Query/Retrieve Level

The legacy TCP/IP-based command framework that defines the C-FIND, C-MOVE, and C-GET operations. Each DIMSE command specifies a Query/Retrieve Level to scope the database search. A C-FIND request at the STUDY level returns study-level attributes; a SERIES level request drills into specific acquisitions. DIMSE requires a persistent association negotiated between the SCU and SCP.

A RESTful alternative to DIMSE C-FIND defined in DICOM Part 18. QIDO-RS uses HTTP GET requests with query parameters to search for studies, series, or instances. The Query/Retrieve Level is implicit in the endpoint path:

• /studies — Search at the Study level
• /studies/{StudyInstanceUID}/series — Search at the Series level
• /studies/{StudyInstanceUID}/series/{SeriesInstanceUID}/instances — Search at the Instance level

The Service Class User (SCU) initiates the query, and the Service Class Provider (SCP) responds with matching records. A single Application Entity can be both an SCU and an SCP. For example, a PACS archive acts as an SCP when receiving a C-FIND from a workstation, but becomes an SCU when forwarding a C-MOVE to a VNA. Role negotiation occurs during Association Negotiation using the Query/Retrieve SOP Class UID.

DICOM enforces a strict Patient → Study → Series → Instance hierarchy. A query at a higher level returns summary attributes; a query at a lower level requires the Unique Keys of all parent levels:

• Patient Root: Queries begin at the Patient level
• Study Root: Queries begin at the Study level (most common)
• Patient/Study Only: A retired model limited to Patient and Study levels Each model defines which Unique Keys must be supplied in the request identifier.

DICOM supports relational queries that span multiple levels in a single C-FIND request. By including attributes from different levels, an SCU can ask complex questions like: 'Find all CT studies for Patient X that contain a series with slice thickness less than 1mm.' The SCP must support relational-query behavior as declared in its Conformance Statement. Not all PACS implementations support this advanced feature.

A C-MOVE operation requires three Application Entities: the SCU issuing the command, the SCP holding the data, and a Destination AE that receives the images via a separate C-STORE sub-operation. The Query/Retrieve Level of the C-MOVE determines what is moved—an entire study, a single series, or individual instances. The Destination AE must be configured in advance on the SCP, adding a layer of operational complexity absent in C-GET.