Renal Denervation Using Externally Focused Therapeutic Ultrasound

Inclusion Criteria: * At least 18 years of age * Average systolic blood pressure at least 160 mmHg * Refractory, stable hypertension despite being treated with at least three hypertensive drugs * Two functioning kidneys, defined as eGFR \>= 45 ml/min * At least one renal artery on each side which is greater than 4 mm. Exclusion Criteria: * History of nephrectomy or hydronephrosis * Renal stenosis \> 50% * Renal stent * Ambulatory blood pressure monitoring 24 hour average systolic blood pressure \<= 135 mmHg * Kidney stones which are symptomatic and/or \> 1 cm * History of abdominal surgery within the past 6 months * Heterogeneities in the kidneys (cysts or tumors) * Residual pyelonephritis * History of myocardial infarction, unstable angina pectoris, or cerebrovascular accident within the last 6 months * Hemodynamically significant valvular heart disease * Implantable cardioverter defibrillator, pacemaker, neurostimulator or other device incompatible with MRI * Body weight \> 150 kilograms * Target treatment depth \> 14 cm from the skin line * Pregnant, nursing or intends to become pregnant during the trial period