Understanding and using DICOM, the data interchange standard for biomedical imaging

Abstract

The Digital Imaging and Communications in Medicine (DICOM) Standard specifies a non-proprietary data interchange protocol, digital image format, and file structure for biomedical images and image-related information. The fundamental concepts of the DICOM message protocol, services, and information objects are reviewed as background for a detailed discussion of the functionality of DICOM; the innovations and limitations of the Standard; and the impact of various DICOM features on information system users. DICOM addresses five general application areas: (1) network image management, (2) network image interpretation management, (3) network print management, (4) imaging procedure management, (5) off-line storage media management. DICOM is a complete specification of the elements required to achieve a practical level of automatic interoperability between biomedical imaging computer systems--from application layer to bit-stream encoding. The Standard is being extended and expanded in modular fashion to support new applications and incorporate new technology. An interface to other Information Systems provides for shared management of patient, procedure, and results information related to images. A Conformance Statement template enables a knowledgeable user to determine if interoperability between two implementations is possible. Knowledge of DICOM's benefits and realistic understanding of its limitations enable one to use the Standard effectively as the basis for a long term implementation strategy for image management and communications systems.

Figure 1

This figure depicts three different implementations of the DICOM protocol scheme. A DICOM network connection exists between the application software programs (peer DICOM application entities) of imaging devices. In configuration 1 the application software generates the DICOM command request (RQ) and command response (RSP) messages that flow from one device to another. In configuration 2 a separate DICOM Message Service Element (DIMSE) protocol machine generates the command messages on behalf of the application software. The DIMSE protocol machine is the DICOM service provider (DSP). The application software is the DICOM service user (DSU). Configuration 3 uses separate modules for all communications and applications functions. A second layer of DICOM messages is used within each device. These are the DIMSE service primitives: the Request Primitive (REQP), Indication Primitive (INDP), Response Primitive (RSPP), and Confirmation Primitive (CFMP). A freestanding DSU module generates the DIMSE service primitives on behalf of the application software. Even though the protocol machine and DICOM service user module may be implemented in various ways, the external command messages are identical for all configurations.

Figure 2

Image transfer. The scanner initiates routine image transfers. DICOM does not specify the behavior of the scanner device; the scanner may begin sending images whenever it is ready. This may be done automatically as individual images are completed during a scan procedure, or it may be done at some later time after all images of a procedure have been acquired and the scanner operator has initiated the transfer by activating a “send images” key on the scanner console. When the scanner is ready to send, it sends images one by one to the workstation. The scanner initiates a DICOM communications session (called an “association”) with the workstation for the transfer of each image. Various details are negotiated during association establishment, so that the workstation can prepare to handle the image that it is about to receive.

Figure 3

Image query and retrieval. Acting on a specific request entered by the user a the workstation console, the workstation software provides to the scanner a query request message, asking for image records that have values matching a set of query keys. The scanner returns a list of matching images. Now, having knowledge of the identification numbers of the images, the workstation user selects the pertinent images from the displayed list and enters a “retrieve images” command at the keyboard. The workstation software then sends a message to the scanner, listing the image identification numbers and requesting the scanner to send the images. The scanner sends the requested images, one at a time, to the workstation, using the DICOM Storage Service, as illustrated in Figure 2.

Figure 4

Reliable storage. Storage Commitment is an extension of the basic DICOM Storage Service illustrated in Figure 2. After sending a set of images to an archive device, the scanner operator sends a Storage Commitment Request message to the archive. The purpose of the message is two-fold. First, the message requests the archive device to verify that all of the intended images have been received. Second, the message requests that the archive device assume responsibility for the safe-keeping of the images, so that the scanner can, for example, delete its local copies of the images. If all is well, then the archive returns a confirmation message to the scanner. If there is a problem with one or more images or with the entire operation, then the archive device returns an appropriate error message to the scanner operator.

Figure 5

Network printing. In preparation for printing, the workstation first establishes a communications session with the printer (a Print Management Association, established in a similar manner as the Image Storage Association illustrated in Figure 2). At association establishment, various details are negotiated. Then the workstation informs the printer of the desired layout of the printed output so that the printer can prepare the appropriate format to receive the images. The workstation then sends the images, one by one, to the printer. The printer returns status notifications to the workstation so the user can monitor the progress of the print job.

Figure 6

Obtain a worklist. The modality worklist is an itemized list of imaging procedures that are scheduled to be performed on a particular scanner or on any scanner of the specified imaging modality type. The original requests for imaging procedures are first received and processed by an information system. The order entry and/or scheduling application of the information system schedules procedures and prepares worklists for the appropriate imaging equipment. At periodic intervals (determined by department policy) the scanner polls the information system for an updated worklist. Upon receiving a worklist request, the information system sends the current worklist to the scanner. The DICOM Worklist Management service class does not specify how the information system is notified that a worklist item is complete. This is done by another DICOM service that manages Performed Procedure Steps.