Radiography – Part 2: Digital methods

This series of articles has been put together by the Practitioner Committee to describe the principles of different NDT methods and techniques as an introduction to practitioners...

Part 1 in the radiography series of articles was published in the December 2021 issue of NDT News and focused on the physical principals concerning the generation, propagation and dispersion of primary radiation from both X-ray and radioisotope sources and its passage through metals.

Part 2 concentrates on the detection of radiation and formation of the latent image by methods other than photographic film.

The technical capabilities and limitations of these techniques are presented, as are the benefits claimed and the cost considerations, particularly the initial outlay and ongoing IT costs.

Computed radiography Computed radiography (CR) is a method used to generate digital images in X-ray radiography that uses photo-stimulated luminescence imaging plates to store the X-ray exposure in a latent image. The first systems were developed in the 1980s for the medical profession, before the systems migrated to the industrial world.

There are three basic steps involved in computed radiography:

Image acquisition, employing a source of radiation, X or gamma, but employing an imaging plate (IP) instead of film;
Processing of the imaging plate using a CR reader; and
Performing a review of the image on a computer.

The process works by the same basic method as conventional radiography: a component is placed on a CR panel with an imaging plate inside and is exposed to radiation. This creates a latent image on the imaging plate by the process of photo-stimulated luminescence.

The CR panel is then placed in a digitiser, where the latent image is scanned and a digital image is produced. A laser scans the panel, which then releases the stored energy into visible light; the light is then measured and converted into a digital image. After the image is scanned, the CR panel is then exposed to a strong light, which clears the latent image by releasing any additional stored energy.

The digital image can be viewed on a PC, where special software is used to manipulate the image to achieve the best results.

Applications for computed radiography

Computed radiography is considered to be an excellent option for corrosion mapping of small bore pipes and has some capability for assessing corrosion under insulation (CUI).
In-service inspections are possible with computed radiography.
Se-75 performance meets the existing relaxed requirements for this radioisotope.

Advantages of computed radiography

There is no requirement for film dark rooms, improving workflow.
There is no need for film processors and chemicals, reducing the environmental impact.
Acquired images can be manipulated and enhanced.
Shorter exposure times are claimed, increasing productivity.
The imaging plate can be reused time and time again (600-1000 times as a guide).
A high dynamic range means fewer reshoots are required, increasing productivity.
The images can be easily stored and shared, preserving information and offering the potential for remote assessment.
Image quality requirements for radiography of castings is based on contrast sensitivity verified by an image quality indicator (IQI).
A signal-to-noise ratio (SNR) of up to 500 is achievable with a large number of frames or integrations, assisting radiographic interpretation.

Disadvantages of computed radiography

Utilising computed radiography incurs expensive initial costs, with high equipment costs (viewing stations) and the requirement for a sizeable server capacity for the storage of large archives.
The imaging plate is prone to damage if it is bent (imaging plates are inflexible and have limited curvature capability).
When radiographing the same component repeatedly, ghosting can be a problem.
A lower energy of <120 kV is used for longevity of the imaging plates.
The brightness, sensitivity and spatial resolution are limited.
This is a two-stage process (an indirect method) in which the latent image degrades over hours, with a 25% loss after 8 hours.
Background noise increases due to natural radiation, scatter or X-ray fluorescence (screens) and imaging plates should be erased before use if used infrequently.
Background can be reduced by using harder radiation closer to the 120 kV limit and tube filtering to remove lower-energy X-rays.
A lower photo multiplier sensitivity and longer exposure time improves image quality.
The lack of precision in the manufacturing process limits reproducibility from one plate to another.
Extra costs are incurred against imaging checks.
During weld inspection, difficulty can be experienced in achieving Class B due to low contrast sensitivity.

Real-time radiography Real-time radiography is a term used to describe a form of radiography that allows images to be captured and viewed in real time and is commonly used in the everyday world, for example in airports and in industries where mass production is employed.

Because image acquisition is almost instantaneous, X-ray images can be viewed as the part is moved and rotated on a manipulator.

Indirect
The process works by using a source of penetrating radiation and, instead of using film or an imaging plate, an image intensifier is employed that converts the radiation into light by using materials that fluoresce (scintillate) when struck by radiation. The more radiation that reaches the intensifier, the more light is given off.

Then, a special charge-coupled device (CCD) camera, which utilises an image sensor to register visible light as an electronic signal, captures an image that can then be viewed on a PC.

Direct
This method does not use a scintillating material; instead, it converts the absorbed X-rays directly into an electrical charge.

This technique uses a special flat-panel detector, which employs a semiconductor material that produces electron-hole pairs in proportion to the incident X-ray intensity. This charge is converted into a pixel, which forms the digital image.

Computed tomography Computed tomography uses a real-time inspection system that continuously takes up to 3600 two-dimensional images (slices) of sections of a component; these slices are called tomographic images.

Then, special software is used to convert these two-dimensional images into three-dimensional images, which can be manipulated to study the component’s structure.

Applications
Computed tomography was originally developed for medical imaging, but now has a wide range of industrial applications, including the location and measurement of volumetric flaws in three dimensions, failure analysis, metrology, product evaluation, packaging integrity assessment, assembly analysis and reverse engineering.

Advantages

Computed tomography has good flaw sensitivity and related positional information, assuming a favourable orientation of any planar defects or features.
It may validate or supplement more standard radiographic techniques and provide more precise information on indications via magnification and digital processing.
Its large dynamic range provides one-shot capability for sections of varying thickness.

The next article in the series will focus on alternating current field measurement (ACFM) and will appear in the June 2022 issue.

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