Analysis of the coaxial probes' sensing volume influence on the measurement of dielectric properties of lung tissue: a numerical and experimental study

Since the ending of the previous century, many studies dedicated to the characterization of the dielectric properties of biological tissues were conducted; however, there are still many gaps in the knowledge of the dielectric properties of certain tissue types and data variability among different studies. The main technique for tissue characterization is the open-ended coaxial probe which includes different-diameter probes which in turn have a different sensing volume. Among biological tissues, the lung is the most challenging to characterize due to its heterogeneity. Therefore, the aim of this paper was to investigate how the sensing volume of different open-ended coaxial probes influences the dielectric properties (i.e. the real (ɛ') and imaginary (ɛ’') part of the complex permittivity) measured in heterogeneous media, using lung tissue as a representative case study. First, the measurement set-up with two different open-ended coaxial probes was simulated using a software for electromagnetic simulations to understand the behaviour of the open probe technique in presence of non-homogeneous materials. Afterwards, physical measurements on ex-vivo lung tissue were performed with both probe types. Lastly, the hypothesized reason for heterogeneous properties of lung tissue was used to interpret the experimental results and correlate them with the simulated measurement results.