Digital Computed Radiography

by Kevin Clear, EWI Project Engineer

Film Radiography vs Computed Radiography
Film radiography is currently the dominant volumetric nondestructive evaluation (NDE) technique used throughout the world. Film is lightweight, flexible, and suitable for diverse applications with a proven track record of more than 100 years. However, film does have disadvantages. It requires significant amounts of time for processing that must be done in a darkroom using chemicals that must be disposed of in compliance with environmental regulations. In addition to a limited shelf life, processed film requires a storage area in which temperature and humidity are controlled, complicating long-term archiving. In contrast, digital radiography requires none of the above. Radiographic images can be generated, optimized, analyzed, stored, and distributed in digital format (equipment seen in Figure 1).

For these reasons, many NDE organizations are considering making the transition from film radiography to digital radiography. One form of digital radiography is digital computed radiography (CR). CR is a technique that captures an analog radiographic image in flexible storage phosphor plates for later readout and display. The technique is possible due to photostimulable luminescence (PSL). PSL is a phenomenon in which a phosphor that has ceased emitting light, because of the removal of the stimulus, once again emits light when excited by light with a longer wavelength. The phenomenon allows the user to view an x-ray image without setting foot in a darkroom. Instead of taking film into a darkroom for developing, the imaging plate is run through a computer scanner to read and digitize the image. The image can then be viewed and enhanced to aid interpretation using software that has functions very similar to conventional image-processing software, such as contrast, brightness, and zoom.

Unlike film, CR plates can be cleared and reused thousands of times. Clearing of an imaging plate is done by placing the plate under room-level fluorescent light for 30-45 seconds. CR reduces the time and expense to produce a radiograph and results in a digital record that can be easily transferred and archived. This quick processing time can increase the productivity of RT operations.

CR can also reduce disruption to other trades while acquiring RT images. Because lower x-ray energies are required to produce images equal to conventional film for a given application, the size of the safety exclusion zone can be reduced.

Challenges
One challenge when transitioning to CR is ensuring that the requirements of the applicable codes and specifications are met. Any transition must be done while meeting regulatory requirements from such governing bodies as API, ASME, and ASTM. For many organizations several different codes and specifications are applicable to their business. Thus, a detailed understanding of the requirements of all of these codes is important for many companies to make the transition.

Another challenge of CR is selection of the proper equipment and training RT technicians in the proper use of the equipment. There are many systems to choose from with different capabilities and features. As with any advanced NDE technology, technicians should be trained in the proper use and protection of the equipment. For example, the phosphor plates can be damaged if not handled properly.

To help our members assess the relative merits of CR for specific applications, EWI assessed several systems and purchased a Kodak Industrex ACR-2000i digital computed radiography system. EWI has also completed a Cooperative Research Program (CRP) project to develop expertise in using the equipment; to review the applicable codes; and to develop an image library comparing film to digital radiography.

Summary
Computed Radiography can produce images that rival those taken via conventional film radiography and can be relied upon for many critical applications. The benefits of CR are numerous: lower source energies allow reduced exclusion zones; images can be developed quickly without a dark room; images can be archived and transferred digitally. These benefits can save a company both time and money. EWI has expertise to help members evaluate the merits of CR for specific applications, specify CR equipment, assess code requirements, and develop a CR implementation program.

For more information, please contact Kevin Clear, kclear@ewi.org, 614.688.5243.

Reference
NDT Technician – Vol. 5, Issue 2, April 2006.