Computed radiography (CR) is the digital replacement of conventional X-ray film radiography and offers enormous advantages for inspection tasks – the use of consumables is virtually eliminated and the time to produce an image is drastically shortened. The CR imaging plate (IP) is exposed to radiation just like one would do with conventional film radiography. The IP can be inserted into a flexible film cassette or in a flat rigid cassette.
- Imaging plates are reusable
- No darkroom or chemicals needed
- Reduction in exposure and processing time
- Software-based evaluation and reporting
- Simple digital data exchange and archiving
Instead of film, an imaging plate is exposed to X-ray or gamma radiation. The imaging plate is digitized by the scanner and then erased for immediate reuse. The digital image is then displayed on a computer monitor for evaluation with specialized software.
How exactly does CR technology work?
In computed radiography, when imaging plates are exposed to X-rays or gamma rays, the energy of the incoming radiation is stored in a special phosphor layer. A specialized machine known as a scanner is then used to read out the latent image from the plate by stimulating it with a very finely focused laser beam. When stimulated, the plate emits blue light with intensity proportional to the amount of radiation received during the exposure. The light is then detected by a highly sensitive analog device known as a photomultiplier (PMT) and converted to a digital signal using an analog-to-digital converter (ADC). The generated digital X-ray image can then be viewed on a computer monitor and evaluated. After an imaging plate is read, it is erased by a high-intensity light source and can immediately be re-used – imaging plates can typically be used up to 1000 times or more depending on the application.
NEW POSSIBILITIES WITH DIGITAL IMAGES
- Digital magnification allows better detail viewing
- Image optimization with specialized filters for improved defect detection
- Histogram adjustment for optimal viewing
- Annotation and drawing tools
- Special analysis tools: e.g. for automated wall thickness measurement
- Simple report generation
- Standardized data exchange via the DICONDE format
- Global data exchange via a local network and/or the Internet
What should be considered when deciding for CR?
Basically, CR technology can be considered as the digital replacement of conventional X-ray film. Imaging plates are used with the same radiographic inspections methods and techniques as film and are also available in different system classes (image quality) which have different required exposure times. However, with CR technology it is not just the imaging plate type that affects image quality – the scan settings used by the scanner are also crucial. In particular, the resolution capability of the scanner (basic spatial resolution or SRb) plays an important part in determining image quality.
With this in mind, DÜRR NDT developed the HD-CR 35 NDT – the world’s first scanner that achieves a basic spatial resolution of 30 μm with a laser focus of merely 12.5 μm (when used with high-resolution imaging plates). This performance was confirmed via certification by the Federal Institute for Materials Research and Testing (BAM). With its variable laser focus size, the HD-CR 35 NDT can also be easily adapted for inspection applications with less demanding basic spatial resolution requirements. If only low resolution is required, the CR 35 NDT with a fixed laser focus of 50 μm may also be perfectly suitable.