1. An image formation method, comprising: generating an image estimate with a processing component in communication with an imaging system; simulating an expected projection based on the image estimate; acquiring a set of measured projection data with the imaging system; comparing, with the processing component, the expected projection to the set of measured projection data based on a cost function; determining, with the processing component, a gradient vector based on the cost function that incorporates noise characteristics or other models of the imaging system; implementing a multi-channel preconditioner on two or more channels such that each channel is processed independent of the other channels; applying, with the processing component, the multi-channel preconditioner to the gradient vector; combining, with the processing component, the outputs of the multi-channel preconditioner to generate a transformed gradient vector; and updating, with the processing component, the image estimate using the transformed gradient vector.