by Dirk Dusharme

In 1895, Wilhelm Roentgen’s discovery of X-rays proved to be one of the key findings of the 19th century. Within months of his breakthrough, this “new light” was put to use identifying fractures and locating bullets in gunshot wounds. But although X-rays were initially used for medical purposes, theories about the new technology’s use in nondestructive testing were also examined. Early X-rays of zinc plates, for instance, hinted at the possibility for welding control--an idea that bore fruit during the early 1900s, when X-rays were used to examine boilers.

During the next half-century, X-ray technology--although constantly refined--didn’t change drastically. X-rays were emitted from a source, passed through an object and detected either on film or a fluorescent screen. Contrast and spatial resolutions improved, as did film speeds and control over X-ray sources. Scintillation screens were also used with film to achieve better images at lower dosage.

The next great advance occurred during the 1950s with the advent of the image intensifier. For the first time, clear images could be made available in real time. With image intensifiers, X-rays are picked up on a phosphor screen, focused down to another screen and then viewed either directly or via a high-quality television image tube or CCD camera. Despite image intensifiers’ great performance for real-time imaging, film remained the only option for large image size and good spatial resolution and contrast until recently.

Each of these technologies has its own set of drawbacks, however. X-ray film must be chemically processed, which often means about 20 minutes of lag time between an image’s capture and the technician’s inspection of it. If the film isn’t properly exposed or the angle is wrong, it’s a do-over, and another 20 minutes are lost. If multiple pieces of film must be shot, the time required to examine an object can run into several hours. Moreover, a company must have methods in place and employees trained to safely handle, store and dispose of film-processing chemicals. Notwithstanding film’s very good spatial resolution, it has a nonlinear and somewhat limited contrast range. Add to this the human eye’s limitations to discern no more than 100 or so contrast levels, and obtaining and examining--on just one piece of film--an X-ray of an object with a wide thickness or density range becomes nearly impossible.

For their part, image intensifiers have a limited field of view, and their bulkiness prevents them from being used in all applications. Distortion toward the edge of the image means that only the center portion is useful in some applications. Image intensifiers’ contrast and spatial resolutions don’t compare well to other technologies, either.

Image sharing and archiving is an issue for both film and image intensifiers. This problem is somewhat mitigated with digitized still or video images, or by scanning X-ray film, but archiving using these technologies is very space- and time-intensive.

The digital realm: computed radiography

With the introduction of computed radiography during the 1980s, a giant step forward for X-ray imaging occurred. Until this time, the technology’s analog nature prevented any real automation. Inspection, defect recognition, sorting and the like depended upon human interpretation of film or image-intensifier images. Computed radiography offered the benefits of computer-aided image enhancement and recognition, the ability to store and transmit digital images, and the elimination of film processing and all its associated costs.

Computed radiography works similarly to film-based X-rays, but instead of X-ray film, a storage phosphor screen is irradiated and the latent image stored within it. It’s then taken to a reader, which uses a laser and detector to scan the latent image from the screen. In most cases this technology can be easily retrofitted into film-based systems, eliminating the need for film, chemicals, processing lab, equipment and storage.

The reduced costs in those areas mean a quick return on investment, says Fred Morro, Fuji Corp’s. director of digital radiography products for NDT (www.fujimed.com/ndt/ndt_fcr.html). “We do a cost analysis for each customer, looking at film costs, chemical costs and the cost of chemical disposal,” he says. “It depends on the application, but the ROI can be less than one year.”

Envision Product Design, developer of the CMOS digital flat panel (as illustrated below), estimates that recurring costs such as film, processing and chemical disposal reach about $6,000 per 1,000 X-ray exposures. This doesn’t include costs for film storage or a processing lab.

In terms of performance, computed radiography’s contrast resolution of 12 bits, or 4,096 contrast levels, rivals film, says Morro. And although its spatial resolution doesn’t yet surpass film, it’s more than enough for most NDT applications, he adds. Computed radiography readers for NDT can resolve 5 line pairs/mm (i.e., 100 µm).

Because of its high contrast range, computed radiography has the ability, as do all digital radiography technologies, to capture the entire density range of most objects in one pass, something that’s impossible with film. Computer manipulation then makes it possible to view just the density ranges of interest.

