review article picture archiving and communication system - rbrs

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Modality clusters, whether for sonography, nuclear medicine, CT or MRI, typically are homogeneous groupings of machines connected to share printing, soft copy viewing and storage resources. Teleradiology supports the acquisition, transmission and viewing of images where the points of acquisition and viewing are separated by distance. Teleradiology was first used for on-call review. Today, the technology has matured to the point where primary diagnoses may be performed remotely. For example, teleradiology may be used to send images from remote clinics to radiologists who are at home or at a professional office. While teleradiology supports several different types of applications, those applications all share one characteristic: there is little or no image storage. For that reason, little image management is necessary. Mini-PACS is a localized version of full PACS. Typically, they let users acquire images (as in teleradiology) and distribute them quickly. Mini- PACS also let users store images for a short period of time, usually at the point of use. Mini-PACS may be used in intensive care units where physicians need to keep exam images on file for several days. Film, however, is still the primary longterm storage medium. Full-fledged PACS are different from teleradiology and mini-PACS in two ways. First, full PACS support long-term digital image storage – the electronic archiving of images. Second, PACS support a more flexible distribution of images. Healthcare facilities may move beyond supporting specific departments to managing the flow of diagnostic images to a wider range of physicians. Very often, full-fledged PACS will include one or more teleradiology subsystems that may communicate with central image archives. PACS hardware Digital image acquisition PACS must have images in digital format, be able to store them, and provide access for interpretation by radiologists and review by other physicians. Images often may be directly acquired in a digital format, but sometimes they must be converted to such a format. Radiographic studies already available as digital data include CT, MRI, and sonography. Recently, some tradi- PICTURE ARCHIVING AND COMMUNICATION SYSTEM — DE BACKER et al. 235 tionally film-based imaging techniques (e.g., angiography, fluoroscopy) have also moved to digital image acquisition. However, the main challenge for a digital radiology department remains projection radiography, such as chest and bone films, which still corresponds for at least half of the procedures performed in radiology. Conversion may currently be accomplished through one of three technologies – use of a film digitiser, CR, or digital radiography (DR) (5, 6). Digitisation of plain film radiographs Conversion of conventional plain radiographic film to digital data by means of a digitiser is the least efficient method. However, it may prove useful in radiology departments that have a relatively low volume of studies. In larger departments, it may be useful during the transition from a film-based system to PACS. Traditional radiographic studies may be digitalised so they may be easily compared with the newer digital images. Computed radiography CR is a technique that uses conventional radiographic imaging equipment to obtain digital data. CR uses cassettes, which contain, instead of film, a re-usable plate with photo-stimulable phosphors to store the latent image. When an Xray photon hits a phosphor crystal its electrons are excited to a higher energy level where they become trapped producing a latent image. The image is digitally processed, and the digital image is transmitted to a processing station for further interactive image manipulation and to a PACS reporting or viewing station. Digital images may then be accessed by radiologists, referring physicians and other departments within the facility. They may also be combined with the patient demographic data from the HIS/RIS systems. The image may also be available as a hardcopy via a laser camera. Most current PACS systems rely on digital storage phosphor radiography to provide digital projection radiographs (7). However, storage phosphor radiography is associated with some disadvantages including a limited spatial resolution and a low-detective-quantum efficiency (5). There is still the need for cassette handling, and the life expectancy of the expensive storage plates is shorter than that of conventional film-screen cassettes, due to mechanical strain during storage plate readout. Due to its flexible handling, storage phosphor radiography probably will remain the digital modality of choice for bedside and intensive care imaging. Digital radiography DR uses electronic detectors to convert radiation that passes through the patient directly to a digital image without the need for cassette handling. The future of digital projection radiography will depend on new digital receptors based on amorphous silicon or selenium (8). Dedicated chest imaging systems based on amorphous selenium are currently available with an excellent image quality and signal-to-noise ratio (5, 10). Direct digital radiography system for general radiology based on thin-film transistor technique is now also available (5). Although DR is the most expensive of the three methods, it is also the most practical way to obtain digital data for plain radiographic studies in high-volume departments. A major drawback is the solely stationary use. Digital receptors based on amorphous silicon or selenium may also be expected to provide an adequate solution for digital mammography, which, due to its high spatial resolution requirements and difficult handling (related to the large size of image files) has usually remained film based, even in otherwise fully digital departments (9, 11). Image archiving A typical radiology department creates many gigabytes of image data per day and several terra-bytes (TB, 1012 Byte) of data per annum (12). The volume of archived images is increasing and will continue to rise at a steeper incline than filmbased storage of the past (13). Many filmless facilities have been caught off guard by this increase, which has been stimulated by many factors: investment in new digital and DICOM-compliant modalities will result in more images to be “brought into” the filmless network; CR and DR is becoming more affordable resulting in digital plain film studies; and multi-slice CT technology results in an increasing number of images per study. New multi-slice CT scanners, for example, may generate as many as 800 to 1,000 images per exam.
