Unique Identifier | UID

Unique Identifiers (UIDs) are world-wid unique numbering schemes used to identify everything in DICOM, from the SOP classes to the individual objects. Their roots are registered by a standards organization to prevent duplication. Used by NEMA to identify SOP Classes, syntaxes, etc., and vendors for indentifying SOP Instances.

The two UIDs, i.e. SOP Class and SOP Instance, uniquely identify all DICOM objects such as images, reports, presentation states, etc. The first number identifies its type, and the second one might be considered its serial number.

The organization defining the DICOM SOP class UIDs is the NEMA, which provides the home for the DICOM standards. The vendor’s software is responsible for creating a unique SOP Instance UID upon creating new SOP Instances (e.g. images).

UIDs are typically created through a combination of an internal unique number, such as the MAC address of the network card and a date/time stamp. If, for some reason, there is a duplicate date/time (such as when switching over from daylight savings time to standard time, or when the time clock has been reset to a prior date/time) identical UIDs might occur, which at best will create an exception or at worst jeopardize the PACS database integrity.