Flat panel digital x-ray detectors offer challenge on traditional applications.
Reiner Quad, Mario Gauer and Brian Giambattista PerkinElmer Optoelectronics
Flat panel digital detectors were introduced in the 1990s as an alternative technology to traditional film, computed radiography and image intensifiers for diagnostic medical imaging. As their performance has improved, they have begun to challenge conventional imaging techniques for a greater share of the x-ray imaging market. End users can also expect some infiltration into other market areas, such as industrial inspection.
The technology is based on amorphous silicon (a-Si) fabricated on a glass substrate using thin-film processing. An x-ray scintillator, which converts x-rays to visible photons, is either grown directly on or attached to the a-Si panel. These photons are converted to an electrical charge that is, in turn, converted to a digital value for each of several million addressable pixels on the panel to create the image.
The a-Si technology for flat panel x-ray detection emerged from the flat panel display industry that produced laptop computer screens. This industry drove the development of much of the equipment necessary for basic processing. However, there were substantial developmental issues to resolve before the technology matured enough for medical applications.
Replacing film
The medical imaging industry is quickly becoming a prime target for flat panel digital applications. The idea is to replace traditional film radiography in hospitals to improve work flow and to enhance productivity. For example, hospital technicians can simply position a patient, take an image and check it in a matter of seconds to determine immediately whether the image is valid or requires a retake. Thus, a process that might have taken 10 minutes or more is completed in about half the time. For traditional radiography, hospitals can typically replace two current film rooms with a single digital unit. Among other benefits are the elimination of processing chemicals and the reduction of space requirements, including those for storage of x-ray film.
Medical technology currently using PerkinElmer's 8-in. flat panel detectors is the CyberKnife stereotactic radiosurgery equipment from Accuray in Sunnyvale, Calif. The system delivers precisely targeted radiation doses to a treatment site, such as to a small tumor.
Although a-Si is preferred for flat panel technology, other materials are available, such as amorphous selenium (a-Se). However, many believe a-Si can produce a better-quality image at high speed. A key enabler for this technology is the ability to deposit a-Si inexpensively over a very large area. The result is a full chest x-ray from a single-piece detector rather than from several smaller wafers. Detectors made from several pieces are prone to image defects at the wafer edges.
A sister application within the medical environment is real-time imaging used in diagnosis for cardiology or angiography. For this use, flat panel digital detectors would replace traditional image intensifiers. By injecting a contrast agent, or x-ray-absorbing dye, into the patient’s vascular system, physicians can view the blood flow in real time to locate and alleviate blockages.
Another potential application is called digital subtraction angiography: making an image without a contrast agent and then subtracting it from an image with a contrast agent. This process further highlights the vascular system by removing other elements, such as bone, from the image. However, this procedure might be eliminated, because it may be possible to produce sufficient visibility in just one image using the contrast agent.
Portal imaging also is possible in radiotherapy. The position of the patient is controlled by online image acquisition during cancer treatment with high-energy beams. Because they enable detection of even the smallest contrast differences in bone and tissue, these detectors could help provide treatment at lower applied doses. Imaging at high frame rates also could enable enhanced treatment methods, such as intensity-modulated radiotherapy.
Future view
In diagnostic medical applications, a typical device can capture a 1000 × 1000-pixel image at 30 images per second. There are detectors available that can produce a 2000 × 2000 image, although some speed is traded for image size. Future applications may require three-dimensional imaging at the same high speeds, and these products are just on the horizon. Also, once costs begin to drop for flat panel digital technology, lower-priced portable x-ray systems may be seen wheeling from room to room in medical centers.
Although much of the interest is focused on medical applications of flat panel digital detectors, potential industrial applications could include sorting, inspection and defect recognition. Quickly providing several images from different angles enables fast identification of defective castings or products. The automotive industry is a prime customer for this technology, as are reconstruction applications where several images are acquired to reconstruct a quality 3-D drawing of the goods being inspected.
Basically, there is a lot of wind in the sails of manufacturers of flat panel digital detectors to improve performance and increase image speed over larger areas. Although most of today’s x-ray applications are still using film or computed radiography methods, digital flat panels, in time, will likely write the next chapter in x-ray detection techniques.
Meet the authors
Reiner Quad is chief technology officer, Mario Gauer is a product manager and Brian Giambattista is a chief engineer with PerkinElmer Optoelectronics in Fremont, Calif.