What's the DICOM Standard?
What's the DICOM Standard?
DICOM is an abbreviation of
Digital Imaging and Communications in Medicine. This standard forms the
main part of the PAC system. It was first created by the ACR (American
College of Radiology) and NEMA ( the National Electrical Manufacturers
Association). It was developed according to the NEMA Procedures.
This Standard is developed
in liasion with other Standardization Organizations including CEN TC251
in Europe and JIRA in Japan, with review also by other organizations including
IEEE, HL7 and ANSI in the USA.
The DICOM 3.0 standard evolved
from earlier versions of the ACR-NEMA standards
version 1.0 (1985) and version 2.0 (1988).
The essential demand for a digital imaging standard arose in the early
1980s from the frustration of the ACR representatives with the inaccessiblity
of digital image data produced by CT scanners and MR imagers. These data
were stored on magnetic tapes and flexible disks (floppies) but could not
readily be deciphered by the customers.
The single most significant demand from ACR was that NEMA, as industry
representative, should cooperate in forming a standards committee charged
with addressing this issue. Three working groups (WGs) were formed, each
reporting to the ACR-NEMA standards committee, which was headed jointly
by an ACR representative and a NEMA representative. The original WGs were:
WG I
Hardware and Protocols
WG II
Data Groups
WG III
System Performance
; ;
Specifications
New WGs are
WG IV Data
Compression
WG V
Exchange Media
WG VI Validation
(and Upgrades)
WG VII Multidimensional
Data
WG VIII HIS/RIS/PACS Interface
Other new WGs have since been proposed.
Now, 9 years later, the definition of images
is still the most important information, although it occupies only a fraction
of the bulk contained in the 10 parts of DICOM 3.0 Standard. These are
:
Part1
Introduction and Overview
Part2
Conformance
Part3
Information Object Definitions
Part4
Service Class Specifications
Part5
Data Structure and Semantics
Part6
Data Dictionary
Part7
Message Exchange
Part8
Network Communication
Part9
Point-to-Point Communication
Part10
Media Storage and File Format
Diagnostic images, particularly series of CT, MR,
and ultrasound images, may be 100 MByte (100 million bytes) or larger.
Sophisticated technology is needed in order to fetch these images fast
from storage and present them conveniently to the radiologist. A fast network
is needed if these images are remotely stored. by contrast, all other clinical
and demographic information can be expressed by less than 0.1% of the data
quantity needed for images. Robust, reliable and affordable technology
exists that can handle such quantities. Furthermore, well-established radiology
information systems (RIS) and hospital information systems (HIS) exist
and provide information and department management functions.
In response to the published version 2.0 of the
ACR-NEMA standard, users requested a network version. It also became evident
that medical informatics should be addressed on a broader basis. The developments
in Europe initiated by the Comite Europeen de Normalisation (CEN) through
its Technical Committee 251 (TC 251) required a matching design. A crucial
requirement was adherence to language and structure of OSI.
In 1991, NEMA joined HISPP, the ANSI Healthcare
Information Standards Planning Panel, which coordinates efforts toward
a comprehensive healthcare informatics standard in the United States.
Therefore, the ACR-NEMA standard, version 2.0 was
rewritten in order to become conformant with ISO and OSI.
THE CONCEPT OF AN OPEN SYSTEM
The DICOM standard is patterned after OSI, the Open
System Interconnection of ISO. The key feature of OSI is communication
between heterogeneous systems. It was developed as a generalized model
based on the experience with ARPANET and CYCLADES. "Openness" is established
when participating parties agree on a communication protocol. The "message"
that is transmitted between the communicating partners ("nodes")
is expressed in a specified form. The DICOM standard specifies this form
through the tansfer syntax that defines the coding of the information.
The message itself is accompanied by instruction elements appropriate to
the communication channel (for instance, which communication "stack" will
be used).
A nontechnical parallel would be a communication
between two mathematicians with no common conventional language. This would
require double translations in each direction; faulty "coding" (misunderstandings)
would probably occur.
Aside from the enormous amount of detail and complexity
of the OSI standard, the essential feature of the standard is the transportable
message in well-defined form, the capability of the sending node to generate
this message, and the corresponding capability of the receiving node
to decipher or parse this message. The sending and receiving nodes need
not use the same operating system or the same application program. They
can be, and in many cases will be, heterogeneous.
Internet which evolved from ARPANET, is an example
of such an open communication system. According to some sources it connects
more than 2 million computers and accomodates more than 15 million users.
It offers greater functionality than DICOM but simpler coding.