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Review of the American College of Radiology-National Electrical Manufacturers' Association standards activity
Computer Methods and Programs m Biomedicine, 37 (1992) 305-309
305
~'~ 1992 Elsevier Science Publishers B.V. All rights reserved I)169-261)7/92/$1)5.1)0
C O M M E T 01285
Review of the American College of Radiology-National Electrical Manufacturers' Association standards activity David E. Best ~ Steven C. Horii 2 William Bennett 3 Bob Thomson 4 and David Snavely .s I Eastman Kodak Company, Rochester, NY, USA, -" Georgetown Unit'ersio, Hospital, Washington, DC. USA, Vortech Data, Inc., Richardson, TX, USA. 4 Uni~'erwityof North Carolina, Chapel Hill, NC, USA and 5 National Electrical Mant(facturers Association, Washington. DC, USA
The American College of Radiology and the National Electrical Manufacturers' Association published the ACRN E M A Digital Imaging and Communications Standard in 1985. Implementations are just now becoming available. During this time, working groups of the committee responsible for the standard have been very active. An expanded version of the standard was published in 1988 and a third version, to be known as Digital Imaging and Communications in Medicine (DICOM), is being prepared for publication in 1992. This paper briefly reviews the history of the standard, describes recent activities, outlines the extensions planned for the D I C O M standard, and describes the participation of the committee in international radiological imaging standards activities. Standards: Digital imaging: PACS
1. Introduction
In the early 1980s, as digital images were becoming increasingly important to radiology departments, it became clear that the problem of acquiring images and related information from various manufacturers' equipment was very real. Users" need to acquire direct digital data was important, but many equipment manufacturers were cautious about disclosing their data formats and providing direct access to the digital data. In an effort to begin resolution of this issue, the American College of Radiology (ACR) and the National Electrical Manufacturers' Association (NEMA) formed the ACR-NEMA Digital Imag-
Correspondence: David E. Best, Eastman Kodak Company, 4th Floor, Corporate Place. Rochester, NY 14650, USA. Fax: 716-724-6505.
ing and Communications Standards Committee in late 1982. This committee is composed of radiologists (representing users of medical imaging equipment) and experts from industry (representing manufacturers of medical imaging equipment). The members addressed the problems involved in interfacing different digital imaging equipment. In addition to the ACR, active industrial participants in the committee and its working groups include: Agfa Matrix, DeJarnette Research, DuPont, Kodak, GE Medical Systems, Merge Technologies, Philips Medical Systems, Picker International, Siemens Medical Systems, Sony Medical Systems, 3M, Toshiba Medical Systems, and Vortech Data. A recently formed ACR Steering Committee gives guidance to the standards committee to help assure that the issues of most importance to the users are addressed by the ACR-NEMA corn-
306
mittee. The purposes of the initial standards activity were to:
2. Structure of the standard
2.1. Application layer • Promote communication of digital imaging information, regardless of source format or device manufacturer • Facilitate the development and expansion of picture archiving and communication systems (PACS) that can also interface with other systems of hospital information • Allow the creation of diagnostic information data bases that can be interrogated by a wide variety of devices distributed geographically. Specific goals of the committee were to: allow simple implementations for existing equipment; be applicable for future sophisticated implementations with maximum performance; and not require a PACS, but to enable its implementation. The result of this committee's work was the preparation of a draft of the proposed standard. Following approval by the A C R and the m e m b e r companies, it was distributed at the Radiological Society of North America meeting in 1985 and published as a N E M A Standard ( A C R - N E M A 300-1985) in the same year [1]. This standard is based on the seven-layer ISO-OSI model. It contains a hardware specification for point-to-point communication between two medical imaging devices. A protocol is defined that controls communication across the hardware link. A data format specification and data dictionary are provided. The standard also includes a specification of required and optional image descriptive information that is communicated with images to facilitate accurate radiological interpretation of the image. The purpose of the standard is to permit communication of medical images and related information between two image devices. It is not a communications network standard, a PACS standard, or a database standard. However, it is intended to enable implementation of these types of systems in a radiology department. The major focus of the standard is to allow communication of two-dimensional medical images. However, additional specifications are provided to allow communication of graphics, text, and a minimal set of commands across the interface.
