1. An image formation method, comprising: generating an image estimate; simulating an expected projection based on the image estimate; comparing the expected projection to a set of measured projection data acquired by an imaging system based on a cost function; determining 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 the multi-channel preconditioner to the gradient vector; combining the outputs of the multi-channel preconditioner to generate a transformed gradient vector; and updating the image estimate using the transformed gradient vector.