The dielectric properties of biological tissues at radio and microwave frequencies are strongly correlated with tissue water content. Localized, in vivo measurement of permittivity and conductivity should therefore provide useful clinical information in diseases involving abnormal hydration, such as lymphoedema. We have developed an open-geometry sensor for segmental hydration studies based on a flat cavity resonator operating at 300 MHz, and have demonstrated that the changes in its resonant frequency and Q-factor were significantly greater when it was applied to a swollen, oedematous finger, compared to an uninjured finger of similar size. The resonant sensor was calibrated with reference liquids in vials inserted through holes in its cavity plates, and we found that a modified resonant cavity perturbation formula, with coefficients empirically optimized by means of a genetic algorithm, yielded good agreement with literature values of complex permittivity. However, extending the length of the sample containers leads to measurement artefacts owing to antenna currents with associated radiated energy losses. A detailed simulation of the system with a full-wave solver using Method-of-Moments enabled us to estimate the current distribution and energy balance, and thus take steps towards mitigating these effects and enabling the system to make quantitative in vivo measurements of tissue dielectric properties.
- dielectric measurement
- resonant cavity
- genetic algorithm
- computational electromagnetics
- COMPLEX PERMITTIVITY