1. A method for iteratively reconstructing image data, comprising the steps of: accessing an initial image; defining a plurality of image element subsets of the initial image, wherein some or all of the plurality of image element subsets correspond to voxels aligned in z such that they share the same (x,y) location; iteratively generating updates to the initial image or a subsequent updated image based on the respective image element subsets, wherein at least one of the updates is generated by directly inverting a respective Hessian matrix or an approximation of a respective Hessian matrix associated with a corresponding image element subset; and terminating the iterative generation of updates upon satisfaction of a completion criterion.