Quantification of the Sensing Radius of a Coaxial Probe for Accurate Interpretation of Heterogeneous Tissue Dielectric Data

Abstract

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.

An Overview of Microwave Imaging for Breast Tumor Detection

“…Further, Porter et al [49] observed that histology depth (defined as the depth to which the probe can detect changes in the tissue sample within the measurement uncertainty) varies with frequency, and hence, it should have been included as a con-founder in historic data sets (like Lazebnik et al) where the histology depth was taken as a constant value. In 2018, it was further noted that the probe sensing radius could be smaller than the probe radius and depended on the histology of the tissue sample [48]. MINDER (Minimum Information for Dielectric Measurements of Biological Tissues) model was developed by the team which allows to reproduce measurements, provides ease of interpreting and reusing data, and comparison of data across studies [49].…”

“…Hence, on the whole it can be suggested that, in future, dielectric studies must be taken in accordance with the latest principles by accounting for all confounders to understand the dielectric contrast in exact figures. Reported results that tissue dehydration reduces dielectric values by about 25% within 3.5 hours after excision [45], that sensing depth varies with frequency and hence is not to be taken as a constant [48], that permittivity drops 0.13% per degree Celsius [52], that sensing radius is not restricted to be larger than probe radius [48], etc. must all be taken into consideration during data collection and interpretation to arrive at the final result.…”

Exploiting Tissue Dielectric Properties to Shape Microwave Thermal Ablation Zones

“…The thermal and dielectric properties used for the tissue models are detailed in Table 1 at 2.45 GHz. The dielectric properties were obtained experimentally and loaded into the material library of the CST MWS software: measurements were conducted on ex vivo porcine tissue using a Keysight slim form probe (N1501A) connected to a Keysight E5063A network analyzer (operating frequency range: 100 kHz-8.5 GHz) as described in La Gioia et al [26]. The values related to heat capacity, thermal conductivity and density were obtained from the literature [22] and manually loaded into the material settings of the CST MWS software.…”

An Analysis of Open-Ended Coaxial Probe Sensitivity to Heterogeneous Media

“…In the context of biological material measurement, Meaney et al estimated that the sensing volume of a similar open-ended coaxial probe was ∼0.3–0.4 mm based on measurements of layered media [ 18 ]. La Gioia et al determined the sensing volume was less than 1 mm in one report, and was 0.5 mm for a dielectric contrast of 0.5 in another report [ 12 , 19 ]. While different in approach and definition, our observation that the inclusion influence of each probe decayed to less than 5% of its maximal value by around 0.4 mm from the center conductor parallels the estimates from those prior experimental and theoretical works.…”

“…Later, Meaney et al used an experimental setup with layered media to suggest that the measured permittivity is disproportionately affected by media within 200–400 m (i.e., the measurement volume is less than 1 mm) [ 11 ]. La Gioia et al used a series of simulations and measurements in tissue to demonstrate that the spatial distribution of materials around the probe alters the measured permittivity, and determined that the measurement volume was at most 1 mm [ 12 ]. While there is some qualitative agreement between these studies, a quantitative functional relationship between heterogeneous material permittivity and the effective permittivity measured by a coaxial probe has not yet been established.…”