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).
What Patient, Study, Series and Instance means in practice
The levels are how every query, archive and viewer navigates a dataset. One knee MRI visit is a Study; the sagittal run and the axial run within it are two Series; each slice is an Instance. Patient sits above Study in the standard's information model, which is why a query can ask for everything belonging to one person across visits.
Source
This definition follows NEMA PS3.3 §7 — DICOM Information Model, published by NEMA as part of the DICOM standard.
Related terms
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.
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.
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.
SOP 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.
Related on this site: Browsing studies and series in SoftDicom.