Open-Ended Coaxial Dielectric Probe Effective Penetration Depth Determination

Abstract

We have performed a series of experiments which demonstrate the effect of open-ended coaxial diameter on the depth of penetration. We used a two layer configuration of a liquid and movable cylindrical piece of either Teflon or acrylic. The technique accurately demonstrates the depth in a sample for which a given probe diameter provides a reasonable measure of the bulk dielectric properties for a heterogeneous volume. In addition we have developed a technique for determining the effective depth for a given probe diameter size. Using a set of simulations mimicking four 50 Ω coaxial cable diameters, we demonstrate that the penetration depth in both water and saline has a clear dependence on probe diameter but is remarkably uniform over frequency and with respect to the intervening liquid permittivity. Two different 50 Ω commercial probes were similarly tested and confirm these observations. This result has significant implications to a range of dielectric measurements, most notably in the area of tissue property studies.

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

“…However, recent studies [16], [17], have suggested that there is not a linear proportionality of contribution, specifically, tissues closer to the probe tip contribute to the dielectric measurement more than tissues further from the probe tip. This finding suggests that the contribution of tissues to the dielectric data is not equal to the proportion of volume occupied by those tissues in the Histology Region.…”

“…In order to examine heterogeneous tissue structures, a measurement tank was developed based on the one in [17]. Within the tank, a solid material is placed on the bottom, surrounded by a liquid.…”

“…From the diagram, it is evident that the measurements are performed on samples composed of bilayered material structures. This type of investigation of bulk heterogeneous samples that are composed of homogeneous layers has often been a test scenario in the field of dielectric spectroscopy [17], [28]- [30], and is useful in a wide range of studies including to quantify the sensitivity or specificity of the dielectric probe [28] and to quantify the material properties of the individual layers based on the reflection coefficient measurements [29].…”

“…Recent studies [16], [17] suggest that the tissue closest to the dielectric measurement location (i.e., at the probe tip) contributes with disproportional dominance to the measured dielectric properties. This effect must be taken into consideration in order to have accurate interpretation of dielectric measurements of heterogeneous tissue samples.…”

Investigation of the effect of dehydration on tissue dielectric properties in ex vivo measurements

“…The effective sensing volume of the open-ended coaxial probes for tissue measurements has been widely studied for various applications. A number of studies (Hagl et al 2003, Meaney et al 2014, Meaney et al 2016 assessed the sensing depth of the slim probe (2.2 mm diameter) for tissue measurement applications and reported minimum sample thickness of less than 2 mm for accurate measurements. The average sample size in the study presented in this paper was 20 mm (minor axes) by 25 mm (major axis).…”