ULTRASONIC IMAGING OF THE NONLINEAR PARAMETERS B/A

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The long term objective of this application is to develop a prototype real-time ultrasonic system to image the nonlinear parameter B/A for research purposes. The nonlinearity parameter B/A is a measure of the degree to which the medium, supporting the ultrasonic wave, is nonlinear relative to the mechanical strain produced in response to applied mechanical stress. The B/A parameter reflects properties of the medium largely not observed by other imaging methods, viz., it appears to be sensitive to changes in the structural hierarachy of tissues and to the macromolecular contents. Thus, pathological conditions involving alteration in tissue architecture and changes in large molecular weight constituents would provide images distinguishable from those of the normal state and different other imaging schemata. The nonlinear acoustic interaction between a reflected probe signal and a broadband pump waveform propagating in the opposite direction, produces phase changes in the probe proportional to B/A in the spatial region of interaction. The instantaneous phase change along the received probe is the convolution of the pump waveform with the spatial distribution of B/A along the interaction path. The phase modulated probe can be processed to produce a line image of B/A. Multiple reflections along the propagation path and the need for pump waveforms with sufficient energy over a wide range of wavelengths are two problems to be dealt with in the study.