. Some patients will also be asked to undergo an MRI scan of the heart before the ablation procedure, and again at three months and one year following the procedure. This will allow us to examine the potential role for MRI in imaging scar tissue formed by the ablation, and help us understand the time course of scar formation and changes to that part of the heart following the ablation. 