This study is performed to assess if PET-MR imaging can improve treatment of patients with irregular heart rate and heart failure. Heart failure occurs when the heart muscle is too weak to do his work correctly. Irregular heart rate can be related to numerous diseases. One category of irregular heart rate is called ventricular arrhythmia. It is often seen in patients who have had a heart attack. This type of arrhythmia can be dangerous and can cause sudden death. To prevent these arrhythmias, doctors can perform procedures that burn the source of arrhythmia in the heart muscle. This is called ablation. Unfortunately, ablation does not fix the problem in 100% of patients and some will still have the arrhythmia requiring repeated procedure. The purpose of this study is to find new ways of guiding the doctors performing ablation, hoping to improve the success rate of the procedure.