Numerical simulation of the enhanced heat production in tissue due to the nonlinear character of high-intensity focused ultrasound (HIFU)

Abstract

HIFU is used in thermotherapy to destroy tumors located deep in human tissue by heat. It is well known that HIFU is connected with the generation of higher-frequency components due to nonlinear steepening during propagation. An over-proportional heat generation in tissue occurs with enhanced power input. To obtain fundamental information about an optimal dosage of the applied ultrasound, simulations are necessary. A hot spot in healthy tissue must be avoided and a sufficient heat delivery in the tumor region must be guaranteed. Until now most numerical investigations of thermotherapy with HIFU have been carried out under the assumption of linearity. This means an essential underestimation of heat production. Here an improved model is introduced which enables accurate simulations, including nonlinearities. It consists of a set of nonlinear acoustic equations which ensure an accurate full-wave propagation modeling. A broadband absorption model of typical soft-tissue frequency-power-law character is included to obtain a nonlinear modeled heat source. The thermal behavior is described by a bio-heat-transfers-equation. The complete model is solved by means of a high-order FDTD scheme. Exemplary simulations illustrate the efficiency of this method.