Digital radiography in industry
Industrial radiography has been used for non-destructive testing for decades. While its conventional, silver-based form enables us to obtain a quality image and preserve radiograms (if stored under the right conditions), advances in digital radiography are tending to shake up habits and transform practices.
Although the aeronautics and foundry industries took the plunge ten years ago, the technology was long shunned by some customers, who were concerned about system performance and correlation with silver-based films. The arrival of digital radiography certification for the CIFM proves that the progress achieved over the last few years has been convincing, and positions it as a genuine alternative to silver.
The principle of radiography is to observe differences in absorption: a film is imprinted with ionizing radiation (X or γ) and, after development, becomes a radiogram that can be interpreted by the radiologist. An area presenting a discontinuity, and therefore a lack of material, will let more radiation through. The anomaly then appears as a dark spot on the film (the more radiation the film receives, the darker it is after development). In digital radiography, while the radiation source remains the same (X-rays or γ source), the receiver and acquisition chain change. The image obtained is no longer an amalgam of more or less opaque crystals, but a matrix of pixels, each with a gray-scale value depending on the dose received by it.
Although silver radiography has long since proved its worth, there are many advantages to going digital. First of all, there are no development chemicals, and far fewer consumables are used, which reduces costs, logistics and health risks for operators. Image "development" is shorter: a few seconds with a flat panel, less than two minutes with a photo-stimulable screen, compared with 8 minutes for silver film with a developing machine. Dematerialization also makes it possible to share images quickly, for example, for remote verification of suspicions. Finally, digital images enable the use of tools to aid interpretation: zoom, contrast and brightness modification, digital filters...
Two digital radiography techniques are available on the market today: CR radiography (Computed Radiography), requiring the use of rewritable plates and a reader-digitizer, and DR radiography (Digital Radiography), where images are acquired via a flat panel. While the images obtained and the way in which they are interpreted are virtually identical, each of these techniques has its own characteristics and advantages, and is more suited to one need than another. The choice is made by taking into account control requirements (image quality, accessibility, production rate...) and equipment characteristics (resolution, dimensions, implementation...).
CR Radiography: ideal for the transition from film to digital
Silver film is replaced here by a photo-stimulable screen, which is used in much the same way as conventional radiography. The screen is exposed to radiation in the same way, and then scanned by a scanner. The image plate is flexible, almost as flexible as silver film, and made up of elements of varying size, again like the silver halide grains of conventional film. The finer these elements are, the better the basic resolution of the photo-stimulable screen. The latent image principle is also retained, until the plate is scanned. Although the technology and exposure times differ, the same procedure is used as for silver film, at least as far as exposure is concerned.
Once the image plate has been exposed, it needs to be digitized. It is passed through a digitizer reader, on which the digital gain (which amplifies the signal) and the laser scan pitch (which determines the image pixel size) can generally be set. This laser passes through the plate and communicates data to the system, corresponding to the grayscale value of the image pixel, which is a function of the dose received during exposure. The photo-stimulable screen is then generally erased by the scanner, using intense light. The image and associated metadata are then stored.
While photo-stimulable screens can adapt to the shape of the parts to be inspected and be exposed up to several hundred times, handling can reduce their lifespan. Likewise, the more the image plate is used, the higher the dose required to obtain an image. As they are sensitive to light, they should not be exposed in a very bright environment, to avoid losing information (as mentioned above, bright light is used to erase them).
DR Radiography: from inspection to automation
The detector is a flat panel (flat panel), sensitive to ionizing radiation and connected directly to a PC. Although image acquisition speed depends on the material of the photosensitive layer, it is very fast (a few seconds). This type of receiver is available in site or cabin versions and, as with photo-stimulable screens, several resolutions are available. Pixel pitch is the distance between the centers of two neighboring pixels. The lower this value, the better the image's spatial resolution.
In production, flat panels are used for near-real-time inspection, as with X-ray fluoroscopy using an image intensifier. The number of images per second that the receiver can acquire depends on the scintillator material used. A high acquisition frequency is required for on-line inspection, to guarantee an acceptable cycle time. This control can also be automated (axis control, image acquisition and recording), and sanctioning can be aided or even decided by software (development of algorithms, use of artificial intelligence).
