Application Entity

The first step in DICOM networking with pynetdicom is the creation of an Application Entity by using the AE class. A minimal initialisation of AE requires no parameters:

>>> from pynetdicom import AE
>>> ae = AE()

This will create an AE with an AE title of 'PYNETDICOM'. The AE title can set by supplying the ae_title keyword parameter during initialisation:

>>> from pynetdicom import AE
>>> ae = AE(ae_title='MY_AE_TITLE')

Or afterwards with the ae_title property:

>>> from pynetdicom import AE
>>> ae = AE()
>>> ae.ae_title = 'MY_AE_TITLE'

AE titles must meet the conditions of a DICOM data element with a Value Representation of AE:

  • Leading and trailing spaces (hex 0x20) are non-significant.

  • Maximum 16 characters (once non-significant characters are removed).

  • Valid characters belong to the DICOM Default Character Repertoire, which is the basic G0 Set of the ISO/IEC 646:1991 (ASCII) standard excluding backslash (\ - hex 0x5C) and all control characters (such as '\n').

  • An AE title made entirely of spaces is not allowed.

When creating SCPs it’s also possible to give each SCP its own AE title through the ae_title keyword parameter in AE.start_server().

Creating an SCU

Adding requested Presentation Contexts

If you intend to use your AE as a Service Class User then you need to specify the Presentation Contexts that will be requested during Association negotiation. This can be done in two ways:

  • You can add requested contexts on a one-by-one basis using the AE.add_requested_context() method.

  • You can set all the requested contexts at once using the AE.requested_contexts property. Additional contexts can still be added on a one-by-one basis afterwards.

Adding presentation contexts one-by-one:

>>> from pydicom.uid import UID
>>> from pynetdicom import AE
>>> from pynetdicom.sop_class import CTImageStorage
>>> ae = AE()
>>> ae.add_requested_context('1.2.840.10008.1.1')
>>> ae.add_requested_context(UID('1.2.840.10008.5.1.4.1.1.4'))
>>> ae.add_requested_context(CTImageStorage)

Adding presentation contexts all at once:

>>> from pynetdicom import AE, StoragePresentationContexts
>>> ae = AE()
>>> ae.requested_contexts = StoragePresentationContexts

Here StoragePresentationContexts is a prebuilt list of presentation contexts containing 120 of the most commonly used Storage Service Classes’ supported SOP Classes, and there’s a similar list for all the supported service classes. Alternatively you can build your own list of presentation contexts, either through creating new PresentationContext instances or by using the build_context() convenience function:

>>> from pynetdicom import AE, build_context
>>> from pynetdicom.sop_class import CTImageStorage
>>> ae = AE()
>>> contexts = [
...     build_context(CTImageStorage),
...     build_context('1.2.840.10008.1.1')
... ]
>>> ae.requested_contexts = contexts

Combining the all-at-once and one-by-one approaches:

>>> from pynetdicom import AE, StoragePresentationContexts
>>> from pynetdicom.sop_class import Verification
>>> ae = AE()
>>> ae.requested_contexts = StoragePresentationContexts
>>> ae.add_requested_context(Verification)

As the association Requestor you’re limited to a total of 128 requested presentation contexts, so attempting to add more than 128 contexts will raise a ValueError exception. StoragePresentationContexts consists of 120 of most commonly used Storage Service Classes, therefore you are able to add 8 additional presentation contexts without rasing a ValueError exception.

When you add presentation contexts as shown above, the following transfer syntaxes are used by default for each context:

1.2.840.10008.1.2

Implicit VR Little Endian

1.2.840.10008.1.2.1

Explicit VR Little Endian

1.2.840.10008.1.2.1.99

Deflated Explicit VR Little Endian

1.2.840.10008.1.2.2

Explicit VR Big Endian

Specifying your own transfer syntax(es) can be done with the transfer_syntax keyword parameter as either a single str/UID or a list of str/UIDs:

>>> from pydicom.uid import ImplicitVRLittleEndian, ExplicitVRLittleEndian
>>> from pynetdicom import AE
>>> from pynetdicom.sop_class import CTImageStorage, MRImageStorage
>>> ae = AE()
>>> ae.add_requested_context(CTImageStorage, transfer_syntax='1.2.840.10008.1.2')
>>> ae.add_requested_context(
...     MRImageStorage, [ImplicitVRLittleEndian, ExplicitVRLittleEndian]
... )

>>> from pydicom.uid import ImplicitVRLittleEndian, ExplicitVRBigEndian
>>> from pynetdicom import AE, build_context
>>> from pynetdicom.sop_class import CTImageStorage
>>> ae = AE()
>>> context_a = build_context(CTImageStorage, ImplicitVRLittleEndian)
>>> context_b = build_context(
...     '1.2.840.10008.1.1', [ImplicitVRLittleEndian, ExplicitVRBigEndian]
... )
>>> ae.requested_contexts = [context_a, context_b]

The requested presentation contexts can be accessed with the AE.requested_contexts property and are returned in the order they were added.

