Computed Radiography

[ Computed radiography, a form of digital imaging, is being introduced in radiology departments .Although many articles have been written about the advantages of computed radiography, little has been published regarding the technology's impact on radiographers' day-to day practice.

This article reviews the technology behind computed radiography and examines the tasks that radiographers need to learn in order to process digital images.
Digital imaging brings many advantages to the radiology department. The technology makes it possible for physicians to interpret images from a remote location, allows staff to reduce the amount of storage space required for film archival, gives people simultaneous access to a single image on a computer network and lets clinicians access images digitally rather than waiting for delivery of a hardcopy radiograph. It also has the potential to decrease the number of repeated x-ray studies. These advantages are driving more and more radiology departments toward digital imaging, and specifically toward computed radiography (CR).
Despite the obvious benefits of digital imaging, its introduction to a radiology department can disrupt radiographers' daily routines. Although the standard rules about anatomy and positioning remain unchanged with CR systems, many conventional guidelines about exposure and density are less effective when used with CR. Therefore, it is important for radiographers to understand how CR images are produced and to learn how to adapt their skills to produce quality images using computed radiography.
Computed Radiography Systems
Computed radiography systems are available from several manufacturers. Some vendors offer complete systems that include acquisition and networking capabilities, while others purchase the image acquisition unit from a third party, then add their own equipment to provide networking capability. The information presented in this article is based on clinical experiences with CR systems Fuji models 9000 and AC-3 (Fuji Medical Systems, Stamford, Conn.).
A computed radiography image is captured on an imaging plate. The plate looks like a conventional intensifying screen, and it is placed in a cassette that looks like a conventional screen-film cassette. This, however, is where the similarities end.
The CR imaging plate is composed of a photostimulable phosphor, usually barium fluorohalide crystals doped with europium. Instead of fluorescing when exposed to radiation as intensifying screens do, imaging plates store the energy following an x-ray exposure until the plate is placed in a plate reader. There, it is scanned by a laser that causes the plate to release its stored energy in the form of light. The light is gathered by a photomultiplier tube, where it is converted into digital data. The final image is assembled from this data.
After reading, the imaging plate is flooded with an intense fluorescent light to erase any remaining energy.[1] This cycle continues until the plate wears out. Plate life depends on the model of the plate reader and to what extent the plate is bent during the reading process. Vendors generally guarantee at least 10,000 readings before a platel begins to show wear.

Radiologic technologists whose departments are converting to CR technology should have a solid understanding of all aspects of computed radiography, particularly histogram construction, exposure latitude, mottle, dose calculation, selection of a processing algorithm, postprocessing, collimation and grid selection.
Histograms
When the CR imaging plate releases its stored energy, a photomultiplier tube gathers and amplifies the light and converts it into digital data. The CR system uses this digital information to construct a histogram, or graphic diagram, of the pixel values that exist in the image. Every image has its own unique histogram. Positioning and centering of the body part, pathology and missing or additional anatomy all affect image processing because these factors affect the distribution of the different pixel values present in the histogram
Exposure Latitude and Mottle
left]Computed radiography offers substantially increased exposure latitude compared with conventional screen-film systems.

to be continued

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