Ultrasound applicators with internal cooling for interstitial thermal therapy

The use of internally-cooled, direct-coupled interstitial ultrasound applicators as a means of providing controlled and directed thermal therapy was investigated. Applicators were constructed using tubular ultrasound sources (1.5 - 2.5 mm OD) with active acoustic zones of 90 degree(s), 200 degree(s), 270 degree(s), and 360 degree(s) (single and multiple transducers). Cooling of the inner transducer surface was accomplished by the flow of chilled air or an integrated water mechanism. Thermal performance of the applicators was characterized through high temperature heating trials in vivo (porcine thigh muscle and liver) and in vitro (bovine liver). Both air-cooled and water-cooled applicators produced well- defined angular directional heating, with coagulated zones corresponding to the active sector of the transducer. Axial collimation and control of heating along the length of the applicator was also demonstrated using multiple transducer elements. Thermal penetration and extent of coagulation was reproducible and controlled with sonication time and power, extending radially 12 - 22 mm for 1 - 5 minutes. Directly of lesion shapes (both angular and axial) was found to remain characteristically similar at different heating times. This enhanced thermal penetration and improved control of directional heating with internal cooling shows great potential for treatment of localized tumors in prostate, brain, and liver.