通过工作列表实现数字化网络各系统之间患者信息的一致性

目的 实现图像存储与传输系统(picture archiving and communication system，PACS)、放射学信息系统(radiology information systems，RIS)、影像设备之间患者检查信息的一致性。方法 我院引进GE Signa 1．5T磁共振、数字乳腺，Agfa数字X线摄影、计算机X线摄影及GE CT等数字化医学设备。GE PACS是英文系统，所以前期医院在未解决信息一致性时只通过PACS对检查影像进行保存，通过PACS系统中简单的信息管理工作，并没有真正意义上的worklist。2个月后，我院采用国际上先进的解决方法即通过医学数字成像及通讯(digital imaging and communication in medicine，DICOM)标准的工作列表(worklist)的方法实现信息的一致性。在RIS系统中将患者中文信息转换为英文信息，保存并为worklist提供患者的英文信息。结果 我院在集成PACS和RIS的2年多时间以来，通过worklist来保证RIS与影像设备患者检查信息的一致性，取得了非常好的效果。在所有诊断工作站上，诊断医生通过中文RIS系统，对病人的信息进行编辑、修改、产生。结论 通过worklist实现PACS、RIS、影像设备之间患者检查信息的一致性是可行的。     

小型医学影像存储与传输系统的临床应用

目的 探索小型医学图像存档与通讯系统(minimizing picture archiving and communication system，mini—PACS)在实际工作中的应用，逐步实现科室内的无胶片化管理。方法 建立基于PC机的局域网，连接CT、MR、数字胃肠机、DSA、激光相机等医学影像设备，整合数字图像网络(digital imaging network，DIN)和医学图像诊断系统(medical diagnostic imaging system，MDIS)，组成放射科信息管理系统(radiology information system，RIS)。RIS系统通过其中1台安装双网卡的PC工作站与医院信息系统(hospital information system，HIS)相连。结果 系统在2年多的时间内得到连续使用，放射科信息管理系统得以实现和完善。在现有的数字化影像设备上实现了符合医学数字图像传输标准3．0(digital imaging communication in medicine，DICOM3．0)格式的图像采集、储存、传输、打印、浏览功能。图像和诊断报告信息通过Microsoft Access数据库管理，不同设备上保存的在线图像为3～6个月，所有图像用CD—R光盘刻录，作为离线永久保存，已有32700多份诊断报告存入数据库；HIS终端可有限制地从该系统获得图像和诊断信息。结论 mini—PACS系统投入和运行成本低、维护简单、性能可靠，可基本实现PACS的重要功能，在中、小医院具有良好的应用前景。     

PACS系统的应用体会

目的：介绍我院影像医学存档与通讯系统（ＰＡＣＳ）应用的经验。材料与方法：将所有Ｂ超、ＣＴ、Ｘ线机和一台ＩＢＭ服务器、四台奔腾Ⅱ档次的工作站及惠普光盘塔连接成医学数字影像传输（ＤＩＣＯＭ）网络;ＤＩＣＯＭ服务器与各种图像浏览终端羞以太网络;通过ＨＵＢ连接成ＰＡＣＳ系统。结果：将常规放射图像的模拟信号通过数字转换器转换为数字信号后,与ＣＴ等数字成像系统的数字信号一并输入光盘塔,并进行诊断。将数字图像和诊断报告一起舆到医院各个图像浏览终端,使其可通过ＷＥＢ界面系统及咨询平台进行查询。ＰＡＣＳ投入使用２年来,效果良好。结论：ＰＡＣＳ的应用明显提高了放射科及相关科室的工作效率,方便了工作、教学、科研和会诊。     

