Providing pose information for x-ray projection images

A computer-implemented method of providing pose information for X-ray projection images, the method comprising:

• receiving X-ray projection data, the X-ray projection data comprising an X-ray projection image representing an anatomical structure comprising a plurality of bones, the X-ray projection data being acquired by a projection X-ray imaging system having a corresponding pose with respect to the anatomical structure;
• segmenting the X-ray projection image to identify a plurality of projected sub-regions of the anatomical structure;
• generating a pose metric for the X-ray projection image based on a relative size of two or more of the projected sub-regions in the segmented X-ray projection image, wherein the pose metric represents:
• the pose of the projection X-ray imaging system with respect to the anatomical structure and/or a suitability of the projection X-ray imaging system pose for acquiring the X-ray projection image, and/or feedback for adjusting the projection X-ray imaging system pose in order to acquire a subsequent X-ray projection image representing the anatomical structure;
• and
• wherein at least one of the projected sub-regions represents a gap between two of the bones;
• and/or
• wherein at least one of the projected sub-regions is defined at least in part by:
• an intersection between a gap between two bones and a further bone in the X-ray projection image;
• and
• outputting the pose metric to provide the pose information for the X-ray projection image.