Computed radiography, or CR, is a digital image acquisition and processing system for radiography that uses computers and laser technology. It was developed in the mid-1980s. CR images can be recorded on laser-printed film or transmitted and stored digitally. This technological change has a significant impact on hospital operating costs and efficiency because radiography is the most common method of diagnostic imaging. It accounts for 70% of all imaging procedures, in comparison to 10% for CT scans and 6% for MRIs.
The purpose of CR is to produce accurate radiographic images without the use of film, thereby streamlining the storage, display, and transmission of patient data. Because CR allows the radiographer to correct images immediately following exposure, the need for retake exposures is dramatically reduced. In a CR system, corrections made in the image are relayed to the radiographer through an s number. This value tells the radiographer whether the system had to brighten or darken the image, and to what degree, in order to produce a usable image. The adjusted image can then be printed on a film by a laser printer.
In addition to providing clear diagnostic images that can be adjusted before printing, CR simplifies the process of transmission for purposes of consultation. CR images can easily be sent to other physicians or facilities for consultation via computer networks. Furthermore, CR systems permit considerable reductions in the cost of storage space for diagnostic images. Given the rapid rise in operating costs of full-service radiology departments, many newer facilities and some larger hospitals have installed CR systems.
One problematic aspect of CR is that it requires a higher dose of radiation to produce an image comparable to those produced by the film-screen method. The higher dose is necessary because the plate speed is approximately half that of the current screens used in film-screen combinations. The speed of the plate is directly related to the amount of radiation needed to create the x-ray image. Keeping the patient's exposure to radiation "ALARA," or "as low as is reasonably achievable," has always been one of the goals of radiologic imaging. On the other hand, some radiologists note that patients may receive lower total dosages of radiation from CR imaging because fewer repeat exposures are required.
A second problematic aspect of CR is that although its contrast resolution is better than that of conventional radiographic films, its spatial resolution is not as good. This drawback is especially apparent in mammograms and chest radiographs. One study that compared six different systems for chest radiography found that the CR system performed the least well, even though it was tested under the normal operating conditions for its setting.
Computed radiography is an imaging technology in which a phosphor imaging plate replaces the older combination of film-screen radiography. Phosphors are substances that become luminescent (emit light) when they are excited by ultraviolet light or other forms of radiation. The imaging plate consists of either aluminum or a steel frame with honeycombed carbon fiber on one side. This side is the x-ray attenuating side. Inside is the phosphor imaging plate, which replaces the older film-screen cassette. The plate is contained in standard-size radiographic cassettes that can be used in existing radiographic tables and stands.
The radiographer positions the patient and inserts the imaging plate cassette with the carbon-fiber side facing the x-ray tube. When the phosphor inside the imaging plate is exposed to x rays, its electrons are excited to higher energy levels. The radiographer then places the plate in a scanner, in which a helium-neon laser irradiates the excited electrons in the phosphor. The electrons emit light and return to a lower energy level. The light given off is converted first to an analog electrical signal which is then digitized, or converted into numerical data. The data can be recorded as an image on laser-printed film or transmitted and stored digitally.
Picture-archiving communications systems (PACS)
Picture-archiving communication systems, or PACS, are computer systems that allow several physicians to view radiographic images from multiple locations at the same time, whether in different departments of the same hospital or from remote facilities. CR images can be scanned, reviewed by the technologist for accuracy, and made available within three minutes to both the radiologist and the admitting physician. PACS improve hospital efficiency by eliminating the risk of losing or misplacing x-ray films as well as minimizing the need for storage space.
One of the major challenges in the implementation of computed radiography in any institution is acceptance by the clinical staff. The task of retraining radiologists and technologists as well as ancillary staff can be daunting. Lastly, the increasing complexity of radiographic equipment requires greater cooperation between the engineers and scientists who develop the equipment and the health care personnel who use them. Gradual introduction of CR, along with smooth coordination of the technology experts and the clinical staff, is usually the best way of moving toward full departmental conversion.
An additional complication to implementing CR systems is that they no longer represent cutting-edge imaging technology. Within the past few years, a system called digital radiography, or DR, has been introduced. It resembles CR in that it transmits, displays, and stores images without the use of film, but it differs from CR in that it is strictly digital and does not use cassettes at all. While the advantages of CR include its portability and its lower production cost, DR offers superior contrast resolution, immediate image readout, and considerable time savings. The first productivity studies comparing the two imaging systems have found that a CR examination takes between three and four times as long as an examination using digital radiography. In one hospital near Boston, the average two-view chest radiograph required 9.9 minutes with a CR system but only 2.5 minutes with DR. Most of the time difference appears to be due to the steps required to process the CR cassettes. While some hospitals are using both CR and DR systems, others are concerned that CR technology may not have a long enough future in spite of its lower initial costs to justify implementing it at all.
CR does, however, offer significant advantages in diagnostic quality over conventional film-screen methods. The latest enhancements for CR include energy subtraction, which makes it possible to view bone-only and soft tissue-only images of the chest, and dynamic range control, which makes it possible to study both bone and soft tissue on the same image. Another benefit of the CR system is that the images are available immediately on the monitor for evaluation and post-processing enhancement. In addition, the images can be evaluated by an off-site radiologist with a monitor hooked up to the hospital's system. Lastly, CR is a portable system that can be used at the patient's bedside or in the emergency room as well as in a radiology clinic.
Health care team roles
With computed radiography as with conventional film-screen technology, the exposures can be made by a radiologic technologist or a radiologist. The radiologist will interpret the images as part of the process of differential diagnosis. A PACS system will allow the radiologist to consult with colleagues in other parts of the hospital or other institutions.
Cassette— The thin container that holds the film or the laser plate during radiographic exposure.
Digital radiography (DR)— A newer form of filmless imaging that produces digital images on a computer monitor without the use of cassettes.
Digital recording— A method of recording audio or visual data in which an input wave form is sampled thousands of times per second, and each sample is given a binary numerical value.
Film-screen— The combination of x-ray film and screens used in current conventional methods of radiographic imaging.
Phosphor— A substance that emits light when it is struck by light of a certain wavelength. Some phosphors are luminescent only when struck by ultraviolet light.
Picture-archiving communication system (PACS)— A digitized system for storing CR or DR images that replaces older film storage, manual filing, and manual routing systems.
Screen— The layer lining the cassette that changes the x-ray into light.
DeMaster, Dean R. "Digital radiography offers major productivity gains over computed radiography: Results of a time-motion study." Applied Radiology 30, no. 3 (March, 2001): 28-31.
Manning, D. J., et al. "An ROC evaluation of six systems for chest radiography." Radiography 5, no. 4 (November 1999): 201-209.
Association of Educators in Radiological Sciences (AERS). P. O. Box 90204, Albuquerque, NM 87199-0204. (505) 823-4740. Web site: 〈http://[email protected]〉.
Society for Computer Applications in Radiology (SCAR). 10105 Cottesmore Court, Great Falls, VA 22066-3540. (703) 757-0054. Fax: (703) 757-0454. Web site: htpp://www.scarnet.org. The Society has published the Journal of Digital Imaging since 1988.
Page, Douglas. "Point-counterpoint: Direct and computed radiography coexist, but for how long?" Presentation at SCAR2000, annual meeting of the Society for Computer Applications in Radiology.
Van Metter, Richard, Ph.D. "The Technology Behind Digital Radiography: All Systems are Not Created Equal." 〈http://www.kodak.com〉 (January/February 2001).