1. 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; wherein the fiducial marker system comprises four markers; wherein the four markers are coplanar; wherein none of centers of the four markers is collinear with centers of any other two of the four markers; wherein centers of all the four markers are on vertices of a convex hull of the centers.