Method for updating an initial registration between a 3D image of a region of interest and a set of at least two 2D images (2, 3) of the same region of interest, wherein the 2D images (2, 3) are acquired using different imaging geometries, wherein the registration is described by a set of registration parameters, comprising the steps: - Selecting at least one respective contour point (4 - 7) in the 3D image for each 2D image (2, 3), - Determining a 2D position (8 - 11) of a respective depiction of each contour point (4 - 7) in the respective 2D image (2, 3), wherein the determination of the 2D position (8 - 11) of the respective depiction of each contour point (4 - 7) depends on the or an initial set of registration parameters, wherein this determination is repeated with the resulting set of registration parameters to determine respective updated 2D positions for the depictions and a further resulting set of registration parameters is determined in dependence of the updated 2D positions, - For each contour point (4 - 7) determining at least one condition for the registration parameters that needs to be fulfilled to map the respective contour point (4 - 7) to the respective 2D position (8 - 11) in the respective 2D image (2, 3), - Determining an updated set of registration parameters by solving a set of equations or an optimization problem that depends on the conditions, - wherein the imaging geometries are defined or approximated by a respective central projection, wherein the registration parameters define a position and an orientation of the region of interest relative to the center of projection and to the plane of projection for each imaging geometry and therefore the 3D position of each contour point (4 - 7) relative to the centers and planes of projection, wherein the 2D position (8 - 11) of the depiction of the respective contour point (4 - 7) defines the position of the depiction in the respective plane of projection, wherein the condition limits the possible 3D positions of the respective contour point (4 - 7) to a respective bounding plane (12 - 15) that comprises the center of projection for the imaging geometry of the respective 2D image (2, 3) and the position of the depiction of the respective contour point (4 - 7) in the projection plane of that imaging geometry, - wherein an initial position and orientation of the 3D image relative to the centers and planes of projection is defined by an initial set of registration parameters, wherein a direction of the gradient of the 3D image at the respective 3D position of the contour point (4 - 7) in the 3D image is determined, wherein the bounding plane (12 - 15) is orthogonal to the respective gradient. - Selecting at least one respective contour point (4 - 7) in the 3D image for each 2D image (2, 3), - Determining a 2D position (8 - 11) of a respective depiction of each contour point (4 - 7) in the respective 2D image (2, 3), wherein the determination of the 2D position (8 - 11) of the respective depiction of each contour point (4 - 7) depends on the or an initial set of registration parameters, wherein this determination is repeated with the resulting set of registration parameters to determine respective updated 2D positions for the depictions and a further resulting set of registration parameters is determined in dependence of the updated 2D positions, - For each contour point (4 - 7) determining at least one condition for the registration parameters that needs to be fulfilled to map the respective contour point (4 - 7) to the respective 2D position (8 - 11) in the respective 2D image (2, 3), - Determining an updated set of registration parameters by solving a set of equations or an optimization problem that depends on the conditions, - wherein the imaging geometries are defined or approximated by a respective central projection, wherein the registration parameters define a position and an orientation of the region of interest relative to the center of projection and to the plane of projection for each imaging geometry and therefore the 3D position of each contour point (4 - 7) relative to the centers and planes of projection, wherein the 2D position (8 - 11) of the depiction of the respective contour point (4 - 7) defines the position of the depiction in the respective plane of projection, wherein the condition limits the possible 3D positions of the respective contour point (4 - 7) to a respective bounding plane (12 - 15) that comprises the center of projection for the imaging geometry of the respective 2D image (2, 3) and the position of the depiction of the respective contour point (4 - 7) in the projection plane of that imaging geometry, - wherein an initial position and orientation of the 3D image relative to the centers and planes of projection is defined by an initial set of registration parameters, wherein a direction of the gradient of the 3D image at the respective 3D position of the contour point (4 - 7) in the 3D image is determined, wherein the bounding plane (12 - 15) is orthogonal to the respective gradient.