Steven C. Horii, MD

The information object and the service class are the two fundamental components of DICOM. An understanding of these components makes it possible to comprehend, at least at a functional level, what DICOM does and why it is so useful. Information objects define the core contents of medical imaging, and service classes define what to do with those contents.

The service classes and information objects are combined to form the functional units of DICOM. This combination is called a service-object pair, or SOP. Since DICOM is an object-oriented standard, the combination is actually called a service-object pair class, or SOP class. The SOP class is the elemental unit of DICOM; everything that DICOM does when implemented is based on the use of SOP classes. Figure 5 shows an analogy between constructing a sentence and building a DICOM SOP class.

Combining a service and an information object is straightforward. For example, DICOM defines a series of storage SOP classes (eg, CT image storage SOP class, MR image storage SOP class). The CT information object definition and the storage service class are combined to form the CT image storage SOP class; other storage SOP classes are formed in a similar fashion. Because the SOP classes are referred to as a way to describe DICOM functionality, they carry UIDs. Furthermore, once the attributes in the information object and the variables of the service class are "filled in" by values representing a real patient, a particular piece of imaging equipment, and a resulting image, the SOP class becomes a SOP instance and is assigned its own UID.

The process of DICOM communication involves the exchange of SOP instances with use of DICOM messages. The DICOM message is the communication version of the SOP class; it contains the commands that use or provide the specified service and the data set made up of the properly encoded information object instance.

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