CR and DR

Computed Radiography and Digital Radiography

CR – Computed Radiography – is the term given to the application of radiography where the radiographic film is replaced by a re-usable imaging plate.

DR – Digital Radiography – is a term given to the application of radiography where the radiographic film is replaced by a sensor that provides an immediate digital image of the radiograph. However, DR is also sometimes used as a more generic term which relates to the use of digital technology in radiography inspection. In such instances, it can refer to both the use of digital sensors and the scanning, ie digitisation, of radiographic film.

The CR imaging plate contains photo-stimulable storage phosphors. When radiation impinges on the plate, the phosphor crystals become trapped in a semi-stable higher energy state and the image is retained. The CR cassette is then removed from the inspection site and the plate placed in a CR reader. The CR reader scans a laser over the image, releasing the trapped electrons in the phosphor crystals and emitting visible light. This light is captured, digitised and stored as the radiographic image. The plate is then wiped clean for re-use.

CR has the following advantages:

The phosphors on the plate have an extremely wide dynamic range, giving a high tolerance for varying exposure conditions and the exposure dose. This reduces the need for re-takes.
The increased sensitivity of the plate allows for shorter exposure times or the use of a weaker radiation source, providing dose reduction over that required for film radiography.
No chemical developer is needed.

In DR, a sensor digitises the radiation passing through the component under inspection so that the radiographic image can be displayed immediately on a PC screen. There are three main digital sensors: CCD, CMOS and Amorphous Silicon. The DR sensors produce images in seconds, rather than the minutes required for the CR images, and are therefore particularly suited for real-time radiography. The digital sensors can also have a better resolution than CR plates and most film applications. However, the portability of CR cassettes makes them particularly suited for application in difficult access conditions.

Both CR and DR allow the benefits of digital technology to be applied to radiographic inspections: digital measurement can improve accuracy and reproducibility of results; the image can be digitally enhanced for better interpretation; and images can be easily archived and intergrated into plant databases.

What the hec?! articles are not intended to be the definitive account on the topic or acronym in question. Readers’ comments and contributions are welcomed. Email: ndtnews@bindt.org