All communication is achieved by transmission of messages across the interface. Each message can contain at most, one image or command. To help assure reliable exchange of information, all messages require a response from the receiver of the message. There are four commands that involve images. The Send command is used to transmit an image from one device to a second device. When a device wishes to receive an image from a second device, a Get command is used. A Move command is used when a device wishes to instruct a second device to transmit an image to a third device. A Find command provides a method for one device to inquire about images which are stored in another device. Three other commands are provided to facilitate m a n a g e m e n t of a digital imaging system. The Dialog command provides the means for exchanging unformated and interactive information between two devices. A Cancel command is specified to allow previously initiated exchanges to be stopped before completion. And finally, the Echo command is used to test and verify the end-to-end connection between two devices. Implementation of the commands is optional except for the Send and Echo commands. A device that generates images (source device) is required to be able to initiate a Send command. A device whose normal function involves display, processing, or storage of images (sink device) is required to be capable of receiving and responding to a Send command. All devices are required to implement the Echo command. All of the information contained in a message is organized into numbered groups, which contain related information about the image. The first group is the command group. This group contains information that pertains to the command, such as command type, sender, receiver, and status. All messages contain the command group. Most messages also contain a Data Set, which contains additional groups which contain information about the image. For instance, an image will contain the following groups: Image, Patient, Ac-
307
quisition, Relationship, Image Presentation, and Pixel Data. Within the groups, information is encoded in variable length data elements. There are three types of data elements in the standard. Type 1 elements contain information that is mandatory for image storage or display (e.g., message length and number of pixel rows and columns in the image) . Type 2 elements contain primary items for interpretation of the image such as patient name and age, referring physician, slice thickness, and patient orientation. Type 3 elements are optional and contain secondary items of interpretation of the image. Certain elements of Types 1 and 2 have default values. This minimizes the amount of [teader information that needs to be transmitted in the message. The type of several of the elements is dependent on the modality that generated the image. For example, slice thickness is required for CT, Nuclear Medicine, and MRI images, but not for other modalities.
2.2. Hardware and data exchange layers The physical layer consists of a 50-pin connector with 24 differential circuits and signal ground. Six control circuits are crossed, allowing the two devices to operate as peers. Data transfer is asynchronous, permitting the slower of the two devices to determine the data transfer rate. Prior to writing the specification for this unique physical layer, many existing standards were examined. While some ideas were drawn from these, none in particular was satisfactory for all the requirements defined. In addition, because of antitrust regulations in the United States, N E M A must. operate under certain constraints that prohibit the use of certain proprietary interfaces, or interfaces that would unfairly benefit a single manufacturer. The data link layer specifies that data is sent in frames including a frame desecrator word and checksum. The transport and network layers are combined into a single layer since this is a point-topoint standard and does not need all the services of these layers in the OSI model. This layer specifies that numbered packets are used to send
data. Multiple virtual channels are supported, allowing the possibility of multiple simultaneous messages. The support of multiple channels is optional. Devices are required to support at least one channel. A partial session layer is defined to establish end-to-end communication when multiple virtual channels are used. 3. Implementation of the standard
Commercial systems implementing the full seven layers of the standard are just now becoming available. There are few existing installations using all the layers of the standard, however most major manufacturers are incorporating ACRNEMA data formats in their newer systems. In May, 1989, a workshop for implementors was sponsored by the committee. At this workshop, six manufacturers (DeJarnette Research Systems (Towson, MD), General Electric Medical Systems (Milwaukee, WI), 3M (St. Paul, MN), Siemens Medical Systems (Hoffman Estates, IL), and Vortech Data (Richardson, TX)) brought test hardware implementations of the interface standard to Georgetown University (Washington, DC), and tested all possible pairwise connections. Of 15 possible connections, all were successful in that bidirectional data exchange was demonstrated. In some instances, testing was attempted up to the application layer and was successful. Two working groups of the committee are actively pursuing extensions to the standard. One of the most important extensions will be specifications for use of A C R - N E M A messages on standard networks. These specifications for use on OSI and T C P / I P networks are expected to significantly promote implementations of systems using the standard. Discussion within the committee has started on plans to sponsor a public demonstration of interconnected systems at RSNA, 1993. 4. Other ACR-NEMA standards
4.1. Data compression The ACR-NEMA standard requires that all devices have the capability to send or receive
308
uncompressed data. This is to assure communication at this base level. However, because of the very large amount of data in medical images, there are cases where data compression is desirable. To enable standard communication of compressed image data, a companion standard has been published. This Data Compression Standard (ACR-NEMA PS 2-1989) was published in 1989 [2]. This standard provides a specification for communicating information about the compression method used. It does not require or favor the use of any particular compression method. The choice of the method is made by the user. Supported compression techniques include differential pulse code modulation (DPCM), discrete cosine transform, and the pyramid (and its relative, the S) transform. Public, proprietary, reversible, and irreversible algorithms are supported. The working group that produced the compression standard is currently inactive, but may soon reconvene to consider inclusion of newer compression techniques such as JPEG.