PACS utilization in radiologic research

RATIONALE AND OBJECTIVES: The authors investigated patterns of utilization of the digital picture archiving and communication system (PACS) in radiologic publications in order to ascertain whether there may be a correlation between PACS use and demographic/cultural factors such as geographic location, radiologic specialization, and use of advanced radiologic technologies. MATERIALS AND METHODS: A total of 1,037 articles in 22 randomly selected issues of AJR: American Journal of Roentgenology and of Radiology (1999-2001) were reviewed for the rate of PACS use and the type of PACS used. Articles for which PACS use or nonuse was established were further classified according to originating continent or region, imaging modality, study design (whether retrospective or prospective), and the use of advanced radiologic technology. The use of a PACS was then correlated with these factors. The data were recorded and statistics were prepared by means of statistical software. The nonparametric (chi2) test also was run by using this software. RESULTS: PACS had been used and reported in the preparation of 225 of the 1,037 articles. The type of PACS used was mini PACS (eg, systems using digital imaging and communications in medicine [DICOM] protocols or precursors) in 55 (24%) and department- or hospital-wide PACS in 161 (72%). Most of the articles for which use of a PACS was reported had originated in North America (60%), Europe (22%), or Asia (14%). PACS were used in almost half of retrospective studies and in one-fourth of prospective studies (P < .01). A low correlation was found between utilization of PACS and use of other advanced technologies. CONCLUSION: Although PACS were utilized in the preparation of a substantial proportion of articles published in the two major radiology journals, there was a great disparity in the rate of PACS use among world regions. The proportion of studies originating in North America for which a PACS was used was nearly three times the number of similar studies originating in Europe, and more than four times the number originating in Asia.     

Picture archiving and communication systems: an overview.

Organizational techniques that enable small departments to function efficiently often fail as departments become larger. With the recent growth in imaging technology, the capacity of film-based systems to meet the increasing needs of radiology departments has decreased. Electronic picture archiving and communication systems (PACS) have been developed in an attempt to provide economical storage, rapid retrieval of images, access to images acquired with multiple modalities, and simultaneous access at multiple sites. Input to a PACS may come from digital or analog sources (when the latter have been digitized). A PACS consists primarily of an image acquisition device (an electronic gateway to the system), data management system (a specialized computer system that controls the flow of information on the network), image storage devices (both short- and long-term archives), transmission network (which serves local or wide areas), display stations (which include a computer, text monitor, image monitors, and a user interface), and devices to produce hard-copy images (currently, a multiformat or laser camera). The goals of PACS are to improve operational efficiency while maintaining or improving diagnostic ability.     

Integrating digital systems: commitment and collaboration

One year ago, the radiology department at Ball Memorial Hospital, a 350-bed facility in Muncie, IN, was completely film-based. The need to support a new all-digital, 35-room emergency department (ED) hastened the facility's transition to a digital environment. Today, with the exception of mammography, the hospital's imaging services are now digital. To develop and implement the project, the hospital formed an internal implementation team. An independent consultant was also hired to evaluate the impact of these new technologies and to provide an estimated cost payback. After research, site visits, and vendor demonstrations, the hospital selected a single vendor for its picture archiving and communication system (PACS), digital radiography (DR), computed radiography (CR), and overall project management. The DR system was installed in the ED to provide digital image capture for a full range of trauma exams. The ED also initially began utilizing a Web-based PACS distribution originally implemented for after-hours teleradiology. The majority of the hospital's imaging studies are captured with 2 multi-cassette CR systems that serve 7 exam rooms in the radiology department. The hospital also installed remote operations panels to expedite entry of patient and exam information. Technologists readily embraced both CR and DR systems. The Web distribution system now transmits images to hospital-based computers and to 150 remote referring physicians. The PACS platform automatically e-mails key images and radiology reports to referring physicians. Authorized physicians can also request reports and images on an as-needed basis. The PACS vendor had previously performed multiple integrations with the radiology information system (RIS) vendor; the integration of PACS and RIS was extremely smooth. One of the critical components of a successful conversion is experienced, dedicated management. The hospital retained professional project management services to facilitate implementation and to ensure adequate training for all users.     

Incorporating electronic media into medical student education: a survey of AMSER members on computer and web use in radiology courses. Alliance of Medical Student Educators in Radiology

RATIONALE AND OBJECTIVES: The purpose of this study was to define the current use of information technology in radiology tutorials for medical students. MATERIALS AND METHODS: The authors conducted a Web-based survey of directors of medical school courses in radiology. The survey dealt with the details of the courses and the use of computers and the Web during the courses. RESULTS: There were 48 responses. Most radiology courses were elective (73%) and were offered monthly. Most institutions (79%) had picture archiving and communication systems (PACS) available or were completely filmless. The teaching case presentations, however, often included film images displayed on a view box or by an overhead projector. Computers dedicated to student use were uncommon (28%). The Web was used infrequently as a teaching resource, and a Web site was not available in most courses. Computer technical support was variable and usually provided by the course director. Course directors at institutions with PACS were more likely to use digital technology for case presentations and more likely to use the Web for teaching purposes. CONCLUSION: Despite the widespread use of digital technology and PACS in the field of radiology, digital technology is underused in radiology courses. However, departments with PACS tend to use digital technology more frequently in education than do departments without PACS.     

