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Nonlinear modeling of 3D ultrasound fields of HIFU arraysтезисы доклада
- Авторы: Yuldashev P.V., Khokhlova V.A.
- Сборник: Book of Abstracts of the 19th International Symposium on Nonlinear Acoustics (May 21-24, 2012)
- Тезисы
- Год издания: 2012
- Место издания: Tokyo, Japan
- Первая страница: 63
- Последняя страница: 63
- Аннотация: A current trend in HIFU medical technologies is to use 2D focused phased arrays that enable electronic steering of the focus, formation of patterns of multiple foci, and beam forming to avoid overheating of obstacles (such as ribs) on the way to the focus. Phased arrays can be also used to improve focusing through inhomogeneities of soft tissue using time reversal methods and to track a treatment region that moves due to respiration. In many HIFU applications, the acoustic intensity in situ can reach thousands of W/cm2 which causes nonlinear propagation effects. At high power outputs, shock fronts develop in the focal region and significantly alter the therapeutic effect of HIFU impact on tissue. Acoustic characterization of array fields, particularly at high intensities, is therefore important for the development of HIFU devices. Numerical modeling has been proven to be an important tool to characterize HIFU fields, and to predict corresponding biological effects in tissue. However, modeling of 3D nonlinear fields of therapeutic arrays remains a difficult problem and up to day, most of the results have been obtained only for axisymmetric single element transducers. Difficulties in modeling of 3D nonlinear fields have following reasons. The first is a complex structure of near field and high focusing angles that impose to use precise diffraction models and fine resolution of spatial grid. The second is a strong acoustic nonlinearity in the focus, so high resolution of temporal gird or big number of harmonics in spectral representation must be used. Finally, the 3D geometry of the problem is a difficulty itself. All these factors lead to huge demands in memory size and to long computation time even using modern SMP (shared memory) computers. In this work a numerical algorithm to model nonlinear fields of a therapeutic array in the presence of high amplitude shock was developed. The algorithm is based on the Westervelt equation simplified for the case of forward propagating waves. Numerical solution was obtained using second order accuracy splitting algorithm in which different physical effects (nonlinearity, diffraction, and absorption) are applied sequentially at each step. A spectral representation of acoustic field was chosen to storage the numeric data. Higher harmonics were stored in smaller area than lower harmonics in order to sufficiently reduce memory consumption. Diffraction of each harmonic was calculated using angular spectrum approach. Both high angle and parabolic approximation of diffraction operator are included. Two algorithms were implemented to calculate nonlinear effects. One is a direct harmonic integration for smooth waveforms, and second is a Godunov-type algorithm working in time domain for waveforms with shock fronts. The algorithm was successfully validated using the nonlinear field of single focused element in the parabolic approximation as a benchmark solution, which data was obtained from other papers where axisymmetric sources were considered. To demonstrate capabilities of the developed algorithm, simulations were performed for a 1.2 MHz array consisting of 256 elements of 6.6 mm diameter, randomly distributed on a spherical cup of 68 mm radius and 120 mm focal distance. Acoustic intensity near the elements varied up to 10 W/cm2. Waveforms in the focus and peak positive and negative pressures along propagation axis and in the focal plane were analyzed. The results showed that shock fronts were present in a focal waveform at clinically relevant outputs. [Work supported by NIH EB007643, RFBR 09-02-01530 and 10-02-91062-PICS].
- Добавил в систему: Хохлова Вера Александровна