| An SOP Class is defined as a combination of one or more DICOM Service Elements, which are commands, and an Object, which is defined by an Information Object Definition (IOD). In DICOM, a SOP Instance (e.g. a specific CT image) is the instance of the Class, e.g. a CT Image IOD. Unique Identifiers (UIDs) are used to identify everything in DICOM, from the SOP classes to the individual objects. They are registered by a standards organization to prevent duplication. The two UIDs, i.e. SOP Class and SOP Instance, uniquely identify all DICOM objects such as images, reports, presentation states, etc. Each vendor is required to register an organization root number from a central registration agency such as ANSI in the US, allowing each manufacturer to create their unique numbers. Creators of object instances, such as a CT or MR acquisition unit, generate world-wide unique instance numbers for each individual object. They typically take their organization number as a "root" and add a device specific identifier such as the MAC address (if they have one) and append it with a data-time stamps and potentially some other numbers to make it unique. How does one ensure that a particular manufacturer does not issue the same numbers for instances created from two of its own CT scanners? Actually, there is no absolute guarantee that this will not happen. The DICOM standard clearly requires these instance numbers be unique. However there is no policing agency unfortunately, so it could happen that certain vendors, due to poor implementation, issue an identical instance UID. |