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?

Ventris

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.

Predigestion

In my experience there is only one RT object per SeriesInstanceUID. That is why it seems to me like a SeriesInstanceUID is a good enough identifier. For images, I can see the need for another level if ID to identify each image uniquely. It just seems strange how pathetic Query/Retreive specification really is, and how little you need to support to comply. – Ventris

@Budric-- I've often seen multiple RT structs for a single CT series-- for instance, to have multiple doctors mark the same patient to arrive at some definition of 'ground truth' (see Warfield). – Conventual

Answer 2

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.

Tideway

Answer 3

SOPInstanceUID represent separate uid of the Dicom Image File. Study, series and sopinstace uids are based on data model. StudyUID give you the particular study information. In which different series devided. Series instance uid used for for this. And SOP instance uid represent seperate Dicom image. It's hierarchy structure. I also never used SOPInstanceUID when i developed PACS workstation in Java. As per my experience, Study & Series uids are enough for represent patient's data. But still SOPInstanceUID gives unique identity to dicom image.

Godfree

SOPInstanceUID represents a globally unique DICOM file. That does not directly correlate with a single image. A DICOM file may contain a single image, it may contain multiple images, or it may contain no image at all (SR, for example) – Tideway

Answer 4

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

Templeton

Answer 5

SOPInstanceUId is for image level identification.

Understand it like: A study can have multiple series and a series can have multiple images/DICOM

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So,

to identify study uniquely we use StudyInstanceUID

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to identify series uniquely we use SeriesInstanceUID and

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to identify an image/DICOM uniquely we use SOPInstanceUId

Vaughan

SOPInstanceUID does not identify an image. As commented above - SOPInstanceUID represents a globally unique DICOM file. That does not directly correlate with a single image. A DICOM file may contain a single image, it may contain multiple images, or it may contain no image at all (SR, for example). – Tideway

Answer 6

Series reference is not specific enough. In the case of structure sets the Reference SOP Instance UID ties the contours in the structure set to the specific slice in the dataset. It's not enough to just reference the series because you have to ensure that the contour is exactly aligning with a slice.

Diplomat