X-ray images are formed as shadows of the interior of the body. In an x-ray department, radiologists still examine and diagnose x-ray images on illuminated view boxes, much like a century ago. Recent technological developments are revolutionizing these procedures. Because it is not yet practical to focus x rays, an x-ray detector has to be larger than the body part to be imaged. Thus a practical difficulty in making an x-ray detector is the need to image a large area. The key digital technology permitting an advance in medical x-ray applications is the flat-panel active matrix array, originally developed for laptop-computer displays.
Currently, in taking an x-ray image, a radiological technologist must load a film into a filmâscreen cassette; carry the cassette to the examination room; insert the cassette into the x-ray table; position the patient; make the x-ray exposure; carry the cassette back to the processor to develop the film; and check the processed film for any obvious problems to ensure that the film is suitable for making a medical diagnosis. This laborious process can take several minutes during which time the patient has to remain undressed and the x-ray room is engaged. In contrast, the ideal x-ray imaging system would, immediately after the patient's x-ray exposure, provide a high-quality radiograph on a video monitor. If the physical form of the detector could be similar to a filmâscreen cassette, it could be used with little modification of the x-ray room. Digital x-ray images would provide several advantages: less handling; more convenient patient management; immediate image viewing; computer-aided diagnosis [1, 2]; and more convenient storage on computer disks rather than in archaic film stacks. Such improvements are important, but must not reduce image quality or increase radiation exposure.
Piecemeal approaches to digital x-ray imaging have been pursued for a number of years and have highlighted an interest in the general concept of the fully digital radiology department. In this context, flat-panel imagers are of great interest as they have the potential to solve most of the problems associated with the acquisition of high-quality digital radiographs. Despite the technical issues still surrounding their implementation, they remain the only approach so far identified that has the potential to be all things to all radiologists. The rest of this chapter will describe some of the history behind their development as well as their fabrication, design, evaluation, and configuration into digital x-ray imaging systems for specific clinical tasks.
4.1.1 General requirements of x-ray imaging systems
It is important to recognize, in proposing to build a new x-ray system for medical applications, that projection imaging has been performed for over a hundred yearsâwhen Professor Roentgen first imaged Frau Roentgen's hand in the basement of their house. Since then tens of thousands of individual radiologists working empirically, but competitively, have developed and optimized the standard methods of image acquisition and interpretation. The requirements of any new imaging system will therefore be very similar to those developed by this tried and tested formula.