The general organization of the Basic Directory IOD is introduced in this Section. A simple example is also provided to illustrate the application of this organization.
The Basic Directory IOD organization is based on a hierarchy of Directory Entities. At the origin of this inverted tree is a root Directory Entity. Each Directory Entity includes one or more Directory Records, which in turn may each reference a lower level Directory Entity.
Directory Records serve to reference objects stored in the Files of the File-set. The organization of the Directory is depicted by the Basic Directory IOD entity/relationship model presented in Figure F.2-1.
Each Directory Record, irrespective of the Directory Entity it is included in, contains four types of information:
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A reference to a lower level Directory Entity or Referenced Directory Entity. This reference may be absent if such a lower level Directory Entity does not exist for an instance of a Directory Record;
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A reference to a File of the File-set in which is stored a "Referenced Object" (formally called in DICOM a Referenced SOP Instance). This reference may be absent if no File is referenced. Files may be referenced directly by their File ID;
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A set of "selection Keys", specific to a Referenced Object, which will allow its selection among all the records included in a given Directory Entity;
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A mechanism to chain the various Directory Records that belong to the same Directory Entity.
This generic content of a Directory Record is further specialized based on its specific type in the context the Basic Directory IOD Information Model specified in Section F.4 (e.g., a Study Record, a Series Record, etc.). A Directory Entity may include Directory Records of different Types. By standardizing a number of specific Directory Records (see Section F.5) in the context of the Basic Directory IOD Information Model, one allows the definition of a variety of directory contents while maintaining a framework for interoperability.
To facilitate the management and update of the Directory Information a number of rules are defined:
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Any Lower-Level Directory Entity shall be referenced by at most one higher-level Directory Record. Not allowing multiple higher-level Directory Records to reference the same Lower-Level Directory Entity simplifies the management of the deletion (or inactivation) of Directory Records and Lower-Level Directory Entities and associated Directory Records
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Any Directory Record shall belong to a single Directory Entity. This rule and the above rule, makes the Basic Directory IOD itself strictly hierarchical
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All files referenced by a Directory shall be present in the same File-Set to which the directory belongs
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Non-DICOM files that are not referenced by the Directory may be included in the File-set space. The means of access to such Files and the semantics associated with their absence from the Directory is beyond the scope of the DICOM Standard
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If a DICOMDIR contains a Directory Information Module, all DICOM Files of the File-set shall be referenced by a Directory Record
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Any File of the File-set shall be directly referenced by at most one Directory Record of the Directory.