Abstract

High-intensity focused ultrasound (HIFU) has widespread applications not only for various cancer diagnosis but also for the treatment. It is also a relatively new noninvasive and painless cosmetic treatment procedure for skin lifting and tightening. In this method, complete necrosis of the abnormal cells can be realized by concentrating the ultrasound beam in a small targeted area which results in a rapid temperature rise in the focal region. Since the ultrasound beams are focused on a specific tissue region, negligible disruption has occurred to nearby healthy tissue. The current model aims to determine the effects of varying transducer frequency over the thermal response in the focal zone of a tissue phantom with multi-physics modelling. Helmholtz pressure acoustic equation, Pennes' bio-heat equation and Stefan Boltzmann equation are used as the governing equations for this investigation and are solved by using Galerkin weighted residual finite element method. First, the transducer is kept active for 1 second, and transient analysis has been done up to 5 min. Results of these investigations are shown via the surface plot of temperature throughout the domain for 3 different frequencies. The maximum temperature rise is obtained for 3 MHz frequency which is about 1 K. Further simulation is done by keeping the transducer turned on for a prolonged period with two different frequencies. In that case, almost 41 K temperature rise is recorded which is sufficiently high for the permanent destruction of the abnormal cells. Also, it is observed that in general, higher temperature rise can be obtained with increased source frequency, but at a substantially higher frequency, some acoustic energy can penetrate the obstruction imposed by the perfectly matched layers (PML) resulting in the loss of acoustic energy.