Alessandra La Gioia

Open-Ended Coaxial Probe Technique for Dielectric Measurement of Biological Tissues: Challenges and Common Practices

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.

Modeling of the dielectric properties of biological tissues within the histology region

The dielectric properties of biological tissues characterize the interaction of electromagnetic fields with the human body. As such, accurate knowledge of these properties is vital to the design and development of electromagnetic medical technologies. Despite their importance, the reported dielectric properties of key tissues have been inconsistent across studies. The natural heterogeneity of tissue has been identified as a contributing factor to these inconsistencies. In order to attribute dielectric properties to heterogeneous tissues, histological analysis is conducted to determine which tissue types contribute to the dielectric measurement. However, the Histology Region, i.e., the volume of the tissue sample that undergoes histological analysis, and the region corresponding to the dielectric measurement, has not been well-defined. Thus, instead of reducing uncertainties, more questions have been raised about the accuracy of data: if the Histology Region is not identified correctly, then the corresponding dielectric measurement does not represent the actual tissues involved. In this work, we examine the longitudinal extent of the Histology Region (i.e., the histology depth) for various heterogeneous samples composed of phantoms and biological tissues. This study highlights the fact that the relationship between the volume of tissue in a sample and the contribution of that tissue to the measured dielectric properties is not linear. Assuming that this relationship is linear may be a significant source of error in dielectric data. Further, we model, for the first time, the nonlinear relationship between the contribution of individual tissues to the dielectric measurement and the volume that each tissue occupies within the bulk sample. This work enables prediction of the permittivity of a sample with longitudinal heterogeneities, given knowledge of the constituent tissues of the sample, and provides the basis for modeling of all types of heterogeneities. These results will contribute to the minimization of uncertainties in the dielectric measurement of heterogeneous tissues.

Significance of heterogeneities in accurate dielectric measurements of biological tissues

Accurate knowledge of the dielectric properties of biological tissues is necessary for the design and development of electromagnetic medical technologies; these properties quantify the accuracy and efficacy of system operations. Despite the pressing need, the dielectric properties reported in the literature have suffered from inconsistencies mainly attributed to differences in measurement procedures. In this work, a key source of uncertainty, heterogeneous tissue composition within the sensing region of the dielectric probe, is investigated for biological samples composed of porcine muscle and fat. In particular, the contribution of tissues within the sensing depth to measured dielectric data is quantified and the assumption of equal impact of all tissues within the sensing depth is examined. This study demonstrates quantitatively that tissues at different depths below the measurement site do not contribute proportionally to the measured properties, thus suggesting that new analysis methods need to be developed to account for heterogeneous tissue samples in dielectric measurement data. This improved understanding of how heterogeneous tissues within the sensing region affect dielectric measurements facilitates future studies to reduce uncertainty and improve the quality of collected dielectric data of biological tissues.

Examination of the sensing radius of open-ended coaxial probes in dielectric measurements of biological tissues

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.

Comparison of in-vivo and ex-vivo dielectric properties of biological tissues

Accurate knowledge of the dielectric properties of biological tissues are crucial for various applications such as assessment of specific absorption rate and safety of electromagnetic medical devices. Most of the availably dielectric data in literature is based on ex-vivo measurements on biological tissue samples obtained from various animal models. This study investigates the differences in the in-vivo and ex-vivo dielectric properties of biological tissues, and variance over several animal species. The dielectric data is obtained from literature and organized to a unified format for comparison. The analysis shows considerable variations not only between in-vivo and ex-vivo dielectric properties of various tissues but also between various animal species.