Digital Imaging and Communications in Medicine

DICOM Explorer

Learn the DICOM standard — its file structure, data elements, VRs, modalities, and SOP classes — then upload real .dcm files to inspect them live.

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What is DICOM?File Structure Data Elements Value Representations Common Tags Modalities SOP Classes Info Hierarchy Network Sequences

What is DICOM?

DICOM (Digital Imaging and Communications in Medicine) is the international standard for medical images and related information. It defines the formats for medical images that can be exchanged with data and quality necessary for clinical use.

Originally developed by the American College of Radiology (ACR) and the National Electrical Manufacturers Association (NEMA) in 1985 as ACR-NEMA, it became DICOM 3.0 in 1993. Today it is maintained by the Medical Imaging & Technology Alliance (MITA).

DICOM is not just an image format — it is a complete communication protocol that defines how devices send, receive, print, and store medical images.

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Universal Adoption

Used by virtually every hospital, imaging center, and PACS worldwide.

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Patient Safety

Embeds patient identity with every image to prevent mix-ups.

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Vendor-Neutral

CT from Siemens can be read by software from any vendor.

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Network Protocol

Defines how devices talk: C-STORE, C-FIND, C-GET, and more.

DICOM File Structure

A DICOM file has a specific binary layout. Every conformant file starts with a 128-byte preamble, a 4-byte "DICM" magic marker, and then a series of data elements called the File Meta Information (group 0002), followed by the dataset itself.

0–127

128 bytes

File Preamble

Optional. Ignored by DICOM readers but may be used by specific applications.

128–131

4 bytes

DICM Prefix

Literal bytes "D", "I", "C", "M" — identifies the file as DICOM.

132+

Variable

File Meta Information (Group 0002)

Transfer Syntax UID, Media Storage SOP Class UID, implementation details. Written in Explicit Little Endian.

After meta

Variable

Data Set

All DICOM data elements for this SOP Instance — patient, study, series, image, pixel data, etc.

Data Element Encoding

Explicit VR (recommended)

Group (2B)Element (2B)VR (2B)Length (2-4B)Value

Includes the VR as 2 ASCII bytes, making the file self-describing. Used in Transfer Syntaxes like Explicit Little Endian (1.2.840.10008.1.2.1).

Implicit VR (legacy)

Group (2B)Element (2B)Length (4B)Value

VR is omitted — readers must look up the data dictionary. Default Implicit Little Endian (1.2.840.10008.1.2) is being phased out.

Data Elements & Tags

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Tag

Every data element is identified by a tag — two 16-bit numbers written as (GGGG,EEEE) where GGGG is the group and EEEE is the element number. For example, (0010,0010) is Patient Name.

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Value Representation (VR)

Specifies the data type and format of the value — e.g. DA (date), UI (UID), US (unsigned short). There are 27 VR types defined by the DICOM standard.

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Value Multiplicity (VM)

How many values the element can hold. VM=1 means a single value; VM=1-n means one or more values separated by backslash (\).

Tag Group Ranges

(0002,xxxx)File Meta Information

(0008,xxxx)Identifying Information

(0010,xxxx)Patient Information

(0018,xxxx)Acquisition Parameters

(0020,xxxx)Relationship / Study / Series

(0028,xxxx)Image Presentation

(0040,xxxx)Modality Worklist / Procedure

(7FE0,xxxx)Pixel Data

(FFFE,xxxx)Item / Sequence Delimiters

Value Representations (VR)

VRs define the data type for each element. There are 27 defined VR types.

Application EntityIdentifies an application (16 chars max)

Age StringAge formatted as nnnD/W/M/Y (e.g. 025Y)

Attribute TagA pair of 16-bit unsigned integers (group, element)

Code StringA string of chars from a restricted character set (16 chars max)

DateYYYYMMDD format

Decimal StringDecimal representation of a number as string

Date TimeConcatenated date and time string

Floating Point SingleSingle precision binary floating point number

Floating Point DoubleDouble precision binary floating point number

Integer StringInteger as a string

Long StringCharacter string (64 chars max)

Long TextCharacter string (10240 chars max)

Other Byte StringString of bytes

Other Double StringString of 64-bit IEEE floats

Other Float StringString of 32-bit IEEE floats

Other Word StringString of 16-bit words

Person NameFamily^Given^Middle^Prefix^Suffix

Short StringShort character string (16 chars max)

Signed LongSigned 32-bit integer

Sequence of ItemsNested sequence of dataset items

Signed ShortSigned 16-bit integer

Short TextShort text (1024 chars max)

TimeHHMMSS.FFFFFF format

Unlimited CharactersCharacter string (unlimited length)

Unique Identifier (UID)ISO 8824 UID syntax — 0-9 and dots only

Unsigned LongUnsigned 32-bit integer

UnknownUnrecognised VR — treated as OB

Universal Resource IdentifierURI or URL

Unsigned ShortUnsigned 16-bit integer

Unlimited TextUnlimited character text

Common DICOM Tags

These are the most frequently encountered tags, grouped by their purpose.

