DICOM and PACS Glossary
19 terms, each traced to the section of the DICOM standard that defines it.
Medical imaging has a vocabulary problem: the words that matter — C-STORE, AE Title, transfer syntax — are defined precisely in a standard almost nobody reads, and loosely everywhere else. Every definition here links to the part and section of the DICOM standard it came from, so you can check it rather than take our word for it.
The standard and the file
What DICOM is, and how a single image is stored on disc.
DICOMDigital Imaging and Communications in Medicine
DICOM is the international standard that defines how medical images and their associated patient and acquisition data are formatted, stored and exchanged between imaging devices.
.dcm file (DICOM Part 10 file format)
A DICOM file stores one image or object using the Part 10 format: a 128-byte preamble, the four-byte prefix "DICM", and then the data elements — and the .dcm extension is a common convention, not a requirement of the standard.
DICOMDIR
DICOMDIR is the index file at the root of a DICOM media set, such as a patient CD or USB export, that lists the patient, study, series and instance records the media contains so a viewer can browse it without opening every file.
DICOM tag
A DICOM tag is the pair of numbers — a group number and an element number — that identifies one specific data element inside a DICOM file, such as (0010,0010) for Patient's Name.
Pixel Data
Pixel Data is the DICOM data element, tag (7FE0,0010), that holds the actual image content of an instance, encoded according to the file's transfer syntax.
Patient, Study, Series and Instance
DICOM organises imaging data in a four-level hierarchy: a Patient has Studies (one imaging visit each), a Study has Series (one acquisition run each), and a Series has Instances (the individual images or objects).
SOP ClassService-Object Pair Class
A SOP Class is a DICOM-defined pairing of an Information Object Definition — what the data represents — with a Service, meaning what can be done with it, such as the CT Image Storage SOP Class.
Networking: sending and receiving studies
The services devices use to move studies between each other.
PACSPicture Archiving and Communication System
A PACS is a system that stores, retrieves, distributes and displays medical images, connected to imaging modalities and viewing workstations over a DICOM network.
AE TitleApplication Entity Title
An AE Title is the name a DICOM device identifies itself by on the network, used together with its IP address and port to address it when another device opens an association.
C-STORE
C-STORE is the DICOM network service used to transfer a composite object — an image or similar instance — from one device to another, and it is the operation behind "sending a study" to a PACS.
C-ECHO (DICOM Verification)
C-ECHO is the DICOM network service that tests whether two devices can communicate, without transferring any image data — the equivalent of a ping at the DICOM layer.
Storage SCPStorage Service Class Provider
A Storage SCP is the role a device plays when it accepts incoming DICOM images, listening for C-STORE requests and storing what it receives — the role a PACS or an archiving viewer performs.
Storage SCUStorage Service Class User
A Storage SCU is the role a device plays when it initiates a C-STORE request to send DICOM images to a Storage SCP — the role a CT or MR console performs when it pushes a finished study to a PACS.
Query/Retrieve (C-FIND and C-MOVE)
Query/Retrieve is the pair of DICOM services used to search what a remote device holds and then pull matching studies from it: C-FIND performs the search and C-MOVE transfers the results.
Modality Worklist
A Modality Worklist is the DICOM service that lets an imaging device query a scheduled list of patients and procedures from a worklist provider, so demographics do not have to be typed in again at the scanner.
DICOMweb
DICOMweb is the collective name for DICOM's RESTful web services — WADO-RS for retrieving, QIDO-RS for querying and STOW-RS for storing — which carry DICOM data over HTTP instead of the traditional DICOM network protocol.
Encoding and privacy
How pixel data is encoded, compressed, and stripped of identifiers.
Transfer Syntax
A transfer syntax is the set of encoding rules — byte ordering, explicit or implicit value representation, and any compression — that a DICOM object is written in, and it is negotiated between two devices before they exchange data.
Lossless compression (DICOM)
Lossless compression reduces the size of DICOM Pixel Data while allowing the exact original pixel values to be reconstructed, unlike lossy compression, which permanently discards image information to achieve a smaller file.
De-identification (DICOM anonymisation)
De-identification is the removal or replacement of patient-identifying data elements in a DICOM object so that the images can no longer be linked to an individual through that metadata.
How these definitions are sourced
Each entry cites the part and section of the DICOM standard, published by NEMA, that defines the term, and links to it. Where a term is industry usage rather than something the standard defines — "patient CD", for example — it is left out of this glossary rather than given a citation it does not have.
More terms are being added as each one is verified the same way.