Scaleability, performance, and fault-tolerance of PACS architectures

Three data-base architectures may be distinguished among Picture Archiving and Communication Systems (PACSs): 1) Configurations with logically and physically centralized data-base and file server, 2) systems with physically distributed file servers and a logically centralized data-base, and 3) installations with logically and physically distributed data-bases and file servers. A brief overview of these architectures and their scaleability, performance, and fault-tolerance is given. A PACS for an existing large university hospital is designed for the first as well as the second architecture using given image production data and workflow. We evaluate the fault-tolerance of the two architectures. By modeling the work-flow and employing queuing theory, solutions with practically realizable data transfer requirements are found for both architectures. With today's performance and cost of computers, storage, and information management technologies, the second and third architectures are preferably implemented, depending on the size of the installation. The architectures offer almost unlimited scaleability, very high fault-tolerance, and optimized workflow. We describe a modem commercial PACS that adheres to the open-systems concept and consists of software application programs that run, independent of specific computer and network components, on off-the-shelf hardware and under standard multi-platform operating systems and utilize commercial data-base management systems and network managers. The system is based on the second architecture with multiple islands of functionality, each with servers and archive modules and a physically distributed data-base. Our PACS architecture supports browser technology: Workstations use the data-base to determine the location of needed information and then, through the image browser, mount the appropriate file server for access. The architecture supports a concept similar to domain name server (DNS) directory services on the Internet. The system can be expanded to enterprise-wide installations with a logically distributed data-base. Openness, scaleability, and longevity of a PACS also strongly depend on the architecture of software applications in the operating and tool-set environment as well as on the distribution of image processing tasks across a PACS. These issues are discussed in the last section of our paper. We are presenting an image processing strategy that provides a consistent rendering of image gray-scale and spatial resolution throughout the entire PACS.