Computed Radiography (CR) uses very similar equipment to conventional radiography except that in place of a film to create the image, an imaging plate is used. Hence, instead of taking a film into a darkroom for developing in chemical trays, the imaging plate is run through a computer scanner to read and digitize the image. The image can then be viewed and enhanced using software that has functions very similar to conventional image-processing software, such as contrast, brightness, and zoom.

Differences from Direct Radiography

Computed radiography is commonly distinguished from Direct Radiography (DR). In the same way that a CR system requires a short burst of radiation, so does a DR system. The difference is that on exposure a DR system will almost instantly display the image on the screen in front of the radiographer, therefore removing any need for processing. Post production can of course be performed on DR images in the same way that CR images can.

This is NOT to 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 most people are familiar with, as this is what is used in airport security systems.

Imaging plate

The imaging plate 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, a scanning laser beam causes the electrons to relax to lower energy levels, emitting light that is measured to compute the digital image. Imaging plates can be re-used thousands of times if they are handled carefully, although handling under industrial conditions 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, and the plate is then ready for re-use. In the software, the scanned image is encrypted so that the original data is kept secure and can not be tampered with. Reusable phosphour plates are environmentally safe.

Industrial applications

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.

Medical applications

• Computed Radiography systems are the most common in medical applications because of their low cost. (compaired to other similar technologies)

Plate based DR usually starts at around $150,000.00, while CR starts at around $100,000.00. DR (plate detector) systems must be dedicated to a single X-ray Generator due to the detector to workstation interface. CR IP's can be used in multiple X-ray sites and the IP's processes through a single reader (scanner), similar to a film processor.

Advantages

• No silver based film or chemicals; instead a computer is used.
• Exposure times are a fraction of typical film exposures, which reduces occupational exposure to radiographers in field applications and is more economical.
• Image acquisition is much faster; commonly one minute instead of seven minutes, as with conventional radiography.
• 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. Computed radiography often requires fewer re-shots due to under- or over-exposure.
• 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.

Disadvantages

• CR is still not an approved method for most 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.

• Imaging plates (IP's) are quite expensive and can be easily damaged, if the system being used requires manual handling of the IP's. Theoretically, IP's 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.

See also

• Digital radiography

References

• Computed Radiography in NDT Applications
• Digital Applications of Radiography

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