The Table Top Position attributes of the Table Position Sequence (0018,9406) specify the geometrical position of the Table in the three spatial directions (i.e. Vertical, Longitudinal and Lateral) relative to the Table Top plane (see Figure C.8.19.6-3). The absolute reference point to which the Table positions are related is arbitrarily defined by the manufacturer.
The Table Angle attributes of the Table Position Sequence (0018,9406) specify the rotation and tilt of the Table Top Plane with respect to a plane arbitrarily defined by the manufacturer (usually the horizontal plane).
The Table Top Position attributes allow to describe the incremental translation of the Table top between frames of the same Multi-frame image, and between frames of different images, provided that the Table Angles are not modified between these frames.
When the table angles are modified between two frames, the Table Position Sequence (0018,9406) does not allow to characterize the relationship between the two table positions in an absolute reference coordinate system. For this purpose, the X-Ray Isocenter Reference System Macro has to be used.
Note: The incremental table translation may be used, in conjunction with the Positioner Position Sequence attributes (0018,9405), for simple 2D-2D registration applications (object tracking, pixel shift…), assuming that the patient position is fixed on the table. For more complex registration applications, and in order to properly handle the changes in the table angles, it is recommended to use the X-Ray Isocenter Reference System Macro attributes.
Figure C.8.19.6-3Table Position Vectors