The DICOM File Format provides a means to encapsulate in a file the Data Set representing a SOP Instance related to a DICOM IOD. As shown in Figure 7-1, the byte stream of the Data Set is placed into the file after the DICOM File Meta Information. Each file contains a single SOP Instance.
Figure 7-1File-set and File Format
The File Meta Information includes identifying information on the encapsulated Data Set. This header consists of a 128 byte File Preamble, followed by a 4 byte DICOM prefix, followed by the File Meta Elements shown in Table 7.1-1. This header shall be present in every DICOM file.
The File Preamble is available for use as defined by Application Profiles or specific implementations. This Part of the DICOM Standard does not require any structure for this fixed size Preamble. It is not required to be structured as a DICOM Data Element with a Tag and a Length. It is intended to facilitate access to the images and other data in the DICOM file by providing compatibility with a number of commonly used computer image file formats. Whether or not the File Preamble contains information, the DICOM File content shall conform to the requirements of this Part and the Data Set shall conform to the SOP Class specified in the File Meta Information.
Notes: 1. If the File Preamble is not used by an Application Profile or a specific implementation, all 128 bytes shall be set to 00H. This is intended to facilitate the recognition that the Preamble is used when all 128 bytes are not set as specified above.
2. The File Preamble may for example contain information enabling a multi-media application to randomly access images stored in a DICOM Data Set. The same file can be accessed in two ways: by a multi-media application using the preamble and by a DICOM Application that ignores the preamble.
The four byte DICOM Prefix shall contain the character string "DICM" encoded as uppercase characters of the ISO 8859 G0 Character Repertoire. This four byte prefix is not structured as a DICOM Data Element with a Tag and a Length.
The Preamble and Prefix are followed by a set of DICOM Meta Elements with Tags and Lengths as defined in Table 7.1-1.
Table 7.1-1DICOM File Meta Information
|Attribute Name||Tag||Type||Attribute Description|
|File Preamble||No Tag or Length Fields||1||A fixed 128 byte field available for Application Profile or implementation specified use. If not used by an Application Profile or a specific implementation all bytes shall be set to 00H. File-set Readers or Updaters shall not rely on the content of this Preamble to determine that this File is or is not a DICOM File.|
|DICOM Prefix||No Tag or Length Fields||1||Four bytes containing the character string "DICM". This Prefix is intended to be used to recognize that this File is or not a DICOM File.|
|File Meta Information Group Length||(0002,0000)||1||Number of bytes following this File Meta Element (end of the Value field) up to and including the last File Meta Element of the Group 2 File Meta Information|
|File Meta Information Version||(0002,0001)||1||This is a two byte field where each bit identifies a version of this File Meta Information header. In version 1 the first byte value is 00H and the second value byte value is 01H. Implementations reading Files with Meta Information where this attribute has bit 0 (lsb) of the second byte set to 1 may interpret the File Meta Information as specified in this version of PS 3.10. All other bits shall not be checked. Note: A bit field where each bit identifies a version, allows explicit indication of the support of multiple previous versions. Future versions of the File Meta Information that can be read by verson 1 readers will have bit 0 of the second byte set to 1|
|Media Storage SOP Class UID||(0002,0002)||1||Uniquely identifies the SOP Class associated with the Data Set. SOP Class UIDs allowed for media storage are specified in PS 3.4 of the DICOM Standard - Media Storage Application Profiles.|
|Media Storage SOP Instance UID||(0002,0003)||1||Uniquely identifies the SOP Instance associated with the Data Set placed in the file and following the File Meta Information.|
|Transfer Syntax UID||(0002,0010)||1||Uniquely identifies the Transfer Syntax used to encode the following Data Set. This Transfer Syntax does not apply to the File Meta Information. Note: It is recommended to use one of the DICOM Transfer Syntaxes supporting explicit Value Representation encoding to facilitate interpretation of File Meta Element Values. JPIP Referenced Pixel Data Transfer Syntaxes are not used. (See PS 3.5 of the DICOM Standard).|
