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.