DICOM

DICOM (Digital Imaging and Communications in Medicine) is a standard for handling, storing, printing, and transmitting information in medical imaging. It includes a file format definition and a network communications protocol. The communication protocol is an application protocol that uses TCP/IP to communicate between systems. DICOM files can be exchanged between two entities that are capable of receiving image and patient data in DICOM format. The National Electrical Manufacturers Association (NEMA) holds the copyright to this standard.^[1] It was developed by the DICOM Standards Committee, whose members^[2] are also partly members of NEMA.^[3]

DICOM enables the integration of scanners, servers, workstations, printers, and network hardware from multiple manufacturers into a picture archiving and communication system (PACS). The different devices come with DICOM conformance statements which clearly state the DICOM classes they support. DICOM has been widely adopted by hospitals and is making inroads in smaller applications like dentists' and doctors' offices.

DICOM is known as NEMA standard PS3, and as ISO standard 12052:2006 "Health informatics -- Digital imaging and communication in medicine (DICOM) including workflow and data management".

Parts of the DICOM standard

The DICOM standard is divided into related but independent parts:^[4]

The links below are to the 2009 version published in January 2010. Additions to the standard (Supplements and Change Proposals) since that publication are available through the DICOM Web site.

• PS 3.1: Introduction and OverviewPDF (241 KB)
• PS 3.2: ConformancePDF (6.46 MB)
• PS 3.3: Information Object DefinitionsPDF (6.96 MB)
• PS 3.4: Service Class SpecificationsPDF (1.07 MB)
• PS 3.5: Data Structure and EncodingPDF (1.43 MB)
• PS 3.6: Data DictionaryPDF (7.32 MB)
• PS 3.7: Message ExchangePDF (1.97 MB)
• PS 3.8: Network Communication Support for Message ExchangePDF (901 KB)
• PS 3.9: Retired (formerly Point-to-Point Communication Support for Message Exchange)
• PS 3.10: Media Storage and File Format for Data InterchangePDF (406 KB)
• PS 3.11: Media Storage Application ProfilesPDF (1.14 MB)
• PS 3.12: Storage Functions and Media Formats for Data InterchangePDF (593 KB)
• PS 3.13: Retired (formerly Print Management Point-to-Point Communication Support)
• PS 3.14: Grayscale Standard Display FunctionPDF (2.88 MB)
• PS 3.15: Security and System Management ProfilesPDF (1.00 MB)
• PS 3.16: Content Mapping ResourcePDF (3.08 MB)
• PS 3.17: Explanatory InformationPDF (3.28 MB)
• PS 3.18: Web Access to DICOM Persistent Objects (WADO)PDF (291 KB)

History

Front page of ACR/NEMA 300, version 1.0, which was released in 1985

DICOM is the third version of a standard developed by American College of Radiology (ACR) and National Electrical Manufacturers Association (NEMA).

In the beginning of the 1980s it was almost impossible for anyone other than manufacturers of computed tomography or magnetic resonance imaging devices to decode the images that the machines generated. Radiologists and medical physicists wanted to use the images for dose-planning for radiation therapy. ACR and NEMA joined forces and formed a standard committee in 1983. Their first standard, ACR/NEMA 300, was released in 1985. Very soon after its release, it became clear that improvements were needed. The text was vague and had internal contradictions.

In 1988 the second version was released. This version gained more acceptance among vendors. The image transmission was specified as over a dedicated 25 differential (EIA-485) pair cable. The first demonstration of ACR/NEMA V2.0 interconnectivity technology was held at Georgetown University, May 21–23, 1990. Six companies participated in this event, DeJarnette Research Systems, General Electric Medical Systems, Merge Technologies, Siemens Medical Systems, Vortech (acquired by Kodak that same year) and 3M. Commercial equipment supporting ACR/NEMA 2.0 was presented at the annual meeting of the Radiological Society of North America (RSNA) in 1990 by these same vendors. Many soon realized that the second version also needed improvement. Several extensions to ACR/NEMA 2.0 were created, like Papyrus (developed by the University Hospital of Geneva, Switzerland) and SPI, (Standard Product Interconnect, driven by Siemens Medical Systems and Philips Medical Systems).

The first large scale deployment of ACR/NEMA technology was made in 1992 by the US Army and Air Force as part of the MDIS (Medical Diagnostic Imaging Support) program run out of Ft. Detrick, Maryland. Loral Aerospace and Siemens Medical Systems led a consortium of companies in deploying the first US military PACS (Picture Archiving and Communications System) at all major Army and Air Force medical treatment facilities and teleradiology nodes at a large number of US military clinics. DeJarnette Research Systems and Merge Technologies provided the modality gateway interfaces from third party imaging modalities to the Siemens SPI network. The Veterans Administration and the Navy also purchased systems off this contract.

In 1993 the third version of the standard was released. Its name was then changed to DICOM so as to improve the possibility of international acceptance as a standard. New service classes were defined, network support added and the Conformance Statement was introduced. Officially, the latest version of the standard is still 3.0, however, it has been constantly updated and extended since 1993. Instead of using the version number the standard is often version-numbered using the release year, like "the 2007 version of DICOM".

