People often use the two words as if they meant the same thing. They don't. DICOM is a standard: a published set of rules for imaging data and for how devices exchange it. A PACS is a product: the system a hospital buys to store images and get them to the people who read them. A PACS speaks DICOM. That is most of the relationship.
DICOM is the standard
DICOM stands for Digital Imaging and Communications in Medicine. NEMA publishes it as a numbered set of parts, PS3.1 to PS3.22 and beyond. Among other things, it defines:
- how an image and its patient and study details are encoded, as a list of data elements, each identified by a tag (see DICOM tags explained);
- the network services devices use to talk to each other, such as C-STORE to send an object, C-FIND to search, and C-MOVE or C-GET to retrieve;
- the file format for writing DICOM objects to disk or removable media;
- DICOMweb, the HTTP versions of storing (STOW-RS), searching (QIDO-RS), and retrieving (WADO-RS).
Nobody installs DICOM. A device conforms to it, and its vendor publishes a conformance statement saying which parts it supports and in which role.
A PACS is a system
PACS stands for Picture Archiving and Communication System. It is the software and storage that receive images from the scanners, keep them, and send them on to diagnostic workstations, viewers, and other systems. A PACS usually includes a database of studies, an archive, a viewer or reading workstation, and the rules that decide where images go next.
How the two meet in a working department
Follow one CT study:
- The scanner asks the modality worklist for the day's scheduled procedures, using a DICOM C-FIND query. The worklist itself is fed by orders that usually arrive over HL7 from the radiology information system.
- After the scan, the scanner sends each image to the PACS with C-STORE. In that exchange the scanner is the Storage SCU (the side asking for the service) and the PACS is the Storage SCP (the side providing it).
- A radiologist's workstation searches the PACS with C-FIND or QIDO-RS and pulls the study back with C-MOVE, C-GET, or WADO-RS.
Apart from the orders coming in over HL7, each step is a DICOM exchange. The PACS is one of the systems at either end.
DICOM vs PACS at a glance
| DICOM | PACS | |
|---|---|---|
| What it is | A standard | A system |
| Who makes it | NEMA's DICOM Standards Committee | Vendors, or an open-source project |
| You can | Conform to it | Buy, install, and run it |
| Its job | Define imaging data and how to exchange it | Store images and deliver them to readers |
Where VNA and RIS fit
A vendor neutral archive (VNA) is a long-term archive that keeps images in standard DICOM form, so they are not locked into one vendor's PACS. A radiology information system (RIS) handles orders, scheduling, and reports. Both connect to the PACS, the VNA mostly over DICOM and the RIS mostly over HL7.
Sources: the current DICOM standard, in particular PS3.4 (service classes), PS3.7 (message exchange), and PS3.18 (web services). Polyseminar is an independent publisher; see the independence notice.