8 DICOM File Service

The DICOM File Service specifies an abstract view of files from the point of view of a service user in the Data Format Layer. Constraining access to the content of files by Application Entities through such a DICOM File Service ensures independence of the Data Format Layer functions from specific Media Format and Physical Media selections.

Note: This DICOM File Service definition is abstract in the sense that it is only the specification of a boundary. Its focus is limited to the aspects directly related to the access to the data structures of the Media Format Layer (not the specifications of the data structures themselves). Even though the DICOM File Service may be described by means of a number of abstract primitives such as read, write, delete, etc., it is not intended to be the definition of an Application Programming Interface (API).

The DICOM File Service specified for Media Storage offers a basic service, simple enough to be supported by a wide range of commonly available Media Format (or file systems), but rich enough to provide the key functions to effectively manage files and access their content. The following sections specify the minimum mandatory requirements that shall be met by any physical media and associated media format to comply with the DICOM Media Storage model.

Note: It is acceptable that a specific Media Format offers more file services than those specified in the DICOM File Service. Such services may be internal to an implementation (i.e. not visible through the data structures on the Storage Media). Their usage is beyond the scope of the DICOM Standard. However, in cases where such services are reflected in the file structures of the Media format Layer or in the Data Set encoding an Information Object, the extension of such services in a manner that jeopardizes interoperability should not be done (e.g., File IDs longer than specified in the DICOM File Service).

8.1 File-set

The DICOM File Service offers the ability to create and access one or more files in a File-set. A File-set is a collection of files that share a common naming space within which File IDs (see Section 8.2) are unique. No semantics is attached to the order of Files within a File-set.

Notes: 1. The DICOM File Service does not require that Files within a File-set be simultaneously accessible (e.g., sequentially accessed media such as tapes are supported).

2. The DICOM File Service does not explicitly include the notion of distributing a File-set or a File across multiple "volumes/physical medium". However the transparent support by the Media Format Layer of such a feature is not precluded.

A File ID naming space (corresponding to a File-set) shall be associated with an appropriate feature of a Media Format defined structure. This mapping shall be specified in PS 3.12 for each Media Format specification (this is integral to the specification of the relationship between any specific Media Format services and the DICOM File Services defined in this PS 3.10).

Note: An example of such a relationship is to map the File ID naming space to a volume in a personal computer Media Format or a partition in a workstation File System on a removable medium. Another example is to map the File ID naming space to a directory and its tree of subdirectories. In this case it could offer the possibility of supporting multiple File-sets (one per directory) on the same physical medium. Each File-set would have its own DICOMDIR File. To ensure interoperability, PS 3.12 shall specify these specific mapping rules between the directories and the File ID naming space of a File-set (including the rules to unambiguously locate the DICOMDIR File).

A single File with the File ID DICOMDIR shall be included in each File-set.

Each File-set shall be uniquely identified by a File-set UID which shall be registered according to the UID registration rules specified in PS 3.5 of the DICOM Standard. When Files are added or removed from a File-set, the File-set UID shall not change.

A File-set may also be identified by a File-set ID, which provides a simple (but possibly not globally unique) human readable reference. A File-set ID is string of zero (0) to sixteen (16) characters from the subset of the G0 repertoire of ISO 8859 (see Section 8.5). A File-set ID may be associated or mapped to an appropriate identifier at the Media Format Layer.

Notes: 1. Continuing with the personal computer Media Format example used first in the previous note, a File-set ID may be defined to be identical to a volume label.

2. Non-DICOM Files (Files with a content not formatted according to the requirements of this Part of the DICOM Standard) may be present in a File-set. Such files should not contain the DICOM File Meta Information specified in Table 7.1-1 and may not be referenced by the DICOM Media Storage Directory (See Section 8.6).

A File-set Descriptor File (a "readme" file) may also be attached to a File-set. See PS 3.3 for a detailed specification of the Basic Directory IOD.

8.2 File IDs

Files are identified by a File ID that is unique within the context of a File-set. A File ID is an ordered sequence of File ID Components. A File ID may contain one to eight components. Each Component is a string of one to eight characters from a subset of the G0 repertoire of ISO 8859 (see Section 8.5)

Such a structure for File IDs (a sequence of components) allows the DICOM File Service to organize file selection in a hierarchical mode. No conventions are defined by the DICOM Standard for the use of the structure of File IDs components and their content (except for the reserved File ID DICOMDIR, see Section 8.6). Furthermore, no semantics shall be conveyed by the structure and content of such File IDs. This implies that when a File ID is assigned to any File in a File-set, the creating DICOM Application Entity may choose to structure the File ID as it wishes. Any other AE reading existing files or creating new files shall not be required to know any semantics the original creator may have associated with such a structure.

