Digital Radiography in Industry: Digital Detector Arrays in Radiographic Testing by Brad Kraai

Vol. 15,13, No. 31 FOCUS Digital Radiography in Industry: Digital Detector Arrays in Radiographic Testing by Brad Kraai Introduction This article explores DDA systems: application, capability, process controls, Digital detector array (DDA) systems image attributes and evaluation, and within industrial radiography are becoming personnel qualifications—to hopefully very common in high performance, critical promote an increased interest and margin of safety, and endurance test enlightenment for potential users. article inspections. Investment castings, thermal joints, and a wide variety of other Definitions test articles are being routinely inspected, Per ASTM E 2736, Standard Guide for with improved probability of detection Digital Detector Array Radiology, a digital (POD), and much faster throughput detector array is defined as: “an electronic than conventional film systems—with device that converts ionizing or penetrating high levels of user satisfaction. Potential radiation into a discrete array of analog users within the industrial X-ray imaging signals which are subsequently digitized community continue to recognize these and transferred to a computer for display values and exploit this new technology as a digital image corresponding to the for potential applications, but to do so, radiation energy pattern imparted upon the a basic understanding of DDA systems input region of the device. The conversion and application is a necessary foundation. of the ionizing or penetrating radiation Smooth implementation of this novel into an electronic signal may transpire by technology can be challenging, and first converting the ionizing or penetrating consultation should be considered from an radiation into visible light through the use outside, unbiased, reputable organization. of a scintillating material” (ASTM, 2010a). There are several integrators, or vendors, While DDAs can be used for real-time or that are widely recognized as providers of radioscopic techniques, most applications DDA systems development, installation, for critical test articles employ static and service—each having its own merits imaging and evaluation. Figure 1 provides and specialties. a simplified diagram of a DDA. From NDT Technician Newsletter, Vol. 15, No. 3, pp: 1-6. Copyright © 2016 The American Society for Nondestructive Testing, Inc. TNT · July 2016 · 1 FOCUS | Digital Radiography in Industry the responsibility of the potential user, not the integrator or vendor, to ensure that DDA system performance metrics are met for a given target application. X-ray signal Obviously, DDA systems are a considerable capital expenditure, so once the system is designed, built, and installed, it must Test article meet the requirements—resolve the defects reliably—as intended. This cannot be overemphasized. Film images are commonly used as the baseline or referee for a correlation study with the DDA imaging techniques, and will often be Electronics shielding (reduces radiation dose to electronics) required for process approval. Gadolinium oxysulfide or cesium iodide (converts X-ray signal to light signal) High levels of automation are possible Digital data to Amorphous silicon TFT (converts light signal to electronic signals) frame buffer and often used with DDAs. Imaging, from Electronics (captures electronic signals, organizes and converts to digital data) one acquisition to the next, can be mere Glass substrate (stable TFT and electronics structure base) seconds, depending on the type of DDA and acquisition settings, technique, and Figure 1. Simplified diagram of a digital detector array (not to scale). TFT = thin film transistor. level of automation. Fully articulating and programmable robotics within custom- built radiation enclosures are becoming The most popular types of DDAs in use raw or full fidelity image is saved unaltered quite common in turbine blade inspection; contain an initial and indirect conversion for critical applications. Raw image file size a single blade can be imaged with several layer (or scintillator)—typically either primarily depends on the resolution, or views in less than a minute. Other gadolinium oxysulfide terbium doped, pixel pitch, and input region (or array size) automation tactics may include external or cesium iodide thallium doped—that of the DDA, as well as its bit depth. ingress of test articles into the enclosure by converts an X-ray signal to visible light As one can see, DDAs are quite conveyors, or manual loading of numerous or luminescence. This luminescence then sophisticated electronic conversion parts in fixtures or platens. DDA systems enters the amorphous silicon (αSi) thin devices. Yet, they can provide simplicity with turntables for the test article and a film transistor diode array (discrete pixel to the radiographic imaging process, as C-arm—with the X-ray tube on one end locations), whereby the light is converted to consistency and reliability are often the and DDA on the other—are commonly an electronic voltage (bias change) at each result of a properly engineered, tuned, and used for larger test articles. Other pixel, which is subsequently “read out” of applied DDA system. sophisticated systems have incorporated the array in channels and groups during Applications two robots: one for the X-ray tube and the X-ray exposure. This electronic information other for the DDA, while the test article is amplified and then digitized, typically For the purpose of application validation, a remains stationary. System integrators can through several analog-to-digital converters, potential user must evaluate “representative be creative, and depending on the target synchronized, and sent to the frame buffer quality indicators,” or, more simply application, will often readily collaborate within the image processor and system stated, potential test articles with known with the potential user to design and software. conditions or defects (of a minimum size incorporate optimized article handling and Gray values for each pixel’s digital and all types likely to be encountered) for imaging for the intended application. (binary) value are then assigned by a lookup any DDA system under consideration. It must be realized that DDAs are in table, and a corresponding pixel matrix Quite often, DDA systems are designed fact a consumable, and will eventually that represents X-ray attenuation from the and built around a target application or require replacement due to performance initial image acquisition is generated and family: the potential test articles to be issues or degradation. They will and do considered as the raw or full fidelity image inspected with the DDA system will all fail (unpredictably, in most cases) over file. This image file also contains meta-data, be related—similar in material type and time. The cumulative effects of radiation or image tags of process information as material thickness, subject contrast range, exposure and thermal variations within configured by the system integrator. The and inspection standard criteria. It is the device produce effects that may range 2 · Vol. 15, No. 3 from image quality degradation all the resolutions (discrete pixel dimensions recommended 3 × 3 pixel matrix in the way to failure. ere are no guarantees or “pixel pitch”) currently available, the IQI hole decreases in turn. Higher M of a DDA’s usable duration. Inadvertent contrast sensitivity for these devices is factors increase pixel density for a speci c or unintentional exposure to the DDA remarkable—provided noise is controlled. dimension within the image, thereby should be avoided. Recent developments in Due to inherent DDA resolution, promoting higher POD. Higher M factors hardened electronics have increased DDA geometric magni cation (M) techniques applied within techniques will decrease the resistance to exposure e ects. Shielding are often employed, which amplify image eld of view, resulting in decreased of the electronics around the periphery discontinuity size and enhance POD throughput for larger test articles as of the DDA must be provided by the via a higher number of pixels under the compared to the DDA input region. integrator, and should be analyzed by the potential discontinuity or feature within Another primary consideration for any DDA manufacturer for warranty viability. the test article. It is a relatively simple geometrically magni ed technique is image Moreover, users of DDA systems must matter to calculate for a speci c number unsharpness (Uimg), wherein a reduced contemplate detector failure or sub-par of pixels under a known dimension at e ective focal spot size (EFSS) is often imaging contingency and warranty when geometric magni cation, and quite often, necessary at higher magni cations. Uimg negotiating procurement. the calculation is applied to determine calculations take into account the As mentioned previously, the DDA pixel density within an essential image geometric unsharpness (Ug) of the and its supportive software are very quality indicator (IQI) hole dimension. technique, (where [M – 1] × EFSS = Ug), complex. is DDA system complexity e minimum recommendation is three the geometric magni cation factor can be very intimidating, so a word of pixels, so by using the basic spatial (M = source-to-detector distance / source- advice: complexity of the entire DDA resolution (SRb) or e ective pixel size of to-object distance), and the SRb of the system should be held to a minimum— the DDA, and the speci ed IQI hole size DDA. Uimg can also be evaluated by con gurations should be limited to t the

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