Modelling the sensing radius of a coaxial probe for dielectric characterisation of biological tissues

The open-ended coaxial probe is the most common measurement tool used to dielectrically characterize biological tissues. Most healthy and malignant biological tissues are macroscopically heterogeneous, with the exception of a few tissues, such as liver. Heterogeneous biological samples are dielectrically characterized by defining the dielectric properties of each tissue type constituting the sample. In order to accurately characterize a specific tissue type with a coaxial probe, it is fundamental that only the tissue of interest is contained within the probe sensing volume, which is defined by the sensing radius and sensing depth. In the literature, several studies have investigated the sensing depth with bilayer or multilayer heterogeneous tissues. However, in this paper, we examine the sensing radius through concentrically heterogeneous tissues. In particular, samples composed of two different concentric tissues were modeled to estimate the minimum width of the homogenous tissue region required to accurately acquire the corresponding dielectric properties. As recent studies have indicated that the sensing radius depends on the dielectric properties of the interrogated tissue, in this paper, the sensing radius of a coaxial probe has been numerically quantified across a wide range of scenarios, involving different tissues with varying dielectric contrasts. The numerical results indicate that: 1) the sensing radius increases with the contrast in permittivity between the constituent tissues and 2) the sensing radius is highly dependent on the permittivity of the tissue closest to the inner conductor of the probe. Finally, the numerical outcome has been confirmed with dielectric measurements performed on animal tissues.