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 PS3.8 and discussed in PS3.7.
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 PS3.7.
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.
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:
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.
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).
If an Application Entity receives an A-ASSOCIATE indication corresponding to a request that 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.
In other words, every sending AE is required to be able to convert any Data Set 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.