Cavitation bubbles have much potential for emphasizing therapeutic treatments such as high-intensity focused ultrasound (HIFU) treatment, histotripsy, and sonodynamic therapy. Their highly efficient as well as controlled generation is important to utilize them effectively as well as safely. However, producing negative pressure over the cavitation threshold by focused ultrasound is difficult because of the nonlinear propagation combined with the focal phase shift. We have suggested a dual-frequency ultrasound exposure method, in which N- and P-waves emphasizing either the peak negative or positive pressure, respectively, are synthesized by superimposing the second harmonic onto the fundamental frequency. In this study, high-speed camera observation demonstrated that the exposure with N-waves immediately followed by P-waves could generate cavitation bubbles most efficiently in gel phantom. Furthermore, the measured negative and positive pressure distributions of the N- and P-wave fields, respectively, agreed well with the optically observed distributions of cavitation inception and cavitation cloud growth.
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