By the endIdentify the object, service, SCU, and SCP when a modality sends an image to an archive.
Describe the interaction before naming the role
Suppose someone says, 'The PACS is the SCP.' The sentence is incomplete. SCP for which SOP Class, and in which exchange? Start with the action and the information being handled. Then assign the roles for that specific SOP Class. This keeps familiar device names from steering you toward the wrong answer.
Object
The information acted upon, such as a CT Image instance.
Service
The action requested, such as Storage through C-STORE.
Role
The requester is the SCU; the provider is the SCP for that SOP Class.
A CT scanner stores an image in PACS
In a successful C-STORE operation, the CT scanner acts as Storage SCU and sends C-STORE-RQ with a CT Image instance to the PACS archive acting as Storage SCP. The PACS returns C-STORE-RSP status to the CT scanner.
Worked example
The PACS sends a prior study to a workstation
A diagnostic workstation accepts incoming CT images. The PACS opens the exchange and sends a prior CT Image instance with C-STORE.
1
Find the object
The information is a CT Image instance. The object type helps identify the applicable Storage SOP Class.
2
Find the service
The verb is sends, and the operation is C-STORE. This is a Storage interaction.
3
Assign the roles
The PACS invokes Storage, so it is the Storage SCU. The workstation performs the requested storage, so it is the Storage SCP.
Result: In the earlier CT-to-PACS flow, the PACS was the Storage SCP. Here it is the Storage SCU. The product did not acquire a permanent label; the direction of the service request changed.
One application can make two separate claims
A conformance statement records support by SOP Class and role. Receiving an object and sending an object are related capabilities, but they are not the same claim.
Receiving C-STORE
The remote Storage SCU invokes C-STORE
The application acts as Storage SCP
The application returns the C-STORE response
Sending C-STORE
The application invokes C-STORE as the Storage SCU
The application acts as Storage SCU
The peer returns the C-STORE response
Try it
Trace both legs
A CT sends an image to a PACS. The PACS then sends the same image to an AI application that accepts C-STORE. Name the Storage SCU and SCP on each leg before revealing the answer.
Hint: Treat the two C-STORE requests as separate interactions.
On the first leg, the CT is the Storage SCU and the PACS is the Storage SCP. On the second leg, the PACS is the Storage SCU and the AI application is the Storage SCP.
02 / Questions
Answer 4 questions
Choose an answer. The explanation appears after you submit.
Check 1 of 4scenario · easy
A CT modality sends an image to a PACS using C-STORE. What is the modality's role for this Storage SOP Class?
The modality requests the Storage service, so it is the Storage SCU for this interaction. The PACS is the Storage SCP because it performs the requested service.
Check 2 of 4recall · easy
Which description best defines a DICOM SOP Class?
A SOP Class pairs a service with a type of information object. A network port can help reach an application, but it does not define a SOP Class.
Check 3 of 4recall · easy
When describing a DICOM Storage interaction, which distinction makes each participant's responsibility explicit?
A Storage SOP Class pairs a service with a type of information object, and peer Application Entities perform the SCU and SCP roles for that class. Separating the object, service, and role makes each participant's responsibility explicit. The role is not fixed by device type: an archive also sends images to workstations or back to modalities.
Check 4 of 4recall · medium
When checking a Composite SOP Instance before storage transfer, which action tests the information object itself rather than connectivity or association configuration?
Composite SOP Instances transferred by the Storage Service Class conform to a Composite Instance IOD. Inspecting the dataset attributes tests the object, while host reachability, C-ECHO, and AE Title checks test transport or association configuration.
03 / Recall
Answer without looking back
Name the object, service, SCU, and SCP when a CT sends an image to PACS.
The CT Image instance is the object; Storage through C-STORE is the service; the CT is the Storage SCU; and the PACS is the Storage SCP.
DICOMDigital Imaging and Communications in Medicine
The standard used to represent and exchange medical imaging information.
PACSPicture Archiving and Communication System
A system used to store, manage, retrieve, and present medical images and related information.
CTComputed Tomography
An imaging modality that produces cross-sectional images using X-rays and computational reconstruction.
SOP ClassService-Object Pair Class
For DIMSE services, the union of an Information Object Definition and a set of DIMSE services: a Composite IOD with DIMSE-C services, or a Normalized IOD with DIMSE-N services.
SCUService Class User
The role played by a DICOM Application Entity that invokes operations on a specific Association. When its peer invokes a notification, the SCU is the side that receives it and acts on it.
SCPService Class Provider
The role played by a DICOM Application Entity that performs operations and invokes notifications on a specific Association. It performs the operations its peer asks for, and it sends notifications about events to that peer.