ON THE WAY TO DIGITAL RADIOGRAPHY

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1 The 14 th International Conference of the Slovenian Society for Non-Destructive Testing»Application of Contemporary Non-Destructive Testing in Engineering«September 4-6, 2017, Bernardin, Slovenia More info about this article: ON THE WAY TO DIGITAL RADIOGRAPHY Brigitta Kelenföldi 1, József Harnisch 2 1,2 Grimas Ltd. Puli street 2-4., 1214, Budapest, Hungary Address (Street, Post No., City, State), Authors 1 brigitta.kelenfoldi@grimas.hu, 2 josef.harnisch@grimas.hu ABSTRACT The radiography department occupies a large place at most production and NDT service companies. Exposure bunkers, inspection dark room, a stock to store the films are needed. NDT people use a lot of chemicals to make the images visible. If they cannot reach the optimal dose, they need to repeat the shooting. Can you imagine a world, where a quarter of the space is enough for your radiography service? With the development of digitalization we can choose different solutions from analog technology to automatic inspections. What are these steps? The first step on the way to digital radiography is scanning of analog films, where archiving, saving and image manipulation are possible. Second step is the imaging plate technology with a digital image reader and workstation. This is a really productive and professional method to inspect for example welded joints in the field and get results immediately. Next step is a mobile flat panel technology, which shows a live picture about the tested object. A Wi-Fi unit is needed to forward the images to your workstation, and see the results in the imaging software. Finally the last method is X-ray cabins with CT technology in a semi automatic or full automatic version. Key words: digitalize, radiography, CR scanner, Computer Tomography, archive, report 1. Introduction Conventional radiography to radiograph internal macro details of products is a time consuming testing method because of the image developing processes. People who are working in non-destructive testing spends long hours to develop their radiographic images with chemicals in the dark room, then analyse if the product is adequate or not with a special strong light lamp. But thanks to the digitalized and innovative technology of radiography, the endless workdays are over! There are many steps on the way from analogue to full automatic measurements. Digitalization eliminates the disadvantages of conventional radiography. It makes the processes simpler, more accurate and faster, in addition it ensures a better image quality. 2. Analouge Film scanning The first step in digital radiography is analogue film scanning. The developing process is the same like in the analogue method. Optimization of parameters is primary to achieve the best possible image quality. A scanner digitalizes the film after the processing of the analogue film. 73

2 The scanner lights the film with a LED source, then a CCD detector transforms the light signals to digital signals. The scanner is forwarding the image to the computer, on which we have preinstalled the software for processing the image. You can see the theory of analogue film scanning on the Fig.1 [1]. Fig. 1: Theory of analogue film scanning [1] The software provides a possibility to reach a better image quality with the adjustment of contrast, brightness, and the optimization of grey values. With this method it is possible to scan more analogue film at the same time. These images can be stitched to one file. Further advantages of the digital technology against the analogue, that we can write comments near to the critical places, which enables a faster decision-making. With pre-loaded datas after the image manipulation the software can do reports in one second. We can send these reports digitally to the customer. We can store our radiographic images in an external server, CD/DVD or a Winchester, thus ensuring the retrievalability. 3. CR technology (Laser scanner and imaging plates) Images are made on an imaging plate instead of an X-ray film in the case of computed radiography. Imaging plates contain a light sensitive phosphor layer. A special laser scanner creates the image and transfares them to the connected computer or laptop. The widely used softwares are easy-to-use. Everybody can learn the image evaluation, digitalization and archiving quickly. Image enhancement functions are also available, like contrast and brightness optimization, filter application and zoom to see the small details. The following picture (Fig. 2) shows well the differences in resolution between conventional and digital imaging. As the scanning capabilities of scanners developing, the quality of imaging plates are improved, and different application areas are formed. Standard imaging plates are used for high energy X-ray sources. Medium imaging plates with µm resolution and good contrast sensitivity are used for medium energy X-ray sources. The newest development of imaging plates with µm resolution are used for small size products, and low energy X-ray sources. This is an extension of conventional testing, so this type of testing becomes useful for example in micro-electronics, where visibility of small details is a requirement. 74

3 Fig. 2: Comparison of analogue and CR image A lower quality or dark image is not a problem in Computed Radiography, thanks to the digitalization, because grey value, contrast and brightness are easily adjustable in the software. If we optimalize these properties, the result is a sharp image with optimal grey value. The use of the filters provides further opportunities, we can make those defects visible that we didn t see earlier. Defects could be measured, and we can determine the thickness of the products wall (for example: tube) with some click. Documentation is just a click with a predefined, customized report format. The other advantage of digitizing is that the original images can be archived without any data loss, which could be saved on an external server or cloud system, thus ensuring the preservation for a long time and verifiability. Some CR equipment manufacturers developed viewer software packages, too. So it is possible to change files between service companies and customers. This makes their work cost-efficient and reliable. 4. Flat Panel technology Flat Panel or in another name, mobile detector is comprised an electronic units. These units convert the intensity differences which arrives from the radiation to digital image. The Fig. 3 picture shows the theory of flat panel technology. It is a suitable technology to inspect the wall thickness and corrosion process of the continuous operation tubes. There are two possible ways with this method. One of them is a wired solution, where the Flat Panel is connected to the workstation with a cable, and the other one is wireless, where the detector sends the images to the computer through a Wi-Fi unit [4]. Advantages of the Flat Panel technology are reduced exposure, fastness and high resolution. DDA panels are really sensitive. This capability combined with frame averaging can reduce the radiation time, but still receive the sensitivity which is required. Consumables are not needed, so the time when the operator exposures and prepare the next inspection can be saved. DDA panels make it possible to start the measurement and calibration quickly, and the image is immediately visible. DDA panels can reveal the smallest details, cracks and defects. The most common softwares can be a help in the following tasks: measurements of defects, repairs and analysis [4]. 75

