With a billion instances in a repository, the Inventory itself may be on the order of 300 GB (i.e., > 238 bytes) in size. Producing and processing an Inventory of such size may exceed some resource constraints of the creator and/or user application (such as 32-bit indices). The content of an Inventory may therefore need to be divided into more than one SOP Instance.
Note
Because an Inventory object has relatively low information entropy, compression of the Inventory object may substantially decrease its size. Such compression may be applied to the Inventory SOP Instance using the Deflated Little Endian Transfer Syntax (see Section A.5 “DICOM Deflated Little Endian Transfer Syntax (Explicit VR)” in PS3.5), or if the Inventory is stored in a DICOM File Format, the entire file may be compressed (e.g., using ZIP or GZIP). However, generally the instance needs to be fully constructed before it is compressed, and fully uncompressed before it is processed, and inventory applications need to be designed for the full potential size.
Very large repositories may also be partitioned or distributed into (semi-)independent subsystems. Production of an inventory for such distributed subsystems may be performed by parallel processes, which would be facilitated by each process producing a separate Inventory SOP Instance.
The Inventory IOD supports such cases of multiple SOP Instances comprising a single logical Inventory. The IOD supports one SOP Instance incorporating the content of one or more others by reference. The IOD thus has the structure shown schematically in Figure YYYY.3-2. An Inventory SOP Instance may contain links to other Inventory SOP Instances whose content is incorporated by reference, or may contain inventoried Study records, or both. A set of incorporated SOP Instances form a tree structure, with one SOP Instance at the root.
Within any tree (or subtree) of Inventory SOP Instances, the root node specifies the Scope of Inventory and Completion Status for the entire tree, regardless of the value of those Attributes in subsidiary referenced objects. As will be seen in the examples, this is true regardless of the process used to create the Inventory, whether with new objects, or with reference to previously created objects. The root object is the last SOP Instance to be completed in a tree, and it thus contains the final Completion Status for the tree and its Scope of Inventory. Any Completion Status value other than COMPLETE implies that the defined Scope of Inventory is not satisfied with this object as the root of the tree.
It is the responsibility of the creator of the root object for a tree to ensure that the Completion Status value accurately describes the content of the tree relative to the Scope of Inventory at the Content Date and Time for the repository system identified in the General Equipment Module.