Digital Imaging and Communications in Medicine (DICOM)

10.2 Special DICOM File Formats 227
instead of SUBDIR1\SUBDIR2\SUBDIR3\FNAME. However, the use of SOPs
in file names can be problematic as well: SOP strings are long (up to 64 characters),
and if used in several file name components, can easily outgrow what
your software can handle. Be prepared.
No wonder that it has become virtually impossible to recognize a DICOM
file by its name. It can be practically anything: with the “.dcm” extension or
not; with UID naming syntax or a standard eight-character uppercase name.
Moreover, different operating systems and media could have their own requirements
on what a valid file name should be. Which is why looking for the DICM
prefix in a DICOM file header is the only reliable way of telling DICOM files
from other files.
10.2
Special DICOM File Formats
10.2.1
DICOMDIR
DICOMDIR is a very special DICOM file. While all other DICOM files store
their own DICOM data objects, DICOMDIR stores the information about
DICOM files in a given file directory (folder). Thus, DICOMDIR plays the role
of a small DICOM database, or an index of DICOM files, placed in the root
folder of the media.
File indexing
File indexing – creating a special file that contains the information about
the other files in a particular folder – is a very common task in many software
applications. Operating systems, email, and multimedia software all
try to index the files on your computer. File indexing speeds up the access
to the file data and improves searching the files with user-specified search
keys (such as keywords). In particular, if file search key values are stored in
the index file, only the index file needs to be searched.
The flip side is that creating and maintaining the index file takes processing
power, and the index file needs to be updated every time something
changes in the indexed folder.
Consequently, just like any DICOM database (see more in 10.5), DICOMDIR
organizes all directory data into four principal DICOM levels: Patient, Study, Series,
and Image, as shown on Fig. 74. Therefore, for each file in the DICOMDIR
folder, DICOMDIR will record four entries – patient, study, series, and image
information – corresponding to this file.