Nonlinear derating of high-intensity focused ultrasound beams using Gaussian modal sums

Abstract

A method is introduced for using measurements made in water of the nonlinear acoustic pressure field produced by a high-intensity focused ultrasound transducer to compute the acoustic pressure and temperature rise in a tissue medium. The acoustic pressure harmonics generated by nonlinear propagation are represented as a sum of modes having a Gaussian functional dependence in the radial direction. While the method is derived in the context of Gaussian beams, final results are applicable to general transducer profiles. The focal acoustic pressure is obtained by solving an evolution equation in the axial variable. The nonlinear term in the evolution equation for tissue is modeled using modal amplitudes measured in water and suitably reduced using a combination of "source derating" (experiments in water performed at a lower source acoustic pressure than in tissue) and "endpoint derating" (amplitudes reduced at the target location). Numerical experiments showed that, with proper combinations of source derating and endpoint derating, direct simulations of acoustic pressure and temperature in tissue could be reproduced by derating within 5% error. Advantages of the derating approach presented include applicability over a wide range of gains, ease of computation (a single numerical quadrature is required), and readily obtained temperature estimates from the water measurements.

Acoustic holography as a metrological tool for characterizing medical ultrasound sources and fields

“…[21][22][23] Pressures and intensities inferred from these measurements are then derated to estimate in situ values that account for acoustic propagation in tissue rather than water. 24,25 In practice, this basic approach can produce incomplete or erroneous results. Collecting measurements throughout a 3D volume is often impractical, making hydrophone measurements at the high-pressure focus may not be feasible, and using typical derating schemes may not adequately account for nonlinear propagation effects.…”

Enhanced Heat Transfer and Thermal Dose Using Magnetic Nanoparticles During HIFU Thermal Ablation—An In-Vitro Study

“…In order to ensure safety and efficacy of HIFU devices, preclinical evaluations of the thermal and acoustic field generated by the HIFU transducers are necessary. Preclinical testing of HIFU systems has been performed using computational modeling [3][4][5][6][7], ex-vivo [8][9][10][11] or in-vivo [12][13][14][15] animal tissues, and tissue phantoms [16][17][18][19]. The advantage of using phantoms with tissue mimicking material (TMM) is that such test sections can be reused repeatedly without affecting acoustical and thermal properties, which can be made similar to that of human tissue [20].…”