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Microbubbles insonified by ultrasound show nonlinear oscillation and reflect waves including harmonic and subharmonic signals. Subharmonic imaging can be more useful, though occurrence conditions of subharmonics are not clarified enough. In this study, we investigated effects of microbubble interaction on occurrence of subharmonics. The interaction depends on the distance between two microbubbles and their radius ratios. For different values of these parameters, we numerically calculated the Rayleigh-Plesset equations with these interaction terms added and investigated occurrence conditions of subharmonics. Obtained results show that, by decreasing the distance between the bubbles and increasing the radius of one of the bubbles, the threshold of the pressure amplitude and the transmit frequency for generating subharmonics become lower.

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Microbubbles are expected to be applied for ultrasonic therapy. In this research, considering two neighboring bubbles, we added to the Rayleigh-Plesset equation the term of nonlinear influence corresponding to pressure change caused by the neighboring bubble’s oscillation, and numerical simulation was performed. It is known that the natural frequency of a microbubble decreases with increasing the bubbles’ density. This fact agrees with our analytical prediction based on the Rayleigh-Plesset equation. Further, the natural frequency also depends on the diameter ratio of the two bubbles. Our numerical results show that superharmonic response reaches a peak at some distance between bubbles when they are driven at half their resonance frequency with their ratio of the natural frequency being two to one. Numerical simulation also shows that if the two bubbles of the same size exist at a close distance, the occurrence region of the subharmonics is larger than that of a single bubble.

Recently, ultrasound contrast imaging has been improved greatly, because its application of an approach for therapy is urgently required for area of the heart and abdominal. Sonoporation is also one of the applications using microbubbles that attracts attention in the world. There have been so many studies for understanding the mechanism of sonoporation and improving efficiency of ultrasound contrast agents. Nonspherical oscillation is at a stage before the breakup process of microbubbles. Therefore, consideration of a microbubble's nonspherical oscillation is absolutely imperative for its future development. Thus in this thesis, the author regarded microbubbles as two dimensional air-liquid two-phase flow and calculated their dynamics by using Computational Fluid Dynamics (CFD). Numerical calculation of a microbubble's oscillation and deformation was also carried out, for an air bubble and for a shell-coated bubble in ultrasound field. As a result, it was confirmed that the CFD method developed in this study can be useful for analyzing an insonified microbubble showing nonspherical oscillation.