Electromagnetic (EM) medical technologies are rapidly expanding worldwide for both diagnostics and therapeutics. As these technologies are low-cost and minimally invasive, they have been the focus of significant research efforts in recent years. Such technologies are often based on the assumption that there is a contrast in the dielectric properties of different tissue types or that the properties of particular tissues fall within a defined range. Thus, accurate knowledge of the dielectric properties of biological tissues is fundamental to EM medical technologies. Over the past decades, numerous studies were conducted to expand the dielectric repository of biological tissues. However, dielectric data is not yet available for every tissue type and at every temperature and frequency. For this reason, dielectric measurements may be performed by researchers who are not specialists in the acquisition of tissue dielectric properties. To this end, this paper reviews the tissue dielectric measurement process performed with an open-ended coaxial probe. Given the high number of factors, including equipment- and tissue-related confounders, that can increase the measurement uncertainty or introduce errors into the tissue dielectric data, this work discusses each step of the coaxial probe measurement procedure, highlighting common practices, challenges, and techniques for controlling and compensating for confounders.

Achieving a better characterization of human blood conductivity is of high relevance for medical applications. In this study we measured the complex impedance of N = 10 human whole blood samples (from N = 10 oncology patients) at room temperature ( T = 22.6 $\pm\; 0.8\;^\circ \mathrm{C}$ ) and at body temperature ( T = 36.6 $\pm\; 0.4\;^\circ \mathrm{C}$ ). The complex impedance was measured using the measurement setup consisting of a custom made four-electrode probe and a commercially available galvanostat. The measured complex impedance data were used to calculate the conductivity of whole blood over the 631 Hz–100 kHz frequency range. The calculated conductivity data is presented and was compared with the literature data. The data from our study is in good agreement with the data available in the literature. Additionally, full blood counts were provided for N = 8 samples and Pearson correlation coefficient was calculated between the conductivity and blood counts at different frequencies. The three blood count parameters with the highest correlation coefficient are haematocrit (Hct), haemoglobin (Hgb) and red cell count (RBC). The correlation coefficient was shown to decrease as the frequency increases and was the highest at f = 631 Hz, which is the lowest reported frequency. To our knowledge this is the first study to measure low-frequency (i.e. below 1 MHz) conductivity of whole human blood at body temperature using the four-electrode technique. The results of this study represent an important contribution to the literature, which is currently limited in this area and will help further medical device design.

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.

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 minimised by estimating the probe sensing volume, defined by the sensing depth and radius, and characterising the tissue distribution within that volume. While several studies have investigated the sensing depth, this work 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 work 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.

The coaxial probe technique is used to acquire the dielectric properties of biological tissues in the microwave frequency range. To dielectrically characterise heterogeneous samples, the sensing radius of the probe must be known. Thus, in this study, for the first time, both experimental and numerical investigations were conducted to analyse and model the sensing radius dependence on the probe dimensions. The results suggest that: i) the sensing radius increases linearly with the inner radius of the outer conductor and is not affected by the width of the outer conductor; ii) the inner conductor has higher impact than the insulator on the sensing radius; and iii) 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.

The dielectric properties of biological tissues are typically measured using the open-ended coaxial probe technique, which is based on the assumption that the tissue sample is homogeneous. Therefore, for heterogeneous tissue samples, additional post-measurement sample processing is conducted. Specifically, post-measurement histological analysis may be performed in order to associate the measured dielectric properties with the tissue types present in a heterogeneous sample. Accurate post-measurement histological analysis enables identification of the constituent tissue types that contributed to the measured dielectric properties, and their relative distributions. There is no standard protocol for conducting post-measurement histological analysis, which leads to high numbers of excluded tissue samples and inconsistencies in the resulting reported data for heterogeneous tissues. To this extent, this study examines the post-measurement histological process and the challenges in associating the acquired dielectric properties with the different tissue types present in heterogeneous samples. The results demonstrate that the histological process inevitably alters the morphology of samples, thus introducing errors in the interpretation of the dielectric properties acquired from heterogeneous biological samples. Notably, sample size was seen to shrink by up to 90% through the histological process, meaning that sensing volume determined from fresh tissues is not directly applicable to histology images.