Contexts and the Association Requestor#

When acting as the association requestor (usually the SCU), you must propose presentation contexts to be negotiated by the association process. There are a couple of simple rules for these:

  • There must be at least 1 and up to 128 proposed presentation contexts.

  • You can use the same abstract syntax in more than one presentation context.

  • Each presentation context must have at least one transfer syntax.

In pynetdicom this is accomplished through one of the following methods:

  1. Setting the AE.requested_contexts attribute directly using a list of PresentationContext items.

    from pynetdicom import AE, build_context
from pynetdicom.sop_class import Verification

ae = AE()
ae.requested_contexts = [build_context(Verification)]
assoc = ae.associate("127.0.0.1", 11112)

  2. Using the AE.add_requested_context() method to add a new PresentationContext to the AE.requested_contexts attribute.

    from pynetdicom import AE
from pynetdicom.sop_class import Verification

ae = AE()
ae.add_requested_context(Verification)
assoc = ae.associate("127.0.0.1", 11112)

  3. Supplying a list of PresentationContext items to AE.associate() via the contexts keyword parameter.

    from pynetdicom import AE, build_context
from pynetdicom.sop_class import Verification

ae = AE()
requested = [build_context(Verification)]
assoc = ae.associate("127.0.0.1", 11112, contexts=requested)

The abstract syntaxes you propose should match the SOP Class or Meta SOP Class that corresponds to the service you wish to use. For example, if you’re intending to use the storage service then you’d propose one or more abstract syntaxes from the corresponding SOP Class UIDs.

The transfer syntaxes you propose for each abstract syntax should match the transfer syntax of the data you wish to send. For example, if you have a CT Image Storage dataset with a (0002,0010) Transfer Syntax UID value of 1.2.840.10008.1.2.4.50 (JPEG Baseline) then you won’t be able to send it unless you propose (and get accepted) a presentation context with a matching transfer syntax.

Note

Uncompressed and deflated transfer syntaxes are the exception to this rule as pydicom is able to freely convert between these (provided the endianness remains the same).

If you have data encoded in a variety of transfer syntaxes then you can propose multiple presentation contexts with the same abstract syntax but different transfer syntaxes:

>>> from pydicom.uid import ImplicitVRLittleEndian, JPEGBaseline
>>> from pynetdicom import AE
>>> from pynetdicom.sop_class import CTImageStorage
>>> ae = AE()
>>> ae.add_requested_context(CTImageStorage, ImplicitVRLittleEndian)
>>> ae.add_requested_context(CTImageStorage, JPEGBaseline)
>>> for cx in ae.requested_contexts:
...     print(cx)
...
Abstract Syntax: CT Image Storage
Transfer Syntax(es):
    =Implicit VR Little Endian
Abstract Syntax: CT Image Storage
Transfer Syntax(es):
    =JPEG Baseline (Process 1)

Provided both contexts get accepted then it becomes possible to transfer CT Image datasets encoded in JPEG Baseline and/or Implicit VR Little Endian. Alternatively it may be necessary to decompress() datasets prior to sending (as Implicit VR Little Endian should always be accepted).