>>> from pynetdicom import AE
>>> from pynetdicom.sop_class import Verification
>>> ae = AE()
>>> len(ae.requested_contexts)
0
>>> ae.add_requested_context(Verification)
>>> len(ae.requested_contexts)
1
>>> print(ae.requested_contexts[0])
Abstract Syntax: Verification SOP Class
Transfer Syntax(es):
    =Implicit VR Little Endian
    =Explicit VR Little Endian
    =Explicit VR Big Endian

It’s also possible to have multiple requested presentation contexts for the same abstract syntax.

>>> from pydicom.uid import ImplicitVRLittleEndian
>>> from pynetdicom import AE
>>> from pynetdicom.sop_class import CTImageStorage
>>> ae = AE()
>>> ae.add_requested_context(CTImageStorage)
>>> ae.add_requested_context(CTImageStorage, ImplicitVRLittleEndian)
>>> len(ae.requested_contexts)
2
>>> print(ae.requested_contexts[0])
Abstract Syntax: CT Image Storage
Transfer Syntax(es):
    =Implicit VR Little Endian
    =Explicit VR Little Endian
    =Explicit VR Big Endian
>>> print(ae.requested_contexts[1])
Abstract Syntax: CT Image Storage
Transfer Syntax(es):
    =Implicit VR Little Endian

All the above examples set the requested presentation contexts on the Application Entity level, i.e. the same contexts will be used for all association requests. To set the requested presentation contexts on a per-association basis (i.e. each association request can have different requested contexts) you can use the contexts keyword parameter when calling AE.associate() (see the Association page for more information).

Specifying the network port

In general it shouldn’t be necessary to specify the port when acting as an SCU, as by default pynetdicom will use the first port available. To specify the port number manually you can use the bind_address keyword parameter when requesting an association, which takes a 2-tuple of (str host, int port):

>>> from pynetdicom import AE
>>> ae = AE()
>>> ae.add_requested_context('1.2.840.10008.1.1')
>>> assoc = ae.associate("127.0.0.1", 11112, bind_address=("127.0.0.1", 11113))

Note

When using bind_address you may sometimes be unable to immediately reconnect with the same bound address and port due to an exception about the socket or address already being in use. This occurs because the previous TCP connection using the bound socket remains in a TIME_WAIT state which must expire before you are able to re-use the socket.

Association

For information on how request an association with a peer AE when acting as an SCU please see the Association page.

Creating an SCP

Adding supported Presentation Contexts

If you intend to use your AE as a Service Class Provider then you need to specify the Presentation Contexts that will be supported during Association negotiation. This can be done in two ways:

  • You can add supported contexts on a one-by-one basis using the AE.add_supported_context() method.

  • You can set all the supported contexts at once using the AE.supported_contexts property. Additional contexts can still be added on a one-by-one basis afterwards.

Adding presentation contexts one-by-one:

>>> from pydicom.uid import UID
>>> from pynetdicom import AE
>>> from pynetdicom.sop_class import CTImageStorage
>>> ae = AE()
>>> ae.add_supported_context('1.2.840.10008.1.1')
>>> ae.add_supported_context(UID('1.2.840.10008.5.1.4.1.1.4'))
>>> ae.add_supported_context(CTImageStorage)

Adding presentation contexts all at once:

>>> from pynetdicom import AE, StoragePresentationContexts
>>> ae = AE()
>>> ae.supported_contexts = StoragePresentationContexts

Here StoragePresentationContexts is a prebuilt list of presentation contexts containing 120 of the most commonly used Storage Service Classes’ supported SOP Classes, and there’s a similar list for all the supported service classes. Alternatively you can build your own list of presentation contexts, either through creating new PresentationContext instances or by using the build_context() convenience function:

>>> from pynetdicom import AE, build_context
>>> from pynetdicom.sop_class import CTImageStorage
>>> ae = AE()
>>> contexts = [
...     build_context(CTImageStorage),
...     build_context('1.2.840.10008.1.1')
... ]
>>> ae.supported_contexts = contexts

Combining the all-at-once and one-by-one approaches:

>>> from pynetdicom import AE, AllStoragePresentationContexts
>>> from pynetdicom.sop_class import Verification
>>> ae = AE()
>>> ae.supported_contexts = AllStoragePresentationContexts
>>> ae.add_supported_context(Verification)

As the association Acceptor you’re not limited in the number of presentation contexts that you can support.

When you add presentation contexts as shown above, the following transfer syntaxes are used by default for each context:

1.2.840.10008.1.2

Implicit VR Little Endian

1.2.840.10008.1.2.1

Explicit VR Little Endian

1.2.840.10008.1.2.1.99

Deflated Explicit VR Little Endian

1.2.840.10008.1.2.2

Explicit VR Big Endian

Specifying your own transfer syntax(es) can be done with the transfer_syntax keyword parameter as either a single str/UID or a list of str/UIDs:

>>> from pydicom.uid import ImplicitVRLittleEndian, ExplicitVRLittleEndian
>>> from pynetdicom import AE
>>> from pynetdicom.sop_class import CTImageStorage, MRImageStorage
>>> ae = AE()
>>> ae.add_supported_context(CTImageStorage, transfer_syntax='1.2.840.10008.1.2')
>>> ae.add_supported_context(
...     MRImageStorage, [ImplicitVRLittleEndian, ExplicitVRLittleEndian]
... )

