A method of providing an image of an interior of a structure (46) containing regions having different conductivities or admittivities. In one embodiment a series of electrical signal sets (V i ) is applied to a series of electrodes, each signal set generating a rotating electric field. A measured set of electrical signals obtained from the electrodes (44) is used to calculate a conductivity or admittivity distribution representative of the interior of the structure in a region about which the electrodes (44) are positioned. The process of calculating the distribution includes defining a cost function, e.g., the energy function based on the distribution, and a forward model expressing the measured set of electrical signals as a function of the change in voltage values with respect to conductivity or admittivity and requiring that the partial derivative of the difference between the energy function and a function based on the forward model equal zero.