Nonlinear imaging based on Time Reversal Acoustic focusing

Abstract

The lack of success of nonlinear imaging with the use of interacting focused acoustic beams of different frequencies is connected with complex phase structure of acoustic field in the interaction area. This difficulty can be overcome using Time Reversal Acoustics (TRA) focusing because of its ability to concentrate acoustic energy in a small volume with the diameter close to a half wavelength. The proposed Nonlinear TRA imaging is based on scanning through the medium a spot composed by two concentric focused acoustic fields of different frequencies generated by two synchronized TRA focusing systems. Intensity of the combination frequency waves generated in the focal region provides information on nonlinear parameter of medium. The focal area acts as a monopole source for difference frequency waves and there is some fluctuation in the frequency dependence of radiated sound for the sum frequency wave, the radiation source is close to a monopole. Both theoretical modeling and experimental studies confirmed feasibility of the nonlinear TRA imaging. Experiment was conducted in a medium with spatially varying acoustical nonlinearity produced by contrast agents. An order of magnitude increase of the sum frequency component was observed when coinciding focused ultrasonic fields interacted in the focal area created by two synchronized TRA systems.