Microwave imaging: Inversion of scattered near-field measurements

In this paper, the problem of reconstructing the electromagnetic properties of unknown scatterers is treated by means of a spatial-domain technique. T his technique combines the finite-element method and the Polak-Ribiere nonl inear conjugate gradient optimization algorithm. The forward scattering pro blem is solved via the finite-element method, while the inversion is implem ented by minimizing a cost function. This function consists of a standard e rror term and a regularization term. The first one is related to the scatte red near-field measurements, which are obtained by illuminating the scatter er with plane waves from various directions of incidence. The regularizatio n term is introduced in order to cope with the ill-posedness of the inversi on. A sensitivity analysis, which is performed by an elaborate finite-eleme nt procedure, provides the direction required for updating the estimate of the scatterer profile. Significant reduction of the computation time is obt ained by applying the adjoint-state-vector methodology.