Computed Radiography, or CR uses very similar equipment to conventional radiography except that in place of a film to create the image, an imaging plate (IP) made of photostimulable phosphor is used. The imaging plate housed in a special cassette and placed under the body part or object to be examined and the medical x ray exposure is made. Hence, instead of taking an exposed film into a darkroom for developing in chemical tanks or an automatic film processor, the imaging plate is run through a special laser scanner, or CR reader, that reads and digitizes the image. The digital image can then be viewed and enhanced using software that has functions very similar to other conventional digital image-processing software, such as contrast, brightness, filtration and zoom.
Imaging Plate
The CR imaging plate (IP) contains photostimulable storage phosphors, which store the radiation level received at each point in local electron energies. When the plate is put through the scanner, the scanning laser beam causes the electrons to relax to lower energy levels, emitting light that is detected by a photo-multiplier tube, which is then converted to an electronic signal. The electronic signal is then converted to discrete (digital) values and placed into the image processor pixel map.
Imaging plates can theoretically be re-used thousands of times if they are handled carefully. IP handling under industrial conditions, however, may result in damage after a few hundred uses. An image can be erased by simply exposing the plate to a room-level fluorescent light. Most laser scanners automatically erase the image plate after laser scanning is complete. The imaging plate can then be re-used. Reusable phosphor plates are environmentally safe but need to be disposed of according to local regulations.
Medical Applications
Computed Radiography systems are the most common in medical applications because they have proven reliability over more than two decades, flexibility to address a variety of clinical applications and lower costs to take multiple exam rooms digital. DR systems are generally sold as a full x-ray room replacements and tied to a single x-ray generator. CR IPs can be retrofitted to existing exam rooms and used in multiple x-ray sites since IPs are processed through a CR reader (scanner) that can be shared between multiple exam rooms.
Advantages
1、 No silver based film or chemicals are required to process film.
2、 Reduced film storage costs because images can be stored digitally.
3、 Computed radiography often requires fewer retakes due to under- or over-exposure which results in lower overall dose to the patient.
4、 Image acquisition is much faster - image previews can be available in less than 15 seconds.
5、 By adjusting image brightness and/or contrast, a wide range of thicknesses may be examined in one exposure, unlike conventional film based radiography, which may require a different exposure or multiple film speeds in one exposure to cover wide thickness range in a component.
6、 Images can be enhanced digitally to aid in interpretation.
7、 Images can be stored on disk or transmitted for off-site review.
8、 Ever growing technology makes the CR more affordable than ever today. With Chemicals, dark room storage and staff to organize them, you could own a CR for the same monthly cost while being environmentally conscious, depending upon the size of the Radiographic Operation.
Disadvantages
1、 In medical applications, manual handling of the cassette housing the IP is considered a disadvantage versus DR but it also offers more flexibility for patient positioning.
2、 CR is still not an approved method for higher quality radiologic applications (aerospace), due to the possibility of digital manipulation to the captured image, the inherent geometric unsharpness and resultant lower spatial resolution as compared to film (radiographic) images, SNR (signal vs. noise)issues and sensitivity to scattered radiation, and the general lack of procedural consensus among primes and OEM's.
3、 There also are no quality (image resolution)standards for general radiography, only for mammography, however, competition among manufacturers has raised the bar and newer CR technologies with increased detective quantum efficiency (DQE) and higher spatial resolution have emerged.
4、 Imaging plates (IPs) are expensive and can be damaged if the system being used requires manual handling of the IPs. Theoretically, IPs may be reused thousands of times, but constant use will always result in damage to the IP and image artifacts, eventually to the point of necessary replacement.

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