PART III



IIITheoretical
Methodsin Bioelectromagnetism


Chapters 11 and 12 examine theoretical methods
that have been developed for analyzing the source-field relationships of
bioelectric and biomagnetic phenomena. As discussed in Chapter 7, because
bioelectric sources and conductors are volume sources and volume
conductors, the theoretical methods that are used in analyzing electronic
circuits are not applicable in bioelectromagnetism. Therefore, the contents of
Part III are central to theoretical bioelectromagnetism. In Part III, it is shown that the
reciprocity theorem applies to the volume conductor. It serves as the
basis for the lead field theory, which provides a powerful way of
evaluating and interpreting measured signals in terms of their sources. The lead
field theory ties together sensitivity distribution of the measurement of
bioelectric sources, distribution of stimulation energy, and sensitivity
distribution of impedance measurements. These points pertain in both electric
and magnetic applications. The two
chapters of Part III are linked together by the fact that the same
electrophysiological sources generate both bioelectric and biomagnetic
fields. Since the fields behave differently, separate treatments are
necessary. Furthermore, it is important to point out that from a theoretical
point of view, the only difference between bioelectric and biomagnetic
measurements is their different sensitivity distribution in regard to the
bioelectric sources. The lead field theory clearly explains the similarities and
differences between the electric and the corresponding magnetic methods. Because
different instrumentation is employed, there are, of course, certain technical
differences between these methods. Although the ECG and MCG are the vehicles for explaining most of the
theoretical methods discussed in Part III, application of these methods is, of
course, not limited to electro- and magnetocardiography; this generalization is
emphasized where appropriate.