Determining a registration between a 3D image and a set of at least two 2D images

1. A method for determining or updating a registration between a three-dimensional (3D) image of a region of interest and a set of at least two two-dimensional (2D) images of the same region of interest, wherein the at least two 2D images are acquired using different imaging geometries, wherein the registration is described by a set of registration parameters, the method comprising:

selecting at least one respective contour point in the 3D image for each of the at least two 2D images;

determining a 2D position of a respective depiction of each contour point in the respective 2D image;

for each contour point, determining at least one condition for registration parameters that needs to be fulfilled to map the respective contour point to the respective 2D position in the respective 2D image; and

determining an updated set of registration parameters, the determining of the updated set of registration parameters comprising solving a set of equations or an optimization problem that depends on the conditions.

2. The method of claim 1 , 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 a center of projection and to a plane of projection for each imaging geometry and therefore the 3D position of each contour point relative to the centers of projection and the planes of projection,

wherein the 2D position of the depiction of the respective contour point defines the position of the depiction in the respective plane of projection, and

wherein the at least one condition limits the possible 3D positions of the respective contour point to a respective bounding plane that comprises the center of projection for the imaging geometry of the respective 2D image and the position of the depiction of the respective contour point in the projection plane of that imaging geometry.

3. The method of claim 2 , wherein an initial position and orientation of the 3D image relative to the centers of projection and the planes of projection are defined by an initial set of registration parameters,

wherein a direction of a gradient of the 3D image at the respective 3D position of the contour point in the 3D image is determined, and

wherein the bounding plane is orthogonal to the respective gradient.

4. The method of claim 1 , wherein the conditions are conditions for the 3D positions of the contour points in a common coordinate system.

5. The method of claim 4 , wherein determining the at least one condition comprises generating an initial condition for a 3D position of the respective contour point in an initial coordinate system and transforming the at least one condition to the common coordinate system.

6. The method of claim 5 , wherein the initial coordinate system and the common coordinate system are defined by the imaging geometries for different 2D images.

7. The method of claim 4 , wherein an origin of the common coordinate system is a geometric center of the 3D positions of the contour points.

8. The method of claim 1 , wherein the registration parameters define a rigid rotation, translation, or rotation and translation of the region of interest with respect to a given frame of reference.

9. The method of claim 1 , wherein at least one of the conditions depends on a relative rotation between an orientation of the region of interest according to the set of registration parameters or an initial set of registration parameters and according to the resulting set of registration parameters, and

wherein a linear approximation for the relative rotation is used in the condition.

10. The method of claim 1 , wherein at least one of the 2D images of the set is an X-ray image, wherein the 3D image is a magnetic resonance image or a computer tomography image, or a combination thereof.

11. The method of claim 1 , wherein at least one prior set of prior 2D images is used,

wherein the set of registration parameters or an initial set of registration parameters describes the registration of the 3D image to the prior 2D images, and

wherein the set of registration parameters depends on the initial registration parameters and the prior 2D images.

12. The method of claim 1 , wherein determining the 2D position of the respective depiction of each contour point depends on the registration parameters or an initial set of registration parameters, and

wherein the determining of the 2D position of the respective depiction of each contour point is repeated with a 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.

13. A processing unit comprising:

a processor configured to determine or update a registration between a three-dimensional (3D) image of a region of interest and a set of at least two two-dimensional (2D) images of the same region of interest, wherein the at least two 2D images are acquired using different imaging geometries, wherein the registration is described by a set of registration parameters, the determination comprising:

selection of at least one respective contour point in the 3D image for each of the at least two 2D images;

determination of a 2D position of a respective depiction of each contour point in the respective 2D image;

for each contour point, determination of at least one condition for registration parameters that needs to be fulfilled to map the respective contour point to the respective 2D position in the respective 2D image; and

determination of an updated set of registration parameters, the determination of the updated set of registration parameters comprising solving a set of equations or an optimization problem that depends on the conditions.

14. The processing unit of claim 13 , wherein the processing unit is a processing unit of an Xray device.

15. A non-transitory computer-readable storage medium storing instructions executable by one or more processor to determine or update a registration between a three-dimensional (3D) image of a region of interest and a set of at least two two-dimensional (2D) images of the same region of interest, wherein the at least two 2D images are acquired using different imaging geometries, and wherein the registration is described by a set of registration parameters, the instructions stored in the non-transitory computer-readable storage medium comprising:

selecting at least one respective contour point in the 3D image for each of the at least two 2D images;

determining a 2D position of a respective depiction of each contour point in the respective 2D image;

for each contour point, determining at least one condition for registration parameters that needs to be fulfilled to map the respective contour point to the respective 2D position in the respective 2D image; and

determining an updated set of registration parameters, the determining of the updated set of registration parameters comprising solving a set of equations or an optimization problem that depends on the conditions.

16. The non-transitory computer-readable storage medium of claim 15 , 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 a center of projection and to a plane of projection for each imaging geometry and therefore the 3D position of each contour point relative to the centers of projection and the planes of projection,

wherein the 2D position of the depiction of the respective contour point defines the position of the depiction in the respective plane of projection, and

wherein the at least one condition limits the possible 3D positions of the respective contour point to a respective bounding plane that comprises the center of projection for the imaging geometry of the respective 2D image and the position of the depiction of the respective contour point in the projection plane of that imaging geometry.

17. The non-transitory computer-readable storage medium of claim 16 , wherein an initial position and orientation of the 3D image relative to the centers of projection and the planes of projection are defined by an initial set of registration parameters,

wherein a direction of a gradient of the 3D image at the respective 3D position of the contour point in the 3D image is determined, and

wherein the bounding plane is orthogonal to the respective gradient.

18. The non-transitory computer-readable storage medium of claim 15 , wherein the conditions are conditions for the 3D positions of the contour points in a common coordinate system.