Physics Of Image Formation By Microwave Scattering

The complex permittivities of three-dimensional inhomogeneous biological bodies can be extracted from microwave scattering data by inverse scattering approach. A water-immersed microwave system is used to contract the wavelength to millimeter range and to enhance impedance matching with the biological body. Contraction of the wavelength increases the image resolution, while impedance matching promotes the microwave penetration. Scattered fields are measured using an array of 127 dipole elements and a total size of approximately 15cm x 18cm with operating frequency at 3 GHz. Two inverse scattering approaches have been developed. One approach, which has been published earlier, utilizes an inverse scattering theorem which may be considered as a generalization of the Lorentz reciprocity theorem to dissipative media. The other approach, which is presented in this article, takes scattering measurement by an array with various directions of incident wave; the wave equation is converted to a matrix equation by dividing the dielectric body into a number of cells, the dielectric data is then obtained by inverting the matrix equation. In both approaches, uniqueness is assured owing to the dissipativity of the propagation medium.