Suspected infant abuse: radiographic skeletal survey practices in pediatric health care facilities

PURPOSE: To determine current national radiographic skeletal survey imaging practices, including migration to digital technologies, for evaluation of suspected infant abuse. MATERIALS AND METHODS: Of 155 children's health care facilities in the United States in which radiographic skeletal surveys are performed for suspected infant (<1 year old) abuse, 137 (88.4%) agreed to complete a questionnaire. Questions included facility type, imaging department volume, radiographic equipment, and details of skeletal survey imaging practices. Statistical analysis was performed with chi(2), Fisher exact, Pearson correlation, Spearman rank correlation, and Student t tests. RESULTS: One hundred seven completed questionnaires were returned. Forty-seven (43.9%) facilities used screen-film imaging; 60 (56.1%) used digital. Of screen-film users, 25 (53.2%) had already migrated or planned to migrate to digital within 1 year. Of screen-film users, 27 (60.0%) reported use of a high-detail imaging system, while 13 (21.7%) digital users employed a high-resolution technique (P < .001). Eighty-four (78.5%) facilities reported more than 10 images in their protocol, and 45 (42.0%) specified more than 15 images. Only one (0.9%) facility obtained fewer than three images. Upper extremities were imaged separately with at least two exposures in 81 (75.7%) facilities. Lower extremities were imaged separately with at least two exposures in 82 (76.6%) facilities. One hundred five (98.1%) facilities reported acquisition of lateral spinal views. CONCLUSION: Within U.S. pediatric health care facilities, most skeletal surveys in cases of suspected infant abuse include separate frontal views of the appendicular skeleton and frontal and lateral views of the axial skeleton. Imaging protocols and other image quality determinants vary widely, and as U.S. pediatric health care facilities migrate from film-based to digital imaging technology, imaging practices directly applicable to the digital environment are being retained; however, less attention is being paid to technical elements specific to digital imaging that affect high-detail image quality.     

Medical imaging, PACS, and imaging informatics: retrospective

Historical reviews of PACS (picture archiving and communication system) and imaging informatics development from different points of view have been published in the past (Huang in Euro J Radiol 78:163–176, 2011; Lemke in Euro J Radiol 78:177–183, 2011; Inamura and Jong in Euro J Radiol 78:184–189, 2011). This retrospective attempts to look at the topic from a different angle by identifying certain basic medical imaging inventions in the 1960s and 1970s which had conceptually defined basic components of PACS guiding its course of development in the 1980s and 1990s, as well as subsequent imaging informatics research in the 2000s. In medical imaging, the emphasis was on the innovations at Georgetown University in Washington, DC, in the 1960s and 1970s. During the 1980s and 1990s, research and training support from US government agencies and public and private medical imaging manufacturers became available for training of young talents in biomedical physics and for developing the key components required for PACS development. In the 2000s, computer hardware and software as well as communication networks advanced by leaps and bounds, opening the door for medical imaging informatics to flourish. Because many key components required for the PACS operation were developed by the UCLA PACS Team and its collaborative partners in the 1980s, this presentation is centered on that aspect. During this period, substantial collaborative research efforts by many individual teams in the US and in Japan were highlighted. Credits are due particularly to the Pattern Recognition Laboratory at Georgetown University, and the computed radiography (CR) development at the Fuji Electric Corp. in collaboration with Stanford University in the 1970s; the Image Processing Laboratory at UCLA in the 1980s–1990s; as well as the early PACS development at the Hokkaido University, Sapporo, Japan, in the late 1970s, and film scanner and digital radiography developed by Konishiroku Photo Ind. Co. Ltd. (Konica-Minolta), Japan, in the 1980–1990s. Major support from the US National Institutes of Health and other federal agencies and private medical imaging industry are appreciated. The NATO (North Atlantic Treaty Organization) Advanced Study Institute (ASI) sponsored the International PACS Conference at Evian, France, in 1990, the contents and presentations of which convinced a half dozen high-level US military healthcare personnel, including surgeons and radiologists, that PACS was feasible and would greatly streamline the current military healthcare services. The impact of the post-conference summary by these individuals to their superiors opened the doors for long-term support of PACS development by the US Military Healthcare Services. PACS and imaging informatics have thus emerged as a daily clinical necessity.     

