Digital Imaging and Communications in Medicine (DICOM)

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Chapter 9 DICOM Associations
You can also tell that the most important part is Act 1, the actual association
establishment. Now that we know the logic behind it, we can consider the
technicalities.
9.2
Association Establishment
The contents of the DICOM association establishment are shown on Fig. 54.
Do not try to grasp every detail immediately; we will learn everything step by
step in the following sections. Instead, look at the entire structure and try to
think how it fits into the “Act 1” logic.
To begin the process, the association-requesting (calling) AE builds and
sends an A-Associate-RQ message (first part of the diagram), requesting that
the receiving (called) AE start an association. This message is packed with several
Presentation Contexts (our business cards), and additional user information
data, describing the capabilities of the AE initiating the association (user
refers to the requesting AE).
The receiving AE looks at all proposed communication parameters, picks
the most appropriate, and replies with an A-Associate-AC message if the association
is accepted, as shown in our diagram. If none of the proposed parameters
matches the receiving AE profile (for example, you are trying to store
an MR image to a CT scanner), it will reply with an A-Associate-RJ message,
rejecting the association. It could also reply with a more general A-Abort message
to abort associations at any time.
Straightforward, isn’t it? So let’s glance at the contents of the association
establishment message first; then we can revisit the entire association messaging
workflow.
9.3
Abstract Syntax
As already explained, Abstract Syntaxes, present in A-Associate-RQ and
A-Associate-AC messages, are very important for DICOM association establishment.
They describe the services that DICOM applications can render to each
other. In other words, Abstract Syntaxes encode SOPs (see Chaps. 7 and 8, and
the table in Appendix A.2), supported on the communicating AEs. We already
learned that each DICOM SOP has its own Abstract Syntax Name such as “MR
image storage”, “CT image storage”, “MWL”, and so on, which DICOM encodes
with an Abstract Syntax (SOP) UID. For example, 1.2.840.10008.5.1.4.1.1.4 is
the UID for “MR image storage”. Earlier (see 5.5.8) we talked about UIDs and
their importance for identifying object instances. DICOM networking uses