4.2. Exchange media A second companion standard has been approved and is in the process of being published. This is an Exchange Media standard. It permits the exchange of ACR-NEMA messages on 1/2inch magnetic tape. Magnetic tape was considered first since there was no consensus on optical media standards at the time this activity began. The working group that produced the tape standard is currently reviewing a Japanese proposal (ISAC) for a standard for erasable optical (magneto-optical) disk. This work adheres to international standards for physical media and format and the A C R - N E M A data format. This standard is expected to cover the 5.25-inch medium and shows great promise. 5. Standards in other countries
5.1. Japan The Medical Imaging and Processing System (MIPS) committee has approved the MIPS-87 standard, a completely compatible subset of the A C R - N E M A standard. One extension to the
ACR-NEMA standard is the inclusion of 2-byte character code for the Japanese language. This standard has been successfully used at several institutions with systems from various Japanese manufacturers. MIPS compiled information from users and manufacturers and provided input to ACRNEMA version 2.0, which has been adopted as MIPS-89.
5.2. Europe The European Standardization Committee (CEN) has formed a technical committee TC251 with the objective of establishing standards for Medical Infomatics. Working group 4 of this committee is addressing the area of medical images. Joint meetings between TC251 working group 4 and the working groups of the ACR-NEMA committee have been held. These working groups are formulating plans for joint activity that is expected to result in a truly worldwide standard. 6. Future directions
The technology driving radiological imaging is constantly advancing. Standards for interfacing of medical devices must continuously evolve to remain useful. In 1988, ACR-NEMA published version 2.0 of the standard [3]. This revision maintained compatibility with the original standard and added useful features, including control of display devices such as laser printers. Two working groups of the committee are now working on revisions to the standard that will continue to keep the standard current with evolving technology. This revision is expected to be published in 1992 as Digital Imaging and Communications in Medicine (DICOM) version 3.0. The overall goals for version 3.0 of the standard are to: • Provide increased capability to encourage wider implementation • Seek help from users and implementors around the world • Use existing ISO and other standards • Move toward an object oriented description of the standard
309 • Incorporate appropriate features from Standard Product Interconnect which is an extension of the standard developed in Europe by Siemens and Philips [4] • Provide a standard in ISO format that is worthy of worldwide implementation • Maintain compatibility with version 2.(1 where possible • Complete the first draft in early 1992 One of the important specific objectives for version 3.0 of the standard is to extend the standard from point-to-point communication to use of existing standard networks for communication of medical images. Specifications for communication of A C R - N E M A messages over OSI and T C P / 1 P standard networks will be provided. As medical information systems become more complete and complex, it is important that various systems within a hospital be able to communicate with each other. In medical imaging departments this means exchange of information between hospital information systems (HIS), radiology information systems (RIS), and picture archiving and communication systems (PACS). A dedicated working group of the A C R - N E M A committee is working to incorporate methods for this communication in version 3.0. The current standard was intentionally restricted to communication of a single image per message. There are many cases when it is desirable to transmit more than one image at a time. A folder message is being created that will allow communication of multiple images and complete information about the relationships of the images in a single message. Many types of folders can be constructed, including patient, teaching, and study folders. Two types of folders will be specified: folder by reference, where the image data is referenced by the folder (reducing the need for multiple copies of images) and folder by value, where all the data is contained in the folder. One of the strengths of the standard is that it is very flexible and allows implementation of systems that meet specific user needs. However, this flexibility can lead to problems of assuring compatibility of devices from different manufacturers. Version 3.0 of the standard will include specifica-
tions of Service Classes. These will provide common specification for users and providers of services. This will help assure that devices adhering to the standard can talk to each other and perform useful work. Additions to version 3.0 will update the 50-pin protocol, provide improved capability to accommodate new imaging modalities, improve the capability of communication management, correct previous errors, and clarify the standard for implementors.
