Measurements and Analysis of the Microwave Dielectric Properties of Human and Animal Tissues

Microwave thermography, tomography and hyperthermia are potentially useful clinical techniques that are currently at the development stage. They may ultimately assist in the early detection and treatment of breast cancer and other conditions causing a local temperature or tissue permittivity anomaly in the body. Accurate complex permittivity values for human tissues are needed for computer modelling to fulfil the potential of these techniques. A novel open-ended co-axial probe measurement system operating at room temperature was used to investigate the complex permittivity of many animal and human tissues at a frequency of 3 GHz. The probe was calibrated using a bilinear transform method based on an equivalent circuit model of the probe aperture. A quarter-wave choke at the probe aperture provided an area of high impedance around the open-end. This minimised the effects of surface currents, and thus greatly improved measurement repeatability and general probe performance in comparison with ordinary co-axial probes. Mixture equations were used to analyse the measured permittivity values in terms of the permittivities of the other major component materials of biological tissue. By considering all the sources contributing to the microwave permittivity, the amount of bound water in tissues was estimated, and an approximate value for its average relaxation frequency evaluated using an extrapolation technique. Further measurements on partially dehydrated tissue samples extended the range of tissue water content investigated, and permitted better extrapolation of the bound water relaxation frequency. These values were in close agreement with the estimates of previous researchers. To verify the validity of this method, aqueous gelatine solutions were used as simple tissue phantoms. Since the solutions contained comparatively few distinct components, it was possible to use mixture equations again to extrapolate the permittivity of water bound to the gelatine polymer molecules. The method gave highly consistent estimates for the bound water relaxation frequency using either or both of the real and imaginary parts of the measured solution permittivity