Like film, computed radiography screens can be cut or bent. Although the storage plates are more expensive than film--a 14 X 17 in. plate costs about $700--it can be used thousands of times, limited only by mechanical wear caused by handling. This makes it cheaper than film. Also similar to film, storage plates can be used--barring condensation or other moisture issues--for field X-ray inspection under extreme temperature conditions.

One advantage of computed radiography over other digital radiography technologies is that, in most cases, only one screen reader is needed to service an entire lab. The reader is separate from the screen and therefore not an integral part of the image-capture process. This could offer an advantage over other digital technologies, where image acquisition and the reader are integrated--requiring the purchase of an entire system for each concurrent use.

A disadvantage of computed radiography is that, like film, it isn’t real-time. Although faster than film processing, the screen must still be removed from the X-ray station and fed through the reader. Computed radiography represents a huge step forward and is still the prevailing nonfilm technology, but it can’t provide all the benefits of digital X-ray products.

Digital flat panels

During the late 1990s, digital flat panels were introduced. Unlike their film or computed radiography predecessors, they provide digital readout of an X-ray image, making it possible to automate X-ray NDT inspection. Except for the fact that they can’t be cut or bent, digital flat panels are used in much the same way as film or computed radiography and can be left in place while robotic or conveyor systems bring parts to them or reposition parts for multiple views. The operator doesn’t have to change film or storage phosphor plates between shots, and the X-ray image is available seconds after it’s acquired, greatly improving on the productivity offered by film or computed radiography systems.

Currently, two digital flat-panel technologies are battling head-to-head for the market: amorphous selenium (a-Se) and amorphous silicon (a-Si). Outwardly, they both function in the same manner: X-rays are picked up by the panel, which converts them into a digital image that can be read from the plate. Because the panels require no processing, images can be obtained at rates of one image every few seconds up to live video speeds of 30 images per second. Because of their better resolution and increased field of view, flat panel displays with 30 images-per-second speed are ideal replacements for image intensifiers. However, depending upon the manufacturer, the 30 image-per-second frame rate may come at the cost of decreased resolution.

With amorphous selenium technology, X-rays strike an a-Se layer, which converts them directly into an electric charge that’s further converted to a digital value for each pixel. This is called a direct-imaging method. Proponents of a-Se say it offers better spatial resolution than a-Si.

With what are commonly called amorphous silicon panels (a misnomer because even a-Se panels use amorphous silicon), X-rays first strike a scintillation layer. This layer emits photons in direct proportion to the energy of the X-rays striking it. The photons are picked up by the underlying a-Si photo-diode matrix, which converts them to an electric charge. This charge is then converted to a digital value for each pixel. Because of the intermediate step of converting X-rays to light via the scintillation layer, this is called an indirect-imaging method. The scintillation layer is commonly composed of either cesium iodide or gadolinium oxysulfide, with CsI being the preferred material. Proponents of a-Si panels say they offer much faster frame rates, up to 30 images per second, than a-Se panels.

Both technologies offer near-film spatial resolution but with contrast ranges far exceeding film.

The battle between these two technologies--waged mostly on a theoretical level with much talk of modular transfer function, detective quantum efficiency and numerous Einsteinian-looking equations--concerns which offers higher spatial resolution with the best contrast and least noise. Bedford, Massachusetts-based Hologic Inc., the main developer of a-Se flat panels, argues that light generated by the scintillation layer of indirect systems is somewhat scattered before reaching the photodetectors, therefore degrading resolution. By contrast, with a-Se systems, the electrons generated by X-rays striking these panels are picked up directly by the electronics with very little scatter, resulting in better image quality at a higher resolution.

“I would argue that it’s not just theoretical,” says Ken Swartz of Hologic Inc. “There are now a number of published studies that have compared the image quality and productivity advantages of direct and indirect capture detectors.” He points to an article by Ehsan Samei and Michael Flynn in the April 2003 issue of Med. Phys. “An Experimental Comparison of Detector Performance for Direct and Indirect Digital Radiography Systems” in which a Hologic direct-imaging panel was compared to indirect-imaging panels from GE and Philips. The authors conclude that when resolutions smaller than 200 µm are required, a-Se performs better. For resolutions greater than 200 µm, a-Si panels perform better. Swartz also points out that Kodak, Siemens, Philips, Agfa and Instrumentarium have chosen a-Se detectors for difficult medical applications because of their higher-resolution image quality.

