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

Conventional surgical vasectomy may lead to complications including bleeding, infection, and scrotal pain. Noninvasive transcutaneous delivery of therapeutic focused ultrasound has previously been shown to thermally occlude the vas deferens. However, skin burns and inconsistent vas occlusion have presented complications. This study uses bio-heat transfer simulations and thermocouple measurements to determine the optimal ablation dosimetry for vas occlusion without skin burns. A 2-rad ultrasound transducer mounted on a vasectomy-clip-delivered ultrasound energy at 4 MHz to the canine vas deferens co-located at the focus between the clip jaws. Chilled degassed water was circulated through an 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 deferens (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 simulations modeling the effects of variations in power and sonication time on tissue temperatures and coagulation zones. Active skin cooling produces a thermal gradient in the tissue during ablation, allowing sufficient thermal doses to be delivered to the vas without skin burns. However, low-power, long-duration heating produced excessive tissue necrosis due to thermal diffusion, while high power and short heating times reduced the therapeutic window and produced skin burns presumably due to direct ultrasound absorption. In conclusion, both simulations and experiments suggest that a therapeutic window exists in which thermal occlusion of the vas may be achieved without the formation of skin burns in the canine model (power=5-7 W, surface intensity=1.4-1.9 W/cm/sup 2/, time=20-50 s). This range of ablation parameters will help guide future experiments to refine incisionless vasectomy using focused ultrasound.