1. A system comprising: a plurality of receive antennas configured to receive scattered signals from a target object; a receiver coupled to the plurality of receive antennas, the receiver processing the received scattered signals thereby forming measured field data; a memory coupled to the receiver, the memory configured to store the measured field data; and a processor, the processor coupled to the memory and programmed (a) to access the measured field data; (b) to define a plurality of regions from the target object; (c) to initialize estimation method parameters; (d) to calculate field data for each of the plurality of antennas using the estimation method parameters; and (e) to estimate a value for each material parameter of a material parameter set using a conjugate-gradient method to minimize a cost function, the cost function measuring the difference between the measured field data and the calculated field data at each of the plurality of antennas wherein the material parameter set includes a material parameter associated with each of the plurality of regions, and further wherein the conjugate-gradient method includes an average gradient calculated for each of the plurality of regions from a gradient calculated at a plurality of points within each of the plurality of regions.