SOPInstanceUIDs

SOP Instance UID (0008,0018) is the unique identifier assigned to a single SOP Instance in the DICOM information model. It serves as a global reference for that particular object, and when combined with the SOP Class UID (0008,0016) it uniquely identifies the type and instance of a DICOM object, such as an image, a report, or a presentation state.

Format and generation: SOP Instance UIDs are UI values—strings of digits separated by periods that comply with the DICOM UID syntax. They are typically constructed from a UID root assigned to an organization, with software appending components to create a unique identifier. The maximum length is 64 characters, and the value must remain globally unique.

Usage and stability: The SOP Instance UID is stored in the dataset as (0008,0018) and should remain unchanged for the object's lifetime. It is intended to be preserved when the object is transferred, copied, or stored in different systems, and is used to retrieve, reference, or link related data across studies and series.

Uniqueness and policy: UIDs must not be reused for another object while the original instance exists. Facilities typically deploy a UID generator or allocator to ensure uniqueness, and many applications rely on a central or well-managed UID root.

Relation to other identifiers: In DICOM, StudyInstanceUID (0020,000D) and SeriesInstanceUID (0020,000E) group related objects, while the SOP Class UID identifies the object type; the combination with the SOP Instance UID yields a globally unique reference.