While the DICOM standard has achieved a near universal level of acceptance amongst medical imaging equipment vendors and healthcare IT organizations, the standard has its limitations. DICOM is a standard directed at addressing technical interoperability issues in medical imaging. It is not a framework or architecture for achieving a useful clinical workflow. RSNA's Integrating the Healthcare Enterprise (IHE) initiative layered on top of DICOM (and HL-7) provides this final piece of the medical imaging interoperability puzzle.

Derivations

There are some derivations from the DICOM standard into other application areas. This includes

• DICONDE - Digital Imaging and Communication in Nondestructive Evaluation, was established in 2004 as a way for nondestructive testing manufacturers and users to share image data.^[5]
• DICOS - Digital Imaging and Communication in Security was established in 2009 to be used for image sharing in airport security.^[6]

DICOM Data Format

DICOM differs from some, but not all, data formats in that it groups information into data sets. That means that a file of a chest X-Ray image, for example, actually contains the patient ID within the file, so that the image can never be separated from this information by mistake. This is similar to the way that image formats such as JPEG can also have embedded tags to identify and otherwise describe the image.

A DICOM data object consists of a number of attributes, including items such as name, ID, etc., and also one special attribute containing the image pixel data (i.e. logically, the main object has no "header" as such: merely a list of attributes, including the pixel data). A single DICOM object can only contain one attribute containing pixel data. For many modalities, this corresponds to a single image. But note that the attribute may contain multiple "frames", allowing storage of cine loops or other multi-frame data. Another example is NM data, where an NM image by definition is a multi-dimensional multi-frame image. In these cases three- or four-dimensional data can be encapsulated in a single DICOM object. Pixel data can be compressed using a variety of standards, including JPEG, JPEG Lossless, JPEG 2000, and Run-length encoding (RLE). LZW (zip) compression can be used for the whole data set (not just the pixel data) but this is rarely implemented.

DICOM uses three different Data Element encoding schemes. With Explicit Value Representation (VR) Data Elements, for VRs that are not OB, OW, OF, SQ, UT, or UN, the format for each Data Element is: GROUP (2 bytes) ELEMENT (2 bytes) VR (2 bytes) LengthInByte (2 bytes) Data (variable length). For the other Explicit Data Elements or Implicit Data Elements, see section 7.1 of Part 5 of the DICOM Standard.

The same basic format is used for all applications, including network and file usage, but when written to a file, usually a true "header" (containing copies of a few key attributes and details of the application which wrote it) is added.

DICOM Value Representations

Extracted from Chapter 6.2 of

• PS 3.5: Data Structure and EncodingPDF (1.43 MiB)

Value Representation	Description
AE	Application Entity
AS	Age String
AT	Attribute Tag
CS	Code String
DA	Date
DS	Decimal String
DT	Date/Time
FL	Floating Point Single (4 bytes)
FD	Floating Point Double (8 bytes)
IS	Integer String
LO	Long String
LT	Long Text
OB	Other Byte
OF	Other Float
OW	Other Word
PN	Person Name
SH	Short String
SL	Signed Long
SQ	Sequence of Items
SS	Signed Short
ST	Short Text
TM	Time
UI	Unique Identifier
UL	Unsigned Long
UN	Unknown
US	Unsigned Short
UT	Unlimited Text

In addition to a Value Representation, each attribute also has a Value Multiplicity to indicate the number of data elements contained in the attribute. For character string value representations, if more than one data element is being encoded, the successive data elements are separated by the backslash character "\".

DICOM Services

DICOM consists of many different services, most of which involve transmission of data over a network, and the file format below is a later and relatively minor addition to the standard.

Store

The DICOM Store service is used to send images or other persistent objects (structured reports, etc.) to a PACS or workstation.

Storage Commitment

The DICOM storage commitment service is used to confirm that an image has been permanently stored by a device (either on redundant disks or on backup media, e.g. burnt to a CD). The Service Class User (SCU: similar to a client), a modality or workstation, etc., uses the confirmation from the Service Class Provider (SCP: similar to a server), an archive station for instance, to make sure that it is safe to delete the images locally.

Query/Retrieve

This enables a workstation to find lists of images or other such objects and then retrieve them from a PACS.

Modality Worklist

This enables a piece of imaging equipment (a modality) to obtain details of patients and scheduled examinations electronically, avoiding the need to type such information multiple times (and the mistakes caused by retyping).

Modality Performed Procedure Step

A complementary service to Modality Worklist, this enables the modality to send a report about a performed examination including data about the images acquired, beginning time, end time, and duration of a study, dose delivered, etc. It helps give the radiology department a more precise handle on resource (acquisition station) use. Also known as MPPS, this service allows a modality to better coordinate with image storage servers by giving the server a list of objects to send before or while actually sending such objects.

Printing

The DICOM Printing service is used to send images to a DICOM Printer, normally to print an "X-Ray" film. There is a standard calibration (defined in DICOM Part 14) to help ensure consistency between various display devices, including hard copy printout.

Off-line Media (DICOM Files)

The off-line media files correspond to Part 10 of the DICOM standard. It describes how to store medical imaging information on removable media. Except for the data set containing, for example, an image and demography, it's also mandatory to include the File Meta Information.

