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.
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.
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.
Radiographic testing using computed radiography has pros and cons:
- No film or chemicals; instead a computer is used
- Image available faster; commonly one minute instead of seven minutes, as with conventional radiography
- By adjusting image brightness, a wide range of thicknesses can be examined in one shot, unlike conventional radiography, which requires a different exposure time for each thickness in a component. Computed radiography thus requires fewer re-shots due to under- or over-exposure.
- Images can be stored on disk or transmitted for off-site review.
- Due to the high cost of the CR equipment and the training required to operate the CR systems, pricing is typically higher when compared to traditional film radiography. In medical situations though, the break even point with not having to use chemicals and have a darkroom reduce the ongoing cost considerably.