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.