1. A system comprising: one or more transmitters to transmit an excitation energy into an object under observation; one or more detectors to generate projection space data encoding an energy received by the one or more detectors in response to the transmitted excitation energy into the object under observation; a controller to control the one or more transmitters to transmit the excitation energy and the one or more receivers to generate the projection space data; and an image reconstructor having at least one processor to receive the projection space data and to process the projection space data by: computing a first quantity characterizing a difference between the projection space data and predicted projection data, wherein the difference between the projection space data and predicted projection data is associated with a projection gain; recording, in a data storage device, a first data encoding said difference; computing a revised first quantity based on recorded first data from preceding iterations; computing an update value using the revised first quantity; and reconstructing an object space image representing the object under observation using the update value.