Patient

Tag	VR	Name	Description
(0010,0010)	PN	Patient Name	Patient's full name.
(0010,0020)	LO	Patient ID	Primary hospital identification number.
(0010,0030)	DA	Patient Birth Date	Patient's date of birth.
(0010,0040)	CS	Patient Sex	Patient's sex (M, F, or O).
(0010,1010)	AS	Patient Age	Age of the patient at time of Study.
(0010,1020)	DS	Patient Size	Patient's height in metres.
(0010,1030)	DS	Patient Weight	Patient's weight in kg.

Study

Tag	VR	Name	Description
(0008,0020)	DA	Study Date	Date the Study started (YYYYMMDD).
(0008,0030)	TM	Study Time	Time the Study started (HHMMSS.FFFFFF).
(0008,0050)	SH	Accession Number	RIS-generated identifier for the Study.
(0008,0090)	PN	Referring Physician Name	Name of the referring physician.
(0008,1030)	LO	Study Description	Description of the Study.
(0020,0010)	SH	Study ID	User or equipment generated Study identifier.
(0020,000D)	UI	Study Instance UID	Unique identifier for the Study.

Series

Tag	VR	Name	Description
(0008,0021)	DA	Series Date	Date the Series started.
(0008,0031)	TM	Series Time	Time the Series started.
(0008,0060)	CS	Modality	Type of equipment that acquired the data (CT, MR, US, etc.).
(0008,103E)	LO	Series Description	Description of the Series.
(0020,0011)	IS	Series Number	A number that identifies this Series.
(0020,000E)	UI	Series Instance UID	Unique identifier for the Series.
(0018,1030)	LO	Protocol Name	Description of the imaging protocol used for this series.

Image

Tag	VR	Name	Description
(0008,0022)	DA	Acquisition Date	Date the acquisition of data began.
(0008,0023)	DA	Content Date	Date the image content was created.
(0018,0050)	DS	Slice Thickness	Nominal slice thickness in mm.
(0018,0081)	DS	Echo Time	Time in ms between the middle of the RF excitation pulse and the peak of the echo (MRI).
(0018,0087)	DS	Magnetic Field Strength	Nominal field strength of MR magnet in Tesla.
(0018,1164)	DS	Imager Pixel Spacing	Physical distance in the patient between centre of each pixel.
(0020,0013)	IS	Instance Number	A number that identifies this SOP Instance.
(0020,0032)	DS	Image Position (Patient)	x, y, and z coordinates of the upper left corner of the image in mm.
(0020,0037)	DS	Image Orientation (Patient)	Direction cosines of the first row and column with respect to the patient.
(0028,0002)	US	Samples per Pixel	Number of samples per pixel (1 for grayscale, 3 for RGB).
(0028,0004)	CS	Photometric Interpretation	Intended interpretation of pixel data (MONOCHROME2, RGB, etc.).
(0028,0010)	US	Rows	Number of rows in the image.
(0028,0011)	US	Columns	Number of columns in the image.
(0028,0030)	DS	Pixel Spacing	Physical distance in the patient between pixel centres [row, column] in mm.
(0028,0100)	US	Bits Allocated	Number of bits allocated for each pixel sample (typically 8 or 16).
(0028,0101)	US	Bits Stored	Number of bits stored for each pixel sample.
(0028,0102)	US	High Bit	Most significant bit for pixel sample data.
(0028,0103)	US	Pixel Representation	Data representation of pixel samples (0 = unsigned, 1 = two's complement signed).
(0028,1050)	DS	Window Center	Luminance level for windowing display.
(0028,1051)	DS	Window Width	Range of luminance values for windowing display.
(0028,1052)	DS	Rescale Intercept	Intercept value for rescaling pixel values.
(0028,1053)	DS	Rescale Slope	Slope value for rescaling pixel values.
(0028,0008)	IS	Number of Frames	Number of frames in a multi-frame image.
(0020,0052)	UI	Frame of Reference UID	Unique identifier of the coordinate system shared by spatially related images.
(0008,0008)	CS	Image Type	Image characteristics: ORIGINAL\PRIMARY\AXIAL or DERIVED\SECONDARY, etc.
(0018,9004)	CS	Content Qualification	PRODUCT, RESEARCH, or SERVICE — indicates image intended use.

