Evaluation of multielement catheter-cooled interstitial ultrasound applicators for high-temperature thermal therapy

Catheter-cooled (CC) interstitial ultrasound applicators were evaluated for their use in high-temperature coagulative thermal therapy of tissue. Studi es in ex vivo beef muscle were conducted to determine the influences of app lied electrical power levels (5-20 W per element), catheter flow rate (20-6 0 ml min(-1)), circulating water temperature (7-40 degreesC), and frequency (7-9 MHz) on temperature distribution and thermal lesion geometry. The fea sibility of using multiple interstitial applicators to thermally coagulate a predetermined volume of tissue was also investigated. Results of these st udies revealed that the directional shape of the thermal lesions is maintai ned with increasing time and power. Radial depths of the thermal lesions ra nged from 10.7+/-0.7 mm after heating for 4 min with an applied power level of 5 W. to 16.2+/-1.4 mm with 20 W. The axial length of the thermal lesion s is controlled tightly by the number of active transducers. A catheter flo w rate of 20 to 40 ml min(-1) (52.2+/-5.5 kPa at 40 ml min(-1)) with 22 deg reesC water was determined to provide sufficient cooling of the transducers for power levels used in this study. In vivo temperatures measured in the center of a 3-cm-diam peripheral implant of four applicators in pig thigh m uscle reached 89.3 degreesC after 4 min of heating, with boundaries of coag ulation clearly defined by applicator position and directivity. Conformabil ity of heating in a clinically relevant model was demonstrated by inserting two directional CC applicators with a 2 cm separation within an in vivo ca nine prostate, and generating a thermal lesion measuring 3.8 cmx2.2 cm in c ross section while directing energy away from, and protecting the rectum. M aximum measured temperatures at midgland exceeded 90 degreesC within 20 min of heating. The results of this study demonstrate the utility of single or multiple CC applicators for conformal thermal coagulation and high tempera ture thermal therapy, with potential for clinical applications in sites suc h as prostate, liver, breast, or uterus. (C) 2001 American Association of P hysicists in Medicine.