Noninvasive vasectomy using a focused ultrasound clip: Thermal measurements and simulations

Introduction: Conventional surgical vasectomy may lead to complications inc luding bleeding, infection, and scrotal pain. Noninvasive transcutaneous de livery of therapeutic focused ultrasound has previously been shown to therm ally occlude the vas deferens. However, skin burns and inconsistent vas occ lusion have presented complications. This study uses bio-heat transfer simu lations and thermocouple measurements to determine the optimal ablation dos imetry for vas occlusion without skin burns. Methods: A 2-rad ultrasound transducer mounted on a vasectomy-clip-delivere d ultrasound energy at 4 MHz to the canine vas deferens co-located at the f ocus between the clip jaws. Chilled degassed water was circulated through a n attached latex balloon, providing efficient ultrasound coupling into the tissue and active skin cooling to prevent skin burns. Thermocouples placed at the vas, intradermal, and skin surface locations, recorded temperatures during ablation. Procedures were performed with transducer acoustic powers of 3-7 W and sonication times of 60-120 s on both the left and right vas de ferens (n = 2) in a total of four dogs (precooling control, 3 W/120 s, 5 W/ 90 s,7 W/60 s). Measurements were compared with bio-heat transfer simulatio ns modeling the effects of variations in power and sonication time on tissu e temperatures and coagulation zones. Results: Active skin cooling produces a thermal gradient in the tissue duri ng ablation, allowing sufficient thermal doses to be delivered to the vas w ithout skin burns. However, low-power, long-duration heating produced exces sive tissue necrosis due to thermal diffusion, while high power and short h eating times reduced the therapeutic window and produced skin burns presuma bly due to direct ultrasound absorption. Conclusions: Both simulations and experiments suggest that a therapeutic wi ndow exists in which thermal occlusion of the vas may be achieved without t he formation of skin burns in the canine model (power = 5-7 W, surface inte nsity = 1.4-1.9 W/cm(2), time = 20-50 s). This range of ablation parameters will help guide future experiments to refine incisionless vasectomy using focused ultrasound.