Numerical dosimetry at power-line frequencies using anatomically based models

Abstract

We have used the finite-difference time-domain (FDTD) method to calculate induced current densities in a 1.31-cm (nominal 1/2 in) resolution anatomically based model of the human body for exposure to purely electric, purely magnetic, and combined electric and magnetic fields at 60 Hz. This model based on anatomic sectional diagrams consists of 45,024 cubic cells of dimension 1.31 cm for which the volume-averaged tissue properties are prescribed. It is recognized that the conductivities of several tissues (skeletal muscle, bone, etc.) are highly anisotropic for power-line frequencies. This has, however, been neglected in the first instance and will be included in future calculations. Because of the quasi-static nature of coupling at the power-line frequencies, a higher quasi-static frequency f' may be used for irradiation of the model, and the internal fields E' thus calculated can be scaled back to the frequency of interest, e.g., 60 Hz. Since in the FDTD method one needs to calculate in the time domain until convergence is obtained (typically 3-4 time periods), this frequency scaling to 5-10 MHz for f' reduces the needed number of iterations by over 5 orders of magnitude. The data calculated for the induced current and its variation as a function of height are in excellent agreement with the data published in the literature. The average current densities calculated for the various sections of the body for the magnetic field component (H) are considerably smaller (by a factor of 20-50) than those due to the vertically polarized electric field component when the ratio E/H is 377 ohms. We have also used the previously described impedance method to calculate the induced current densities for the anatomically based model of the human body for the various orientations of the time-varying magnetic fields, namely from side to side, front to back, or from top to bottom of the model, respectively.