It has been well known that the treatment time of high-intensity focused ultrasound (HIFU) surgery can be reduced by expanding the focal area per sonication. Previously, a dual-concentric transducer using phase-inverted signals was proposed to axially extend the focal area, but it has suffered from the deep notch point between two focal lobes. In this paper, we propose the improved HIFU transducer with dual-concentric aperture driven by phase-inverted multifrequency signals based on an inversion layer technique. The proposed transducer can generate the expanded focal zone with a significantly reduced level of the notch point between two focal lobes in the axial direction. The performance of the proposed transducer was investigated using finite element analysis simulation. The electrical impedance, one-way impulse response, and acoustic field of the transducer were simulated. Subsequently, the lesion volume was investigated by heat transfer simulation. In the proposed method, the level of the notch point was increased above -6 dB due to various phase interactions between the fundamental and harmonic frequency combinations and the inverted and noninverted frequency combinations. The -6-dB depth of field related to the necrotic lesion size was increased by 141% compared with the conventional single element transducer. Thus, the proposed transducer can be a potential way to enlarge coagulated lesion size resulting in a reduced overall treatment time of HIFU surgery.
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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