Important Concepts


Unique identifiers (UIDs) are a way of identifying a wide variety of items in a way that guarantees uniqueness across multiple countries, sites, vendors and equipment. The UID identification scheme used by DICOM is based on the numeric form of the OSI Object Identification as defined by ISO/IEC 8824 (ITU X.680).

Each UID is composed of two parts, an <org root> and a <suffix>: UID = <org root>.<suffix>

The <org root> uniquely identifies an organisation and is composed of numeric components as defined by ISO/IEC 8824. The <suffix> portion of the UID is also composed of a number of numeric components, however it is generated by the application and must be unique within the scope of the <org root>. If you don’t have an <org root> and you don’t want to use pynetdicom’s (1.2.826.0.1.3680043.9.3811) an <org root> can be obtained for free from the Medical Connections website.

The DICOM <org root> is 1.2.840.10008 and is reserved for use for DICOM defined items and shall not be used for privately defined items. As an example, the official DICOM UID for CT Image Storage is 1.2.840.10008., which makes the <suffix>

Each component of a UID (1, 2, 840, 10008 are all components) must not start with 0 unless the component itself is 0 (e.g. is valid but is invalid) and the maximum length of a UID is 64 total characters.

More information on DICOM UIDs is available in Part 5 of the DICOM Standard.

DICOM Information Model

Information Object Definition (IOD)

An IOD is an object-orientated abstract data model used to specify information about a class of real-world objects that share the same properties. For example, a Patient, a Study, an imaging Series and a piece of imaging Equipment are all real world objects. An IOD, then, is the data model used to define the relationship between the objects (the Patient has one or more Studies, each Study contains one or more Series and each Series is created by a piece of Equipment).

IODs come in two types; composite and normalised IODs. Normalised IODs generally represent only a single class of real-world objects (such as the Print Job IOD). Composite IODs include information about related real-world objects (such as the CT Image IOD which contains objects like Patient, Study, Series, Equipment, etc).

There are many different DICOM IODs and they’re all defined in Part 3 of the DICOM Standard.

SOP Classes

A Service-Object Pair (SOP) Class is defined by the union of an IOD and a DICOM Message Service Element (DIMSE) service group:

  • Composite SOP Classes are the union of Composite IODs and the DIMSE-C service group. An example of a Composite SOP Class is the CT Image Storage SOP Class, which is the union of the CT Image IOD and the DIMSE C-STORE service. A CT Image Storage instance stores information about a single slice of a patient’s CT scan. A complete scan (a Series) is made up of one or more CT Image Storage SOP Class instances, all with the same Study Instance UID and Series Instance UID values but differing SOP Instance UID values (one for each SOP instance within the Series).

  • Normalised SOP Classes are the union of Normalised IODs and DIMSE-N service group. An example of a Normalised SOP Class is the Print Job SOP Class, which is the union of the Print Job IOD and the DIMSE N-EVENT-REPORT and N-GET services. The Print Job SOP Class is an abstraction of a print job containing one or more films to be printed.

The DIMSE-C and DIMSE-N services are defined in Part 7 of the DICOM Standard. Every DICOM SOP class has its own UID that can be found in Part 6.

Service Classes

A DICOM Service Class defines a group of one or more SOP Classes related to a service that is to be used by communicating application entities, as well as the rules that are to govern the provision of the service. Services include storage of SOP Class instances (Storage Service Class), verification of DICOM connectivity (Verification Service Class), querying and retrieval of managed SOP instances (Query/Retrieve Service Class), printing of images (Print Management Service Class) and many others.

The labels Service Class User and Service Class Provider are derived from whether or not an AE uses or provides the services in a Service Class.

Service Classes are defined in Part 4 of the DICOM Standard.

Application Entities

A DICOM Application Entity (AE) is an application that supports the DICOM standard in general, and especially IODs, service classes and dataset encoding/decoding.

In DICOM networking, AEs are identified by their AE Title.

Presentation Contexts

Presentation Contexts are used during the negotiation of an association to provide a method for communicating AEs to agree on a set of supported services. Each Presentation Context consists of an Abstract Syntax and one or more Transfer Syntaxes, along with an ID value.

  • The association requestor may propose multiple presentation contexts per association but is limited to a maximum of 128 proposed contexts.

  • Each proposed presentation context contains one Abstract Syntax and one or more Transfer Syntaxes.

  • The requestor may propose multiple contexts with the same Abstract Syntax

  • The association acceptor may accept or reject each presentation context individually, but only one Transfer Syntax may be accepted per presentation context.

  • The acceptor selects a suitable Transfer Syntax for each accepted presentation context.

A more detailed guide to presentation contexts and how to use them with pynetdicom is available here.

Abstract Syntax

An Abstract Syntax is a specification of a set of data elements and their associated semantics. Each Abstract Syntax is identified by an Abstract Syntax Name in the form of a UID. Abstract syntax names used with DICOM are usually the officially registered SOP class UIDs (and the abstract syntax is therefore the SOP class itself), but the standard also allows the use of private abstract syntaxes. While pynetdicom can handle association negotiation containing private abstract syntaxes the implementation of the associated services/semantics is up to the end user.

Transfer Syntax

A Transfer Syntax defines a set of encoding rules able to unambiguously represent the data elements defined by one or more Abstract Syntaxes. In particular, the negotiation of transfer syntaxes allows communicating AEs to agree on the encoding techniques they are able to support (i.e. byte ordering, compression, etc.).

The official DICOM transfer syntaxes are defined in Part 5 of the DICOM Standard. The Standard also allows the use of privately defined transfer syntaxes. While pynetdicom is able to handle association negotiation containing private transfer syntaxes, the implementation of the associated encoding requirements is the responsibility of the end user.


When peer AEs want to communicate they must first establish an Association.

  • The AE that is initiating the association (the Requestor) sends an A-ASSOCIATE message to the peer AE (the Acceptor) which contains a list of proposed presentation contexts and association negotiation items.

  • The acceptor receives the request and responds with:

    • acceptance, which results is an association being established, or

    • rejection, which results in no association, or

    • abort, which results in no association

An association may be rejected because none of the proposed presentation contexts are supported, or because the Requestor hasn’t identified itself correctly or for a number of other reasons.

The full service procedure for an association is found in Part 8 of the DICOM Standard.

Association Negotiation and Extended Negotiation

Standard association negotiation usually involves the peer AEs agreeing on a set of abstract syntax/transfer syntax combinations through the mechanism provided by presentation contexts. In some cases it may be necessary for communicating AEs to exchange more detailed information about features and services they may optionally require/support. This is accomplished by sending additional user information items during the association request:

  • Asynchronous Operations Window Negotiation

  • SCP/SCU Role Selection Negotiation

  • SOP Class Extended Negotiation

  • SOP Class Common Extended Negotiation

  • User Identity Negotiation

Some of these items are conditionally required, depending on the requested service class (such as SCP/SCU role selection negotiation when the Query/Retrieve service class’ C-GET operation is requested). Association negotiation involving these additional items is usually referred to as extended negotiation.

Extended negotiation items are defined in Part 7 and Part 8 of the DICOM Standard.