Physics, Techniques and ProceduresXeroradiography
the production of an image from an
X-ray exposure using the charged surface of amorphous selenium, a
photoconductor, as the
X-ray detector. In conventional xeroradiography, the image is characterized by a visible edge enhancement, which tends to improve structure visibility over the extent of an image with a wide
dynamic range. Because of this ability to show structure detail over a region with wide
dynamic range, xeroradiography has found applications in chest radiography as well as
mammography and musculoskeletal imaging. An example xeroradiograph is shown in
Fig.1.
The xeroradiographic imaging process consists of several steps:
1. A uniform charge is deposited on the surface of a highly smoothed selenium plate. This xeroradiographic plate consists of a thin sheet of aluminum on which a thin layer of selenium is deposited. A very thin aluminum oxide layer is placed as an electric insulator between the selenium and the conducting aluminum plate. The charge is applied by passing the plate in front of a charging scorotron.
2. The charged plate is placed in a light tight cassette and is exposed to X-rays during the imaging procedure. The selenium acts as a photoconductor and the positive charge discharges in proportion to the X-ray flux incident on the selenium plate. The distribution of X-ray flux is thus recorded as a distribution of latent positive charge on the selenium surface. (See also photoconduction, photoconductive layer.)
3. The latent positive charge distribution is developed or made visible by a process which deposits charged toner particles on the selenium plate in proportion to the remaining latent charge on the plate. Typically, development is performed using a process known as powder cloud development in which an aerosol of charged toner particles is created and a back-bias voltage is applied to attract the toner particles of the desired charge to the plate. A voltage is also applied to a development electrode placed just in front of the selenium plate which serves to remove non-uniformities from the developed image. It is the powder cloud development process that results in the edge enhancement that is characteristic of xeroradiographic images.
4. The toner on the selenium plate is transferred to a paper sheet and permanently fixed within the plastic coating on the paper.
5. The plate is cleaned to remove all remaining toner particles and prepared for re-use. Charging is performed only when the plate is ready for use.
Xeroradiography has been used most extensively for mammography applications, because of the inherent intensity equalization which appeared to allow more uniform detectability. Unfortunately, the dose efficiency for xeroradiography systems is low and for that reason the systems have not achieved high utilization.
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