Digital Imaging and Communications in Medicine (DICOM) 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 holds the copyright to this standard. It was developed by the DICOM Standards Committee, whose members are also partly members of NEMA.
DICOM enables the integration of scanners, servers, workstations, printers, and network hardware from multiple manufacturers into a picture archiving and communication system. 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.
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History
DICOM is the third version of a standard developed by American College of Radiology (ACR) and National Electrical Manufacturers Association (NEMA).
In the early 1980's it was almost impossible for others than the manufacturers of a CT or MR to decode the images that the machines generated. The radiologists wanted to use the images for dose planning for Radiation therapy. ACR and NEMA joint forces and formed a standard committé in 1983. Their first standard, ACR/NEMA 300, was released in 1985. Very soon after it had been released it was clear that it needed improvements to be useful. The text was vague and had internal contradictions.
In 1988 the second version was released. This had a broader acceptance by the vendors. The image transmission was over a dedicated 50-pin DICOM cable. On the annual meeting of RSNA in 1990 the first commercial equipments supporting ACR/NEMA 2.0 was presented by GE Healthcare and a company named Vortech (later purchased by Eastman Kodak). Many soon realized that the second version also needed improvements. Several extensions to ACR/NEMA 2.0 was created, like Papyrus (developed by the University Hospital of Geneva, Switzerland) and SPI, Standard Procduct Interconnect (driven by Siemens Medical Solutions and Philips Medical Systems).
In 1992 the third version of the standard was released. It then changed name to DICOM. New service classes were defined, network support added and the Conformance Statment was introduced. We still have version 3.0 of the standard, but considerable development is done in the standard. Instead of using the version number the standard is often version-numbered using the release year, like "the 2007 version of DICOM".
DICOM Data Format
DICOM differs from other data formats in that it groups information together into a data set. That is, an X-Ray of your chest actually contains your patient ID within it, so that the image is never mistakenly separated from your information.
A DICOM data object consist of a number of attributes, containing such items 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 image, but that image may have multiple "frames", allowing storage of cine loops or other multi-frame data. Image data can be compressed using a variety of standards, including JPEG, JPEG Lossless, JPEG 2000, and Run-length encoding (RLE). LZW (zip) compression could be used for the whole data set (not just the pixel data) but is rarely implemented.
The same basic format is used for all uses, including network and file usage, but when written to a file, then a true "header" (containing copies of a few key attributes and details of the application which wrote it) is prepended.
DICOM Services
DICOM consists of many different services, most of which involve transmission of data over a network, and the file format below is in fact 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 (modality, workstation etc.) uses the confirmation from the service class provider (archive station) to make sure that it is safe to delete the images locally.
Query/Retrieve/Transfer
This enables a workstation to find lists of and then retrieve images or other such objects from a PACS. Because of HIPAA regulations, most DICOM imagery is not transferable because of the patient identifiers that go along with the images. Certain technologies are available now though to accomodate the retrieval and transfer of DICOM and other medical imagery in comliance with [HIPAA] guidelines. The iMedicor Physician's Portal provides physicians the possibility of uploading DICOM imagery into the secure system and transferring those images to other physicians in the portal. This type of portal promotes communication between health organizations beyond what current EMRs and other document management systems provide. iMedicor is a technology provided as a free service to physicians by Vemics, Inc., a publicly traded company involved in Medical, Business, and Educational solutions.
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 also 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, dose delivered, etc. Also known as MPPS, this service renders modality better coordinance with image storage server in a way of giving 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 character names (many people wrongly use 8.3, but this is not legal). No information must be extracted from these names (PS10:6.2.3.2). This is a common source of problems with media created by developers that 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, that 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.
There is also an ongoing media exchange test and connectathon process for CD media and network operation that is organized by the IHE organization.
