Is it true that DICOM "Media Storage SOP Instance UID" = "SOP Instance UID"? Why?

Question

I have two questions when I am reading the DICOM standard:

In a DICOM file, (0002 0003)"Media Storage SOP Instance UID" and (0008 0018) "SOP Instance UID", are they the same? What about (0002 0002) and (0008 0016)? and Why ??

Answer 1

Chris is correct, they are the same. From the dicom standard section C.12.1.1.1:

The SOP Class UID and SOP Instance UID Attributes are defined for all DICOM IODs. However, they are only encoded in Composite IODs with the Type equal to 1. See Section C.1.2.3. When encoded they shall be equal to their respective Attributes in the DIMSE Services and the File Meta Information header (see PS3.10 Media Storage).

As to the reason why these items are duplicated, I can only speculate, but the File Meta Information Header only exists in dicom files (it is not transmitted by an SCP/SCU). When an SCP writes a file from the DICOM data it receives, it has to get the SOP class and instance UIDs from the dataset, so that is the mechanical reason they are the same. As to why these tags and not some others, I am sure there are many reasons, but note that the File Meta Information Header is always readable by any dicom entity as it is always "Little Endian Explicit" even if the following dataset is some weird transfer syntax. So these two fields are always guaranteed to be readable and usable in any valid dicom file (even if the group 8 versions are in an unreadable transfer syntax).

I also tried to look up the condition:

However, they are only encoded in Composite IODs

Almost every IOD is a Composite IOD when I look at the standard:

Normalized IODs

Composite IODs

Answer 2

Yes they are the same. Tags with group 0002 are part of the DICOM P10 header, I assume they are duplicated so they can be quickly read without having to parse the entire file.