The native resolution of flat panels does not allow for very fine spatial resolution values. In most cases, it is necessary to use geometric magnification: the object to be controlled is moved further away from the receiver, enlarging the image obtained. For example, if the part is placed halfway between the transmitter and receiver, the image obtained will be twice the size of the part itself. In this way, indications of discontinuities or anomalies will also be magnified. This technique increases the value of the geometric blur, which can be counter-productive; in such cases, tubes with small foci (generally 0.4 mm) or micro-foci are preferred.
Flat panels are also used for tomography, which X-ray slices of an object to visualize its volume in 3D. This method makes it possible to pinpoint anomalies in a part, and to compare this result with CAD simulations to determine whether these defects could be detrimental in service. In the foundry, tomography can be used when fine-tuning a part, to relocate any imperfections to areas where they will have no impact on the part's service life.
On site, the use of a flat panel allows images to be obtained quickly and the inspected installations to be released as quickly as possible. The receiver is battery-powered and connected to a laptop PC. Images are checked immediately for interpretation or dispatch. While flat panels are usually rigid, new incurvable models are now on the market. These can be used to inspect circular, pipeline-type objects, and follow the shape of the part to be inspected.
Software and interpretation
Each manufacturer offers its own software, which is able to communicate with its hardware. For CR radiography, the software accompanying scanners is generally that of the manufacturer. For DR systems, this may vary: cabin manufacturers, integrating equipment from another brand, offer their own software capable of controlling the mechanics, automation, transmitter and receiver. Each of these software packages has its own interface, and switching from one to the other may upset habits and be a little confusing for operators, but the functionalities remain more or less the same.
Interpretation is similar to that of film: image quality is checked (IQI read on the image, spatial resolution, grayscale), then the print is examined for indications. Working with digital images allows you to zoom in on the image, modify its brightness and contrast, and also apply filters. These can have a number of advantages, and it's important to use the right filter so as not to degrade image quality or mask defects. Some software programs allow you to create your own filters and save them, thus freezing the interpretation conditions. Image files are saved in DICONDE format, containing information about the shooting conditions and controlled objects. This is a raw format that cannot be altered.
Going digital
Digital radiography is the little sister of silver radiography. While in some applications it cannot replace or even equal its predecessor, for reasons of image quality and implementation, in others it can revolutionize the way things are done and change working habits. In the case of large production runs or highly complex parts, DR radiography can be automated, greatly reducing inspection times. The fact that images can be shared immediately means that decisions can be made and doubts dispelled without the need for transport; and if the shot has to be repeated, the inspector is always on site with his equipment.
As with conventional radiography, images need to be kept for a certain length of time, depending mainly on the sector of activity. As digital images can weigh a certain amount depending on image quality (the more pixels, the more data), a robust storage system is required, and double-backup is strongly recommended, if not mandatory. This comes at a price, which must be taken into account when considering a switch to digital.
Standardization and certification
In terms of standardization, there are numerous standards and specifications for digital radiography (ISO, EN, ASTM...) that define the operating procedures and image quality to be achieved, for both DR and CR radiography.
Specific certification for digital radiography already exists in the aeronautics and foundry sectors; a new one is on the way for CIFM (industry, manufacturing and maintenance). Agents in charge of controls will have to have followed a training course as defined by COFREND, and pass a specific examination.
COFREND is organizing this year's event, from 1er July 3, the 10th international symposium on digital radiography, DIR 2025. This event will provide an opportunity for all those involved in digital radiography to meet up, discuss developments in the field and discover the latest innovations. A not-to-be-missed event, the coming months will truly mark the start of digital radiography throughout the French industry.
Actemium NDT Products & Systems is a major player in the field of Non-Destructive Testing in France. For over 50 years, we have been supporting industry players and working with the best partners to offer both standard and customized solutions. With the largest portfolio and in-depth experience in digital radiography, we are the ideal partner to support you in your transition from film. We'll be present at DIR 2025, which we're also sponsoring. We look forward to seeing you there!
