Digital Imaging and Communications in Medicine (DICOM)

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Chapter 10 DICOM Media: Files, Folders, and DICOMDIRs
Table 50 (adapted from DICOM PS3.10) gives an example of the DICOM-
DIR file with sample data. It might appear lengthy, but look closer and you will
see that the list of all DICOMDIR items (patients, studies, series, and images)
is simply inserted into the DICOMDIR object as an SQ sequence elem ent
(0004,1220). For each entry in the DICOMDIR (0004,1220) sequence, the
DICOMDIR object stores two types of data:
1. Entry-specific selection keys. This data type is provided to facilitate item
searches in DICOMDIR. For example, series modality (0008,0060) is one
of the most frequently used selection criteria when searching for series, so
it is wise to have it as a selection key in our DICOMDIR index; we will
know what modalities are present in the given directory. Nearly all DICOM
software usually has some kind of DICOMDIR browser presenting the user
with the list of entries found in DICOMDIR. For example, if you insert a
DICOM CD into a PACS workstation, it will usually show you the list of
patients and studies on the CD, which are extracted from the DICOMDIR
entries. Patient names, study dates, modalities, and other useful data also
come from the selection keys.
2. Basic Directory Information Object. The second type of DICOMDIR entry
data is stored in group 0004 elements. We have not mentioned this group
yet, so it is time to do so. Group 0004 is reserved for the Basic Directory Information
Object and, as you can see from Table 50, stores directory-related
information about the DICOMDIR entries: file IDs, relationships between
the files, and so on. In essence, any DICOMDIR object is an instance of the
Basic Directory IOD, just like any CT image object is an instance of the CT
image IOD. The Basic Directory Information Object is meant to be an abstract
representation of any media directory, something corresponding to a
set of DICOM files, wherever they might be.
The entries listed in the DICOMDIR file do not have to correspond to DICOM
images only, just like DICOM files do not have to store only DICOM images.
Fig. 74 DICOMDIR structure. Each image corresponds to a DICOM file in the
DICOMDIR directory