Equipment

Tag	VR	Name	Description
(0008,0070)	LO	Manufacturer	Manufacturer of the equipment.
(0008,0080)	LO	Institution Name	Institution where the equipment is located.
(0008,1090)	LO	Manufacturer Model Name	Model name of the equipment manufacturer.
(0018,1020)	LO	Software Versions	Software versions of the modality that produced the image.
(0018,1000)	LO	Device Serial Number	Manufacturer's serial number of the device.

SOP Common

Tag	VR	Name	Description
(0008,0016)	UI	SOP Class UID	Uniquely identifies the SOP Class.
(0008,0018)	UI	SOP Instance UID	Uniquely identifies the SOP Instance.

Network / Worklist

Tag	VR	Name	Description
(0040,0275)	SQ	Request Attributes Sequence	Sequence of requested procedure information from the RIS/HIS.
(0040,1001)	SH	Requested Procedure ID	Identifier of the requested procedure.
(0040,0009)	SH	Scheduled Procedure Step ID	Identifier of the scheduled procedure step.
(0040,0006)	PN	Scheduled Performing Physician Name	Physician scheduled to perform the procedure step.
(0040,0007)	LO	Scheduled Procedure Step Description	Description of the scheduled procedure step.
(0040,0004)	DA	Scheduled Procedure Step End Date	Date on which the scheduled procedure step is expected to end.
(0032,1070)	LO	Requested Contrast Agent	Contrast agent requested for the procedure.

Imaging Modalities

The Modality tag (0008,0060) identifies the type of equipment that acquired the image.

CTComputed Tomography

X-ray measurements from multiple angles to produce cross-sectional images.

MRMagnetic Resonance

Strong magnetic fields and radio waves to produce detailed organ images.

USUltrasound

High-frequency sound waves to create images of internal structures.

CRComputed Radiography

Digital X-ray images captured via photostimulable phosphor plates.

DXDigital Radiography

Direct digital X-ray capture (flat-panel detectors).

NMNuclear Medicine

Radioactive tracers to image organ function and physiology.

PTPositron Emission Tomography

Metabolic imaging using positron-emitting radionuclides.

XAX-Ray Angiography

X-ray imaging of blood vessels, often with contrast.

MGMammography

Low-energy X-rays to examine breast tissue.

RFRadio Fluoroscopy

Real-time moving X-ray images.

OTOther

Modality not covered by defined terms.

SRStructured Report

Clinical reports stored in DICOM format.

SOP Classes

A SOP Class (Service-Object Pair Class) combines a DICOM Service (e.g. Storage) with an Information Object Definition (e.g. CT Image). The SOP Class UID uniquely identifies what kind of object this is.

UID	Name
1.2.840.10008.5.1.4.1.1.2	CT Image Storage
1.2.840.10008.5.1.4.1.1.4	MR Image Storage
1.2.840.10008.5.1.4.1.1.128	PET Image Storage
1.2.840.10008.5.1.4.1.1.6.1	Ultrasound Image Storage
1.2.840.10008.5.1.4.1.1.1	Computed Radiography Image Storage
1.2.840.10008.5.1.4.1.1.1.1	Digital X-Ray Image Storage — For Presentation
1.2.840.10008.5.1.4.1.1.7	Secondary Capture Image Storage
1.2.840.10008.5.1.4.1.1.481.1	RT Image Storage
1.2.840.10008.5.1.4.1.1.12.1	X-Ray Angiographic Image Storage
1.2.840.10008.5.1.4.1.1.11.1	Grayscale Softcopy Presentation State

DICOM Information Hierarchy

DICOM organises data in a strict four-level hierarchy. Every instance belongs to a series, every series belongs to a study, and every study belongs to a patient. Unique identifiers (UIDs) link the levels together.

Patient(0010,0020) Patient ID

Top-level entity. One patient can have many studies across years. Identified by Patient ID and Patient Name.

PatientID (0010,0020)PatientName (0010,0010)PatientBirthDate (0010,0030)PatientSex (0010,0040)

Study(0020,000D) Study Instance UID

One clinical visit / request. A chest CT and a bone scan ordered together share a Study Instance UID. Described by Study Date, Time, Description, and Accession Number.

