The Position Reference Indicator (0020,1040) specifies the part of the imaging target that was used as a reference point associated with a specific Frame of Reference UID. The Position Reference Indicator may or may not coincide with the origin of the fixed frame of reference related to the Frame of Reference UID.
For a Patient-related Frame of Reference, this is an anatomical reference point such as the iliac crest, orbital-medial, sternal notch, symphysis pubis, xiphoid, lower coastal margin, or external auditory meatus, or a fiducial marker placed on the patient. The patient-based coordinate system is described in C.22.214.171.124.1.
For a slide-related Frame of Reference, this is the slide corner as specified in C.126.96.36.199 and shall be identified in this attribute with the value “SLIDE_CORNER”. The slide-based coordinate system is described in C.188.8.131.52.
The Position Reference Indicator shall be used only for annotation purposes and is not intended to be used as a mathematical spatial reference.
Note: The Position Reference Indicator may be sent zero length when it has no meaning, for example, when the Frame of Reference Module is required to relate mammographic images of the breast acquired without releasing breast compression, but where there is no meaningful anatomical reference point as such.