But the real test for NDT professionals is which of these technologies will get the job done in an NDT application.

The answer is, “all of them,” according to Scott Thams, president of X-R-I Testing (www.xritesting.com), an independent testing lab that does work for aerospace and automotive industries. “We find that amorphous selenium, amorphous silicon and computed radiography are good enough,” explains Thams. “They’re film-equivalent.” Eighty to 90 percent of X-R-I’s X-ray inspection is done on film--the company spends more than $1 million on X-ray film per year--and Thams considers himself a pretty good judge of how the various technologies stack up against film and each other. “All of these technologies have matured to the point that comparing them is just splitting hairs,” he observes.

Specsmanship aside, manufacturers interviewed for this article did acknowledge that no one technology is the silver bullet; it all depends upon the application. For NDT, arguments about which technology provides the highest-resolution images might not really matter, says PerkinElmer’s Mario Gauer, product leader for flat-panel detectors and image tubes.

“You have to apply the technology to the respective application,” he explains. “For automated defect recognition or 3-D reconstruction, the use of detectors with high dynamic range and excellent signal-to-noise characteristics will reduce the number of required images and, thereby, the cycle time.”

Whether a-Se or a-Si, both camps agree on digital panels’ productivity.

“Our thrust is not so much to replace film as to automate a process,” says Greg Budner, technical account representative for Varian Medical Systems Security & Inspection Products. “We’ve taken a high-volume film application and used the panel with a robot to do many parts. The operator doesn’t have to take 20 minutes to set up and look at a part.”

Budner points to the productivity gains that came about after Varian installed a digital flat-panel X-ray to inspect Ariane V satellite placement rockets. Before installation, inspecting a cylindrical carbon composite part took more than 350 pieces of X-ray film, roughly one shot per degree as the part was incrementally rotated in front of the X-ray machine. Using a digital flat panel, Varian could provide X-ray inspections that greatly reduced inspection time. As the part was rotated, the system captured and stored an X-ray image in seconds before moving to the next position. The system operated at 6 and 9 MeV energy levels and could detect down to 300 µm occlusions with a 2 percent contrast ratio through as much as 1,200 mm of carbon.

Although great for lab or production environments, Thams and others familiar with all the X-ray technologies aren’t convinced that digital flat-panel technologies will stand up as well to field conditions as the more rugged film or computed radiography. “The digital flat panels are fragile, and they’re sensitive to temperature variations,” says Thams. They also need power and cabling at the inspection location where film and computed radiography do not, he explains.

Varian and GE say that environmental concerns aren’t an issue. GE has a thermal stabilization system and claims its units are not sensitive to sunlight, and Varian says its system works fine without thermal stabilization. Both companies have a-Si flat-panel units in use in harsh desert environments inspecting pipelines and unexploded ordnance. On the other hand, a-Se panels were designed for medical applications and normal room temperatures of 50° to 86° F. For temperatures outside this range, active heating or cooling systems are required.

But the real issue for field work isn’t so much environmental but rather the portability of computed radiography and film vs. the data-collection capabilities of digital flat panels, says Mike Bernstein of GE Inspection Technologies.

“For a one-off picture, computed radiography does a nice job,” he says. “It’s portable in terms of what you carry to the pipeline, for instance, and it’s a bit more flexible. But if you’re taking 40 or 50 images, you have to think about the logistics of processing those images.” Compare that to the ability to capture and disposition images instantly and simply move to the next location, and that versatility may offset the portability advantages of film or computed radiography, says Bernstein.

One disadvantage of digital panels, and perhaps a costly one, is that, unlike film or computed radiography, where several technicians can take X-ray images at one time and process them in one film or computed radiography processor, a complete system is required for each X-ray station--at a cost of about $150,000 each--if more than one digital flat-panel X-ray inspection is happening simultaneously. This might be somewhat offset by productivity gains (e.g., fewer stations and lower labor expenses), but it’s a consideration.

