1. An image reconstruction method, the method comprising: 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: computing a gradient of a subset objective function of the objective function; and computing an image update; upon completion of a respective inner loop, updating an image with the image update and updating one or more auxiliary variables, if present; 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.