7. Conclusion The A C R - N E M A standard and the family of related standards continues to grow and evolve. The appearance of implementations from a number of vendors and institutions and successful pairwise testing have shown that the standard can be built, and that it does work. As more institutions install PACS, the continued need for the A C R - N E M A standard has become apparent. The general growth of PACS is one of the reasons for the continued requests for additions to the standard, and will foster its evolution as PACS-wide (rather than point-to-point) interface. Continued cooperative work by Japanese, European, and North American standards organizations will contribute to establishment of a worldwide medical imaging standard that will benefit users and implementors alike.
References [I] American College of Radiology - National Electrical Manufacturers Association Digital Imaging and Communication Standards Committee, ACR-NEMA 300-1985: Digital Imagingand Communications(November, 1985). [2] American College of Radiology National Electrical Manufacturers Association Digital Imaging and Communication Standards Committee, ACR-NEMA PS 2-1989: Data Compression Standard (1989). [3] American College of Radiology - National Electrical Manufacturers Association Digital Imaging and Communication Standards Committee, ACR-NEMA 300-1988: Digital Imagingand Communications (1989). [4] Siemens AG and Philips Medical Systems. SPI, Standard Product Interconnect for Compatibility of Digital Imaging (1987).
305
~'~ 1992 Elsevier Science Publishers B.V. All rights reserved I)169-261)7/92/$1)5.1)0
C O M M E T 01285
Review of the American College of Radiology-National Electrical Manufacturers' Association standards activity David E. Best ~ Steven C. Horii 2 William Bennett 3 Bob Thomson 4 and David Snavely .s I Eastman Kodak Company, Rochester, NY, USA, -" Georgetown Unit'ersio, Hospital, Washington, DC. USA, Vortech Data, Inc., Richardson, TX, USA. 4 Uni~'erwityof North Carolina, Chapel Hill, NC, USA and 5 National Electrical Mant(facturers Association, Washington. DC, USA
The American College of Radiology and the National Electrical Manufacturers' Association published the ACRN E M A Digital Imaging and Communications Standard in 1985. Implementations are just now becoming available. During this time, working groups of the committee responsible for the standard have been very active. An expanded version of the standard was published in 1988 and a third version, to be known as Digital Imaging and Communications in Medicine (DICOM), is being prepared for publication in 1992. This paper briefly reviews the history of the standard, describes recent activities, outlines the extensions planned for the D I C O M standard, and describes the participation of the committee in international radiological imaging standards activities. Standards: Digital imaging: PACS
1. Introduction
In the early 1980s, as digital images were becoming increasingly important to radiology departments, it became clear that the problem of acquiring images and related information from various manufacturers' equipment was very real. Users" need to acquire direct digital data was important, but many equipment manufacturers were cautious about disclosing their data formats and providing direct access to the digital data. In an effort to begin resolution of this issue, the American College of Radiology (ACR) and the National Electrical Manufacturers' Association (NEMA) formed the ACR-NEMA Digital Imag-
Correspondence: David E. Best, Eastman Kodak Company, 4th Floor, Corporate Place. Rochester, NY 14650, USA. Fax: 716-724-6505.
