Microwave imaging of two-dimensional scatterers by inverting scattered near-field measurements

A spatial domain technique for the reconstruction of the electromagnetic properties of unknown scatterers is presented. The new methodology combines the finite element method (FEM) and the Polak-Ribiere nonlinear conjugate gradient optimization algorithm. The forward scattering problem is treated via the FEM, while inversion is implemented by minimizing a cost function, consisting of a standard error term and a regularization term. The first one is related to the scattered near-field measurements, obtained by illuminating the scatterer with plane waves from various directions. A sensitivity analysis, performed by an elaborate finite element procedure, provides the direction required for correcting the scatterer profile. Significant reduction of the computation time is obtained by introducing the adjoint state vector methodology.