Medical Imaging : DICOM Standard

DICOM which stands for " Digital Imaging and Communication in Medicine " is a document which defines a method of communication for the various equipment of digital medical imaging devices/softwares. This standard is now in use by the majority of medical imagering hardware manufacturers. It represents a progress in that it makes it possible to predict the interconnection of various imaging modalities, through a Document of Conformity emitted for each machine/software following this standard. Thus, the standard makes it possible for the equipment to communicate remotely through a network or a media (disk or tapes ). By ensuring the compatibility of the equipment and by eliminating proprietary formats.

The goal is to obtain the images of the patient and all epidemiological information associated with, in an identical format allowing the interconnection and the interaction of the equipments and the transfer of the data.

WHY DICOM FORMAT FOR MEDICAL IMAGES?

1.) Single Identification Of the Produced Images:

A Department of Radiology produces several thousands of images every day, thus, a CT Scanner, working at the rate of 3 examinations per hour produces approximately 150 images per hour, and it is not possible to classify these images in a JPEG or GIF format because it would have a risk of losses of the demographic data of the images, (name of the patient, type of examination, hospital, date of examination, type of acquisition etc...). The DICOM Standard allows to make each produced image single and to associate specific information to them. Thus each image is autonomous, if it is lost, reproduced or re-elected, it is always possible to identify formally its origin, the patient, the date, the series from where it comes, parameters of acquisition etc..

The format is not rigid, it contains obligatory informations and other optional ones. Several single identification numbers UID (" Unique IDentifiers " ) are automatically generated by DICOM modalities and mandatory in each DICOM file or transfer. It cannot exist two identical UID that indicates different information, for example the machine and its localization, the UID of a series of images which is specific to a date, a patient, a study, a hospital and a given machine. This identification is necessary not only for medical and medico-legal reasons, but also to allow the devices for the formation and the management of hospital or imaging databases.

4 mandatory UID's for each DICOM image:

i.) Sop Class Uid  
Identify the type of Service for which the image is intended .
Storage Class Service
Query/Retrieve Class Service.

ii.)Study Authority Uid 
Identify a whole examination, in time and place.

iii.)Series Authority Uid 
Identify a series of images within the examination.

iv.)SOP Authority UID or Image UID 
Identify the image associated with the file

2.) The format is independent of the machines and the communications protocols:

Standard DICOM is used at the applicatif " level " i.e. it allows the communication of a programe another, this imply that connect them of low level, cablages and protocols networks are drawn up.

3.) the format uses a controlled vocabulary:

It is significant that from one machine to another the vocabulary either identical thus, one identifies the data in a universal way the some or machine. DICOM uses standard SNOMED developed by the anatomo-pathologists (Systemized Nomenclature for Medicine).

4.) the standard can be easily revised and modified thanks to the multiplicity of the Reference documents.

Currently standard DICOM 3 is composed of 12 parts to which are added supplements. The drafting of this standard follows strict directives established by the ISO committee.

5.) Format DICOM is adaptable to the different medical specialities :

Thus this format is already adopted by the cardiologists, the dentists, the anatomopathologists and the gastro-enterologists.

UNIVERSALITY OF DICOM

The old equipment often not to standard DICOM. Currently the numerical systems should be completely conforming DICOM, actually the majority of the equipment are it only partially and it is the responsibility for the user to buy open Dicom systems i.e.
i.)connectables by a network TCP/IP Ethernet
ii.)and/or being able to leave information in the form a media numerical (CD Rom, Zip discs, optical discs erasable...).

Indeed two conforming machines DICOM can not be very well connectables for reasons of physical wiring or protocols different networks.

ORGANIZATIONS RESPONSIBLE FOR DICOM

Standard DICOM was emitted by the ACR (American College of Radiology) in partnership with the NEMA (National Electrical Manufacturers Association), it is currently updated by these 2 committees to which joined different committees of international experts the such JRIA Japan, the ANSI in the USA, the CENTC251 in Europe.

The ACR is responsible for the technical and medical instructions, the NEMA is responsible for the publications and the legal problems, to avoid conflicts of interests or a possible infringement with the antitrust loies.

These committees do not have economic interests, one will note that this standard is defined out of France, in English language with the difficulties of translation of a precise technical language.