ing and Communications Standards Committee in late 1982. This committee is composed of radiologists (representing users of medical imaging equipment) and experts from industry (representing manufacturers of medical imaging equipment). The members addressed the problems involved in interfacing different digital imaging equipment. In addition to the ACR, active industrial participants in the committee and its working groups include: Agfa Matrix, DeJarnette Research, DuPont, Kodak, GE Medical Systems, Merge Technologies, Philips Medical Systems, Picker International, Siemens Medical Systems, Sony Medical Systems, 3M, Toshiba Medical Systems, and Vortech Data. A recently formed ACR Steering Committee gives guidance to the standards committee to help assure that the issues of most importance to the users are addressed by the ACR-NEMA corn-
306
mittee. The purposes of the initial standards activity were to:
2. Structure of the standard
2.1. Application layer • Promote communication of digital imaging information, regardless of source format or device manufacturer • Facilitate the development and expansion of picture archiving and communication systems (PACS) that can also interface with other systems of hospital information • Allow the creation of diagnostic information data bases that can be interrogated by a wide variety of devices distributed geographically. Specific goals of the committee were to: allow simple implementations for existing equipment; be applicable for future sophisticated implementations with maximum performance; and not require a PACS, but to enable its implementation. The result of this committee's work was the preparation of a draft of the proposed standard. Following approval by the A C R and the m e m b e r companies, it was distributed at the Radiological Society of North America meeting in 1985 and published as a N E M A Standard ( A C R - N E M A 300-1985) in the same year [1]. This standard is based on the seven-layer ISO-OSI model. It contains a hardware specification for point-to-point communication between two medical imaging devices. A protocol is defined that controls communication across the hardware link. A data format specification and data dictionary are provided. The standard also includes a specification of required and optional image descriptive information that is communicated with images to facilitate accurate radiological interpretation of the image. The purpose of the standard is to permit communication of medical images and related information between two image devices. It is not a communications network standard, a PACS standard, or a database standard. However, it is intended to enable implementation of these types of systems in a radiology department. The major focus of the standard is to allow communication of two-dimensional medical images. However, additional specifications are provided to allow communication of graphics, text, and a minimal set of commands across the interface.
All communication is achieved by transmission of messages across the interface. Each message can contain at most, one image or command. To help assure reliable exchange of information, all messages require a response from the receiver of the message. There are four commands that involve images. The Send command is used to transmit an image from one device to a second device. When a device wishes to receive an image from a second device, a Get command is used. A Move command is used when a device wishes to instruct a second device to transmit an image to a third device. A Find command provides a method for one device to inquire about images which are stored in another device. Three other commands are provided to facilitate m a n a g e m e n t of a digital imaging system. The Dialog command provides the means for exchanging unformated and interactive information between two devices. A Cancel command is specified to allow previously initiated exchanges to be stopped before completion. And finally, the Echo command is used to test and verify the end-to-end connection between two devices. Implementation of the commands is optional except for the Send and Echo commands. A device that generates images (source device) is required to be able to initiate a Send command. A device whose normal function involves display, processing, or storage of images (sink device) is required to be capable of receiving and responding to a Send command. All devices are required to implement the Echo command. All of the information contained in a message is organized into numbered groups, which contain related information about the image. The first group is the command group. This group contains information that pertains to the command, such as command type, sender, receiver, and status. All messages contain the command group. Most messages also contain a Data Set, which contains additional groups which contain information about the image. For instance, an image will contain the following groups: Image, Patient, Ac-
307
quisition, Relationship, Image Presentation, and Pixel Data. Within the groups, information is encoded in variable length data elements. There are three types of data elements in the standard. Type 1 elements contain information that is mandatory for image storage or display (e.g., message length and number of pixel rows and columns in the image) . Type 2 elements contain primary items for interpretation of the image such as patient name and age, referring physician, slice thickness, and patient orientation. Type 3 elements are optional and contain secondary items of interpretation of the image. Certain elements of Types 1 and 2 have default values. This minimizes the amount of [teader information that needs to be transmitted in the message. The type of several of the elements is dependent on the modality that generated the image. For example, slice thickness is required for CT, Nuclear Medicine, and MRI images, but not for other modalities.