|Implementation Class UID||(0002,0012)||1||Uniquely identifies the implementation that wrote this file and its content. It provides an unambiguous identification of the type of implementation that last wrote the file in the event of interchange problems. It follows the same policies as defined by PS 3.7 of the DICOM Standard (association negotiation).|
|Implementation Version Name||(0002,0013)||3||Identifies a version for an Implementation Class UID (0002,0012) using up to 16 characters of the repertoire identified in Section 8.5. It follows the same policies as defined by PS 3.7 of the DICOM Standard (association negotiation).|
|Source Application Entity Title||(0002,0016)||3||The DICOM Application Entity (AE) Title of the AE that wrote this file's content (or last updated it). If used, it allows the tracing of the source of errors in the event of media interchange problems. The policies associated with AE Titles are the same as those defined in PS 3.8 of the DICOM Standard.|
|Private Information Creator UID||(0002,0100)||3||The UID of the creator of the private information (0002,0102).|
|Private Information||(0002,0102)||1C||Contains Private Information placed in the File Meta Information. The creator shall be identified in (0002,0100). Required if Private Information Creator UID (0002,0100) is present.|
Except for the 128 byte preamble and the 4 byte prefix, the File Meta Information shall be encoded using the Explicit VR Little Endian Transfer Syntax (UID=1.2.840.10008.1.2.1) as defined in DICOM PS 3.5. Values of each File Meta Element shall be padded when necessary to achieve an even length, as specified in PS 3.5 by their corresponding Value Representation. The Unknown (UN) Value Representation shall not be used in the File Meta Information. For compatibility with future versions of this Standard, any Tag (0002,xxxx) not defined in Table 7.1-1 shall be ignored.
Values of all Tags (0002,xxxx) are reserved for use by this Standard and later versions of DICOM. Data Elements with a group of 0002 shall not be used in datasets other than within the File Meta Information.
Note: PS 3.5 specifies that Elements with Tags (0001,xxxx), (0003,xxxx), (0005,xxxx), and (0007,xxxx) shall not be used.
Each File shall contain a single Data Set representing a single SOP Instance related to a single SOP Class (and corresponding IOD).
Note: A file may contain more than a single 2D image frame as specific IODs may be defined to include multiple frames.
The Transfer Syntax used to encode the Data Set shall be the one identified by the Transfer Syntax UID of the DICOM File Meta Information.
Notes: 1. The Transfer Syntax used to encode the Data Set cannot be changed within the Data Set; i.e., the Transfer Syntax UID Data Element may not occur anywhere within the Data Set, e.g., nested within a Sequence Item.
2. A DICOM Data Set does not include its total length. The end of the file indication provided by the DICOM File Service (see Section 8.4) is the only indication of the end of the Data Set.
The last Data Element of a Data Set may be Data Element (FFFC,FFFC) if padding of a Data Set is desired when a file is written. The Value of this Data Set Trailing Padding Data Element (FFFC,FFFC) has no significance and shall be ignored by all DICOM implementations reading this Data Set. File-set Readers or Updaters shall be able to process this Data Set Trailing Padding (FFFC,FFFC) either in the Data Set following the Meta Information or in Data Sets nested in a Sequence (See PS 3.5 of the DICOM Standard).
The DICOM File Format does not include file management information in order to avoid duplication with functions related to the Media Format Layer. If necessary for a given DICOM Application Profile, the following information should be offered by the Media Format Layer:
a. File content owner identification;
b. File access statistics (e.g. date and time of creation);
c. Application file access control;
d. Physical media access control (e.g., write protect).
A Secure DICOM File shall contain a single DICOM File encapsulated with the Cryptographic Message Syntax as defined in RFC 3369. Depending on the cryptographic algorithms used for encapsulation, a Secure DICOM File can provide one or more the following security properties:
Data Confidentiality (by means of encryption)
Data Origin Authentication (by means of certificates and digital signatures)
Data Integrity (by means of digital signatures)
In addition, a Secure DICOM File offers the possibility to communicate encryption keys and certificates to the intended recipients by means of key transport, key agreement or symmetric key-encryption key schemes.