StudyInstanceUID (0020,000D)StudyDate (0008,0020)AccessionNumber (0008,0050)StudyDescription (0008,1030)

Series(0020,000E) Series Instance UID

A coherent set of images acquired with the same protocol (e.g. axial CT chest, sagittal T2 MR spine). One study may have many series.

SeriesInstanceUID (0020,000E)Modality (0008,0060)SeriesNumber (0020,0011)SeriesDescription (0008,103E)

Instance (Image)(0008,0018) SOP Instance UID

One individual DICOM object — a single image slice, a structured report, or an entire multiframe dataset. Identified by the globally unique SOP Instance UID.

SOPInstanceUID (0008,0018)SOPClassUID (0008,0016)InstanceNumber (0020,0013)PixelData (7FE0,0010)

UID Structure

Every UID in DICOM follows the ISO 8824 Object Identifier (OID) syntax — a dot-separated sequence of digits, max 64 characters. The root prefix is registered with the standards body; the suffix is generated by the implementation.

1.2.840.10008.5.1.4.1.1.2

ISO root

ISO member bodies

840

USA (ANSI)

10008

DICOM standard

5.1.4.1.1.2

CT Image Storage SOP Class

DICOM Network & Associations

DICOM defines its own networking protocol stack (DIMSE — DICOM Message Service Element) that operates over a TCP/IP connection. Before any data flows, two devices must establish an Association — a handshake that negotiates capabilities.

Association Negotiation

A-ASSOCIATE-RQSCU

Request: lists supported Presentation Contexts (SOP Class + Transfer Syntax pairs)

A-ASSOCIATE-ACSCP

Accept: confirms which Presentation Contexts are accepted

Data TransferSCU↔SCP

C-STORE / C-FIND / C-MOVE messages exchanged

A-RELEASE-RQ/RPEither

Graceful association release

Key Networking Concepts

AE Title

Application Entity Title — up to 16 ASCII chars identifying a DICOM device (e.g. PACS_ARCHIVE, CT_SCANNER_3).

SCU

Service Class User — the initiator/client. Calls the service (e.g. sends C-STORE).

SCP

Service Class Provider — the responder/server. Handles the service (e.g. receives and stores C-STORE).

Presentation Context

A pairing of one Abstract Syntax (SOP Class) with one or more Transfer Syntaxes. Each gets an ID (1, 3, 5…).

PDU

Protocol Data Unit — the binary message unit. A-ASSOCIATE, P-DATA, A-RELEASE, A-ABORT are all PDU types.

DICOM Port

Default TCP port 104 (standard), 11112 (non-privileged alternative). DICOMweb uses 443 (HTTPS).

Modality Worklist (MWL)

Worklist is the mechanism by which a scanner fetches the list of scheduled patients from the RIS before a scan. Without it, technologists must manually type patient demographics at the scanner — a major source of error.

Step 1: RIS creates order

Radiologist / clinician requests a CT chest. RIS creates a Scheduled Procedure Step.

Step 2: Modality queries MWL

CT scanner sends C-FIND to the worklist SCP asking for today's scheduled steps for this AE.

Step 3: Scan & C-STORE

Technologist selects the correct patient. Scanner sends MPPS (N-SET) to notify PACS the scan started, then C-STOREs the images.

Sequences (SQ) & Nested Data

VR SQ (Sequence of Items) is DICOM's way of nesting structured data — similar to an array of objects. Each Item in a sequence is a complete dataset enclosed in Item delimiters.

Sequence Binary Layout

SQ tag + lengthe.g. (0008,1115) Referenced Series Sequence

(FFFE,E000) Item startMarks beginning of one item

DatasetArbitrary DICOM attributes (can include nested SQ)

(FFFE,E00D) Item endMarks end of one item

… repeat for each itemMultiple items = multiple rows of data

(FFFE,E0DD) Seq endSequence delimiter (if undefined length)

Common Sequences

(0008,1115)

Referenced Series Sequence

Links to series referenced by this object.

(0008,1250)

Related Series Sequence

Series acquired in the same frame of reference.

(0040,0260)

Performed Protocol Code Sequence

Codes describing the performed imaging protocol.

(0040,A730)

Content Sequence

Tree of SR (Structured Report) content items.

(5200,9229)

Shared Functional Groups Seq.

Attributes shared by all frames in Enhanced IOD.

(5200,9230)

Per-Frame Functional Groups Seq.

Per-frame geometry and acquisition parameters.

(0008,9092)

Referenced Image Evidence Sequence

References to images that were used to derive this instance.

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