A “normal” DICOM Query, as part of the Query/Retrieve service, is used to ask about the patients, studies, series or images known to a Q/R SCP (normally a PACS). It uses the C-FIND operation (and was originally the only service using that operation, so this query service is often called just C-FIND, despite that fact that the C-FIND operation is also used for other services such as Modality Worklist).

What attributes can be requested in a Query?

There are 2 factors constraining the attributes that can be retrieved in a query:

Those that are stored/indexed by the server and which it chooses to make available through Q/R<

This is because the server is not expected to touch the image itself merely to answer a query - only the information from the PACS index is used.

Those that are appropriate for the “level” of the query (PATIENT, STUDY, SERIES or IMAGE)

DICOM defines 4 types of attribute for each level:

The single unique key for each level This is the PatientID, StudyUID, SeriesUID or InstanceUID

Required Keys

Support for these is mandated in DICOM, and it includes for instance Name at patient level, or Modality at series level. The list is however quite small, and many commonly used attributes such as study description are notably absent.

keys for that level

When supported by an SCP, these are handled just like required keys, and most decent PACS support a list larger than the DICOM minimum. There are 2 variations on the usage of optional attributes - those that are explicitly defined in Part 4 of DICOM in the Q/R section, and “any other” optional attribute that the SCP recognises which could be all other attributes of the instances at the appropriate “level”. This level is determined by looking at the level of the Module in which they are defined in Part 3, but this can sometimes be ambiguous, especially if an attribute is defined in different modules at different levels in different SOP_Classes!

Special Attributes

These are requests for data which is not part of the instances (e.g. images) stored on the PACS, but which is instead generated on the fly by the PACS, and includes for instance the number of series or images in a study. All these special keys are optional for the PACS.

Unlike for some other DICOM services (e.g. MWL) DICOM does not for this service distinguish between keys which are mandatory for matching, and those that must be returnable

Only attributes from the current “Level” can be requested and returned in normal DICOM Queries. The SCU is required to specify the Unique keys in the Query for all levels above the current level, so in the case of a Series Level Patient Root Query, Patient ID and Study UID are needed along with any Series levels Keys.

These rules are relaxed for Relational Queries, but these are not supported by many servers.

Summary of Attributes for Different Query Roots and Levels

Root Level Unique Key Required Keys Optional Keys Special Keys



Patient ID

  • Patient Name
  • Date of Birth
  • Time of Birth
  • Sex
  • Other Patient Ids
  • Other Patient Names
  • Ethnic Group
  • Referenced Patient Sequence
  • Patient Comments
  • Number of Studies
  • Number of Series
  • Number of Instances
for this patient



Study Instance UID

  • Study Date
  • Study Time
  • Accession Number
  • Study ID
  • Referring Physician’s Name
  • Study Description
  • Procedure Code Sequence
  • Issuer of Patient ID
  • Patient’s Age
  • Patient’s Size
  • Patient’s Weight
  • Occupation
  • Additional Patient History
  • Other Study Numbers
  • All other Attributes at Study Level
  • Modalities in Study
  • SOP Classes in Study
  • Number of Series
  • Number of Instances
for this study
STUDY STUDY Study Instance UID
  • Patient ID
  • Patient Name
  • Study Date
  • Study Time
  • Accession Number
  • Study ID
All attributes in above two cells



Series Instance UID

  • Modality
  • Series Number
  • All Other Attributes at Series Level
  • Number of Instances for this series



Instance UID

  • Instance Number
  • SOP Class UID
  • Alternate Representation Sequence
  • Related General SOP Class UID
  • Concept Name Code Sequence
  • Content Template Sequence
  • All Other Attributes at composite object instance Level



Important Notes

  • As a study may contain multiple modalities, modality is a series level attributes, not study level (though there is an optionally supported “Modalities in Study” special attribute at study level)