title: Measurement and modelling the sensitivity of tetrapolar transfer impedance measurements.
abstract: Finite element method (FEM) modelling of a small disk in a homogeneous saline medium showed that the sensitivity distribution for tetrapolar transfer impedance measurements was dependant on the ratio, σdisk/σsaline, and not absolute conductivity values. In addition, the amplitude of the negative sensitivity regions between the drive and receive electrodes decreased non-linearly with σdisk/σsaline for σdisk/σsaline < 1, eventually becoming zero. This non-linear behaviour determined the limit of the assumption of a small change in conductivity in Geselowitz's lead theorem with 0.5 <σdisk/σsaline <1.5 for the measurements reported. The modelling supported the design of a sensitivity measurement system using an insulating support and a metal disk in a saline filled tank. Measurements were shown to give good agreement with sensitivity predictions from Geselowitz's lead theorem. Replacing the homogeneous medium in the FEM model with layers of different conductivity parallel to the plane of the electrodes changed the sensitivity distribution when the thickness of the layers adjacent to the electrodes were less than ½ the electrode spacing. A layer of greater conductivity over a layer of lesser conductivity next to the electrodes gave a peak in the sensitivity distribution and extended regions of negative sensitivity further into the tissue.
authors.affiliation: Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK.
authors.forename: E
authors.initials: E
authors.lastname: Naydenova
authors.affiliation: Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK.
authors.forename: S
authors.initials: S
authors.lastname: Cavendish
authors.affiliation: Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK; Department of Clinical Physics and Bioengineering, University Hospital, Coventry CV2 2DX, UK . Electronic address: Adrian.Wilson@warwick.ac.uk.
authors.forename: A J
authors.initials: AJ
authors.lastname: Wilson
mesh_terms: D017097:Electric Impedance; D020342:Finite Element Analysis
keywords: Impedance; Measurement; Modelling; Sensitivity