Introduction to the DICOM Standard

1.Overview of DICOM

The contents of the DICOM standard go far beyond a definition of an exchange format for medical image data. DICOM defines：

• data structures (formats) for medical images and related data,
• network oriented services, e. g.（面向网络的服务）
  • image transmission,
  • query of an image archive (PACS),（图像存档查询）
  • print (hardcopy), and
  • RIS - PACS - modality integration
• formats for storage media exchange, and
• requirements for conforming devices and programs.（设备和程序开发的一致性）

2.DICOM data structures

A DICOM image consists of a list of data elements (so-called attributes) which contain a multitude of image related information:

• patient information (name, sex, identification number),
• modality and imaging procedure information (device parameters, calibration, radiation dose, contrast media), and
• image information (resolution, windowing).

For each modality, DICOM precisely defines the data elements that are required, optional (i. e. may be omitted) or required under certain circumstances (i. e. only if contrast media was used). This powerful flexibility is at the same time, however, one crucial weakness of the DICOM standard because practical experience shows that image objects are frequently incomplete. In such objects, required fields are missing or contain incorrect values. These problems can lead to subsequent problems when exchanging data.

3.DICOM network sevices

The DICOM network services are based on the client/server（客户端/服务器） concept. In case two DICOM applications want to exchange information, they must establish a connection and agree on the following parameters:

• who is client and who is server,
• which DICOM services are to be used, and
• in which format data is transmitted (e. g. compressed or uncompressed).

Only if both applications agree on a common set of parameters, the connection can and will be established. In addition to the most basic DICOM service "image transmission" (or in DICOM terminology: "Storage Service Class") there are number of advanced services, e. g.:

• The DICOM image archive service ("Query/ Retrieve Service Class") allows to search images in a PACS archive by certain criteria (patient, time of creation of the images, modality etc.) and to selectively download images from this archive.
• The DICOM print service ("Print Management Service Class") allows to access laser cameras or printers over a network, so that multiple modalities and workstations can share one printer.
• The DICOM modality worklist service allows to automatically download up-to-date worklists including a patient's demographic data from an information system (HIS/RIS) to the modality.

4.Media exchange（媒介交换）

In addition to the exchange of medical images over a network, media exchange has become another focus which has been integrated into the DICOM standard in 1996. Fields of application are for example the storage or cardiac angiography films in cardiology or the storage of ultrasound images. In order to make sure that DICOM storage media are really interchangeable, the standard defines so-called "application profiles" which explicitly define

• images from which modalities may be present on the medium (e. g. 'only X-Ray Angiography images'),
• which encoding formats and compression schemes may be used (e. g. only uncompressed or loss-less JPEG),
• which storage medium is to be used (e. g. 'CD-R with ISO file system').

Aside from the image files, each DICOM medium contains a so-called "DICOM directory".This directory contains the most important information (patient name, modality, unique identifiers etc.)for all images which are captured on the medium. With the necessary help of this directory, it is possible to quickly browse or search through all images on the medium without having to read the complete image files - which would for instance take a couple of minutes when reading from a CD.