The File ID used to access a File through the abstract DICOM File Service is not necessarily the sole file identifier. The interchange Media Format (file system) may allow multiple file names to address the same physical file. Any use of alternate file names is beyond the scope of the DICOM Standard.

Notes: 1. A DICOM File ID is equivalent to the commonly used concept of "path name" concatenated with a "file name". An example of a valid DICOM File ID with four components shown separated by backslashes is: SUBDIR1\SUBDIR2\SUBDIR3\ABCDEFGH

2. As specified in the DICOM Storage Media Model, no semantics is attached to File ID content and structure as it relates to the DICOM Information Objects stored in these files. If used, the hierarchical structure simply provides a means to organize the Files of a File-set and facilitate their selection.

3. The DICOM File Service does not specify any "separator" between the Components of the File ID. This is a Value Representation issue that may be addressed in a specific manner by each Media Format Layer. In DICOM IODs, File ID Components are generally handled as multiple Values and separated by "backslashes". There is no requirement that Media Format Layers use this separator.

4. DICOM files stored on interchange media may have an alternate file name or link that uses less restricted file names, such as a filename extension (e.g., “.dcm” in accordance with RFC 3240).

8.3 File Management Roles and Services

When DICOM Application Entities participate in the exchange of information by the interchange of Storage Media, they perform through the DICOM File Service a number of Media Storage Services:

a. M-WRITE, to create new files in a File-set and assign them a File ID;

b. M-READ to read existing files based on their File ID;

c. M-DELETE to delete existing files based on their File ID;

e. M-INQUIRE FILE-SET to inquire free space available for creating new files within the File-set;

f. M-INQUIRE FILE to inquire date and time of file creation (or last update if applicable) for any file within the File-set.

A DICOM Application Entity may take one or more of the following three roles:

a. File-set Creator (FSC). Such an Application Entity, exercises this role by means of M-WRITE Operations to create the DICOMDIR File (see Section 8.6) and zero or more DICOM Files;

b. File-set Reader (FSR). Such an Application Entity, exercises this role by means of M-READ Operations to access one or more Files in a File-set. A File-set Reader shall not modify any of the files of the File-set (including the DICOMDIR File);

c. File-set Updater (FSU). Such an Application Entity, exercises this role by means of M-READ, M-WRITE, and M-DELETE Operations. It reads, but shall not modify, the content of any of the DICOM files in a File-set except for the DICOMDIR File. It may create additional Files by means of an M-WRITE or delete existing Files in a File-set by means of an M-DELETE.

Note: Although a File-set Updater (FSU) may include the functions corresponding to a File-set Creator (FSC) and a File-set Reader (FSR), it is not required that implementations supporting an FSU role also support an FSC or an FSR role..

The use of the concept of roles in DICOM Conformance Statements will result in a more precise expression of the capabilities of implementations supporting DICOM Media Storage. Conforming implementations shall support one of the following choices:

a. File-set Creator,

b. File-set Reader,

c. File-set Creator and File-set Reader,

d. File-set Updater,

e. File-set Updater and File-set Creator,

f. File-set Updater and File-set Reader,

g. File-set Updater, File-set Creator and File-set Reader.

Based on the roles supported by a DICOM Application Entity, the DICOM File Service shall support the Media Operations defined in Table 8.3-1.

Table 8.3-1Media Operations and roles

FSC Mandatory Not required Not required Mandatory Not required
FSR Not required Mandatory Not required Not required Mandatory
FSC+FSR Mandatory Mandatory Not required Mandatory Mandatory
FSU Mandatory Mandatory Mandatory Mandatory Mandatory
FSU+FSC Mandatory Mandatory Mandatory Mandatory Mandatory
FSU+FSR Mandatory Mandatory Mandatory Mandatory Mandatory
FSU+FSC+FSR Mandatory Mandatory Mandatory Mandatory Mandatory

Notes: 1. Media Preparation is outside the scope of this Part of the DICOM Standard. However it is assumed to be performed by the FS Creator.

2. The DICOM File Service does not require that file update capabilities (e.g., append) be supported by every Media Format Definition selected. The non-support of such file update capabilities to the DICOMDIR File may simply result in having to delete and create a new file in order to keep the directory information consistent.

