1. A processor-implemented image reconstruction method, the method comprising: on an imaging system or a remote client in communication with the imaging system, for a given set of scan data acquired by the imaging system determining an objective function of an inner loop nested within an outer loop; iteratively processing the inner loop using ordered subsets until the inner loop is determined to be complete, wherein iteratively processing the inner loop comprises: applying a preconditioner; and computing an image update; upon completion of a respective inner loop: computing an update direction for the respective outer loop, wherein the update direction is determined by combining a plurality of inner loop update directions from the inner loops encompassed by the respective outer loop; computing a step size for the respective outer loop; and updating an image; determining if the image is converged: if the image is not converged, proceeding to a next outer loop, determining a next objective function of a next inner loop, and iteratively processing the next inner loop; and if the image is converged, ending the image reconstruction, generating a final image, and displaying a final image on a display of the imaging system or the remote client.