ultrasound applicators with internal watercooling for highpowered interstitial thermal therapy internal watercooling of directcoupled ultrasound us applicators for interstitial thermal therapy hyperthermia and coagulative thermal therapy was investigated implantable applicators were constructed using tubular us sources 360 angular acoustic emittance approximately 7 mhz of 10 mm length and 15 18 22 and 25 mm outer diameter od directional applicators were also constructed using 22 mm od tubes sectored to provide active acoustic sectors of 90 degrees and 200 degrees a watercooling mechanism was integrated within the inner lumen of the applicator to remove heat from the inner transducer surface high levels of convective heat transfer 21003800 wm2k were measured for practical water flow rates of 2080 mlmin comparative acoustic measurements demonstrated that internal watercooling did not significantly degrade the acoustic intensity or beam distribution of the us transducers watercooling allowed substantially higher levels of applied electrical power 45 w than previous designs with aircooling or no cooling without detriment to the applicators hightemperature heating trials performed with these applicators in vivo porcine liver and thigh muscle and in vitro bovine liver showed improved thermal penetration and coagulation radial depth of coagulation from the applicator surface ranged from 12 to 20 mm for 15 min of sonication with 28w applied power higher powers 41 w demonstrated increased coagulation depths approximately 9 mm at shorter times 15 s thermal lesion dimensions angular and axial expanse produced with directional applicators were controlled and directed and corresponded to the active zone of the transducer these characteristic lesion shapes were also generally unchanged with different sonication times and power and were found to be consistent with previous coagulation studies using aircooled applicators the implementation of watercooling is a significant advance for the application of ultrasound interstitial thermal therapy usitt providing greater treatment volumes shorter treatment times and the potential for treatment of highly perfused tissue with shaped lesions