Investigation of histology radius for dielectric characterisation of heterogeneous materials

Knowledge of the dielectric properties of biological tissues is fundamental in the design of novel electromagnetic-based medical devices. Tissue dielectric properties are typically measured using the open-ended coaxial probe technique, which is designed for homogeneous samples. Histological analysis may be conducted to associate the measured dielectric properties to different tissue types within heterogeneous samples. However, the histology radius (i.e., the radial extent of the tissue sample that undergoes histological analysis) has not been consistently defined in the literature; therefore, this parameter may be a source of error in dielectric data. For this reason, we investigate the histology radius of various heterogeneous samples. Dielectric measurements were conducted over the frequency range of 0.5 to 20 GHz on radially heterogeneous tissue-mimicking materials and biological tissues, with different dielectric properties and contrasts. The experimental results were validated with numerical simulations and indicate that: i) the histology radius does not exceed the probe radius; ii) the dielectric properties of radially heterogeneous tissues depend on the spatial distribution of each material within the histology radius; and iii) the bulk dielectric properties of concentric heterogeneous tissues highly depend on the properties of each constituent material within the histology radius. This study supports consistent identification of the histology radius and provides a basis for rigorous interpretation of the dielectric properties of heterogeneous tissues.