IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 48, NO. 12, DECEMBER 2001 1453 Noninvasive Vasectomy Using a Focused Ultrasound Clip: Thermal Measurements and Simulations Nathaniel M. Fried*, Yegor D. Sinelnikov, Bharat B. Pant, William W. Roberts, and Stephen B. Solomon Abstract—Introduction: 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. Methods: A 2-rad ultrasound transducer mounted on a vasec- tomy-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 bal- loon, 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 ( 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. Results: Active skin cooling produces a thermal gradient in the tissue during ablation, allowing sufficient thermal doses to be deliv- ered to the vas without skin burns. However, low-power, long-du- ration 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. Conclusions: 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 ca- nine model (power 5–7 W, surface intensity 1.4–1.9 W/cm , time 20–50 s). This range of ablation parameters will help guide future experiments to refine incisionless vasectomy using focused ultrasound. Index Terms—Ablation, temperatures, ultrasound, vas deferens, vasectomy. I. INTRODUCTION S URGICAL sterilization is currently the most common method of contraception in the United States [1]. Male sterilization (vasectomy) has a higher success rate, lower Manuscript received March 14, 2001; revised July 21, 2001. This work was supported by Transurgical, Inc. under an external research grant. Asterisk indi- cates corresponding author. *N. M. Fried is with the Department of Urology, Johns Hopkins Medical School, Baltimore, MD 21224 USA (e-mail: nfried@jhmi.edu). Y. D. Sinelnikov and B. B. Pant are with Transurgical, Inc., Setauket, NY 11733 USA. W. W. Roberts is with the Department of Urology, Johns Hopkins Medical School, Baltimore, MD 21224 USA. S. B. Solomon is with the Departments of Urology and Radiology, Johns Hop- kins Medical School, Baltimore, MD 21224 USA. Publisher Item Identifier S 0018-9294(01)10238-7. (a) (b) Fig. 1. (a) Diagram and (b) photograph of the experimental setup. The claws of the vasectomy clip were co-located with the vas deferens, and thermocouples were placed at the vas, intradermal (subsurface skin), and skin surface (at the skin-balloon interface). Ultrasound gel and the inflated balloon provided efficient coupling of energy into the tissue. Chilled, degassed water circulating through the balloon provided cooling of the transducer and skin surface. complication rate, is less expensive, and easier to perform than female sterilization (tubal ligation) [1], [2]. Despite these advantages, female sterilization is more commonly performed. Fear of complications related to surgical vasectomy was frequently cited as the reason for a couple choosing tubal ligation [1]–[3]. Presumably, these fears may be overcome if a noninvasive, safer, and faster method of vasectomy were to be developed. Previous studies have demonstrated that thermal ablation of the human vas deferens with electrocautery results in successful occlusion [4]. Preliminary work in our research group has also demonstrated that therapeutic focused ultrasound may be used for noninvasive or incisionless vasectomy [5], [6]. In these early studies, a hand-held ultrasound probe applied to the skin sur- face delivered focused ultrasound energy to the vas deferens. To 0018–9294/01$10.00 © 2001 IEEE