DCMTK doesn't use UN when Value Representation is unknown?

[ander833]

Hi,

If I have a data set stored with implicit transfer syntax and
containing an element with several possible value representationsand,
and I convert it to explicit transfer syntax, I would expect that
element to get value representation UN. This is my understand of the
DICOM standard part PS 3.5 - 2008 6.2.2 Unknown (UN) Value
Representation, at least:

(Note 3:) "This VR of UN is needed when an explicit VR must be given to a Data
Element whose Value Representation is unkown (e.g. store and
forward). UN is a means to explicitly indicate that the Value
Representation of a Data Element is unkown."

DCMTK appears to simply pick one of the possible value
representations, however. For example:

Code: Select all

$ dcmdump implicit.dcm 
  ...
# Dicom-Data-Set
# Used TransferSyntax: LittleEndianImplicit
(5400,1010) ox fd\60\01\b7    (ox = Either OB or OW)

$ dcmconv +te implicit.dcm toExplicit.dcm  (Convert to explicit VR)

$ dcmdump toExplicit.dcm
  ...
(5400,1010) OB fd\60\01\b7    (Not UN. Picked OB, but might have been OW.)

Is my understand of the standard wrong, or is this a bug?

Regards,

Andreas