Disclosed herein is an X-ray imaging system, comprising: a first X-ray source; an image sensor, spaced apart from the first X-ray source; a table configured to be positioned between the first X-ray source and the image sensor and configured to accommodate a person for imaging in the X-ray imaging system; and a fiducial marker system; wherein the fiducial marker system comprises three markers, centers of the three markers not being collinear; wherein at least one of the three markers is distinct. The X-ray imaging system may be used to model the 3D shape and track the 3D motion of an object (e.g., a surgical instrument) in the person.