DICOM Transfer Syntax Negotiation

Transfer Syntax Negotiation in DICOM is relatively simple, but often misunderstood. It is important to understand this fully before attempting to take control using DicomObjects.

The transfer syntaxes available for network transmission depend on what is agreed between the applications during initial negotiation (and at no other time. The steps are as follows:

  1. The SCU sends the SCP a list of “presentation contexts”, each of which has a SOP class and one or more transfer syntaxes. Normally each SOP class is offered only once, with a list of transfer syntaxes, though it is allowable to offer a SOP class more than once with a different list of offered transfer syntaxes.

  2. For each presentation context, the SCP chooses just one of the transfer syntaxes from each of those offered, or it may reject a presentation context entirely if it doesn’t accept either the SOP class or any of the offered transfer syntaxes (Only support for default Implicit VR Little Endian is mandatory in DICOM SCPs)

  3. When the SCU sends an image (C-STORE) or uses any other DICOM services, it must use one of the negotiated presentation contexts which apply to that Image or Service SOP class (e.g. CT Image, or Study Root C-FIND). So, in the “normal” situation where an SCU offers each SOP class once, with a list of all the transfer syntaxes it supports, the choice of transfer syntax is effectively made by the SCP, not the SCU (though if a lossy format is selected, only the SCU controls quality).

For details of how this works in DicomObjects, see one of the following pages: