| Research Abstract |
The purpose of the present work is to realize an imaging system for the distribution of the acoustic nonlinearity parameter B/A in a small volume sample such as biological media utilizing a high frequency focused sound. The previously presented B/A measurement method was modified to be applicable to high frequencies, and its validity was experimentally demonstrated at relatively low frequencies first. For the modified method, a new focusing transducer was experimentally produced utilizing the partial inversion of polarization by heat treatment of LiNbO_3 plate in addition to the appropriately designed electrode pattern. By adapting a 30-point asteroid electrode twisted according to the radial distance with 36 deg maximum at the outer fringe, a Gaussian beam to facilitate the analysis can be radiated. The sending and receiving sensitivities with a Gaussian distribution with regard to the radial distance were obtained for the second order resonance as well as the first order resonance. B
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onding to the solid acoustic lens, a focused Gaussian beam was produced by this piezoelectric plate. Utilizing the transducer, the imaging system for the fundamental and second harmonic sounds was constructed. The sound wave emanated from the transducer set on an x-y stage was two dimensionally scanned, for instance, by 20x20 steps with an interval of 0.15 mm on the sample, which was closely set by vacuum suction on the optically polished end-surface of a tungsten rod. The sound transmitting through the sample before and after the reflection at the rod end was received. When the 3x3-mm^2 area of biological samples with 1-mm thickness was imaged, the amplitude of nonlinearly generated second harmonic sometimes differed in spite of the same fundamental amplitude. This suggests non-uniform distribution of the B/A value. Since the present system is not complete yet, the imaging of B/A must be realized by the reform. On the other hand, the imaging utilizing nonlinear component of the sound can be quite different from the images of linear component. This difference was discussed by analyzing the propagation of second harmonic sound, taking into consideration the enhanced nonlinearity and diffraction effect of the focused sound. The result showed that the phase shift accompanied by the diffraction could lead to the different images of the second harmonic sound from that of the fundamental. Less
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