Dicom: What's the point of SOPInstanceUID tag?

Question

There is already a unique enough identifier - the SeriesUID. So why have this one?

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 SOPInstanceUID. However any of the DICOM SCPs - such as find/move - don't work with SOPInstanceUID, they work with the SeriesUID. So what gives? Do I have to load the whole study to find all the referenced objects?

Answer 1

As you probably know, DICOM has a hierarchy of identifiers for each individual SOP (Service Object Pair) Instance (Patient ID / Study Instance UID / Series Instance UID / SOP Instance UID). This hierarchy is built into the Query/Retrieve mechanism in DICOM, and is also used to identify specific SOP Instances.

In the specific case you're mentioning, I believe there could be the possibility of multiple RT Structure Sets within a Series/Study. The individual SOP Instance must be referenced so that you know which Structure Set the RT Plan is referencing.

As for products supporting retrieving by SOP Instance UID, unfortunately, relational queries are not widely supported in DICOM Query/Retrieve SCPs, as you've discovered, and some DICOM servers do not support Image level queries. In this specific case, you could query at the series level specifically for the RTSTRUCT modality, and only retrieve the Series that have this modality, thus narrowing down which data you need to download to just the RT Structure Sets.

Answer 2

SOP Instance UID : Represent your a unique Identifier for IOD, Its a TYPE 1 tag must present with value.

For Example :

Each DICOM Image has unique identifier