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The coaxial probe technique is used to acquire the dielectric properties of biological tissues in the microwave frequency range. To dielectrically characterize heterogeneous samples, the sensing radius of the probe must be known. Thus, in this article, for the first time, both experimental and numerical investigations were conducted to analyze and model the sensing radius dependence on the probe dimensions. The results suggest that 1) the sensing radius increases linearly with the inner radius of the outer conductor and is not affected by the width of the outer conductor; 2) the inner conductor has higher impact than the insulator on the sensing radius; and 3) although the sensing radius depends on the dielectric properties of the investigated samples, the trend of the sensing radius relative to the probe dimensions is the same across different samples. Furthermore, a method for predicting the sensing radius, through use of neural networks, is proposed. peer-reviewed

Accurate tissue dielectric measurements are crucial for the development of electromagnetic diagnostic and therapeutic devices that are designed based on estimates of the dielectric properties of diseased and healthy tissues. Although the dielectric measurement procedure is straightforward, several factors can introduce uncertainties into dielectric data. Generally, uncertainties are higher in the dielectric measurement of heterogeneous tissues, due to the fact that there is no standard procedure for acquiring and interpreting the dielectric data of heterogeneous tissues. Uncertainties related to tissue heterogeneity can be minimized by estimating the probe sensing volume, defined by the sensing depth and radius, and characterizing the tissue distribution within that volume. While several studies have investigated the sensing depth, this paper focuses on examining the sensing radius. Both dielectric measurements and numerical simulations with heterogeneous porcine tissues in the microwave range of 0.5-20 GHz have been conducted to quantify the sensing radius and the dielectric contribution of each tissue within the sensing volume. Experiments demonstrate that the sensing radius, which depends on the individual dielectric properties of the constituent tissue types, can be smaller than the probe radius. This paper further quantitatively demonstrates that the dielectric contribution of a particular tissue depends on both its location within the sensing volume and its dielectric properties. This study provides fundamental knowledge for accurately interpreting dielectric data of heterogeneous tissues, with the aim of supporting medical device development. peer-reviewed

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. This work has been developed in the framework of COST Action MiMed (TD1301). peer-reviewed

A number of emerging electromagnetic diagnostic and therapeutic devices are designed based on estimates of benign and malignant tissue dielectric properties at different frequencies and temperatures. Accurate tissue dielectric measurements are crucial for the development of these technologies. Although the dielectric measurement procedure is straightforward, several factors can introduce uncertainties and errors into dielectric data. These errors or confounders can be strictly related to the acquisition system or to the intrinsic properties of the investigated tissues. Generally, uncertainties are higher in the dielectric measurement of diseased tissues, due to their heterogeneity and complex structure and composition. These confounders can be minimized by clearly defining the measurement sensing volume, and characterizing the tissue distribution within that volume. The volume is defined by sensing depth and radius. In this work, early-stage experiments are presented to investigate the sensing radius for biological heterogeneous tissues, with the aim of providing more accurate dielectric measurements to support medical device development. The research leading to these results has received funding from the European Research Council under the ERC Grant Agreement BioElecPro n. 637780: “BIOELECPRO”. This work was also supported by the Irish Research Council (grant numbers RCS1325 and RCS1377) and the Hardiman Research Scholarship. This work has been developed in the framework of COST Action MiMed (TD1301). non-peer-reviewed