Challenges facing validation of noninvasive electrical imaging of the heart

Noninvasive imaging of regional cardiac electrophysiology remains an elusive target. Such imaging is still in its infancy, particularly in comparison to structural imaging modalities such as magnetic resonance imaging (MRI), x-ray computed tomography (CT), and ultrasound. We present an overview of noninvasive ECG imaging, and the challenges and successes of the various techniques across a range of applications. Unlike MRI and CT, reconstructing cardiac electrophysiology from remote body surface measurements is a highly ill-posed problem. We therefore first review the theoretical considerations and associated algorithms that are used to address this issue. We then focus on the important issue of validation, and review and contrast recent advances in this area. Efforts to validate ECG inverse procedures using a modeling-based approach are addressed first. We then discuss various experimental studies that have been conducted to provide appropriate data for robust validations. We present new data that are simultaneously recorded from dense arrays of electrodes on the epicardium and body surface of anesthetized pigs during sinus rhythm, ventricular pacing, and regional ischemia. These data have been obtained specifically to help validate inverse ECG procedures, and form a useful supplement to recent clinical validation studies. Finally, clinical applications and outstanding issues regarding noninvasive imaging of regional cardiac electrophysiology are addressed.