Fundamental Ultrasonic Wave Research Articles

Low-Frequency Elastic Wave Imaging by Adaptive Combination of Fundamental and Tissue Harmonic Ultrasonic Waves

An ultrasonic imaging of a low-frequency elastic wave which propagates inside the soft tissue-like medium is proposed. This method uses both the fundamental ultrasonic wave and the tissue harmonics ultrasonic wave which is produced by the nonlinear characteristic of the medium in order to reconstruct the wavefront of the low-frequency vibration. The accuracy of elastic wave velocity estimation of the proposed method is evaluated by numerical analysis. Examples of two-dimensional maps which show the elastic wave propagation inside the medium are shown.

Nonlinearity Considerations in Bragg Imaging of Ultrasonic Waves

Two fundamentally different techniques exist for the study of the diffraction of light by finite amplitude ultrasonic waves: (1) Diffraction of light at normal incidence can be used to determine the ultrasonic waveform. (2) Diffraction of light incident at the Bragg angle gives detailed information of a completely different nature. Each diffraction order contains an image of the ultrasonic wavefront. Therefore, objects placed within the ultrasonic field are imaged in the diffraction orders. When a finite amplitude ultrasonic wave is used, an ultrasonic wave transmission filter allows one to distinguish the images produced by the second harmonic from those produced by the fundamental. The image produced by the second harmonic is twice as large as that produced by the fundamental and appears at the position of the fundamental's second order. In addition, at large amplitudes, multiple scattering of light by the fundamental ultrasonic wave can produce higher Bragg diffraction orders. These higher orders contain multiple images. Two images appear in the second order, three in the third, etc. [Research supported by the Office of Naval Research.]

Bragg Imaging with Finite Amplitude Ultrasonic Waves

Light passing through ultrasonic waves of finite amplitude at the angle at which Bragg diffraction is dominant produces diffraction orders which contain images of the ultrasonic wavefront, with multiple images appearing in the higher orders. These multiple images are attributed to multiple scattering of light by the fundamental ultrasonic wave. The harmonics of the ultrasonic wave arising from nonlinearity of the propagating medium are isolated by use of a transmission filter plate. The images produced by them are larger than those produced by the fundamental.