Another consideration in NDT applications is the issue of dead pixels. Dead pixels or even dead lines (i.e., rows of pixels) are inherent in this technology but can be replaced artificially based on surrounding pixels (i.e., interpolation). Nonetheless, in some industries, this might not be acceptable. By analogy, consider that in aerospace, inspection codes require that film be reshot if there’s an artifact (scratch, pin hole, bubble, etc.) on the X-ray film itself within the region of interest, says Thams. Bernstein doesn’t see this as an issue and points out that with GE’s flat panels a dead pixel is only 100 µm square, while film artifacts are much larger.

This argument does point to one last consideration: industry acceptance of nonfilm X-ray inspection. Currently, various industry groups and standards organizations are looking at how to change inspection specifications in order to accommodate the digital X-ray technology, says Thams. He says that the reason 80 to 90 percent of his X-ray work is still done in film is because companies still specify testing requirements in terms of it.

Although there currently is not a common industrial standard, Bernstein indicates many companies have already defined quality specifications for the implementation of nonfilm X-ray. “GE Aircraft Engines has allowed the use of both image intensifiers and digital detectors for years,” he says. “In fact, at one of our manufacturing facilities, digital detectors have been used to acquire and disposition more than 30 million images since the mid-1980s.”

Other technologies

With all the buzz over a-Si and a-Se flat panels, some other flat-panel technologies might be overlooked but shouldn’t be, such as the CMOS X-ray. This interesting new product from Alaska-based Envision Product Design (www.cmosxray.com) is a scanning flat panel. It consists of a linear X-ray detector array and drive system housed within a flat panel 3 in. thick. The panels can measure up to 24 X 36 in. and are used in the same fashion as the ones described earlier.

With this technology, as the scanner traverses the panel (think document scanner), X-rays pass through a slot that collimates them before they strike the scintillating material. The material is deposited on the ends of a fiber optic bundle that runs the length of the array (see illustration to the right). In an arrangement that protects the X-ray-sensitive CMOS detectors from X-ray damage, the fiber optic bundle leaves the scan head at a right angle to the X-rays and connects to the CMOS detectors, which are housed beneath tungsten and lead shielding. The system can be built to operate with high-energy X-ray systems up to 10 MeV.

Alternatively, Envision sells CMOS linear arrays up to 6 ft long that can be fixed in one place while parts are moved past robotically or on a conveyor system. The image on this issue’s cover (minus the rider) was taken by moving the motorcycle in front of a 54 in. linear array. The 54 X 84 in. image took 110 seconds to acquire.

Envision flat panels and linear arrays have spatial resolutions of 80 µm and contrast resolution of 12 bits, or 4,096 gray levels.

Envision also sells a conventional 4 X 4 in. flat-panel CMOS array capable of 50 µm resolution and 12 bits of contrast.

As mentioned, one of the arguments against digital flat panels for fieldwork is the necessity for power and cables. Israel-based Vidisco Ltd. (www.vidisco.com) has addressed this issue with its Flat FoX portable and wireless a-Si systems. According to company literature, the entire system, including a 150 kV pulsed X-ray source, is contained in one case and can be used anywhere in the field without an AC power supply. The equipment can also be used with most existing heavy-duty industrial X-ray sources. It captures a 12- to 16-bit image at 127 to 400 µm resolution on an 11 X 16 in. a-Si panel. The system will operate from a battery for two hours. A wireless option allows the X-ray source to be triggered remotely and/or image transmission delivered up to 200 m.

Conclusion

Although computed radiography is still the technology of choice for film replacement, look for digital flat panels, probably a-Si, to become the next key player in the NDT X-ray market. The ability to capture images and store or transmit them in real time opens up possibilities for NDT professionals that don’t exist with film or computed radiography. GE, Varian, Agfa, Siemens, PerkinElmer and others will drive the technology to the next level. Though most digital flat-panel manufacturers say they offer a productivity tool rather than film-replacement technology, advances in digital X-ray technology--such as improved contrast and spatial resolutions, lower noise, improved ruggedness and lower cost--could chip away at the film and computed radiography market.

About the author

Dirk Dusharme is Quality Digest’s technology editor. Letters to the editor regarding this article can be e-mailed to letters@qualitydigest.com.