geometric complexity, and differing electrical properties. However, detailed anatomically based models of the hu-A hybrid finite-difference technique, for modelling the induction in compact isolated heterogeneous conductors by external low-man body at the millimeter resolution level are now feasifrequency time-harmonic electric fields, is described. A quasi-static ble, thanks to data from the Visible Human Project (http:// finite-difference time domain code is used to model the induction www.nlm.nih.gov/research/visible/visiblehuman.html) at in a low-resolution model of the conductor. In particular, this code the U.S. National Library of Medicine. Recently measured solves the external problem and yields the surface charge density conductivity data suitable for low frequencies are also now induced on the body. Although the internal fields are also computed, the surface charge density can be interpolated onto the surface of available [14]. The major limitation on whole-body induca higher-resolution model of the conducting body. These are then tion modelling is then the choice of numerical technique, used as the source terms for a scalar-potential finite-difference code and the availability of sufficient computer resources.
capable of solving the interior problem at an enhanced resolution, Finite-difference methods can be used [12,13] to solve yet with a similar computational load. This paper describes the the interior and exterior problems simultaneously, subject theory for integrating the two techniques. It demonstrates the validity of the technique, as applied to a human body model exposed to the use of appropriate grids and boundary condition. to a 60-Hz uniform electric field. Extremely good agreement is dem-
The finite-difference time-domain (FDTD) [24,17,21] onstrated for the low-resolution internal fields computed by the two technique can also be used to solve the full problem. For codes. The feasibility of the interpolation procedure and associated example, plane-wave induction calculations in humans at higher-resolution internal calculations are then illustrated. แฎ 1997 10 MHz have been reported [16,15], but straightforward Academic Press application to lower frequencies is hampered by the timestepping stability criterion.