Section 10 Transfer Syntax

A Transfer Syntax is a set of encoding rules able to unambiguously represent one or more Abstract Syntaxes. In particular, it allows communicating Application Entities to negotiate common encoding techniques they both support (e.g., byte ordering, compression, etc.). A Transfer Syntax is an attribute of a Presentation Context, one or more of which are negotiated at the establishment of an Association between DICOM Application Entities. This Association negotiation is specified in PS 3.8 and discussed in PS 3.7.

The selection of a Transfer Syntax applies to the encoding rules for the Data Set portion of a DICOM Message only. All DICOM Standard and Private Transfer Syntaxes implicitly specify a fixed encoding for the Command Set portion of a DICOM Message as Specified in PS 3.7.

This part of the DICOM Standard defines standard DICOM Transfer Syntaxes and assigns a unique Transfer Syntax Name to each one. The standard DICOM Transfer Syntaxes are specified in Annex A. The DICOM notation for Transfer Syntax names is the notation used for UIDs (see Section 9).

The organization responsible for the definition and registration of DICOM Transfer Syntaxes is NEMA. NEMA guarantees uniqueness for all DICOM Transfer Syntax Names.

Privately defined Transfer Syntax Names may also be used; however, they will not be registered by NEMA. Organizations that define private Transfer Syntax Names shall follow the registration process defined in Section 9.2.

10.1 DICOM default transfer syntax

DICOM defines a default Transfer Syntax, the DICOM Implicit VR Little Endian Transfer Syntax (UID = "1.2.840.10008.1.2"), which shall be supported by every conformant DICOM Implementation. This implies that:

a) If an Application Entity issues an A-ASSOCIATE request, it shall offer the DICOM Implicit VR Little Endian Transfer Syntax in at least one of the Presentation Contexts associated with each offered Abstract Syntax.

Note: Offering Abstract Syntax (AS1) in two Presentation Contexts with Transfer Syntaxes (TS1) and (TS2) is not valid, but offering AS1-TS1, AS1-TS2 and AS1-TSD is valid because the DICOM Default Little Endian Transfer Syntax (TSD) is present in at least one of the Presentation Contexts that are based on Abstract Syntax (AS1).

b) If an Application Entity receives an A-ASSOCIATE indication corresponding to a request which follows the requirements specified in Section 10.1 a), every Presentation Context related to a given Abstract Syntax cannot be rejected in an A-ASSOCIATE response for the reason that none of the Transfer Syntaxes are supported.

Both of these requirements, a) and b), are waived when the Application Entity sending the pixel data has only access to the pixel data in lossy compressed form and a Transfer Syntax that uses a pixel data reference is not offered.

Requirement b) to accept the default Transfer Syntax is waived if a Transfer Syntax that uses a pixel data reference is offered.

Note: In other words, every sending AE is required to be able to convert any dataset it is going to transmit into the default Transfer Syntax, regardless of the form in which it originally received or stored the data set, except in the single case of when it received it in a lossy compressed form. In that exceptional case, the sending AE is permitted to propose only the lossy compressed Transfer Syntax appropriate to the lossy form that was received.

In particular, this waiver does not apply to data sets received in a lossless compressed form, which means that any AE receiving a data set in a lossless compressed Transfer Syntax that needs to re-send the data set is required to be able to decompress it in order to support (at least) the default Transfer Syntax.

10.2 Transfer syntax for a DICOM default of lossless JPEG compression

DICOM defines a default for lossless JPEG Image Compression, which uses a subset of coding Process 14 with a first-order prediction (Selection Value 1). It is identified by Transfer Syntax UID = "1.2.840.10008.1.2.4.70" and shall be supported by every DICOM implementation that chooses to support one or more of the lossless JPEG compression processes. This implies that:

a) If an Application Entity issues an A-ASSOCIATE request where any offered Abstract Syntaxes is associated in one or more Presentation Context with a JPEG lossless compression Transfer Syntax, at least one of the Presentation Contexts which include this Abstract Syntax, shall include the DICOM Default Lossless JPEG Compression Transfer Syntax and the DICOM Default Transfer Syntax (uncompressed).

Note: Offering Abstract Syntax (AS1) in two Presentation Contexts with Transfer Syntaxes JPEG lossless (JL1) and (JL2) is not valid, but offering AS1-JL1, AS1-JL2, AS1-TSD, and AS1-JLD is valid because the DICOM Default JPEG Lossless Transfer Syntax (JLD) and the DICOM Default Transfer Syntax (TSD) are present in at least one of the Presentation Contexts which are based on Abstract Syntax (AS1).

b) If an Application Entity that supports one or more lossless JPEG Transfer Syntax receives an A-ASSOCIATE indication corresponding to a request which follows the requirements specified in Section 10.2 a), every Presentation Context related to a given Abstract Syntax cannot be rejected in an A-ASSOCIATE response for the reason that the DICOM Default lossless JPEG Transfer Syntax is not supported.

10.3 Transfer syntaxes for a DICOM defaults of lossy JPEG compression

DICOM defines defaults for Lossy JPEG Image Compression, one for 8-bit images and the other for 12-bit images. JPEG coding Process 1 (identified by Transfer Syntax UID = "1.2.840.10008.1.2.4.50") is used for 8-bit images. JPEG coding Process 4 (identified by Transfer Syntax UID = "1.2.840.10008.1.2.4.51") is used for 12-bit images. This implies that:

a) If an Application Entity issues an A-ASSOCIATE request where any offered Abstract Syntaxes is associated in one or more Presentation Context(s) with a JPEG lossy compression Transfer Syntax, at least one of the Presentation Contexts that include this Abstract Syntax, shall include the appropriate DICOM Default Lossy JPEG Compression Transfer Syntax.

