Effects of phase aberration on tissue heat generation and temperature elevation using therapeutic ultrasound

The effects of inhomogeneous propagation media on tissue heating patterns and steady-state temperature distributions are investigated analytically for continuous-wave therapeutic ultrasound using a random-phase screen model. Formulas for the statistical moments of the on-axis heat distribution, total heat deposited in a target volume, and axial temperature elevation are derived. Based on the statistical moments, we propose figures of merit to quantitatively assess therapeutic performance for a range of experimental parameters. The analysis relates statistical properties of the aberrator to those of heat generation in tissue. The mean on-axis heat distribution is substantially reduced in the focal zone, while its variance is increased at other axial positions. The amount of heat delivered to a target volume depends on the location of the target and the size of the volume. Compared to the results for homogeneous media, the greatest reduction in temperature increase occurs in the focal zone. Distortions in the temperature elevation patterns are greatest when the perfusion in tissue is large.