Four-Electrode Tetrapolar Systems

If the effect of the zones proximal to the current carrying electrodes and the polarization impedance of the electrodes themselves are to be reduced, the four-electrode system is preferred. Such four-electrode systems correspond to a two-port, four-terminal network equivalent (see Section 8.1). Because there are two ports, these systems actually measure transfer parameters between the ports. This means that if for instance impedance is measured to 0 O, this does not necessarily imply high-conductivity tissue, but rather no signal transfer from CC to PU electrodes. 

As a four-electrode system, the measured segment is determined by the position of the two PU electrodes R and R, or more exact by the position of their electrolyte/salt bridge 

The spectrum shows two dispersions plus an inductive dispersion, which drives the phase positive at 1 MHz. Both the current density field Jcc and the reciprocal current density field J cc of the PU electrodes, are unknown. 

Aliau-Bonet and Pallas-Areny (2012, 2013) have studied the effect of body capacitance to ground in four-electrode bioimpedance measurements and found that stray capacitance between the measured body and ground could lead to inductive artifacts and resonance behavior. 

Four-electrode systems are commonly used on humans both with skin surface electrodes or on probes meant for different body cavities. It could also be used by implantable devices 

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The four-electrode tetrapolar system of Figure 10.10 is very different from the two-electrode mono-and bipolar systems (Grimnes and Martinsen 2007). It measures transfer impedance because the potential is recorded at a separate PU port and not at the CC port. If the two ports are far apart, no signal will be transferred from the CC to the PU port and the transfer impedance will be virtually 0 2. 

Figure 6.22 Illustration showing current paths and sensitivity in a simulation of a tetrapolar impedance measurement system. Four electrodes on top of the model and a spherical object in the center of the model below the electrodes (a) Current paths for the two current-carrying electrodes are the dark lines to the left, and the current paths for the reciprocal currents are the gray to the right, (b) Sensitivity distribution in a slice through the model in the same level as a spherical object inside the model. The darkest regions indicate negative sensitivity. Courte of Fred Johan...

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