1. A method of providing an image of an interior of a structure containing multiple regions having different conductivities or admittivities which influence transmission of electrical signals through the structure, comprising: providing a series of electrodes along a plane passing through the structure, with the electrodes positioned along an external surface of the structure; providing a series of electrical signal sets for input to the electrodes, each signal set comprising an electrical signal pattern for input to each of the electrodes; sequentially applying each of the electrical signal sets to the electrodes, application of each signal set generating a rotating electric field within the structure; obtaining, in response to application of each signal set, a measured set of electrical signals from the electrodes; calculating, with the measured set of electrical signals, a conductivity or admittivity distribution representative of the interior of the structure in a region of the structure about which the electrodes are positioned; and determining image data representative of the interior of the structure including two or more regions having different conductivities or admittivities, wherein the process of calculating the conductivity or admittivity distribution includes defining an energy function based on the conductivity 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 and requiring that the partial derivative of the difference between the energy function and a function based on the forward model equal zero.