Optimizing the shape of ultrasound transducers for interstitial thermal ablation

Heat deposition by interstitial routes, especially with ultrasound-based instruments, is becoming a valuable therapeutic option for the treatments of sites, which are difficult to access from outside of the body. The active part of most interstitial ultrasound applicators described in the literature is logically tubular to induce cylindrical volumes of coagulation necrosis. Because the pressure generated by such tubular transducers falls off rapidly with radial distance, we previously proposed using a rotating plane transducer. For a plane wave, the pressure fall-off is only due to attenuation, which makes deeper lesions and shorter treatment times possible. This work represents an advance in the development of ultrasound applicators designed for interstitial applications. This new applicator used a rotating slightly focused transducer. A brief theoretical analysis resulted in the choice of a long focal distance of 22 mm to obtain a nearly constant pressure all along the therapeutic depth. To experimentally validate this focal distance, pressure measurements were made in a tissue mimicking liquid phantom and the results were compared with those obtained with a plane transducer. In vitro experiments showed that necrosis could be induced at a depth of 15 mm. In the same conditions, the greatest depth attained with a plane transducer was only 10 mm. Because each individual lesion is narrower, more lesions and more time are required to necrose a cylindrical volume. The main advantage of this new type of applicator is that it can be used to induce necrosis at a greater depth without varying either the frequency, the intensity or the transducer cooling efficiency.