Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

(14 x 17 inch) film should be digitized to a pixel size of at least 200 /lm (1,750 X2,150 pixel matrix). Most CCD and laser digitizers meet these requirements.The role of film digitizers in PACS is likely to be transient because of thedecreasing use of screen-film detectors. However, it is likely be a decade or longerbefore film's role in recording and displaying images will be but a footnote in afuture edition of this book.Digital Imaging and Communications in MedicineIn the past, imaging equipment vendors have used proprietary formats for patientdata and digital images, thereby hindering the transfer, storage, and display ofimages acquired on equipment from various vendors and making it difficult tobuild PACS and teleradiology systems. Some facilities solved this problem by purchasingall equipment from a single vendor. To help overcome these problems, theACR and the National Electrical Manufacturers' Association (NEMA) jointly sponsora standard called Digital Imaging and Communications in Medicine (DICOM)to facilitate the transmission of medical images and related data. DICOM specifiesstandard formats for information objects, such as "patients," "studies," and"images." DICOM also specifies standard services that may be requested regardingthese information objects, such as "find," "move," "store," and "get." UsuallyDICOM is used as an upper-layer protocol on top ofTCPIIP.Today, the products of most major vendors of medical imaging and PACSequipment conform to the DICOM standard. A DICOM conformance statement isa formal statement, provided by a vendor, that describes a specific implementationof the DICOM standard. It specifies the services, information objects, and communicationsprotocols supported by the implementation. In reality, even when theconformance statements issued by two vendors indicate the ability to provide a particularservice, interoperability is not assured. Vendors should be contractuallyobligated to provide necessary functions, and verification testing must be performedafter installation.Computer networks, discussed in detail at the beginning of this chapter, are used tolink the components of a PACS. A PACS may have its own network, or it may sharenetwork components with other information systems. The network may provideimages only to interpretation workstations, or it also may provide them to displaystations in intensive care units and in the emergency department. The bandwidthrequirements depend on the imaging modalities and their work loads. For example,a network medium adequate for nuclear medicine or ultrasound may not be adequatefor other imaging modalities. Network traffic typically varies in a cyclic fashionthroughout the day. Network traffic also tends to be "bursty." There may beshort periods of very high traffic, separated by long periods of low traffic. Networkdesign must take into account both peak and average bandwidth requirements andthe delays that are tolerable over each segment of the network. Network segmentation,whereby groups of imaging, archival, and display devices that communicatefrequently with each other are placed on separate segments, is commonly used toreduce network congestion. Different network media may be used for each networksegment. For example, lOBase-T Ethernet is likely to be sufficient for a segment