236 JBR–BTR, 2004, 87 (5) Storage requirements also are affected by disaster recovery initiatives and state retention mandates. Disaster recovery and data storage require a backup of imaging data that may be quickly recovered in case of disaster. This means that a second copy of the data must be stored, which is most easily accomplished electronically. Retention requirements vary by nations; Belgium, for example, mandates up to a 30-year retention for certain types of data. There are two basic approaches to archiving – single tear and multitier (13). Each has benefits. With a single tear approach, all the data are stored on a single media that may be accessed very quickly. A redundant copy of the data is then stored onto another less expensive media. This is usually a removable media. In this approach, the on-line storage is increased incrementally as volume grows. In a multi-tier approach, a hierarchical architecture, based on access speed and cost, is set up in three stages with different storage media depending on the amount and duration of storage and the expected retrieval frequency. The first stage is on-line storage, where information stored is available to the user immediately. This is typically reserved for data needed during a single episode of patient care and is readily accessible to radiologists and clinicians. The second stage is short-term storage intended to provide rapid access to studies for comparison with current studies, and the third stage is deep archival storage intended for long-term storage of prior images. The traditional approach to medical image archiving was tiered. This means that images accessed most frequently were stored on high-cost fast-access media, and those less frequently accessed were shifted to lower-cost slower-retrieval media (5, 13). Fast but expensive redundant array of inexpensive disks (RAID) systems are appropriate for shortterm storage of up to several days or weeks. For inpatients, the RAID should ideally be large enough to hold all current and relevant previous films for the entire hospital stay (14). A RAID is a multitude of hard disk drives where the information is written to all the disks so that it is distributed over the entire RAID. This means that if any of the disks were to fail, only a small proportion of the data would be lost. In addition, the RAID management software has an error-correcting algorithm, which is able to rewrite any lost data with a high degree of accuracy, so that data loss is very unlikely. Optical disk jukeboxes, with a storage capacity of a single jukebox typically between 0.5 and 1 TB, usually provide uncompressed on-line storage only for a maximum of 1 or 2 years. Thereafter, many current PACS concepts still rely on off-line long-term storage of optical disks with the necessity to manually reenter older disks into the system when necessary. With a capacity of 20 TB or more, tape-based storage systems provide an easy, cost-effective, and safe way of long-term storage even for a large radiology department (15, 16). Tape is still the top performer in terms of cost and capacity for data storage. Data security of modern data tapes, with a bit loss rate of less than one unrecoverable hard error for every 1017 bits and an expected data life of 30 years, is now comparable to optical technology (7). However, tape still has one of the slowest data access times making it more appropriate for deep archiving and redundancy. Reliability is also of concern with tape media, as it is sensitive to data loss when exposed to magnets. Based on data safety considerations, optical write-once read-multiple (WORM) technology over rewritable magneto-optical (MO) disks and tapes has been proposed for storage of radiological images (15). However, regardless of whether write-once or rewritable media are used for storage of medical data, the archive setup and software must ensure data integrity and as far as possible prevent fraudulent manipulation of imaging data. Moreover, data integrity must be protected not only during long-term storage, but also during the entire chain from data acquisition to the long-term archive (17, 18). As more historical imaging data will become filmless, there may be a trend toward on-line RAID storage of all images as the long-term or primary archive. This trend toward online storage may be facilitated by a predicted continuous, even dramatic decrease in hard disk cost-permegabyte. For that reason, on-line hard disk archiving may become a viable storage solution for the primary archiving of images. Redundant storage and disaster recovery may then be addressed with a back-up archive on a jukebox or shelf-managed media (13). Once an image has been acquired onto the PACS archive, it can never be lost and is always accessible. Even if the image is not on the short-term archive, it will still be accessible when fetched from the long-term archive within minutes, and available for viewing on the ward workstation. Pre-fetching is the process whereby previous images on a patient are automatically retrieved from the long term archive onto the short term server, prior to the acquisition and viewing of the current imaging examination on that patient. This software is commonly sufficiently sophisticated to take the form of an “intelligent prefetch”. This means that only a configurable number of examinations from the same modality and the same body part as that currently being imaged are pre-fetched from the long term archive; it is usually only these that will be of relevance in making a diagnostic comparison, and there is no point in unnecessarily overloading the PACS network and short term server with data irrelevant to the current clinical problem. The efficiency of a PACS archive also strongly depends on the way permanent storage is organized. If images are written chronologically on a first-in-first-out basis to WORM media, later image retrieval will be tedious, since images of a single patient will be spread over several disks. Long term archive will be organized more efficiently if images of a single patient are first kept in temporary storage and are later written conjointly to the long-term archive, e.g. after the patient has been dismissed from hospital. With rewritable media, such as MO or tape, it is possible to regularly reorganise storage to keep all imaging data of a patient together (5, 19). The amount of data to be stored may be reduced substantially by using appropriate compression techniques. Image compression also has the potential of drastically reducing network performance requirements. Reversible and lossless compression is often used in long-term storage and allows from 2:1 tot 5:1 compression rates. About 15:1 compression for X-ray images, and 10:1 for MR and CT images, may be obtained with some data loss but with essentially no visible change in image quality. Irreversible compression ratios of up to 40:1 without clinically relevant image degradation have been reported (20-23). However, with any kind of irreversible compression, there is at least the theoretical risk of losing important image information, and
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