Enterprise-wide PACS: beyond radiology, an architecture to manage all medical images

RATIONALE AND OBJECTIVES: Picture archiving and communication systems (PACS) have the vocation to manage all medical images acquired within the hospital. To address the various situations encountered in the imaging specialties, the traditional architecture used for the radiology department has to evolve. MATERIALS AND METHODS: We present our preliminarily results toward an enterprise-wide PACS intended to support all kind of image production in medicine, from biomolecular images to whole-body pictures. Our solution is based on an existing radiologic PACS system from which images are distributed through an electronic patient record to all care facilities. This platform is enriched with a flexible integration framework supporting digital image communication in medicine (DICOM) and DICOM-XML formats. In addition, a generic workflow engine highly customizable is used to drive work processes. RESULTS: Echocardiology; hematology; ear, nose, and throat; and dermatology, including wounds, follow-up is the first implemented extensions outside of radiology. CONCLUSION: We also propose a global strategy for further developments based on three possible architectures for an enterprise-wide PACS.     

Hospital-wide distribution of nuclear medicine studies through a broadband digital network

Nuclear medicine provides a good environment for the evaluation of picture archiving and communication systems (PACS) because of the relatively small quantity of digital data that are generated, leading to reduced requirements for storage, display, and transmission compared with those found in radiology. The PACS in nuclear medicine is characterized by use of a single computer as a central storage, display, and analysis node. Images are acquired with use of small, low-cost computers attached to each camera. This network configuration offers advantages of convenience, but with great reliance on a single computer. A campus-wide picture network is under development at Washington University employing broadband cable television technology supplemented by baseband Ethernet (Digital Equipment Corp, Maynard, MA) components. All areas of diagnostic radiology and nuclear medicine are connected via a PACS testbed project. A radiology information system, supporting over 250 terminals, provides digital tracking of patients and report generation and retrieval. A new image workstation is under development in conjunction with Digital Equipment Corp. This system will permit display in multiple windows of report information and images from various modalities. A lung scan demonstration project is now beginning that is designed to test the value of a PACS in nuclear medicine. Digitally acquired chest radiographs will be displayed on an image workstation in nuclear medicine along with digital ventilation and perfusion lung scans. It is hoped that time-consuming logistic bottlenecks now encountered in lung scan interpretation will be reduced.     

Profile of medical student teaching in radiology: teaching methods, staff participation, and rewards

RATIONALE AND OBJECTIVES: The purpose of this study was to collect demographic information about radiology departments and rewards for teaching activities, as well as the impact of new digital imaging methods on teaching. MATERIALS AND METHODS: Two surveys were conducted of directors of medical school clerkships in radiology. The initial survey focused on numbers of staff and students, courses taught, and perception of rewards for teaching. The follow-up survey more specifically addressed teaching methods. RESULTS: Sixty-nine (50%) of the initial surveys sent to 139 departments and 46 (39% of a total of 119) of the follow-up surveys were returned. Clerkship directors spent an average of 9 hours per week teaching and performing administrative tasks, with most given no additional time off. Eighty-four percent of departments provide either no or insignificant rewards for teaching. Many departments have integrated the use of computers in teaching, and most have computers that students use during the radiology course. At the same time, digital imaging and picture archiving and communication systems (PACS) are used, or will be used within 1 year, in most departments. CONCLUSION: Clerkship directors receive little compensation in terms of time and rewards for medical student teaching. Teaching methods are evolving in response to the increasing use of computers, digital imaging, and PACS for at least part of the workload in most radiology departments.     

