  Objective: High-intensity focused ultrasound (HIFU) therapy can be used for
non-invasive treatment of kidney (renal) cancer, but the clinical outcomes have
been variable. In this study, the efficacy of renal HIFU therapy was studied
using nonlinear acoustic and thermal simulations in three patients. Methods:
The acoustic simulations were conducted with and without refraction in order to
investigate its effect on the shape, size and pressure distribution at the
focus. The values for the attenuation, sound speed, perfusion and thermal
conductivity of the kidney were varied over the reported ranges to determine
the effect of variability on heating. Furthermore, the phase aberration was
studied in order to quantify the underlying phase shifts using a second order
polynomial function. Results: The ultrasound field intensity was found to drop
on average 11.1 dB with refraction and 6.4 dB without refraction. Reflection at
tissue interfaces was found to result in a loss less than 0.1 dB. Focal point
splitting due to refraction significantly reduced the heating efficacy.
Perfusion did not have a large effect on heating during short sonication
durations. Small changes in temperature were seen with varying attenuation and
thermal conductivity, but no visible changes were present with sound speed
variations. The aberration study revealed an underlying trend in the spatial
distribution of the phase shifts. Conclusion: The results show that the
efficacy of HIFU therapy in the kidney could be improved with aberration
correction. Significance: A method is proposed by which patient specific
pre-treatment calculations could be used to overcome the aberration and
therefore make ultrasound treatment possible.
