1. A method for imaging an object, the method comprising: (a) acquiring a plurality of background images related to non-uniformity of a grating system from all view angles by an imaging process without the object in place wherein the grating system comprises a steppable source grating that steps in a direction transverse to a plane to which the source grating confirms; (b) acquiring a plurality of raw projection images from all view angles of the object by differential phase contrast (DPC) imaging process that uses an x-ray source, a detector, and the grating system to acquire a plurality of intensity images from which the plurality of raw projection images are derived, said plurality of intensity images being acquired by stepping the steppable source grating for each of the plurality of raw projection images; (c) acquiring a plurality of corrected projection images from all view angles of the object comprising calculating each corrected projection image by removing a background image of the plurality of background images from a raw projection image of the plurality of raw projection images at each of the view angles; (d) performing three-dimensional (3D) computed tomography reconstruction of the object using the plurality of corrected projection images from all view angles; and (e) displaying images of the object derived from the three-dimensional (3D) computed tomography reconstruction.