Radiologists' reading times using PACS and using films: one practice's experience

RATIONALE AND OBJECTIVES: To measure the change in radiologists' productivity in terms of interpretation time per examination when using picture archiving and communication system (PACS) workstations in a particular private practice, Valley Radiologists, Ltd, as part of a feasibility study and subsequent business plan to implement a digital enterprise. MATERIALS AND METHODS: Time to process a series of exams was measured for 18 radiologists during an uninterrupted period of a working day. Radiologists in the practice served in multiple locations. The data were analyzed in aggregate and by modality (plain film, ultrasound, computed tomography, and magnetic resonance imaging). Average time per exam, with and without PACS, was measured for each modality. Regression analysis was used to determine the independent effect of PACS on radiologist productivity. RESULTS: The mean time to process an exam was 1.4 minutes (SE = 0.04) for plain film, 1.96 minutes (SE = 0.14) for ultrasound, 5.08 minutes (SE = 0.44) for computed tomography, and 6.83 minutes (SE = 0.31) for magnetic resonance imaging. Regression results indicate that PACS had no effect on the time taken to read a series of exams. CONCLUSIONS: When considering a PACS purchase or implementation, decrease in radiologists' time to process an examination may not be realized. In this specific practice setting, we did not find evidence that PACS workstations alone, without any other changes in workflow design, improved radiologists' interpretation time.     

The challenge of PACS in a small hospital

Memorial Hospital is located in-North Conway, New Hampshire. Year round, the 35-bed hospital serves mainly tourists and retirees to the area. The imaging department wanted to integrate its services within an existing community network, meet the needs of a transient population and resolve staff utilization and storage problems. Conversion from film to digital in the radiology department took advantage of digital x-ray and PACS. Since the hospital was already using digital technology for CT, MR, ultrasound and fluoroscopy, it made sense to include plain film imaging in the digitization process Memorial Hospital faced a number of challenges. Those in decision-making positions lacked a general knowledge about PACS, and, in particular, about PACS in similarly sized facilities. The hospital also lacked experience working with vendors. A timeline was critical as Memorial Hospital prepared for winter, its busiest season. The facility decided on a phased-in project with no immediate HIS/RIS interface, and computerized radiography with hard-copy films was implemented immediately. The facility has made the transition from a conventional imaging department to a PACS environment. It attributes its success to the way it involved those who would be affected by any future changes in the planning and decision-making processes. Memorial Hospital expects to expand its services and streamline its archival capabilities in the near future.     

The value teleradiology represents for Europe: A study of lessons learned in the U.S.

Pathology and demography have combined to fuel exponential demand for advanced medical imaging. To support this demand, radiology must move beyond traditional department or modality-based picture archiving and communication systems (PACS) to solutions that ensure access regardless of location. This article delineates underlying reasons for the growth in demand for access to medical imaging in both Europe and the United States. It explains why teleradiology/PACS is critical to support this growth in Europe. It discusses the benefits of and barriers to its widespread implementation as discovered in Canada and the U.S. and how these lessons learned relate to Europe.The article establishes the technological imperatives for teleradiology/PACS and presents three real-world case studies of successful data sharing and shared workflow models via single imaging implementations.

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CML HealthCare: Geographically spanning Canada and the United States with 129 sites performing nearly 5 million plus annual exams.

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Shields MRI: 29 facilities, including 3 Radiation Oncology centers, across an area 4 times the size of Switzerland.

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MRA/Novant: 40 radiologists working in a complete subspecialty reporting environment.

Finally, it provides a high-level list of selection criteria for teleradiology/PACS and examines how industry trends affecting the U.S. are important baseline considerations to the success of teleradiology/PACS in Europe.     

Receiver-operating-characteristic study of chest radiographs in children: digital hard-copy film vs 2K x 2K soft-copy images.

Two methods are commonly used to visualize digital radiologic imaging data: (1) hard-copy viewing, in which the digital data are used to modulate the intensity of a laser beam that exposes an analog film and (2) soft-copy viewing, in which the digital data are converted to an analog video signal and presented on a CRT monitor. The film method allows new digital imaging systems to be easily integrated into conventional radiologic management and viewing methods. The second method, soft-copy viewing, allows digital imaging data to be managed and viewed electronically in a picture archiving and communication system (PACS). These PACS systems are hypothesized to have improved operational efficiency and enhanced image-analysis capabilities. The quality of soft-copy images is still not widely accepted. This article reports on the results of a large-scale receiver-operating-characteristic study comparing observers' performance in detecting various pediatric chest abnormalities on soft-copy 2048 x 2048K byte displays with their performance with digital laser-printed film from computed radiography. The disease categories studied were pneumothorax, linear atelectasis, air bronchogram, and interstitial disease. The selected data set included 239 images; 77 contained no proved abnormality and 162 contained one or more of the abnormalities mentioned. Seven pediatric radiologists participated in the study, two as judges and five as observers. Our results show no significant difference between viewing images on digital hard copy and soft copy for the detection of pneumothoraces and air bronchograms. A slight performance edge for soft copy was seen for interstitial disease and linear atelectasis. This result indicates that computed chest radiographs in children viewed in a soft-copy PACS environment should result in diagnoses similar to or slightly more accurate than those obtained in a laser-printed film-based environment.     

