CREATING A COMMON COMMUNICATION STANDARD

A short history lesson

ACR/NEMA made several attempts in the late 1980's and early 1990's, to produce a standard for transferring medical image data between different types of equipment in the diagnostic environment.

Many changes were made to the standard to accommodate the ever increasing requirements of diagnostic equipment and practices, the improved technology used for networking and computing, and early experiences with the ACR/NEMA standard.

The current release, DICOM Version 3.0 (1993), supports many different diagnostic imaging modalities; these include CR, MR, CT, Nuclear Medicine, Ultrasound, X-ray Angiography, and a special modality called 'Secondary Capture'. DICOM 3 provides:

• A specification for transferring medical images and related information between computers
• Emphasis on cross-vendor connectivity
• Use of existing communication standards

DICOM RT is the popular name for the extension of the DICOM 3.0 standard which handles the Radiotherapy modality. It was developed to extend the current DICOM 3.0 standard to include the RT modality, rather than produce a completely new standard

Why do we need DICOM by Merge?

DICOM by Merge offers us a standardized way of addressing many of the general communication requirements within the healthcare environment and should be the primary communication method. Use of proprietary interfaces should be reserved only:

• For where there is no internationally agreed standard for that purpose
• To achieve a performance not possible using an international standard
• To maintain backwards compatibility with older equipment

Overview of DICOM by Merge

All information is grouped under a unique patient identity. A study generally contains one or more series which, in turn, contain one or more images. The images in a series will usually consist of one or more images taken from a single modality such as CT or MR.

Each of the levels of information will have attributes that describe factors relating to the information, for example, diagnosis notes, scaling values for the images or other important details required by staff or equipment used in diagnosis.

For each modality, specific useful attributes define characteristics or qualities that are often grouped together and referred to as an 'object.'

Attributes may be:

• mandatory
• only required under certain conditions
• completely optional

In order for two systems to transfer information both systems must support the same modality object.

The definition of what data must be transferred, in order for it to be received and be usable at the receiving end, must be compatible.

A DICOM conformance statement defines which

• modality
• services
• network protocols (e.g. TCP/IP)

are supported and defines how each is used by the application. This is required to determine whether all essential information is provided, retained or used correctly and to determine theoretically whether two devices can transfer information accurately.

The system providing the information must have implemented the transmitting services (the service class user) and the receiving system must have implemented the receiving and storage services (the service class provider).

It is quite usual for two devices to be able to transfer information in one direction, but not in the other. One example of this is that most CT devices can send CT images to a treatment planning system although few treatment planning systems can send images back to the CT device.

Five new objects define the RT modality

• RT Image
• RT Dose
• RT Structure Set
• RT Plan
• RT Treatment Record (draft)

The first four have been published as Supplement 11 to the standard; the RT Treatment Record (Supplement 29) is due to be officially published soon.

RT Image

Its scope includes all the normal RT images, DRRs, portal images, simulator images and radiograms

RT Plan

Its scope is the geometric and dosimetric data for course of external beam treatment or brachy-therapy.

RT Dose

Its scope is the total dose distributions from the planning system; dose matrix, dose points, isodose curves and dose volume histograms.

RT Treatment Record

Its scope is to record all the treatment session data.

RT Structure Set

Its scoe is the patient related structures identified from diagnostic data, CT, virtual simulation and treatment planning system.