Magnetic fields produced by models of biological current sources

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Abstract
As a means to understand the magnetic fields produced by electrical sources in biological organisms, the fields produced by special sources which approximate the biological sources are calculated. Two categories of sources are considered. The first consists of current dipoles which are aligned in a particular direction; the second consists of randomly oriented dipoles. Although the field is produced by both the source directly and the current generated in the volume conductor by the source, only expressions for the field produced by the source alone are developed. This is done because in certain special volume conductor shapes, such as a semi-infinite volume, an infinite slab, and a sphere, the current flowing in the volume conductor gives zero contribution to the field component perpendicular to the surface of the conductor and greatly simplifies the expressions for this component. Approximations to these special shapes exist on the human body and allow development of simple expressions for the fields produced by the sources. These expressions are used to produce field plots which can be compared with measured fields to estimate the sources of the measured fields. The expressions indicate that under certain circumstances, some sources in the body will produce a small or zero magnetic field. The fields produced by aligned and random dipole sources are shown to have different rates of decrease with distance from the source.

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