Note: 1. Offering Abstract Syntax (AS1) in two Presentation Contexts with Transfer Syntaxes JPEG lossy (JL1) and (JL2) is not valid, but offering AS1-JL1, AS1-JL2 and AS1-JLD is valid because the DICOM Default JPEG Lossy Transfer Syntax (JLD) is present in at least one of the Presentation Contexts that are based on Abstract Syntax (AS1).2. The DICOM Default Transfer Syntax (uncompressed) may be offered if the sender has access to the original pixel data in an uncompressed or lossless compressed form.

b) If an Application Entity that supports one or more Lossy JPEG Transfer Syntaxes receives an A-ASSOCIATE indication corresponding to a request which follows the requirements specified in Section 10.3 a), every Presentation Context related to a given Abstract Syntax cannot be rejected in an A-ASSOCIATE response for the reason that the DICOM Default lossy JPEG Transfer Syntax is not supported.

10.4 Transfer Syntax for DICOM RLE Compression

DICOM defines the RLE Compression (see Annex G). This implies that:

a) If an Application Entity issues an A-ASSOCIATE request where any offered Abstract Syntaxes is associated in one or more Presentation Contexts(s) with RLE compression Transfer Syntax, at least one of the Presentation Contexts which include this Abstract Syntax, shall include the DICOM Default Transfer Syntax (uncompressed).

10.5 Transfer syntax for a DICOM default of Lossless and LOSSY (Near-Lossless) JPEG-LS compression

One Transfer Syntax is specified for JPEG-LS Lossless Image Compression, and one Transfer Syntax is specified for JPEG-LS Lossy (Near-Lossless) Image Compression. The JPEG-LS Lossless Transfer Syntax shall be supported as a baseline if the JPEG-LS Lossy (Near-Lossless) Transfer Syntax is supported.

10.6 Transfer syntax for JPEG 2000 compression

One Transfer Syntax is specified for JPEG 2000 Image Compression (Lossless Only), and one Transfer Syntax is specified for JPEG 2000 Image Compression. Either of these may be negotiated separately and there is no default or baseline specified (other than described in section 10.1).

Notes: 1. All JPEG 2000 codecs are required by ISO/IEC 15444-1 to support both reversible and irreversible wavelet and multi-component transformations. The reason for specifying two separate Transfer Syntaxes in DICOM is to allow an application to request the transfer of images in a lossless manner when possible. The JPEG 2000 Image Compression Transfer Syntax allows for either lossless or lossy compression to be used at the sender’s discretion.

2. No baseline using other compression schemes is required.

3. When the pixel data has been received in the JPEG 2000 Image Compression Transfer Syntax, since it may have been lossy compressed, the waiver of the requirement in Section 10.1 to support the DICOM default Transfer Syntax still applies.

In addition, one Transfer Syntax is specified for JPEG 2000 Multi-component Image Compression (Lossless Only) with Multi-Component Transformation Extensions, and one Transfer Syntax is specified for JPEG 2000 Multi-component Image Compression with Multi-Component Transformation Extensions. Either of these may be negotiated separately and there is no default or baseline specified (other than described in section 10.1).

Note: JPEG 2000 codecs that support the Part 2 JPEG 2000 Multi-Component Transformation Extensions are required to support all the multi-component extensions as described in Annex J of ISO/IEC 15444-2. This includes both array based transformations and the 9-7 and 5-3 wavelet transformations that are also used in Part 1 of JPEG 2000. This also includes component reordering, component collections and application of more than one multi-component transformation in succession.

10.7 Transfer syntax for MPEG2 MP@ML Image compression

One Transfer Syntax is specified for MPEG2 MP@ML Image Compression.

10.8 Transfer syntax for JPIP REFERENCED PIXEL DATA

Two Transfer Syntaxes are specified for JPIP Referenced Pixel Data.

The persistence of the references in objects transferred with one of these transfer syntaxes is not defined. That is, applications should make no assumptions as to the timeframe when the referenced pixel data will be available. Due to the indeterminate time that the URL remains valid, it may be inappropriate to cache the URL. Because the pixel data may not have been retrieved in its entirety or full fidelity, it may be inappropriate to use this transfer syntax for the purpose of permanent storage or to reference such instances in Storage Commitment, Modality Performed Procedure Step and General Purpose Performed Procedure Step service classes.

These transfer syntaxes shall not be used for media storage defined by PS 3.10.

10.9 Transfer syntax for MPEG2 MP@HL Image compression

One Transfer Syntax is specified for MPEG2 MP@HL Image Compression.

10.10 Transfer Syntax for MPEG-4 AVC/H.264 HIP@LEVEL4.1 IMAGE COMPRESSION

One Transfer Syntax is specified for MPEG-4 AVC/H.264 High Profile / Level 4.1 Image Compression and one Transfer Syntax is specified for MPEG-4 AVC/H.264 BD-compliant High Profile / Level 4.1. Transfer Syntax MPEG-4 AVC/H.264 High Profile / Level 4.1 corresponds to the ITU-T H.264 standard’s profile and level specifications. Transfer Syntax MPEG-4 AVC/H.264 BD-compliant High Profile / Level 4.1 corresponds to a restricted set of spatial and temporal resolutions described Table 8-4. This Transfer Syntax limits the ITU-T H.264 High Profile / Level 4.1 to HD video formats that are supported by Blu-ray TM (BDRWP 2.B).