The development of therapeutic applications of ultrasound depends notably on the availability of high-performance transducers. New piezocomposite technologies offer performances that have proved to be particularly well adapted for such applications thanks to high power density generation with high efficiency. Moreover this technology enables a wide variety of shapes and the design of array transducers for electronic focusing, scanning and steering of the beam. This article details these advantages as well as other interests such as a large bandwidth or the MRI compatibility allowing the imaging / therapy association. Furthermore, the feasibility of highly focused transducers and complex array structures will be illustrated through various examples.
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.
This new method of intraductal tumor destruction by high-intensity US during ERCP is feasible and can induce objectively measurable tumor necrosis. Long-term follow-up will determine whether this method is curative in some cases and if it can reduce the need for biliary stent placement.
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