1. A method for imaging an object, the method comprising: (a) acquiring a background image related to non-uniformity of a grating system by an imaging process without the object in place; (b) acquiring a raw projection image of the object by differential phase contrast (DPC) imaging process that uses a source, a detector, and the grating system; (c) wherein the grating system comprises a steppable source grating; (d) acquiring a corrected projection image of the object by removing the background image from the raw projection image; (e) acquiring a plurality of additional corrected projection images of the object at different view angles and performing three-dimensional (3D) computed tomography reconstruction of the object; and (f) displaying images of the object derived from the reconstruction.