2.2. Hardware and data exchange layers The physical layer consists of a 50-pin connector with 24 differential circuits and signal ground. Six control circuits are crossed, allowing the two devices to operate as peers. Data transfer is asynchronous, permitting the slower of the two devices to determine the data transfer rate. Prior to writing the specification for this unique physical layer, many existing standards were examined. While some ideas were drawn from these, none in particular was satisfactory for all the requirements defined. In addition, because of antitrust regulations in the United States, N E M A must. operate under certain constraints that prohibit the use of certain proprietary interfaces, or interfaces that would unfairly benefit a single manufacturer. The data link layer specifies that data is sent in frames including a frame desecrator word and checksum. The transport and network layers are combined into a single layer since this is a point-topoint standard and does not need all the services of these layers in the OSI model. This layer specifies that numbered packets are used to send
data. Multiple virtual channels are supported, allowing the possibility of multiple simultaneous messages. The support of multiple channels is optional. Devices are required to support at least one channel. A partial session layer is defined to establish end-to-end communication when multiple virtual channels are used. 3. Implementation of the standard
Commercial systems implementing the full seven layers of the standard are just now becoming available. There are few existing installations using all the layers of the standard, however most major manufacturers are incorporating ACRNEMA data formats in their newer systems. In May, 1989, a workshop for implementors was sponsored by the committee. At this workshop, six manufacturers (DeJarnette Research Systems (Towson, MD), General Electric Medical Systems (Milwaukee, WI), 3M (St. Paul, MN), Siemens Medical Systems (Hoffman Estates, IL), and Vortech Data (Richardson, TX)) brought test hardware implementations of the interface standard to Georgetown University (Washington, DC), and tested all possible pairwise connections. Of 15 possible connections, all were successful in that bidirectional data exchange was demonstrated. In some instances, testing was attempted up to the application layer and was successful. Two working groups of the committee are actively pursuing extensions to the standard. One of the most important extensions will be specifications for use of A C R - N E M A messages on standard networks. These specifications for use on OSI and T C P / I P networks are expected to significantly promote implementations of systems using the standard. Discussion within the committee has started on plans to sponsor a public demonstration of interconnected systems at RSNA, 1993. 4. Other ACR-NEMA standards
4.1. Data compression The ACR-NEMA standard requires that all devices have the capability to send or receive
308
uncompressed data. This is to assure communication at this base level. However, because of the very large amount of data in medical images, there are cases where data compression is desirable. To enable standard communication of compressed image data, a companion standard has been published. This Data Compression Standard (ACR-NEMA PS 2-1989) was published in 1989 [2]. This standard provides a specification for communicating information about the compression method used. It does not require or favor the use of any particular compression method. The choice of the method is made by the user. Supported compression techniques include differential pulse code modulation (DPCM), discrete cosine transform, and the pyramid (and its relative, the S) transform. Public, proprietary, reversible, and irreversible algorithms are supported. The working group that produced the compression standard is currently inactive, but may soon reconvene to consider inclusion of newer compression techniques such as JPEG.
4.2. Exchange media A second companion standard has been approved and is in the process of being published. This is an Exchange Media standard. It permits the exchange of ACR-NEMA messages on 1/2inch magnetic tape. Magnetic tape was considered first since there was no consensus on optical media standards at the time this activity began. The working group that produced the tape standard is currently reviewing a Japanese proposal (ISAC) for a standard for erasable optical (magneto-optical) disk. This work adheres to international standards for physical media and format and the A C R - N E M A data format. This standard is expected to cover the 5.25-inch medium and shows great promise. 5. Standards in other countries
5.1. Japan The Medical Imaging and Processing System (MIPS) committee has approved the MIPS-87 standard, a completely compatible subset of the A C R - N E M A standard. One extension to the
ACR-NEMA standard is the inclusion of 2-byte character code for the Japanese language. This standard has been successfully used at several institutions with systems from various Japanese manufacturers. MIPS compiled information from users and manufacturers and provided input to ACRNEMA version 2.0, which has been adopted as MIPS-89.
