Theoretical and experimental design of site-specific applicators and heating protocols for interstitial ultrasound thermal therapy

Abstract

Theoretical and experimental approaches were used to develop and evaluate site-specific designs of internally cooled direct coupled (ICDC) and catheter-cooled (CC) ultrasound applicators for thermal coagulation of disease in the prostate, liver, brain, and uterus. The diameter of an interstitial applicator can influence its clinical practicality and effectiveness as well as application site. One purpose of this study was to determine whether the use of larger ultrasound transducers and the inherent increase in applicator size could be justified by potentially producing larger lesion diameters. A second purpose was to explore how the response of tissue acoustic attenuation to heating effects lesion size and preferred applicator configuration. Four applicator configurations and sizes were studied using ex vivo tissue experiments in liver and beef and using acoustic and biothermal simulations. Transmission attenuation measurements showed a 6 to 8 fold increase in baseline tissue attenution inside interstitial ultrasound lesions. Formation of these high attenuation zones in lesions reduced potential lesion size. Larger applicators produced lesions with radial penetration depths superior to their smaller counterparts at power levels in the 20-40W /cm range. The higher cooling rates along the outer surface of the larger diameter applicators due to their greater surface area was a dominant factor in increasing lesion size. The higher cooling rates pushed the maximum temperature farther from the applicator surface and reduced the formation of high acoustic attenuation tissue zones. Acoustic and biothermal simulations matched the experimental data well and were applied to model these applicators within sites of clinical interest such as prostate, uterine fibroid, brain, and normal liver. Lesions of 3.9 to 4.7cm diameter were predicted for moderately perfused tissues such as prostate and fibroid and 2.8 to 3.2cm for highly perfused tissues such as normal liver. Feedback control to reduce maximum tissue temperatures helped to decrease formation of sound-blocking high attenuation zones. This work was supported by a gift from the Oxnard Foundation and Johnson & Johnson.