Since the purpose of this scenario is to overlap the two volumes without additional spatial registration, the spatial location of the anatomy of interest in both volumes needs to be the same. To keep the two volumes spatially registered, the reconstruction application will use the table position of both rotations to correct the table movement with respect to the Isocenter, thus creating both volumes with the same spatial origin and axis, i.e., same patient coordinate system.

Therefore, it is recommended to encode both instances with the same FoR UID, equal to the Frame of Reference UID of the XA projection images. If the originating XA images do not contain a Frame of Reference UID, the reconstruction application will create the FoR UID equal for the two reconstructed volumes.

This module encodes the patient orientation with respect to the table. It is supposed to contain the same values in both 3D volumes, since the patient does not move between the two rotational acquisitions.

The detailed size of the volume element (Pixel Spacing for row/column dimension of each slice and Slice Thickness for the distance of slices) depends on the reconstruction algorithm and is not necessarily identical to the related sizes in the source (projection) image(s).

This macro encodes the position of the 3D slices relative to the patient.

It is assumed that the patient does not move on the table between the two rotational acquisitions, but the table moves with respect to the Isocenter. Although the spatial trajectory of the X-Ray Detector relative to the Isocenter of the two rotational acquisitions is the same, the two volumes contain a different region of the patient.

To allow spatial registration between the two volumes, the position of the slices of the two volumes need to be defined with respect to the same point of the patient. As the patient does not move on the table, the reconstruction application will define the patient origin as a fixed point on the table, so that the 3D slices of the two volumes are all related to the same fixed point on the table (i.e., same point of the patient) by the Attribute Image Position (Patient) (0020,0032).

The volume is positioned in the spatial coordinates identified by the frame of reference, which is common to the two volumes. Therefore, the position of the slices of both volumes is defined with respect to the same patient origin.

The slices can be oriented in any relation wrt. the patient coordinate system. The plane orientation is expected to be the same for the two volumes; however it could be different without compromising the registration.