3-D analysis of magnetic stimulation to human cranium

Magnetic stimulation is a useful tool for the nerve stimulation because of its non-invasive and painless characters. It is, however, difficult localize a stimulating point because the induced current is wide spread compared to electric stimulation. It is the disadvantage of the magnetic stimulation. In order to apply magnetic stimulation to a human body effectively, it is very important to investigate how the stimulation occurs in a human body, especially in a brain, from a point of engineering and medical view. Several homogeneous nerve models applied for magnetic stimulation have been proposed [1-4]. From these models, it is considered that the negative space differentiation of the electric field induced by magnetic stimulation, which is called the virtual cathode, is the main factor of depolarization the nerve fiber. The point of its maximum absolute value is known as the maximum stimulating point. Using an inhomogeneous volume conductor, Liu and Ueno proposed that when induced current flowed from a low conductivity volume conductor to a high conductivity one, the virtual cathode appeared at the interface [5]. In this study, we carried out 3-D analysis to calculate distribution of the electric field induced in a homogeneous volume conductor as the first step. In the second step, we constructed a new detailed human cranial model which was divided into 5 areas (skin, bone, muscle, eye and brain) and calculated the eddy current, electric field and virtual cathode after the model was stimulated with an 8-figure coil. In this model, each area was assumed to have different conductivity.