Generation of Nonlinear Signals by Rectangular Ultrasound Sources in Biological Media

V.A.Khokhlova, A.E.Ponomarev (Department of Acoustics, Faculty of Physics, Moscow State University, Moscow, Russia); M.A.Averkiou (Philips Ultrasound, Bothell, WA, USA); L.A.Crum (Center for Industrial and Medical Ultrasound, Applied Physics Laboratory, University of Washington, Seattle, WA, USA)

e-mail: vera@acs366b.phys.msu.su

A numerical solution of the KZK-type parabolic nonlinear evolution equation is presented for finite-amplitude sound beams radiated by rectangular sources.  The initial acoustic waveform is a short tone burst, similar to those used in diagnostic ultrasound.  The generation of nonlinear signals (higher harmonic components) and their spatial structure are investigated for media similar to tissue with various frequency dependent absorption properties.  Nonlinear propagation in a thermoviscous fluid with quadratic frequency law of absorption is compared to that in tissue with nearly linear frequency law of absorption.  The algorithm is based on that used by Lee and Hamilton [JASA, 1995; 97: 906-917] to model circular sources, which was recently generalized for two dimensional sources without axial symmetry [JASA, 1999; 105: 1208].  The diffraction integral is adapted in the time-domain for two dimensions with the implicit backward finite difference (IBFD) scheme in the near field and with the alternate direction implicit (ADI) method at longer distances.  Arbitrary frequency dependence of absorption is included in this model and solved in the frequency domain using FFT technique.  The results of simulation may be used to better understand the nonlinear beam structure for tissue harmonic imaging in modern medical diagnostic scanners. Work was supported by CRDF and RFBR.


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