Fiducial-less tracking with non-rigid image registration

A method of registering a first image of an object with data representative of a second image of the object, the object including one or more structures, the method comprising:

• reconstructing from the data at least one 2D image by processor, wherein the data comprise 3D scan data;
• estimating one or more 2D local motion fields within a region shown by the at least one 2D image based on a comparison between the at least one 2D image and the first image,
• wherein the first image comprises a 2D x-ray projection image, and wherein the act of estimating the one or more 2D local motion fields within the region comprises maximizing a similarity measure between the at least one 2D image and the 2D x-ray projection image, for each of a plurality of image resolution levels;
• constructing a full 3D motion field from the estimated 2D local motion fields;
• and
• determining from the full 3D motion field a set of non-rigid transformation parameters that represent the difference in a position and orientation of at least one structure as shown in the first image, as compared to the position and orientation of the at least one structure as shown in the at least one 2D image;
• wherein the object comprises an anatomical region, and the one or more structures comprise at least one reference structure and at least one target.