Cardiologic intervention for the treatment of tachyarrhythrnias has evolved from pharmacological therapy to surgical based elimination of arrhythmogenic loci and circuits to catheter based ablation procedures. Ablation via percutaneously placed catheters has become the standard form of therapy for many tachyarrhythmias, surpassing pharmacologic and surgical methods. Despite these major advances, catheter ablation has major limitations effecting both efficacy and safety. The inability to accurately identify anatomic targets and create transmural, continuous lesions limits efficacy. Many of the complications associated with this procedure are due to its invasive nature. In addition, the procedure is performed under fluoroscopy guidance that during a prolonged procedure exposes the patient and physician to a significant dose of ionizing radiation. In this research we will test a hypothesis that an ultrasound phased array with full three dimensional beam steering capability placed in the esophagus and guided and monitored by magnetic resonance imaging can induce accurately targeted transmural myocardial lesions without the problems associated with the intracardiac catheter placement. To accomplish this we plan to: First, perform in vivo animal tests with our linear phased arrays to explore the sonication parameters and methods for inducing adequate tissue coagulation. Second, perform a computer simulation study to explore the array design and sonication methods for trans-esophageal cardiac ablation. Third, develop and test the phased array applicators both in ex vivo and in vivo animal tissues, and fourth, develop MR thermometry methods that can monitor and guide cardiac muscle coagulation. Finally, we plan to develop a complete sonication system and test it in animal experiments. The ability to image in real time the true anatomy without exposure to ionizing radiation and to precisely target and create transmural lesions non-invasively would be a major revolution in the treatment of cardiac arrhythmias. This could result in significant cost savings and lead to a cure of thousands of patients.