Abstract: A method of estimating a position of a foreign object in the body, comprising: (a) providing a 3D data set of at least one blood vessel; (b) acquiring at least one 2D projection image of the vessel including the object; (c) registering the 2D projection image of the vessel to the 3D data set; and (d) using the registration to estimate a 3D position of said object restricted to be in a blood vessel, according to said 3D data set.