A quasi-power theorem for bulk conductors: comments on rheoencephalography

A special case of Tellegen's theorem, the quasi-power theorem, can be readily developed for bulk conductors, and a bulk conductor analog of Cohn's theorem follows which allows prediction of sensitivity of impedance plethysmographic measurements to tissue impedance changes in a system of bulk conductors. An example addressing the effects of scalp and cranium on the inference of regional blood flow in brain tissue begs additional questions about the origins of the rheoencephalogram.