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Noninvasive vasectomy using a focused ultrasound clip: thermalmeasurements and simulations
Fried, N.M.
Sinelnikov, Y.D.
Pant, B.B.
Roberts, W.W.
Solomon, S.B.
Dept. of Urology, Johns Hopkins Univ. Sch. of Med., Baltimore, MD;
This paper appears in: Biomedical Engineering, IEEE Transactions on
Publication Date: Dec 2001
Volume: 48,
Issue: 12
On page(s): 1453-1459
ISSN: 0018-9294
References Cited: 20
CODEN: IEBEAX
INSPEC Accession Number: 7124318
Digital Object Identifier: 10.1109/10.966604
Posted online: 2002-08-06 23:59:24.0
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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/cm2,
time=20-50 s). This range of ablation parameters will help guide future
experiments to refine incisionless vasectomy using focused ultrasound
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