Effect of Nonlinear Wave Propagation on Temperature Rise in High Frequency Imaging

The existing safety standards for diagnostic ultrasound devices were developed for low frequency systems (1 to 10 MHz) under the assumption that nonlinear effects are negligible. However, the standards are now being used for high frequency systems (up to 70 MHz) while nonlinear effects are increasingly evident as the working frequency is increased. The goal of this study is to determine if the coverage of nonlinear effects in safety standards in terms of thermal effects is adequate for high frequency systems (>10 MHz). The investigated cases are marginally acceptable according to current safety standards. The safety of each case is then re-assessed after including nonlinear effects in numerical simulations. The thermal results show that strong higher order harmonics can significantly increase the local heat deposition rate. However, at the same time, they lead to an increase in the rate of axial and radial heat conduction, thereby reducing the net impact on the steady-state temperature rise. Also, the effect of higher order harmonics can be significant on the temperature rise at the focal point, while the maximum temperature rise always occurs close to the skin surface. We conclude that current safety standards are adequate in assessing thermal effects in B-mode imaging at high frequencies despite ignoring the extra heat deposited by higher order harmonics.