Quantitative in vivo measurements of inner ear tissue resistivities. I. In vitro characterization

An in vivo resistivity measurement system, based on the 4-electrode reflection-coefficient technique that nondestructively measures the complex resistivity of cochlear tissues, is described. Details of the theory and instrumentation used for noninvasive measurement of resistivity are presented. In vitro experiments both characterize the accuracy of the proposed resistivity measurement system and establish general criteria for ensuring that a particular theoretical model accurately represents the experimentally measured geometry. 2 Idealized geometries (2-layer planar and 2-layer spherical) are measured experimentally; error analyses using experimental results describe the maximum error with which the experimental system noninvasively estimates resistivity from experimental reflection coefficient measurements. The precise accuracy of a noninvasive resistivity estimate depends on both the variability for experimentally measuring the reflection coefficient of a particular geometry and the average value of the measured reflection coefficient. For example, 2-point measurements of an in vitro 2-layer planar interface allow noninvasive estimation of complex resistivity with total errors of less than 1%. In addition to characterizing accuracy of resistivity estimates for different in vitro geometries, 2 general criteria were established.