Service Object Pair (SOP Class)

Service Object Pair, combination of a services such as Store,Retrieve, and an Object such as an Images. The class defines rules on how to use these; a Class is negotiated as a capability between devices during negotiation of the Association. Certain functionality consisting of a combination of a Service and an Object, which make up the Pair – e.g. CT Image Storage is an example of an SOP Class.

Objects in DICOM are called Information Objects. Instead of discussing images, DICOM talks about Service Classes, Information Objects and Service Object Pair (SOP) Classes.

A SOP Class is defined as a combination of one or more DICOM Service Elements (DIMSE), which is a command, and an Object, which is defined by an Information Object Definition (IOD). Examples of Service Elements are Store, Get, Find, Move, etc. Examples of Objects are CT images, MR images, but also include schedule lists, print queues, etc. For example, the SOP Class CT Image Storage, is defined as the combination of the Store DICOM Service element (or command) and a CT Image object. A device supporting the CT Storage SOP Class will generate CT images according to the Class definition, and use the Store command to exchange them.

Each DICOM compatible device is required to describe its functionality, its supported SOP classes and Transfer syntaxes in a document called the DICOM Conformance Statement.