Development of high intensity focused ultrasound simulator for large-scale computing

High intensity focused ultrasound (HIFU) has been developed as a noninvasive therapeutic option. HIFU simulations are required to support the development of the HIFU device as well as the realization of noninvasive treatments. In this study, an HIFU simulator is developed that uses voxel data constructed from computed tomography scan data on the living human body and signed distance function (SDF) data to represent the object. The HIFU simulator solves the conservation equations of mass and momentum for mixtures with the equation of state for each medium. The numerical method is the finite-difference time-domain method. A high-order finite-difference method based on Lagrange interpolation is implemented to reduce numerical phase error. This approach reproduces wave propagation to an nth order of accuracy. Representation of the sound source by volume fraction, which is obtained from the SDF using a smoothed Heaviside function, provides around 1.66th order of accuracy in the spherical wave problem. As a realistic application, transcranial HIFU therapy for a brain tumor is modeled, where tissue inhomogeneity causes not only displacement of the focal point but also diffusion of the focused ultrasound. Even in such cases, focus control using phase delays, which are pre-computed based on the time-reversal procedure, enables correct focal point targeting as well as improved ultrasound focusing.