Influence of ribs on the nonlinear sound field of therapeutic ultrasound.

This article theoretically and experimentally studies the influence of ribs on the nonlinear sound field in ultrasound thermal therapy. A three-dimensional (3-D) numerical algorithm is developed to solve the Khokhlov-Zabolotaskaya-Kuznetsov equation in the frequency domain for description of the asymmetrical nonlinear ultrasound field. Two kinds of rib configurations are used to simulate the typical situations in the therapeutic treatment of liver tissues. Both numerical simulations and experimental measurements are made to evaluate the effect of the rib cage on the nonlinear ultrasound field. Results indicate that (1) when the ultrasound propagates through the space between the ribs, the position of the maximum amplitude of the fundamental component is shifted toward the transmitter surface up to 20 mm in some configuration; the maximum sound intensity can be either enhanced to 106% or decreased to about 20% compared with the value in the free field and the beam width in situ is affected by the sound diffraction; and (2) in the case of a rib positioned in the way of ultrasound propagation, the sound energy is significantly decreased and severe interference is observed. Satisfactory agreement between the theoretical prediction and measured results demonstrates the validity of the theoretical model and its solution.