Ultrasound applicators with internal water-cooling for high-powered interstitial thermal therapy

Internal water-cooling of direct-coupled ultrasound (US) applicators for in terstitial thermal therapy (hyperthermia and coagulative thermal therapy) w as investigated. Implantable applicators were constructed using tubular US sources (360 angular acoustic emittance, similar to 7 MHZ) Of 10 mm length and 1.5, 1.8, 2.2, and 2.5 mm outer diameter (OD), Directional applicators were also constructed using 2.2 mm OD tubes Sectored to provide active acou stic sectors of 90 degrees and 200 degrees. A water cooling mechanism was i ntegrated within the inner lumen of the applicator to remove heat from the inner transducer surface. High levels of convective heat transfer (2100-380 0 W/m(2)K) were measured for practical water flow rates of 20-80 mL/min, Co mparative acoustic measurements demonstrated that internal water-cooling di d not significantly degrade the acoustic intensity or beam distribution of the US transducers, Water-cooling allowed substantially higher levels of ap plied electrical power (>45 W) than previous designs (with air-cooling or n o cooling), without detriment to the applicators. High-temperature heating trials performed with these applicators in vivo (porcine liver and thigh mu scle) and in vitro (bovine liver) showed improved thermal penetration and c oagulation, Radial depth of coagulation from the applicator surface ranged from 12 to 20 mm for 1-5 min of sonication with 28-W applied power. Higher powers (41 W) demonstrated increased coagulation depths (similar to 9 mm) a t shorter times (15 s). Thermal lesion dimensions (angular and axial expans e) produced with directional applicators were controlled and directed, and corresponded to the active zone of the transducer, These characteristic les ion shapes were also generally unchanged with different sonication times an d power, and were found to be consistent with previous coagulation studies using air cooled applicators. The implementation of water cooling is a sign ificant advance for the application of ultrasound interstitial thermal ther apy (USITT), providing greater treatment volumes, shorter treatment times, and the potential for treatment of highly perfused tissue with shaped lesio ns.