Glossary

DICOM Tag

A unique 32-bit identifier composed of a Group and Element number used to address a specific data attribute, such as Patient Name (0010,0010), within a DICOM data set.

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DATA ELEMENT IDENTIFIER

What is a DICOM Tag?

A DICOM Tag is the fundamental addressing mechanism for every piece of data within a DICOM object, enabling precise, machine-readable identification of attributes from patient demographics to pixel spacing.

A DICOM Tag is a unique 32-bit identifier composed of a Group Number and an Element Number (written as (GGGG,EEEE) in hexadecimal) that addresses a specific data attribute within a DICOM data set. For example, the tag (0010,0010) universally identifies the Patient's Name, while (0028,0030) specifies the Pixel Spacing. This standardized addressing scheme, defined in the DICOM Data Dictionary (Part 6 of the standard), ensures that any compliant system can parse and interpret the exact meaning of every data element without ambiguity, regardless of the imaging modality or vendor that created the file.

Tags are categorized by their Group Number: even-numbered groups are reserved for the DICOM Standard, while odd-numbered groups are designated for Private Tags used by vendors to store proprietary information. Each tag is further defined by its Value Representation (VR)—a two-character code like 'DA' for Date or 'UI' for Unique Identifier—which dictates the data type and encoding rules for the element's value. The combination of a tag's identifier, its VR, and its value length forms the complete data element structure that underpins all DICOM interoperability, from network transfers using DIMSE commands to RESTful queries via DICOMweb.

TAG CLASSIFICATION

Standard vs. Private DICOM Tags

Comparison of standard DICOM data elements defined in the DICOM standard versus proprietary private tags defined by equipment vendors.

Feature	Standard Tags	Private Tags
Definition Source	DICOM Part 6 Data Dictionary	Vendor-specific implementation
Group Number Range	Even groups (0008, 0010, 0028, etc.)	Odd groups (0009, 0011, 0013, etc.)
Interoperability
Public Documentation
Reservation Mechanism	Not required; globally defined	Private Creator Data Element (gggg,0010-00FF)
Risk During Anonymization	Known PHI locations; predictable removal	May contain hidden PHI; requires vendor knowledge
Example	(0010,0010) Patient's Name	(0009,1001) Siemens Private Tag
Required for Conformance

DICOM TAG FUNDAMENTALS

Frequently Asked Questions

Essential questions and answers about the structure, parsing, and clinical significance of DICOM Tags, the atomic data identifiers that form the backbone of medical imaging interoperability.

A DICOM Tag is a unique 32-bit unsigned integer identifier that addresses a specific data element within a DICOM data set. It is canonically represented as two 16-bit hexadecimal numbers enclosed in parentheses and separated by a comma, formatted as (GGGG,EEEE). The first component, the Group Number (GGGG), broadly categorizes the type of information—for example, Group 0010 contains patient demographic data. The second component, the Element Number (EEEE), specifies the exact attribute within that group, such as 0010 for Patient Name. Together, the tag (0010,0010) unambiguously identifies the Patient Name attribute across all DICOM-compliant systems. This hierarchical addressing scheme is the fundamental mechanism that allows parsers to navigate the structured, tag-length-value encoded byte stream of a DICOM file or network message without relying on positional offsets.

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DICOM DATA STRUCTURE

Related Terms

Core concepts for understanding how DICOM Tags organize and reference medical imaging data within the standard's hierarchical information model.

Value Representation (VR)

A two-character code defining the data type and format of a DICOM Tag's value. This ensures correct parsing by any receiving system.

'PN' (Person Name): A structured string with caret (^) delimiters for family and given name components.
'DA' (Date): A fixed 8-character string in YYYYMMDD format.
'US' (Unsigned Short): A 16-bit binary integer.
'SQ' (Sequence of Items): A container for nested data sets, allowing hierarchical structures. The VR is critical for serialization; an Explicit VR Transfer Syntax encodes the VR directly in the data stream, while an Implicit VR syntax requires the parser to look it up from the DICOM Data Dictionary.

DICOM UID

A globally unique identifier used to definitively reference a specific SOP Instance, Study, or other DICOM entity. UIDs are registered with an ISO 8824 object identifier tree to guarantee global uniqueness.

Format: A series of numeric components separated by periods (e.g., 1.2.840.10008.5.1.4.1.1.2 for a CT Image Storage SOP Class).
Uniqueness: A UID must never be reused for a different object, even across different institutions or vendors.
Role: Tags like SOP Instance UID (0008,0018) and Study Instance UID (0020,000D) are the primary keys that link all images and data within a patient's longitudinal record.

SOP Class

The fundamental unit of DICOM interoperability, defined as the union of a specific Information Object Definition (IOD) and a DIMSE Service Group.

Information Object Definition (IOD): An abstract data model specifying the mandatory and optional Tags for a type of object (e.g., a CT Image IOD requires Tags like KVP (0018,0060)).
Service Group: Defines the operations allowed on the IOD (e.g., Storage, Query/Retrieve).
Example: The CT Image Storage SOP Class combines the CT Image IOD with the Storage Service, allowing a CT scanner to send images to a PACS archive.

Transfer Syntax

A set of encoding rules that define how a DICOM data set, including all its Tags, is serialized into a byte stream for network transmission or file storage.

Byte Ordering: Specifies Little Endian or Big Endian byte ordering for binary values.
VR Encoding: Determines if the Value Representation is included explicitly in the data stream (Explicit VR) or must be inferred from a dictionary (Implicit VR).
Compression: Defines the image compression algorithm, such as JPEG Lossless or JPEG 2000, which is applied to the pixel data referenced by Tag (7FE0,0010). The Transfer Syntax UID is stored in the DICOM File Meta Information header.

DICOM File Meta Information

The mandatory header at the beginning of a DICOM Part 10 file that provides the essential context for reading the rest of the data set.

File Preamble: A 128-byte field often used for compatibility with non-DICOM file viewers.
DICOM Prefix: The four-character string 'DICM' that identifies the file as a DICOM file.
Critical Tags: Contains Group 0002 elements, including the Transfer Syntax UID (0002,0010) and Media Storage SOP Class UID (0002,0002). Without this header, a parser cannot correctly interpret the subsequent data elements.

DICOM Controlled Terminology

A set of standardized, coded concepts and value sets defined in DICOM Part 16 that provide unambiguous semantics for machine-readable clinical data.

Purpose: Replaces free-text descriptions with coded tuples (Code Value, Coding Scheme Designator, Code Meaning) to enable reliable automated analysis.
Usage: Used extensively in DICOM Structured Reports and acquisition protocol Tags to ensure that a finding like 'Mass' has the same coded meaning across all vendors.
Example: The code (T-04000, SRT, "Breast") unambiguously identifies the anatomical location, eliminating the variability of natural language strings.

About the author

Prasad Kumkar

CEO & MD, Inference Systems

Prasad Kumkar is the CEO & MD of Inference Systems and writes about AI systems architecture, LLM infrastructure, model serving, evaluation, and production deployment. Over 5+ years, he has worked across computer vision models, L5 autonomous vehicle systems, and LLM research, with a focus on taking complex AI ideas into real-world engineering systems.

His work and writing cover AI systems, large language models, AI agents, multimodal systems, autonomous systems, inference optimization, RAG, evaluation, and production AI engineering.

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