Immittance theory

Immittance theory is based upon sinusoidal excitation and sinusoidal response. In relaxation theory (and cell excitation studies), a step waveform excitation is used, and the time constant is then an important concept. If the response of a step excitation is an exponential curve, the time constant is the time to reach 63% of the final, total response. Let us for instance consider a series resistor-capacitor (RC)-connection, excited with a controlled voltage step, and record file current response. The current as a function of time I(t) after the step is I(t) = (V/R)e , file time constant x = RC, and I( oo) = 0. 

A one-port (dipolar) electrode system measures immittance. The two electrodes function both as CC and PU electrodes. A two-port four-electrode system measures transmittance (transfer immittance) for example, with current injected in one port and voltage recorded at the other port (the black box, Section 7.1). The electrode pairs of current injection and voltage recording may be interchanged if the reciprocity theorem is valid, the transmittance is the same. For the reciprocity theorem to be valid, there are no constraints on geometry, only on, for example, system linearity as outlined in Section 8.1.3. The reciprocity theorem is not based on geometry but on network theory, and is therefore treated in Section 8.1. 

In black box theory the excitation (input) and output ports must be defined, the transmission direction must be defined. A network is reciprocal if the ratio between excitation and response remains unaltered when the ports of excitation and response are interchanged. Then the transfer immittances are equal, Y12 = Y21 and Z12 = Z21. Tissue is reciprocal only if it is... 

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