Digital computerized radiography (DCR) combines conventional radiography with digitized computer imaging. Three basic elements of radiography include a radiation source, the object being evaluated, and a sensing material (Fig. 1). The object in the diagram is a plate of uniform thickness that contains an internal flaw with absorption characteristics different from those of the surrounding material. Radiation from the source is absorbed by the plate as the radiation passes through it; the flaw and surrounding material absorb different amounts of radiation. Thus, the intensity of radiation that impinges on the sensing material in the area beneath the flaw is different from the amount that impinges on adjacent areas. This produces an image, or shadow, of the flaw on the sensing material.
In conventional radiography, the sensing material is film. In digital radiography, film is replaced by phosphor imaging plates, which reduce exposure times by -75% compared with film, and significantly reduce the radioactive source strength needed. Phosphor plates allow images to be captured by the radioactivity of iridium-192, selenium-75, cobalt, and X-ray generators. For Ir-192,3 to 9 curies of radiation is sufficient. This allows for smaller barriers, virtually eliminating the need to evacuate radiography inspection areas.
Exposures can be taken safely within 10 to 15 feet of personnel without interruption. In addition, the phosphor imaging plates can be re-used up to 30,000 times, eliminating film costs. Digital radiography provides immediate information without the delay and expense of film development, and digital image enhancement improves resolution and dynamic range.
Current technology
After exposure, the imaging plates are sent through an electronic scanner that places the image on the computer screen in less than 90 seconds. The image can then be reduced, magnified, rotated, or inverted between a positive and negative image for optimum viewing. The software package has the ability to adjust the contrast and density thresholds, making it possible to evaluate a multitude of thicknesses from one exposure. This feature alone has virtually eliminated the need for re-shots and multiple film speeds. Densities of each area of concern can be controlled for the best possible viewing, enhancing definition and contrast to interpret any and all discontinuities with precision and accuracy.
A built-in analyzing feature allows taking measurements and evaluating wall thicknesses with the click of a mouse. To analyze thickness of an object such as a pipe via ultrasonic transmission can take hours and still overlook areas of thinning or pitting. On the other hand, with DCR the picture is taken, scanned, and measured in less than 15 minutes, with great clarity and resolution.
The PC software allows for insertion of annotations and an actual digital picture for better overall documentation. Results can be printed, e-mailed, or inserted into documents for others to examine.
Electronic images can be archived for efficient storage and retrieval. Digital technology enhances the quality and quantity of records, which allows for rapid and accurate access and quick transmission. The resolution of the image exceeds all other forms of documentation and allows mass quantities to be stored in minimum space, with high resolution constant. Images are archived on CD's or a hard drive, retrievable and transmittable effortlessly to others. This allows multiple inspectors/engineers the efficiency of real-time evaluation of the same radiographic image no matter where they are. A new technology known as a digitizer converts X-ray film already in storage to a computerized image. This reduces storage costs and still maintains the quality of the image for future evaluation.
Because of digital computed radiography, inspections that were once impractical or unachievable can now be successfully completed. For example, DCR enables inspection of in-process piping under insulation in close proximity with personnel, large bore piping, valves (Fig. 2), and castings. The detection of corrosion under insulation, while in process, or the internal inspection of smaller pressure vessels via digital radiography has proven to provide significant cost savings.
Future technology
Engineers at Quality Inspection Services predict that, within the next few years, robotics combined with an enhanced phosphoric imaging plate will change the world of digital computerized radiography. A robotic arm will be able to transmit a radiograph that surrounds the object under inspection, and then the software will create a realtime, 3-D radiographic image on a computer. This capability will allow NDE inspectors to see the object in a virtual reality X-ray image, enabling the inspector to view the entire object with one shot.
ASME, AWS, and API have all accepted digital radiography for code acceptance. The military is making strides in doing the same, and is currently evaluating it in other areas such as corrosion monitoring, composites, and new fabrications. The advantages of digital technology combined with radiography, along with the safety aspects it brings, will expand its applications throughout industry.
Brian Caccamise; Lori Jacobs
