1. A system for estimating an electromagnetic material parameter averaged over a target object, the average electromagnetic material parameter for use in forming an image of the target object, the system comprising: a plurality of receive antennas 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 to access the measured field data; to define a plurality of regions from the target object; to initialize estimation method parameters; to calculate field data for each of the plurality of antennas using the estimation method parameters; to evaluate 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; and to estimate a material parameter set using the evaluated cost function and applying a conjugate-gradient method, wherein the material parameter set includes a material parameter for 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.