The first part of the process is to detect the collimated edges of the X-ray beam. To display a useful image, it is necessary to process the data from the reader. Effectively, the raw CR image would be equivalent to a filmРІР‚вЂњscreen image with a film РћС– ( Ch.

This is in contrast to the film, for which the greyscale is compressed into a narrower range. If the light signal were directly converted to greyscale on an image monitor such that, for example, black was assigned to a signal level of 10 000 in the figure and white was set at level 1, as shown by the bar above the graph, the image seen would be very flat and display minimal contrast. range of doses that can be imaged, is very much greater than for conventional radiography. It can be seen how the latitude of the CR system, i.e. Superimposed on the plot is the characteristic curve of a filmРІР‚вЂњscreen system. This is illustrated in Figure 5.6, in which the log of the light intensity from the screen is plotted against the log of the X-ray dose. Photostimulable phosphors have a very wide dynamic range, being able to record photon intensities varying by a factor of about 10 000:1. 5.3.2 Computed radiography image processing Stacking readers are particularly useful for readers serving more than one X-ray room. To achieve the fastest throughput, stacking readers are available in which several cassettes (at least four) may be placed in a queue for automatic feed into the reader. The time for a CR reader to extract the image from the plate is generally between about 30 and 45 s, and the faster readers are capable of reading 100 or more plates per hour. Most phosphors used for CR emit light at the blue end of the spectrum and need a scanning laser emitting red light for simulation.Р”Р‡ollowing the read cycle, the residual signal from the plate is erased by exposing it to a bright light source. While there is repeated scanning across the plate, it is progressively moved through the scanning beam so that the complete pattern of light intensities can be extracted. The position of the light-emitting centre is determined from the time at which the light is received. Above the plate, there is an array of optical fibres to direct the emitted light to one or more photomultiplier tubes to measure its intensity. Scanning is achieved using a rotating mirror.

In the reader, the plate is removed from the cassette and scanned by a laser beam (see Fig. The signal from the imaging plate is read in a CR reader. For this reason, X-ray equipment used for conventional radiography can be used for CR, making the transition fromanalogue to digital radiography straightforward. The plate is inserted into a light-tight cassette, also similar in appearance and with the same dimensions as that used for filmРІР‚вЂњscreen radiography.

The imaging plate thus formed is similar in appearance to the intensifying screen used in conventional radiography. A surface coat protects the phosphor from physical damage. The phosphor in a powdered form is mixed with a binder or adhesive material and laid down on a base with a thickness of about 0.3 mm. The material is commonly barium fluorohalide doped with europium (BaFX:Eu), in which the halide (X) is a combination of bromide and iodide, typically 85% and 15%, respectively. It is referred to as a photostimulable phosphor. CR uses a storage phosphor that requires light input to release the trapped energy in the form of light that is proportional to the X-ray intensity. Penelope Allisy-Roberts OBE FIPEM FInstP, Jerry Williams MSc FIPEM, in Farr's Physics for Medical Imaging (Second Edition), 2008 5.3 COMPUTED RADIOGRAPHY 5.3.1 Imaging plates and readersР”В¬omputed radiography (CR) is the most common method of producing digital radiographic images and the first technology that was commercially available.

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