Recently, high intensity focused ultrasound (HIFU) has been used for non-invasive surgery of prostate, uterus, and brain. However, a HIFU therapy is suffered from relatively long surgical time mainly due to small focal area per each sonication. In order to solve this problem, a HIFU therapy using multi-frequency was suggested by several researchers, and they demonstrated that this technique can increase the area of the coagulated lesion due to enhanced cavitation effect compared to single-frequency HIFU [1–3]. To generate multi-frequency especially dual-frequency, dual-element and dual-layer HIFU transducers have been developed and provided an expanded lesion size [1–3]. In this study, we present an alternative technique of making dual-frequency HIFU transducer using inversion layer technique. Generally, a single layer piezoelectric element can excite the strong fundamental resonance (f0) and the weak odd-order harmonic resonance (3f0) [4]. In the inversion layer technique, on the other hand, a piezoelectric component consisting of two piezo-ceramic plates bonded together with opposite poling directions and different thicknesses can produce the relatively strong even-order harmonic (2f0) in addition to the fundamental resonance [5]. Additionally, only a pair of electrode at the outside of the each piezo-ceramic plate is required to stimulate dual-frequency ultrasound while two pairs of electrodes are typically required for conventional dual-element and dual-layer transducers [2,3]. A specially designed prototype HIFU transducer was built, and we verified that the dual-frequency ultrasound was successfully generated through electrical impedance and pulse-echo response measurements.

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