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J. R. Cox, G. J. Blaine, R. L. Hill, R. G. JostBiomedical Computer Laboratory and Mallinckrodt Institute of RadiologyWashington University, 700 South Euclid Avenue, Saint Louis, Missouri 63110AbstractA preliminary design study has been carried out for a distributed picture archiving andcommunication system for the Mallinckrodt Institute of Radiology. The study develops designequations for three layers of a picture network and examines the estimated flow of digitalimages between a multiplicity of picture sources, picture archives and picture viewingstations. Application of these data to the design equations leads to some preliminary con-clusions. One network architecture consistent with these conclusions is discussed.IntroductionThe layered approach to the design of computer networks has been well developed in thecontexts of distributed data processing and office automation. We suggest an analogousapproach for picture archiving and communication systems (PACS). By studying the firstthree layers (physical, picture link, picture network) several design equations appropriateto PACS can be obtained. These equations combined with estimates of the flow of digitalimages, provide a methodology for the prediction of the performance of various PACS designs.The rapid response requirements of a department of radiology coupled with the quantity ofdata, suggest architectures that differ markedly from the popular store -and -forward approachto computer networks. A two -level interconnect utilizing multiple broadcast channels isproposed and discussed. Although this architecture is neither the only alternative nor new,the methodology and the example of its use illustrate a means for analysis and comparisonof design alternatives.The advent of digital radiographic images has revealed the prospect of a picture network.Early reports of the design of such systems have appeared,1,2 optical disks for archivingimages have been prototyped3r4 and methods for image compression have been suggested5.Advantages of PACS have been widely recognized, but the present level of development ofcomputer networks is inadequate for rapid response, high volume systems. Careful designwill be required if the anticipated advantages are to be achieved. This study attempts tomake a contribution to design methods for PACS.The basic form of a PACS (Figure 1) includes three classes of nodes interconnected by ahigh bandwidth digital network. The first class of nodes includes all image sources:computed tomography (CT), nuclear medicine, ultrasound, digital subtraction angiography andeventually chest and other forms of radiology. The picture archive may be centralized ordistributed, but seems likely to be heavily dependent on the new technology of optical disksto provide the dense packing of information required. Picture viewing stations that incor-porate image processing responsive to the radiologist's needsare the third class of nodeson the network. The functional characteristics of these stations are just emerging, butrapid response seems to be a high priority requirement whether the image to be viewed is ina distant archive or has already been retrieved but needs to be processed before viewing.PACS engineering design is in its infancy, but one conclusion is clear. Desian issuespreviously studied in the context of computer networks must be reviewed. Specificationsfor PACS are unique and new models for the networks required must be developed.Modeling a picture archiving and communication system (PACS)We adopt the layered approach to networks. This approach reduces design complexity andconforms to modern practice. The Reference Model of Open Systems Interconnection (OSI)proposed by the International Standards Organization (ISO) has seven lavers6, but in thefollowing we consider only the first three layers. The first is the physical layer corre-sponding to ISO terminology. Since this study is aimed at picture archiving and communi-cation exclusively, we shall call the next two layers the picture link layer and the picturenetwork layer.Physical layerThe design issues for this layer concern the transmission of raw bits over a coaxialcable, a microwave link or an optical fiber. Picture service will recuire wide bandtransmission hence neither telephone lines or twisted pair cable are considered. |
