Improved Sensing Volume Estimates for Coaxial Probes to Measure the Dielectric Properties of Inhomogeneous Tissues

Abstract

Accurate knowledge of the dielectric properties of biological tissues is fundamental for the design of electromagnetic medical devices. The open-ended coaxial probe technique is commonly used to measure the complex permittivity of tissues at microwave frequencies. Since the open-ended coaxial probe technique provides an average complex permittivity for the tissues within its sensing volume, when measuring heterogeneous tissues, a post-measurement histological analysis is often conducted to accurately associate the measured dielectric properties to the tissue content. Subsequently, it is crucial to quantify the sensing volume of the probe accurately. Sensing volume is generally defined as a cylindrical volume (rectangular cross-section) based on sensing depth and sensing radius. In this article, the sensing volume is modified based on the field distribution of the probe. Since in practice defining the sensing depth and sensing radius for a given probe is straightforward, the resulting sensing volume is defined as an ellipsoid based on sensing depth and sensing radius values. Considering a finite heterogeneity in a homogeneous background, it is shown that the ellipsoidal sensing volume is a more accurate representation than the typical cylindrical volume for open-ended coaxial probes, and therefore, results in a more accurate dielectric characterization of heterogeneous tissues.