Dear all,

How should one interpret the following statement from PS3.5 2022a:

[...]
The Unknown (UN) VR shall only be used for [...] Standard Data Elements previously encoded as some DICOM VR other than UN using the DICOM Default Little Endian Transfer Syntax (Implicit VR), and whose Value Representation is currently unknown,
[...]

It is pretty clear that for attribute such as B1rms (0018,1320) an implementation may use the VR UN when writing out as Explicit Little Endian from an input in Implicit Little Endian since it was added recently in the standard.

However, the statement is a little less clear about attributes such as SOP Class UID (0008,0016) or SOP Instance UID (0008,0018) to name a few. An implementation cannot pretend that the Value Representation is currently unknown for those.

In other word, should PS 3.6 - Registry of DICOM Data Elements define the minimal set of attributes that all implementations must be aware of ?

Thanks for comments,

On Thursday, 10 February 2022 at 00:37:12 UTC-7, Mathieu Malaterre wrote:
> In other word, should PS 3.6 - Registry of DICOM Data Elements define the minimal set of attributes that all implementations must be aware of ?

If we set aside certain 0002 elements from PS 3.10, which only apply to files, it seems that there is no minimal set.
Consider that PS 3.5 states that any element can be UN except for the following:
> The UN VR shall not be used for Private Creator Data Elements (i.e., the VR is equal to LO, see Section 7.8.1).
> The UN VR shall not be used for File Meta Information Data Elements (any Tag (0002,xxxx), see PS3.10).

Apart from these, the standard allows the VR of any Data Element to be "currently unknown".

So I would say that the "minimal set of attributes" is dependent only on the requirements of the implementation. Such implementation-specific details belong in the conformance statements rather than in the standard itself.