Multiple Overlay Planes and Curves are often associated with a single Image (see PS 3.3 of the DICOM Standard). Standard Data Elements with even Group Numbers (5000-501E,eeee) represent Curves, while elements with even Group Numbers (6000-601E,eeee) represent Overlay Planes. Both of these ranges of Group numbers are known as Repeating Groups. This use of group numbers is a remnant of versions of the standard prior to V3.0 that associated a semantic meaning with particular Groups.
In each of these ranges of Group Numbers, Standard Data Elements that have identical Element Numbers have the same meaning within each Group (and the same VR, VM, and Data Element Type). The notation (50xx,eeee) and (60xx,eeee) are used for the Group Number in Data Element Tags when referring to a common Data Element across these groups (see PS 3.6). Groups (50xx,eeee) and (60xx,eeee) are called Repeating Groups because of these characteristics. Repeating Groups shall only be allowed in the even Groups (6000-601E,eeee) and even Groups (5000-501E,eeee) cases. In the future, Data Elements with VRs of SQ shall be used to serve a similar purpose. Note: Private Groups in the odd Groups (5001-501F,eeee) and (6001-601F,eeee) may still be used, but there is no implication of repeating semantics, nor any implied shadowing of the standard repeating groups.
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