When two systems negotiate for a connection they must agree on the presentation context. The presentation context is always the combination of a SOP Class and one or more Transfer Syntaxes. It is possible to specify a list of Transfer Syntaxes for each SOP Class when a device starts its DICOM negotiation.The Transfer Syntax defines how the information is exchanged and how the data is represented. It specifies the byte ordering (Little Endian or Big Endian), whether or not the VR is specified as part of the DICOM message exchange (Implicit or Explicit VR), and compression.
The VR for each data element specifies the encoding of the data, and is similar to a format or type definition for software languages. It is important to specify the data VR because there are different types of attributes that each requires its own agreement on encoding and interpretation. The data dictionary lists all attributes and VRs.
VRs are of two kinds – Explicit VR and Implicit VR. The Implicit VR Little Endian Transfer Syntax is the default Transfer Syntax that each device has to support independently for each SOP Class. This is similar to an agreement of a common language among devices. Data compression is specified as part of the Transfer Syntax. There are two basic groups of compression schemes in use: lossless, which is used when the original image can be completely reconstructed from the compressed data, and lossy compression, causing some degree of information loss.