Dependence of apparent resistance of four-electrode probes on insertion depth

The apparent resistance of a finite-thickness layer measured with a four-el ectrode plunge probe depends on the electrode insertion depth, electrode sp acing, and layer thickness, as well as the resistivity ratio of an underlyi ng layer A physical model consisting of air, a saline solution layer, and a n agar layer simulates the real situation of resistivity measurement. The s aline layer represents the finite-thickness layer whose resistivity is to b e measured by a plunge electrode probe, and the agar layer represents an un derlying perturbing layer. A micropositioner controls the insertion depth o f the four electrodes into the saline solution. With the apparent resistanc e measured on a semi-infinite-thickness layer of saline solution as standar d, measurement results show decreasing apparent resistance and increasing e rror with increasing electrode insertion depth. This information is importa nt for correct measurement of myocardial resistivity in vivo and in vitro.