3. If the content of a file needs to be updated or changed by an FSU, it is considered by this Part of the DICOM Standard as an M-DELETE Operation followed by an M-WRITE Operation. The FSU is responsible for ensuring the internal consistency of the File and its conformance to PS 3.10 and the specific SOP Class stored, exactly as if the FSU was creating a new File. In particular, if an FSU implementation needs to update the file content but is not able to recognize and fully process the content of the File Preamble (See Section 7.1), it may consider setting the first four bytes of the Preamble to "DICM" followed by 124 bytes to 00H. This would avoid introducing inconsistencies between the content of the File Preamble and the remainder of the file content. An example of this situation may occur when a TIFF IFD 0 Offset in the File Preamble points at a further TIFF IFD embedded in the DICOM Data Set, and the update operation changes the location of this embedded TIFF IFD.

8.4 File Content Access

The DICOM File Service offers the ability to access the content of any File of a File-set. The File content is an ordered string of zero or more bytes, where the first byte is at the beginning of the file and the last byte at the end of the File.

Note: This File content definition as an ordered string of bytes is related to the view provided at the DICOM File Service level. It may not correspond to the physical ordering of bytes of data on a specific medium.

The DICOM File Service shall manage the delimitation of the end of the File by ensuring the user of the File Service that read access beyond the last byte will be detected and reported to the DICOM File Service user. This delimitation function is performed by the Media Format Layer.

The DICOM File Service shall offer the ability:

a. for an FSR or FSU to perform an M-READ to read zero or more bytes of the content of a File;

b. for an FSC or FSU to perform an M-WRITE to write one or more bytes making the content of a File.

Note: The DICOM File Service does not require any specific capability for the selective read access or write access of the content of a file (e.g., seek or append). However it does not restrict specific Media Format definitions to support such features.

8.5 Character Set

File IDs and File-set IDs shall be character strings made of characters from a subset of the G0 repertoire of ISO 8859. The following characters form this subset:

A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z (uppercase)

1, 2, 3, 4, 5, 6, 7, 8, 9, 0 and _ (underscore)

Notes: 1. This is the character set defined for Control Strings (Value Representation CS - see PS 3.5 of the DICOM Standard) except that SPACE is not included.

2. This character set is selected to limit characters in File IDs and File-set IDs to those that do not conflict with reserved characters and delimiters in the file systems defined in PS 3.12. Component delimiters or other required demarcations defined in PS 3.12 are not part of File IDs or File-set IDs

8.6 Reserved DICOMDIR File ID

A single File with a File ID, DICOMDIR, shall exist as a member of every File-set. This File ID is made of a single Component (see Section 8.2 for the File ID structure). It contains the DICOM Media Storage Directory (see PS 3.3 for detailed specification of the Basic Directory IOD) which includes general information about the whole File-set. This general information is always present, but optionally the directory content may be left empty in environments where it would not be needed. If the DICOMDIR File does not exist in a File-set, the File-set does not conform to PS 3.10 of the DICOM Standard. The DICOMDIR shall not reference Files outside of the File-set to which it belongs.

Notes: 1. An example of the content of the DICOMDIR File may be found in Annex A.

2. If one chooses to map the origin of a File-set to a specific directory node in a specific Media Format, the File IDs, including the DICOMDIR File IDs, would be relative to this directory node path name.

The DICOMDIR File shall use the Explicit VR Little Endian Transfer Syntax (UID=1.2.840.10008.1.2.1) to encode the Media Storage Directory SOP Class. The DICOMDIR File shall comply with the DICOM File Format specified in Section 7 of this Standard. In particular the:

a. SOP Class UID in the File Meta Information (header of the DICOMDIR File) shall have the Value specified in PS 3.4 of this Standard for the Media Storage Directory SOP Class;

b. SOP Instance UID in the File Meta Information (header of the DICOMDIR File) shall contain the File-set UID Value. The File-set UID is assigned by the Application Entity which created the File-set (FSC role, see Section 8.3) with zero or more DICOM Files. This File-set UID Value shall not be changed by any other Application Entities reading or updating the content of the File-set.

Notes: 1. This policy reflects that a File-set is an abstraction of a "container" within which Files may be created or read. The File-set UID is related to the "container" not its content. A File-set in the DICOM File Service is intended to be mapped to a supporting feature of a selected Media Format (e.g., volume or partition).

2. The Standard does not prevent the making of duplicate copies of a File-set (i.e., a File-set with the same File-set UID). However, within a managed domain of File-sets, a domain specific policy may be used to prevent the creation of such duplicate File-sets.