Eddy-Current Simulations Using an Independent Impedance Method in Anisotropic Biological Tissues

A widely applied method for simulating the induced eddy currents in biological tissues is the impedance method (IM). This method has recently been extended to the independent impedance method (IIM), which identifies and solves a linear system of independent equations, resulting in acceleration of computations. Both IM and IIM assume isotropic material properties, even though some tissues have directionally dependent characteristics. Therefore, we formulate IIM toward space-dependent anisotropic material properties. In this paper, the method is applied to the simulation of transcranial magnetic stimulation (TMS) for both isotropic and anisotropic spherical head models. The numerical results show that anisotropy has a nonnegligible effect on the induced currents, yielding maximum differences of about 40% in the skull and 19% in gray matter. The method is validated by comparing the results with finite element method (FEM) solutions. This study leads to a more realistic numerical modeling of the eddy currents due to the incorporation of anisotropy.