Alternatives to standard CR detector technology is an automated CR changer, which scans a fixed imaging plate in a manner similar to most DR devices, or the purchase of digital radiography technology itself. The obvious benefit of DR is the “direct” acquisition of the image without user intervention, which frees a substantial portion of the technologist’s time otherwise required to handle and process CR cassettes. DR costs are typically much higher than CR, and usually require the purchase of an x-ray system as well.
DR exceeds CR in system speed and detection efficiency, especially those systems that employ thin film transistor (TFT) flat panels. Advances in system designs and medical x ray converter materials and readout methods are closing the gap among digital detectors in terms of detection efficiency. While patient dose is an important consideration, it is more important to obtain images of the appropriate quality and detail, which most detectors can deliver, albeit at a slightly higher dose for a given signal-to-noise ratio.
The primary categories of DR detectors are chargecoupled device (CCD), complementary metal oxide
semiconductor (CMOS), and flat panels. There are notable differences among these detectors in terms of signal acquisition, such as line-scan CCD arrays versus fully two-dimensional acquisition CCDs with one or several CCD “chips” for image acquisition and conversion.
From a physics standpoint, DR detections can be further subdivided into “indirect” and “direct” acquisition of the x-rays transmitted through the patient. An indirect DR detector first converts x-ray photons into light photons through an absorption and conversion process, with the emitted light distribution then converted into a proportional charge via a photodiode detector. A direct detector eliminates the x-ray-to-light conversion process through the use of a semiconductor detector that directly converts x-rays into a corresponding electronic charge.
Most of the interest in DR detectors is currently focused on systems based on thin film transistor (TFT) technology. TFT detectors have arisen out of the multibillion dollar investment in TFT displays, which have produced a large field of view display that produces a transmitted light pattern through LCD cells that can change their transmissive characteristics based upon input signals. The active matrix detector uses an array of TFT switches that combine with storage capacitors to acquire locally generated charges resulting from the absorption of x-rays. Image acquisition is performed with active readout capabilities and externalized electronics without user intervention. The active matrix array detector elements for conventional radiography have a size range of 100 to 200 microns, which is similar to the effective pixel size of other digital detectors based upon CR or CCD technologies.
The indirect and direct TFT detectors look alike, and perform similarly in terms of image quality, system speed, and positioning flexibility. They typically find a home in a dedicated chest room or an outpatient environment that requires high throughput and minimal positioning. Many improvements are overcoming positioning problems with interesting equipment designs, automated positioning aids, and multiple detectors per room. Although digital radiography systems in general have higher initial costs than CR, the efficiency provided by direct acquisition and display and increased patient throughput can potentially result in reduced long-term costs.
 

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