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Computed radiography (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 is housed in a special cassette and placed under the body part or object to be examined and the 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.
Differences from Direct Radiography
Computed radiography (CR) is often distinguished from Direct Radiography (DR). CR and DR have many similarities. Both CR and DR use a medium to capture x-ray energy and both produce a digital image that can be enhanced for soft copy diagnosis or further review. Both CR and DR can also present an image within seconds of exposure. CR generally involves the use of a cassette that houses the imaging plate similar to traditional film-screen systems, whereas DR typically captures the image directly onto a flat panel detector without the use of a cassette. Image processing or enhancement can be applied on DR images as well as CR images due to the digital format of each. There are many different types of DR detectors in use in medicine and industry. Each type has its own merits and distinctions and may be applied to certain imaging requirements based on these attributes.
CR and DR should not be confused with fluoroscopy, where there is a continuous beam of radiation, and the images appear on the screen like on a TV. This is the system many people are familiar with, where the image of the article being x-rayed is viewed in real time on a monitor or display. Many people think airports use fluoroscopes for baggage inspection, when in fact LDAs (Linear Diode Arrays) are universally used to generate static images of luggage content. LDAs are also used in a wide variety of other screening and imaging applications, and are also presented in a digital format. Fluorosopes until recently have used a device called an image intensifier to enhance the analog output of the real time x-ray image from a fluorescent screen, viewing the I.I output with a video or CCD camera and digitally enhancing the video to reduce the noise inherent in the system; the latest fluroscopes now use flat detectors read at up to 60 frames per second to yield a real-time image via a dedicated imaging computer.
Imaging plate (IP) or Flat Panel Detector (FPD)
The CR imaging plate (IP) contains a photostimulable storage phosphor layer (typically 0.1 to 0.3 mm thick), which stores the radiation dose as a latent image within the phosphor layer as elevated electron energies. When the IP is then transported through the scanner (read out), the scanning laser beam causes the electrons to relax to lower energy levels (photostimulated luminescence), emitting light that is detected by a photo-multiplier tube, which is clocked at a specific resolution or pixel capture frequency, this signal then being converted to an electronic signal and significantly amplified. The electronic signal is then quantized via an ADC to discrete (digital) values for each pixel and placed into the image processor pixel map.
Imaging plates can theoretically be re-used thousands of times if they are handled carefully and under certain radiation exposure conditions. IP handling under industrial conditions often results in damage after a few hundred uses. Mechanical damage such as scratches and abrasions are common, as well as radiation fatigue or imprinting due to high energy applications. An image can be erased by simply exposing the plate to a room-level fluorescent light - but more efficient, complete erasure is required to avoid signal carry-over and artifacts. Most laser scanners automatically erase the IP (current technology uses red LED lighting) 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 due to the composition of the PSL phosphor, which contains the heavy metal Barium.
Common applications for computed radiography include:
- corrosion surveys on pipes, often through insulation;
- Examination of valves for erosion;
- Information shots on industrial components; e.g. checking to see if a valve is closed properly, or checking for obstructions in valves and pipes;
- Examination of boiler water walls;
- Weld examination for all AWS & ASME code applications including Sect. III & XI
- Automotive casting inspection
- High pressure braze joint inspection (aerospace)
- Wax pattern core integrity verification in investment casting foundries
- Best when used with selenium-75 for small core piping due to internal scatter created by the wavelength of iridium-192
- Code work for Nuclear Applications on all size piping
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 in the form of a portable detector starts at around $65,000, while a basic low volume CR can start as low as $20,000 (but higher volume, hospital-grade applications can be higher.) DR systems are generally sold as a full x-ray room replacements and tied to a single x-ray generator. But are also commonly sold as a DR Panel which simply takes the place of where the cassette is placed. These can be either wireless or teathered. 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.
- No silver based film or chemicals are required to process film.
- Reduced film storage costs because images can be stored digitally.
- Computed radiography often requires fewer retakes due to under- or over-exposure which can result in lower overall dose to the patient, if you assume a moderate amount of retakes. CR can require up to 30% less dose than film.
- Image acquisition is much faster - image previews can be available in less than 10 seconds.
- 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.
- Images can be enhanced digitally to aid in interpretation.
- Images can be stored on disk or transmitted for off-site review.
- 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.
- 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.
- 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 to noise ratio) issues, sensitivity to scattered radiation, and the general lack of procedural consensus among primes and OEMs.
- There also are no quality (image resolution)standards for general radiography, only for mammography (21 CFR 900.12 (e)), however, competition among manufacturers has raised the bar and newer CR technologies with increased detective quantum efficiency (DQE) and higher spatial resolution have emerged.
- 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.
- ALLPRO NDT - CR Systems
- Willick Engineering Company, Inc.
- VMI, Virtual Media Integration
- Dürr Dental
- Dürr NDT - CR Systems
- Dürr Medical
- Computed Radiography in NDT Applications
- Digital Applications of Radiography
- Philips Medical
- Siemens Healthcare
- Konica Minolta
- Kodak Dental
- Carestream Health
- Agfa Healthcare
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