Dielectric properties of biological tissue in medical applications

Abstract

Accurate knowledge of the dielectric properties of organ tissues is useful for many medical applications. Numerous studies have been published characterising various tissues but large variability in literature data still exists. Also, there exist conflicting ideas as to whether in-vivo dielectric properties differ from those measured from ex-vivo samples. Additionally, there exists a lacuna in knowledge of dielectric properties characterising biological tissues above 20 GHz. In this work, these lingering questions were addressed by initially conducting a thorough validation of the open-ended coaxial probe measurement system together with a complete assessment of the uncertainties in the measurements. Following that, Ex-vivo measurements on muscle, liver and kidney were conducted and wideband dielectric models were derived from 500 MHz to 40 GHz. Also, the differences between in-vivo dielectric measurements and those measured ex-vivo are investigated by conducting in-situ measurements for up to six minutes after animal death. This study showed that for the frequency range under study (500 MHz - 40 GHz) the differences are statistically significant but still lie within the uncertainty values. Therefore, it can be concluded that it is possible to substitute in-vivo measurements with ex-vivo, given that the organ is kept well hydrated. Additionally, the dielectric measurement techniques for solid/semisolid materials are reviewed. In particular, the conventional reflection and transmission waveguide measurement techniques are presented together with various inversion algorithms. This resulted in a comprehensive library of waveguide methods which can be used for the characterisation of various solid materials. Following accurate implementation of these methods using experimental and numerical studies on standard materials, dielectric measurements on cortical bone were conducted. This proved to be very challenging, mainly because of the extensive sample preparation required to fit the material in the waveguide aperture. Therefore, an alternative method using open ended waveguide is proposed for further validation. The implementation of this method is outlined, describing the forward analytical solution required to solve the inverse problem.