Digital Imaging and Communications in Medicine (DICOM)

10.1 DICOM File Format 225
10.1.3
Data Object
The DICOM data object goes right after the 0002 group and stores the actual
DICOM data; our usual DICOM data object, as we have used it so many times
with DICOM networking. DICOM object group numbering starts with group
0008 (see the DICOM Data Dictionary), so it is fairly easy for an application to
identify where the file meta information (group 0002) ends and the data object
(group 0008) begins.
If you are writing a DICOM application, be careful at this point. For example,
you are required to encode file meta information (group 0002) with
explicit VR syntax, but this is not necessarily true for the DICOM data (groups
0008 and higher). If your DICOM application does not switch to the VR syntax
used for the data object encoding (as indicated in the “Transfer Syntax”
(0002,0010) field), it will fail to read the data. For this very reason, and to support
DICOM compression formats (using explicit VR encoding), DICOM recommends
using explicit VR encoding throughout the entire DICOM file; that
is, for the DICOM data part as well. 30
Finally, the very end of a DICOM file can be padded with “Data Set trailing
padding” (FFFC,FFFC) elements to achieve a certain length. Is this really needed?
The DICOM standard says not. If your application runs into this padding element,
ignore it and gracefully exit because you have reached the end of the DICOM file.
Most DICOM files and software do not use (FFFC,FFFC) elements.
This very much concludes the review of the DICOM file format. Easily,
99.9% of the DICOM files you ever run into will follow this layout, and the
remaining 0.1% will probably be broken or corrupted.
10.1.4
DICOM File IDs and Names
We have mentioned a few times that one cannot identify a DICOM file by its
name. Nevertheless, part PS3.10 prescribes a couple of simple rules that standard
DICOM file names should follow.
According to these rules, all DICOM files are labeled with DICOM File IDs:
unique file identifiers, essentially corresponding to the file names. File ID consists
of up to eight components. In traditional terms, the very last component
corresponds to the short file name and the preceding components correspond
to the folders containing the file path (see Fig. 73). As a result, the file ID corresponds
to the full name of the file. Each component can include only the
30 At the same time, DICOM default encoding (Transfer Syntax) is Implicit Little Endian.
See how explicit and implicit syntaxes have become equally important. You can
never rely on using just one of them.