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 for each of a plurality of object space voxels, the first quantity characterizing a logarithm of a ratio between a value of the projection space data and a corresponding value of predicted projection data; recording, in a data storage device, a first data encoding said logarithm of the ratio; computing a revised first quantity based on recorded first data from preceding iterations and one or more projection gains; computing an update value using the revised first quantity; and reconstructing an object space image representing the object under observation using the update value.