DICOM restricts the filenames on DICOM media to 8 characters (some systems wrongly use 8.3, but this does not conform to the standard). No information must be extracted from these names (PS3.10 Section 6.2.3.2). This is a common source of problems with media created by developers who did not read the specifications carefully. This is a historical requirement to maintain compatibility with older existing systems. It also mandates the presence of a media directory, the DICOMDIR file, which provides index and summary information for all the DICOM files on the media. The DICOMDIR information provides substantially greater information about each file than any filename could, so there is less need for meaningful file names.

DICOM files typically have a .dcm file extension if they are not part of a DICOM media (which requires them to be without extension).

The MIME type for DICOM files is defined by RFC 3240 as application/dicom.

The Uniform Type Identifier type for DICOM files is org.nema.dicom.

There is also an ongoing media exchange test and "connectathon" process for CD media and network operation that is organized by the IHE organization.

Application areas

Modality	Description
AS	Modality of type Angioscopy - Retired
BI	Modality of type Biomagnetic Imaging
CD	Modality of type Color Flow Doppler - Retired 2008
CF	Modality of type Cinefluorography - Retired
CP	Modality of type Colposcopy - Retired
CR	Modality of type Computed Radiography
CS	Modality of type Cystoscopy - Retired
CT	Modality of type Computed Tomography
DD	Modality of type Duplex Doppler - Retired 2008
DG	Modality of type Diaphanography
DM	Modality of type Digital Microscopy - Retired
DS	Modality of type Digital Subtraction Angiography - Retired
DX	Modality of type Digital Radiography
EC	Modality of type Echocardiography - Retired
ECG	Modality of type Electrocardiograms
EM	Modality of type Electron Microscope
ES	Modality of type Endoscopy
FA	Modality of type Fluorescein Angiography - Retired
FS	Modality of type Fundoscopy - Retired
GM	Modality of type General Microscopy
HC	Modality of type Hard Copy
LP	Modality of type Laparoscopy - Retired
LS	Modality of type Laser Surface Scan
MA	Modality of type Magnetic Resonance Angiography (retired)
MG	Modality of type Mammography
MR	Modality of type Magnetic Resonance
MS	Modality of type Magnetic Resonance Spectroscopy - Retired
NM	Modality of type Nuclear Medicine
OT	Modality of type Other
PT	Modality of type Positron Emission Tomography (PET)
RD	Modality of type Radiotherapy Dose (a.k.a. RTDOSE)
RF	Modality of type Radio Fluoroscopy
RG	Modality of type Radiographic Imaging (conventional film screen)
RTIMAG	Modality of type Radiotherapy Image
RP	Modality of type Radiotherapy Plan (a.k.a. RTPLAN)
RS	Modality of type Radiotherapy Structure Set (a.k.a RTSTRUCT)
RT	Modality of type Radiation Therapy
SC	Modality of type Secondary Capture
SM	Modality of type Slide Microscopy
SR	Modality of type Structured Reporting
ST	Modality of type Single-Photon Emission Computed Tomography (retired 2008)
TG	Modality of type Thermography
US	Modality of type Ultrasound
VF	Modality of type Videofluorography - Retired
VL	Modality of type Visible Light
XA	Modality of type X-Ray Angiography
XC	Modality of type External Camera (Photography)

DICOM transmission protocol port numbers over IP

DICOM have reserved the following TCP and UDP port numbers by the Internet Assigned Numbers Authority (IANA):

• 104 well-known port for DICOM over TCP or UDP. Since 104 is in the reserved subset, many operating systems require special privileges to use it.
• 2761 registered port for DICOM using Integrated Secure Communication Layer (ISCL) over TCP or UDP
• 2762 registered port for DICOM using Transport Layer Security (TLS) over TCP or UDP
• 11112 registered port for DICOM using standard, open communication over TCP or UDP

The standard recommends but does not require the use of these port numbers.

References

1. ^ DICOM brochure
2. ^ MEMBERS of the DICOM STANDARDS COMMITTEE
3. ^ NEMA Members
4. ^ "6.1 DIMSE Services". Digital Imaging and Communications in Medicine (DICOM) Part 1: Introduction and Overview. National Electrical Manufacturers Association. 2006. pp. 11. ftp://medical.nema.org/medical/dicom/2009/08_01pu.pdf. 
5. ^ http://www.astm.org: If a Picture Is Worth 1,000 Words, then Pervasive, Ubiquitous Imaging Is Priceless
6. ^ http://www.nema.org: Industrial Imaging and Communications Section

See also

• OsiriX - Image processing application dedicated to DICOM images
• GDCM - DICOM library for medical files

External links

• The latest DICOM specification
• DICOM Standard Status (approved and proposed changes)
• Brief introduction to DICOM
• Introduction to DICOM using OsiriX
• Medical Image FAQ part 2 - Standard formats including DICOM.
• Medical Image FAQ part 8 - Contains a long list DICOM software.
• Collection of DICOM images (clinical images and technical testpatterns)
• Example of an applet based DICOM Viewer