4 5. Computer Tomography Fig. 3: Imaging technology with Flat Panel detector [4] A CT scan produces a two-dimensional density map of a cross-sectional image slice of specific areas, allowing the user to see inside the object without cutting. The CT image is taken from many different viewing angles which are reconstructed using a computer. The method of the CT process can be seen in the Fig. 4 picture [2]. Fig. 4: Computed Tomography process [2] The building blocks of a CT system are X-ray source, detector, rotational product stage and workstation with reconstruction software. There are different types of detectors in CT technology. One of them is a line detector, which can make 2D line projections with a lot of slices on 360. It takes a lot of time, while you inspect eg. a cast part with a line detector, but you will receive an image with the best resolution ever. The second one is the flat panel technology, what I mentioned in the previous chapter [2]. A computer reconstructs the slices to a 3D model with filtered back projection after acquiring the 2D X-ray images. When creating a CT scan, there is a transformation between pixels and voxels. Pixel is a picture element in 2-dimensions whereas a voxel is a volume element in 3- dimensions. Each pixel represents the new volume block and the volume is displayed by accumulated layers. Once we have created a 3D model, determining the surface allows us to make measurements and create CT-based geometries and features. Once the surface has been 76

5 determined, the entity of points which identify the inner and the outer surfaces is called a point cloud. This dataset can be used by measurement and reverse engineering software tools [2]. Fig. 5: Porosity analysis with CT inspection Modern CT systems work with a 5 dimensional NC movement to place the product to the best position. The system can be programmed with coordinate datas to find always the same position. It is an useful option when you need to inspect the same products, and you are aware of the location of the defects. Inspection can be fast and automated in this case. It could be a good solution for foundries, where all molded parts are needed to inspect, and they do not have so much time. 6. Image quality Fig. 6: CT inspection of electronic elements Many factors influances the final perceived image quality of a computed radiograph. The three most important parameters are signal to noise ration (SNR), contrast to noise ratio (CNR) and resolution. Many of these factors compete with each other. An improvement in one parameter can degrade another, resulting in degraded performance. Some of these factors are a 77

6 function of the system design, and others can be controlled by the radiographer. In digital radiography the exposure parameters have less effect to the quality than in the analogue technology [3]. Fig. 7: Effect of Signal to Noise ratio on the image sharpness [3] On Fig. 7 you can see the influence of image noise on detailed visibility. Left diagram shows an ideal case, where every edge is sharp and visible. In reality every digitalization process has a signal noise, which can influence the detectable defect size. You can see in the right diagram, that a big noise level hided the notch. For similar exposures, unsharp systems achieve a higher signal to noise ration than sharp ones, but have lower performance for detection of fine flaws than sharp systems. Therefore, SNR is normalized by the basic spatial resolution [3]. The second most important property is contrast. The higher the contrast the possible detectable defect size becomes smaller. You can see this effect on the Fig. 8. An increasing level of noise relative to contrast diminishes the ability to distinguish circles in the image [3]. Fig. 8: Effect of contrast on detectable defect size [3] 78

7 The EN 462-5/ ASTM E2002 duplex wire IQI is intended to provide a means for measuring spatial resolution as independently as practicable from the system contrast sensitivity limitations [3]. Fig. 9: Duplex IQI [3] A duplex wire IQI consists of 13 pairs of wires with the space equal to the wire diameter. Elements 1 through 3 are of tungsten and elements 4 through 13 are of platinum. The 13 elements are mounted in a rigid plastic holder. Duplex wire IQI can be used for all type of material. It helps to determine the resolution of the radiographic image digitally. According to standards the first unresolved wire pair shall be taken for determination of the un-sharpness value. This is the first wire that is projected with a dip between the wires of less than 20% [3]. These three factors determine the class of our digital images in all technology. If we can reach a good image quality with an optimal digital image manipulation and filters, we receive much more details as using analogue films [3]. 7. Conclusions Nowadays the technology performance duplicates yearly and it has an effect on industrial material testing too. I showed you 4 possible ways to evaluate, save, archive and report digitally. These steps make easier the work of the non-destructive testers and provide a reliable impression to the customers. The conclusion is that all of these techniques can be a suitable solution on their market. The most important thing is the circumstances of the tests. If it is determined, the solution will be found. 8. References [1] Lariviére GmbH Presentation about Pacsess NDT [2] Yxlon International Presentation about Computed Tomography Introduction to the Basics [3] Carestream NDT Presentation about Computed Radiography Image Quality [4] 79