The current state-of-the-art technique for ventricular tachycardia (VT) ablation utilizes electroanatomic mapping (EAM) and site-specific electrograms to guide delivery of ablation lesions. Although EAM can assist in distinguishing viable tissue from scar in infarcted areas, it does not represent a true gold standard for differentiating scar from viable or healthy myocardium. The limitations of EAM are reflected in the frequent recurrences (~30%) of VT after ablation. The basic insight of this proposal is that 3D imaging of mitochondrial membrane potential (MP) and the size of the extracellular space (ECS) provides a more comprehensive assessment of the arrhythmogenic substrate than EAM. We propose PET/MRI to map both mitochondrial MP and ECS for use in the detection of left ventricular (LV) scar. Our approach provides a full 3D map of myocardial membrane potential, in contrast to the 1-2mm depth beneath the endo- and epicardial surfaces afforded by EAM. The method is noninvasive and well suited to clinical translation for study of a variety of diseases, including ventricular arrhythmias, cardiomyopathy, hypertrophy, skeletal muscle disorders, etc.