Digital Imaging and Communications in Medicine (DICOM)

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Chapter 7 DICOM SOPs: Basic
(Fig. 51), our digital archive (typically considered a server) SCP for CT scanner
plays the role of a CT image storage SCU (typically considered a client) when
it sends images to another CT image storage SCP (such as workstation). As in
nearly all DICOM SOPs, the SCU initiates the communication and the SCP
replies to it; therefore, we cannot so easily assign the client/server labels as we
might like.
For another example, think about a PACS archive application using DICOM
as its internal language. To find an image in its database, it will issue a C-Find-
Rq to itself and will reply to it with a C-Find-Rsp, thus being a C-Find SCU and
SCP at the same time.
Another popular expectation is that SCP is somewhat more advanced than
SCU and includes SCU as a subfunction. This is not at all the case. SCPs and
SCUs can often rival in functional complexity, and being one does not imply
being the other; once again, they are really two independent roles that any
DICOM device can play. As a result, each DICOM unit can be SCU, SCP, both,
or none with respect to any particular SOP Class. The DICOM Conformance
Statement for the unit should provide exact specifications on this matter.
This brings us to our final observation. The concept of SOPs, SCUs, and
SCPs is the most essential for any practical implementation of DICOM. Only
SOP SCUs and SCPs define exactly how any DICOM entity should interact,
what it should process, and what information it should supply to other entities
over the DICOM network. SOPs define the DICOM profile of any DICOMcompatible
device, from the largest CT scanner to the smallest DICOM file
viewer. Consequently, any DICOM Conformance Statement issued by a device
manufacturer for its DICOM device or application is nothing but a list of supported
SOPs and their SCU/SCP roles. Traditionally, reading the conformance
statements is the point where many PACS administrators get really lost. In fact,
most do not read them at all, and are overly trusting of DICOM manufacturers
that “it is gonna work okay for me, right?”.
Fig. 51 DICOM application roles: digital archive acting as C-Store SCP and SCU
at the same time