Investigations of the behaviour of microwave thermography antennas when coupled to layered tissue regions

Test structures providing realistic simulations of the layering of body tissues have been used to investigate the radiometric response to be expected from common skin-fat-muscle regions. The electric field distributions in these models have been measured by the nonresonant perturbation method at 3.25 GHz, the mid-frequency of the range used for the radiometric measurements. Investigations have been made of the response patterns of dielectric loaded rectangular TE₁₀-mode and cylindrical TE_ll-mode radiometry antennas coupled to uniform media and models of layered tissue regions. Sucrose solutions of 30% to 50% concentration have been used for skin and muscle simulation. At 3 GHz sucrose solutions conveniently provide dielectric properties close to those of medium to high water content tissues. Sheets of thermo-compressed fabric-phenolic resin insulating material have been used for low permittivity fat layer simulation. These provide a relative permittivity of 3.7 and a loss factor of 0.24. Measurements have also been made using porcine skin-fat layers, with the electric field distribution being measured in sucrose solution below the fat. The field perturbers used are hollow glass spheres of about 2 mm diameter, and low permittivity dielectric sheets 1 to 2 mm thick. A linear heterodyne reflectometer is used to measure the amplitude and phase of the reflection coefficient produced by the presence of the perturber in the fields of the test structure