Dr. Stewart’s letter is very relevant in the context of new X-ray imaging modalities. Since the discovery of X-rays, film (or glass plate in the early days) has been used as the base of the radiographic image. In this way, the plastic film base is not only carrying the X-ray sensitive emulsion, but also the image after the film is processed.
The introduction of CCD and complementary metal oxide semiconductor sensors, or CMOS, and photo-stimulable phosphor sensor systems in the early 1980s implied a fundamental change in this principle. Image capture and image display are done separately from now on; the sensor and the display device (that is, the computer monitor) are two different components of the imaging chain.
Xeroradiography has characteristics of film-based imaging, as well as the digital sensor technology. It uses a photoconductive insulating medium, which is able to store the latent electrostatic-charge pattern produced by the X-rays. This is very similar to the latent image produced in the X-ray–sensitive emulsion of film.
The second step is the transfer of the latent image onto plastic-coated paper through a process comparable to that of a photocopying-type machine. That means that the image itself is physically detached from the sensor that was used to capture the image information. This is analogous to what happens in digital imaging when the image is stored in the computer and subsequently displayed on the monitor.
In spite of these similarities, there is also a principal difference. The way a digital radiograph is displayed on the screen can be optimized and changed by means of image processing tools after it has been archived. This is achieved, of course, without actually changing the original image data, which are stored in the image database (when the software is designed the proper way).
This is different from a Xeroradiographic image, which cannot be changed after it is printed. Therefore, it lacks some of the most important features of a real digital image. That is why I did not mention Xeroradiography in my article.
Having said this, I agree completely with Dr. Stewart in reminding us of Xeroradiography because, until the 1980s, it was one of the few technologies, if not the only technology, competing with some success with film-based imaging. As for why it did not succeed as a new imaging modality, I think it was probably too far ahead of its time. Potential users were reluctant to pay a large amount of money for a device that was so much different from common film-based imaging. There are other examples of good technologies that were not adopted in time by enough users to enable further improvements. Fortune was more favorable for digital radiography, as we know now.
