Modelling of Tissue Dielectric Contribution Within the Sensing Radius of a Coaxial Probe

The dielectric properties of biological tissues are fundamental for the design of electromagnetic medical devices. Tissue dielectric properties are generally measured using the open-ended coaxial probe technique. While the procedure to dielectrically characterise homogeneous biological tissues is straightforward; rigorous dielectric characterisation of heterogeneous tissues requires conducting a post-measurement histological analysis to accurately associate the measured dielectric properties to the tissue content. To this extent, it is crucial to quantify the sensing volume of the probe, consisting of the sensing radius and sensing depth, and examine how different tissue types contribute to the acquired dielectric signal depending on their distribution within the sensing volume. Since a few studies have modelled the dielectric contribution of tissue samples presenting longitudinal heterogeneities, in this work, the dielectric contribution of tissues within radially heterogeneous samples is modelled for the first time. A nonlinear relationship is found between the contribution of individual tissues to the acquired dielectric data and the volume that each tissue occupies within the radially heterogeneous sample. This work enables prediction of the permittivity of a sample with radial heterogeneities, based on knowledge of the constituent tissues, and provides the basis for accurate dielectric characterisation of heterogeneous samples.