The X-Ray projection pixel calibration macro of the PS 3.3 C.220.127.116.11 specifies the attributes of the image pixel size calibration model in X-Ray conic projection, applicable to the Enhanced XA SOP Class.
In this model, the table plane is specified relative to the Isocenter. As default value for the attribute Distance Object to Table Top (0018,9403), the half distance of the patient thickness may be used.
Oblique projections are considered in this model by the encoding of the attribute Beam Angle (0018,9449), which can be calculated from Positioner Primary Angle (0018,1510) and Positioner Secondary Angle (0018,1511) as follows:
For Patient Positions HFS, FFS, HFP, FFP:Beam Angle = arcos( |cos(Positioner Primary Angle)| * |cos(Positioner Secondary Angle)| ).
For Patient Positions HFDR, FFDR, HFDL, FFDL:Beam Angle = arcos( |sin(Positioner Primary Angle)| * |cos(Positioner Secondary Angle)| ).
The resulting pixel spacing, defined as Δ Px * SOD / SID , is encoded in the attribute Object Pixel Spacing in Center of Beam (0018,9404). Its accuracy is practically limited to a beam angle range of +/- 60 degrees.