DICOM: What's the point of SOPInstanceUID tag?

Question

DICOM already provides a unique enough identifier for the Series (e.g. Series Instance UID), so why also include one on the lower level objects (e.g. SOPInstanceUID)?

What I find really annoying is the fact that when referencing other objects - for example when RTPlan object references RTStruct object via ReferencedStructureSetSequence / ReferencedSOPInstanceUID - it's done using the SOP Instance UID. However any of the DICOM SCPs - such as find/move - don't work with SOP Instance UID, they work with the Series Instance UID. So what gives? Do I have to load the whole Series to find all the referenced objects?

Answer 1

This question was from quite a while ago, but I thought I'd add that, ignoring QR altogether, a SeriesInstanceUID is a globally unique identifier for a single series. SOPInstanceUID is a globally unique identifier for a DICOM file. A series can have multiple DICOM files, so each would share that same SeriesInstanceUID, but each file would have it's own SOPInstanceUID.