Filmless Evaluation of the Mechanical Accuracy of the Isocenter in Stereotactic Radiotherapy

BACKGROUND AND PURPOSE: The Winston-Lutz test verifies the mechanical accuracy of the isocenter in stereotactic radiotherapy. A lead ball inside a small beam is exposed to film applying different combinations of the gantry angle and the table angle. The increasing replacement of films by digital images requires alternative imaging methods. The suitability of two different electronic portal imaging systems and of a system based on digital luminescence radiography was investigated. MATERIAL AND METHODS: The imaging systems included the portal imaging devices BEAMVIEW PLUS and OPTIVUE1000 (both Siemens Medical Solutions, Erlangen, Germany) and the luminescence system KODAK ACR 2000 RT (Eastman Kodak Comp., Rochester, NY, USA). 6-MV photons from the linear accelerators PRIMUS and ONCOR (both Siemens Medical Solutions) were applied. First, only the small beam covering the lead ball was exposed. Second, an additional bigger open beam part in a certain distance to the small beam was applied. RESULTS: For all three investigated imaging systems, which are using preprocessing imaging software, only for the beam arrangement with additional open beam parts, the lead ball could be detected inside the small beam. Only for the application of a dosimetric software tool to the luminescence system, the metal ball inside the small beam became visible without an additional open beam part. CONCLUSION: Applying the proposed beam arrangements, the Winston-Lutz test can be done by digital and filmless imaging systems, thereby saving time as well.     

Teleradiology: opportunities, problems, implementation

With the introduction of computerized tomography and magnetic resonance imaging in the early 1970s, computers became integral to the imaging process. Shortly thereafter, scanners that create digitized images from film were introduced and teleradiology--images transmitted in real time--became possible. In the early 1980s, the idea of a picture archiving and communications system (PACS) began to develop. It promised to retrieve, connect and store every kind of image, from x-ray to CT, and render film obsolete. However, inflated expectations and inadequate technology hindered PACS implementation. Digital imaging offers the following benefits over film-based systems: - Less space is needed to store imaging studies. - Digital imaging files can be faster and easier for referring physicians to retrieve than film and are not susceptible to loss and damage. - Digital images can be enhanced, contrasted, colored and otherwise manipulated to maximize available information. - There are no chemicals to dispose of. While telemedicine promises to increase the efficiency and effectiveness of medical professionals, wide-scale implementation faces the following obstacles: - It has been difficult to establish a uniform standard that allows file transfer among systems made by different vendors. - There are unresolved legal questions about "interstate" medical practice as it occurs in teleradiology and about standards of care and image quality. - Any system available on a network is vulnerable to unauthorized users who may invade the database or operation of the system, and it is very difficult to detect fraud--data that has been tampered with--in digital records. - Connections to remote locations depend on local telephone lines, which may be slow. Other options are available, but they may be too expensive for facilities in the rural areas that need them the most. - Digital imaging equipment is still very costly to acquire and install. The future of telemedicine rests now with those who implement it. Once instituted, it will surpass traditional medicine by reducing the expense of storage and misplaced information and allowing faster and more accurate diagnoses.     

PACS: the silent revolution

More than 15 years ago the idea of a Picture Archiving and Communication System (PACS) and a filmless hospital was created. In a PACS environment images are acquired, read, communicated and stored digitally. After many years of unsuccessful attempts and prototype installations, the necessary hardware components for a successful PACS installation are now readily available. However, software development is still lagging behind. Only very recently, software developers have realized that it is not sufficient for PACS software to store, communicate and display images, but that PACS software should effectively support the radiologist in the task of interpreting and communicating imaging findings through context-dependent default display arrangements, work-flow management, radiological and hospital information systems integration, and computer-assisted diagnosis. This review examines hard- and software requirements for efficient PACS operation, analyses costs and benefits, and discusses future developments. Received: 26 October 1998; Revision received: 11 January 1999; Accepted: 4 February 1999