On the Sensitivity of Bevelled and Conical Coaxial Needle Probes for Dielectric Spectroscopy

Dielectric spectroscopy measures the permittivity of a material in a wide frequency band for analysis and characterisation with many applications in biomedical engineering. It is typically performed by measuring the reflection coefficient of the material under test using an open flat-ended coaxial probe. However, probes with a flat end cannot cut through biological tissues and thus, can only be deployed for ex-vivo measurements. Bevelled and conical-ended coaxial probes can overcome this limitation as they can be integrated into existing surgical tools for in-vivo measurements. The geometry of the probe strongly affects the measurement accuracy and this effect must be modelled precisely before deployment. Although there has been significant research using flat-ended probes, there is very limited research investigating other probe geometries. In this paper, a closed-form model of a bevelled and a conical end coaxial probe is presented for the first time. The model is based on the analytical solution of aperture admittance. The accuracy of the model is validated using both simulation and experimental results with a relative error of less than 1% for a wide range of permittivity values and frequencies. Using the obtained model, the sensitivity of conical and bevelled probes is analysed and compared. The results indicate that bevelled probes have a higher sensitivity than conical probes for tissue measurement and thus are the preferable probe geometry for in-vivo deployment.