>>> from pydicom.uid import ImplicitVRLittleEndian, ExplicitVRBigEndian
>>> from pynetdicom import AE, build_context
>>> from pynetdicom.sop_class import CTImageStorage
>>> ae = AE()
>>> context_a = build_context(CTImageStorage, ImplicitVRLittleEndian)
>>> context_b = build_context(
...     '1.2.840.10008.1.1', [ImplicitVRLittleEndian, ExplicitVRBigEndian]
... )
>>> ae.supported_contexts = [context_a, context_b]

The supported presentation contexts can be accessed with the AE.supported_contexts property and are returned in order of their abstract syntax UID value:

>>> from pynetdicom import AE
>>> from pynetdicom.sop_class import Verification
>>> ae = AE()
>>> len(ae.supported_contexts)
0
>>> ae.add_supported_context(Verification)
>>> len(ae.supported_contexts)
1
>>> print(ae.supported_contexts[0])
Abstract Syntax: Verification SOP Class
Transfer Syntax(es):
    =Implicit VR Little Endian
    =Explicit VR Little Endian
    =Explicit VR Big Endian

For the association Acceptor it’s not possible to have multiple supported presentation contexts for the same abstract syntax, instead any additional transfer syntaxes will be combined with the pre-existing context:

>>> from pydicom.uid import ImplicitVRLittleEndian, ExplicitVRLittleEndian
>>> from pynetdicom import AE
>>> from pynetdicom.sop_class import CTImageStorage
>>> ae = AE()
>>> ae.add_supported_context(CTImageStorage, ExplicitVRLittleEndian)
>>> ae.add_supported_context(CTImageStorage, ImplicitVRLittleEndian)
>>> len(ae.supported_contexts)
1
>>> print(ae.supported_contexts[0])
Abstract Syntax: CT Image Storage
Transfer Syntax(es):
    =Explicit VR Little Endian
    =Implicit VR Little Endian

All the above examples set the supported presentation contexts on the Application Entity level, i.e. the same contexts will be used for all SCPs. To set the supported presentation contexts on a per-SCP basis (i.e. each SCP can have different supported contexts) you can use the contexts keyword parameter when calling AE.start_server() (see the Association page for more information).

Handling SCP/SCU Role Selection Negotiation

Depending on the requirements of the service class, an association Requestor may include SCP/SCU Role Selection Negotiation items in the association request and it’s up to the association Acceptor to decide whether or not to accept the proposed roles. This can be done through the scu_role and scp_role keyword parameters, which control whether or not the association Acceptor will accept or reject the proposal:

>>> from pynetdicom import AE
>>> from pynetdicom.sop_class import CTImageStorage
>>> ae = AE()
>>> ae.add_supported_context(CTImageStorage, scu_role=False, scp_role=True)

If either scu_role or scp_role is None (the default) then no response to the role selection will be sent and the default roles assumed. If you wish to accept a proposed role then set the corresponding parameter to True. In the above example if the Requestor proposes the SCP role then the Acceptor will accept it while rejecting the proposed SCU role (and therefore the Acceptor will be SCU and Requestor SCP).

To reiterate, if you wish to respond to the proposed role selection then both scu_role and scp_role must be set, and the value you set indicates whether or not to accept or reject the proposal.

There are four possible outcomes for the role selection negotiation, depending on what was proposed and what was accepted:

  • The proposed roles aren’t acceptable and the context is rejected

  • The Acceptor acts as the SCP and the Requestor the SCU (default)

  • The Acceptor acts as the SCU and the Requestor the SCP

  • Both Acceptor and Requestor act as SCU and SCP

Handling User Identity Negotiation

An association Requestor may include a User Identity Negotiation item in the association request with the aim of providing the Acceptor a method of verifying its identity. Possible forms of identity confirmation methods are:

  • Username

  • Username and password (sent in the clear)

  • Kerberos service ticket

  • SAML assertion

  • JSON web token

By default, all association requests that include user identity negotiation are accepted (provided there’s no other reason to reject) and no user identity negotiation response is sent even if one is requested.

To handle a user identity negotiation yourself you should implement and bind a handler to the evt.EVT_USER_ID event. Check the documentation to see the requirements for implementations of the evt.EVT_USER_ID handler.

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

from my_code import some_user_function

def handle_user_id(event):
    """Handle evt.EVT_USER_ID."""
    # Identity verification code is outside the scope of pynetdicom
    is_verified, response = some_user_function(event)

    return is_verified, response

handlers = [(evt.EVT_USER_ID, handle_user_id)]

ae = AE()
ae.add_supported_context(Verification)
ae.start_server(("127.0.0.1", 11112), evt_handlers=handlers)

Specifying the bind address

The bind address for the server socket is specified by the address parameter to start_server() as (str host, int port).

>>> from pynetdicom import AE
>>> ae = AE()
>>> ae.add_supported_context('1.2.840.10008.1.1')
>>> ae.start_server(("127.0.0.1", 11112))

Association

For information on how to start listening for association requests from peer AEs and how to handle their service requests see the Association page.

References