Digital Imaging and Communications in Medicine (DICOM)

226
Chapter 10 DICOM Media: Files, Folders, and DICOMDIRs
following characters: uppercase letters from A to Z, digits from 0 to 9, or the
underscore character.
The DICOM file ID naming convention looks like a bit disoriented attempt
to marry file names and DICOM UIDs. DICOM UIDs with UI (see 5.5.8)
VR format are used to identify all unique strings, but they do not use letters
or underscores; they can contain only digits separated by periods, such as
1.2.840.10008.1.1. DICOM file IDs, on the contrary, are delineated by separators
(usually slashes, although DICOM does not specify any particular character),
and their eight 8-character components with 7 separators in between can
exceed the 64-character length of UIDs.
Finally (and this is for you, my dear DICOM software developers), as you
should remember, the typical file ID component separator, the backslash (\),
also happens to be an official DICOM wildcard character (see 5.3.2), meaning
“logical or”. In other words, according to DICOM, file ID “SUBDIR1\SUB-
DIR2\SUBDIR3\FNAME” would in any other DICOM string mean “SUBDIR1
or SUBDIR2 or SUBDIR3 or FNAME”, which of course is a completely different
thing. Breaking backslashed file names into separate “or’d” components is
one of the most widespread file-name-related bugs in DICOM software. The
only simple remedy to use forward slashes (/) in all file names.
Then comes the question of the legendary “.dcm” extension, which is commonly
attached to DICOM file names by countless DICOM programs. In operating
systems, each file extension can be associated with a particular fileprocessing
program, so .dcm comes in very handy in this respect: clicking on
a .dcm file in your file manager will immediately launch your DICOM application.
On the other hand, according to the File ID syntax from Fig. 73 there is
no .dcm. Moreover, in various parts of the DICOM standard the .dcm file name
extension is either prohibited or required. I guess the intriguing question of
using .dcm is still debated in the corridors at DICOM WG meetings.
As a result of all this, people taking DICOM File ID conventions seriously
should be prepared to see many wonderful surprises in the real world. Probably
for this very reason, many DICOM implementations simply use DICOM SOP
UIDs (uniquely identifying each DICOM object instance) as file names, and
you may very frequently see something like:
1.2.840.10008.234.2354.437345.79086/1.2.840.10008.8.4568.243.09.dcm
Fig. 73 DICOM file ID