5.2. Europe The European Standardization Committee (CEN) has formed a technical committee TC251 with the objective of establishing standards for Medical Infomatics. Working group 4 of this committee is addressing the area of medical images. Joint meetings between TC251 working group 4 and the working groups of the ACR-NEMA committee have been held. These working groups are formulating plans for joint activity that is expected to result in a truly worldwide standard. 6. Future directions
The technology driving radiological imaging is constantly advancing. Standards for interfacing of medical devices must continuously evolve to remain useful. In 1988, ACR-NEMA published version 2.0 of the standard [3]. This revision maintained compatibility with the original standard and added useful features, including control of display devices such as laser printers. Two working groups of the committee are now working on revisions to the standard that will continue to keep the standard current with evolving technology. This revision is expected to be published in 1992 as Digital Imaging and Communications in Medicine (DICOM) version 3.0. The overall goals for version 3.0 of the standard are to: • Provide increased capability to encourage wider implementation • Seek help from users and implementors around the world • Use existing ISO and other standards • Move toward an object oriented description of the standard
309 • Incorporate appropriate features from Standard Product Interconnect which is an extension of the standard developed in Europe by Siemens and Philips [4] • Provide a standard in ISO format that is worthy of worldwide implementation • Maintain compatibility with version 2.(1 where possible • Complete the first draft in early 1992 One of the important specific objectives for version 3.0 of the standard is to extend the standard from point-to-point communication to use of existing standard networks for communication of medical images. Specifications for communication of A C R - N E M A messages over OSI and T C P / 1 P standard networks will be provided. As medical information systems become more complete and complex, it is important that various systems within a hospital be able to communicate with each other. In medical imaging departments this means exchange of information between hospital information systems (HIS), radiology information systems (RIS), and picture archiving and communication systems (PACS). A dedicated working group of the A C R - N E M A committee is working to incorporate methods for this communication in version 3.0. The current standard was intentionally restricted to communication of a single image per message. There are many cases when it is desirable to transmit more than one image at a time. A folder message is being created that will allow communication of multiple images and complete information about the relationships of the images in a single message. Many types of folders can be constructed, including patient, teaching, and study folders. Two types of folders will be specified: folder by reference, where the image data is referenced by the folder (reducing the need for multiple copies of images) and folder by value, where all the data is contained in the folder. One of the strengths of the standard is that it is very flexible and allows implementation of systems that meet specific user needs. However, this flexibility can lead to problems of assuring compatibility of devices from different manufacturers. Version 3.0 of the standard will include specifica-
tions of Service Classes. These will provide common specification for users and providers of services. This will help assure that devices adhering to the standard can talk to each other and perform useful work. Additions to version 3.0 will update the 50-pin protocol, provide improved capability to accommodate new imaging modalities, improve the capability of communication management, correct previous errors, and clarify the standard for implementors.
7. Conclusion The A C R - N E M A standard and the family of related standards continues to grow and evolve. The appearance of implementations from a number of vendors and institutions and successful pairwise testing have shown that the standard can be built, and that it does work. As more institutions install PACS, the continued need for the A C R - N E M A standard has become apparent. The general growth of PACS is one of the reasons for the continued requests for additions to the standard, and will foster its evolution as PACS-wide (rather than point-to-point) interface. Continued cooperative work by Japanese, European, and North American standards organizations will contribute to establishment of a worldwide medical imaging standard that will benefit users and implementors alike.
References [I] American College of Radiology - National Electrical Manufacturers Association Digital Imaging and Communication Standards Committee, ACR-NEMA 300-1985: Digital Imagingand Communications(November, 1985). [2] American College of Radiology National Electrical Manufacturers Association Digital Imaging and Communication Standards Committee, ACR-NEMA PS 2-1989: Data Compression Standard (1989). [3] American College of Radiology - National Electrical Manufacturers Association Digital Imaging and Communication Standards Committee, ACR-NEMA 300-1988: Digital Imagingand Communications (1989). [4] Siemens AG and Philips Medical Systems. SPI, Standard Product Interconnect for Compatibility of Digital Imaging (1987).