Theoretical and experimental high-frequency nonlinear ultrasound propagation through multilayered media

We present a new model of nonlinear ultrasound propagation through multilayered liquid/tissue media. This model includes the effects of diffraction, attenuation, and nonlinearity, with refraction and energy conservation at layer boundaries. It is capable of simulating pulsed and continuous wave propagation from sources of arbitrary geometry and excitation. Using this model, the acoustic field of a high-frequency circular focused transducer (1.5 mm aperture radius, f# 2.5) was simulated for two configurations, water only, and water-tissue phantom-water, and compared to the field measured with a high-frequency needle hydrophone, for 10 different source amplitudes of a transmitted Gaussian pulse (f₀=20 MHz, fractional bandwidth=35%), ranging from 0.05 to 1 MPa. Very good agreement was found between simulated and measured fields in terms of relative amplitudes of the fundamental, second and third harmonics, beam widths and depth of fields, given the uncertainty in the experimental transmitted pulse amplitudes and hydrophone sensitivity above 40 MHz. Our model is capable of simulating realistic finite-amplitude propagation from high-frequency transducers through multilayered biological media. Its remarkable accuracy and efficiency makes it a very useful tool for the study of nonlinear ultrasonic fields, its well as for transducer design optimization.