NONLINEAR MODELING OF THERAPEUTIC ULTRASOUND
We describe experimental finite element modeling of tissue ablation
by focused ultrasound. Emphasis is on nonlinear coupling of high
intensity sound, temperature, and tissue properties. The numerical
basis for modeling nonlinearity is an incrementally linear, timedomain,
finite element algorithm solving the electromechanical and
bioheat equations in 2D/3D inhomogeneous elastic and acoustic
media. Nonstandard modeling issues examined include harmonic
generation/absorption and focal “bubble” evolution with consistent
sound and thermal redistribution. The nonlinear pressure-density
relation generates harmonics that increase absorption and heating,
particularly in the focal zone. In the tissues modeled, harmonic
heating is negligible for peak focal intensities of a few kW/cm2.
As the focal hot spot ablates tissue it may also generate “bubbles.”
Prefocal growth of a bubbly region is modeled using a simple boiling
threshold and strong coupling between the scattered ultrasound and
temperature redistribution as the region spreads. Generally, these
experiments are intended to develop a more comprehensive
modeling basis for quantifying tissue ablation phenomenology.