Digital Imaging and Communications in Medicine (DICOM) is a technical standard for the digital storage and transmission of medical images and related information. It includes information object definitions (i.e. message formats); service definitions; a file format definition, which specifies the structure of a DICOM file; and a network communication protocol that uses either TCP/IP or HTTPS to communicate between systems. The primary purpose of the standard is to facilitate communication between the software and hardware entities involved in medical imaging, especially those that are created by different manufacturers. Entities that utilize DICOM files include components of picture archiving and communication systems (PACS), such as imaging machines (modalities), radiological information systems (RIS), scanners, printers, computing servers, and networking hardware.
The DICOM standard has been widely adopted by hospitals, large clinics, and the medical software industry. It is sometimes used in smaller-scale applications, such as dentists' and doctors' offices. DICOM is also used in veterinary medicine and in research.
The National Electrical Manufacturers Association (NEMA) holds the copyright to the published standard,DICOM brochure, nema.org. which was developed by the DICOM Standards Committee (which includes some NEMA members). It is also known as NEMA standard PS3 and as ISO standard 12052:2017: "Health informatics – Digital imaging and communication in medicine (DICOM) including workflow and data management".
Applications
DICOM is used worldwide to store, exchange, and transmit medical images. DICOM has been central to the development of modern radiological imaging; DICOM incorporates standards for imaging modalities such as radiography, ultrasonography, computed tomography (CT), magnetic resonance imaging (MRI), and radiation therapy. DICOM includes protocols for image exchange (e.g., via portable media such as DVDs), image compression, 3-D visualization, image presentation, and results reporting.
History DICOM is a standard developed by the American College of Radiology (ACR) and the National Electrical Manufacturers Association (NEMA).
In the beginning of the 1980s, it was difficult 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 sought the images for dose-planning in radiation therapy. ACR and NEMA collaborated and formed a standard committee in 1983. Their first standard, ACR/NEMA 300, entitled "Digital Imaging and Communications", 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 2 pair cable (EIA-485). 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 based at 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 from this contract.
In 1993 the third version of the standard was released. Its name was then changed to "Digital Imaging and Communications in Medicine", abbreviated DICOM. New service classes were defined, network support added and the Conformance Statement was introduced. Initially the DICOM standard was referred to as "DICOM 3.0" to distinguish it from its predecessors. DICOM has been constantly updated and extended since 1993, with the intent that changes are backward compatible, except in rare cases where the earlier specification was incorrect or ambiguous. Officially there is no "version" of the standard except the current standard, hence the "3.0" version number is no longer used. There are no "minor" versions to the standard (e.g., no such thing as "DICOM 3.1") and there are no current plans to develop a new, incompatible, version of the standard (i.e., no "DICOM 4.0"). The standard should be referenced without specification of the date of release of a particular published edition, except when specific conformance requirements are invoked that depend on a retired feature that is no longer documented in the current standard.
While the DICOM standard has achieved a near universal level of acceptance among 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. The Integrating the Healthcare Enterprise (IHE) initiative layered on top of DICOM (and HL-7) defines profiles to select features from these standards to implement transactions for specific medical imaging interoperability use cases.
Though always Internet compatible and based on transport over TCP, over time there has been an increasing need to support port 80 HTTP transport to make use easier within the web browser. Most recently, a family of DICOM RESTful web services have been defined to allow mobile device friendly access to DICOM objects and services, which include WADO-RS, STOW-RS and QIDO-RS, which together constitute the DICOMweb initiative.
Derivations
There are some derivations from the DICOM standard into other application areas. These include DICONDE (Digital Imaging and Communication in Nondestructive Evaluation) that was established by ASTM International and DICOS (Digital Imaging and Communication in Security).
DICONDE
DICONDE was established in 2004 as a way for nondestructive testing manufacturers and users to share image data. DICONDE can be used for computed radiography, digital radiography, computed tomography, ultrasonic testing, eddy-current testing, and thermographic testing. DICONDE is used worldwide to store, send, and exchange data from nondestructive material testing. Unlike DICOM in the medical field, the adoption of DICONDE has been slower due to the lack of regulatory requirements for manufacturer-independent interoperability. DICONDE is also gaining importance in the context of Industry 4.0 due to its extensive capabilities for networking systems from different manufacturers.
DICOS
DICOS was established in 2009 to be used for image sharing in airport security, cargo scanning, and other security applications.
The standard is published by the National Electrical Manufacturers Association (NEMA) as NEMA IIC 1, and was developed in cooperation with the U.S. Department of Homeland Security's Science and Technology Directorate and the Transportation Security Administration (TSA).
Data format
DICOM groups information into data sets. For example, a file of a chest x-ray image may contain 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, being merely a list of attributes, including the pixel data). A single DICOM object can have only one attribute containing pixel data. For many modalities, this corresponds to a single image. However, 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, lossless JPEG, 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 has rarely been 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 that wrote it) is added.
Image display To promote identical grayscale image display on different monitors and consistent hard-copy images from various printers, the DICOM committee developed a lookup table to display digitally assigned pixel values. To use the DICOM grayscale standard display function (GSDF),http://medical.nema.org/Dicom/2011/11_14pu.pdf images must be viewed (or printed) on devices that have this lookup curve or on devices that have been calibrated to the GSDF curve.
Value representations 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 "\".See Table 6.2-1 of PS 3.5
Services DICOM consists of services, most of which involve transmission of data over a network. The file format for offline media is a later addition to the standard.
Store The DICOM Store service is used to send images or other persistent objects (structured reports, etc.) to a picture archiving and communication system (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 picture archiving and communication system.
Modality worklist