1. Apparatus for providing markers on a 2D X-ray projection image, for use in conjunction with C-arm calibration apparatus for 3D reconstruction in an imaging system, comprising; a plurality of radiation transparent planes intersecting at an intersection line; and radiation opaque markers disposed on said transparent planes and arranged in respective patterns such that detection of coplanar markers on said 2D X-ray projection image is possible without ambiguity even though said patterns be superimposed on said 2D X-ray projection image.