Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Patent
1998-07-20
1999-07-27
Westin, Edward P.
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
25037008, 2502081, H01L 310224, G01T 124
Patent
active
059294493
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to imaging systems and in particular to a flat panel detector for radiation imaging.
BACKGROUND ART
X-ray imaging systems are widely used in medical diagnosis and industrial and security inspection environments. One well known prior art x-ray imaging system is commonly referred to as an x-ray image intensifier ("XII") system. The XII system includes a large image tube that converts a low intensity x-ray image into a visible image. Incident x-rays are transmitted through a low absorbing window, then absorbed by an input phosphor screen and converted into a light image. On the inner surface of the input phosphor screen is a photocathode which converts the light into photoelectrons. The photoelectrons are accelerated and focused by an electrical static lens. The focused photoelectrons bombard an output phosphor screen and are converted into an optical image. A charge-coupled device ("CCD") or a camera tube is coupled to the output phosphor screen to convert the light image into an electronic video signal.
However, the XII system suffers from a number of problems due to the multiple conversion stages, resulting in a reduction in image resolution and image contrast as well as pincussion distortion caused by the magnification error of the electrical static lens. Moreover, the XII system is complex and bulky.
To overcome the problems associated with the XII system, alternative x-ray imaging systems employing flat-panel radiation image sensors have been proposed. For example, U.S. Pat. No. 4,382,187 to Fraleux et al. and U.S. Pat. No. 4,689,487 to Nishiki et al. disclose early designs of large area flat-panel radiation image sensors for use in radiation imaging systems. These flat-panel sensors are responsive to incident x-rays and generate output signals representative of a radiation image.
U.S. Pat. No. 5,079,426 to Antonuk et al. discloses a direct-detection x-ray image sensor incorporating an amorphous silicon thin film transistor ("TFT") switch and photodiode array. X-rays are detected by a phosphor screen that is placed on the top of the TFT switch and photodiode array. When x-rays interact with the phosphor film, light photons are generated and converted into electronic charges by the photodiode array. The charges are read out via the TFT switches to generate an image. However, problems exist with this sensor. Because the sensor employs a phosphor screen to detect the x-rays, blurring occurs due to the fact that the light photons are emitted in all directions and are scattered inside the phosphor screen. This results in a poor image resolution. Although higher resolution can be obtained by increasing the thickness of the phosphor film, this is done at the expense of signal gain.
In an article entitled "New solid-state image pickup devices using photosensitive chalcogenide glass film," by T. Tsukada et al., published in the Proceedings of IEEE International Electron Devices Meeting, 1979, pp.134-136, a solid state image sensor is disclosed including a photoconductive selenium film deposited on a n-channel MOSFET switch array made from crystalline silicon. Although this image sensor is suitable for some imaging applications, it is not suited for large area radiation imaging applications due to the difficulties in fabricating a large sensor array on a crystalline silicon wafer.
In an article entitled "Digital radiology using self-scanned readout of amorphous selenium," authored by W. Zhao et al. presented at COMP91, Canadian Organization of Medical Physicists, Winnipeg, Manitoba, Canada, Jun. 19, 1991, a flat-panel x-ray image sensor is disclosed. The image sensor includes a thick amorphous selenium film (a-Se) on a two-dimensional TFT switch array. The TFT switches are arranged in rows and columns to form a two-dimensional imaging system. Gate lines interconnect the TFT switches in each row while source lines interconnect the TFT switches in each column. The thick selenium film is deposited directly on top of the TFT switch array and a top electrode is deposit
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W. Zhao et al. "Digital Chest Radiology Using Self-Scanned Readout of Amorphous Selenium", Medical Physics, vol. 18, No. 5, Sep./Oct. 1991, p. 1073.
T. Tsukada, et al., "New Solid-State Image Pickup Devices Using Photosenstive Chalcogenide Glass Film", Proceedings of IEEE International Electronic Devices Meeting, 1979, pp. 134-136.
1294339 Ontario, Inc.
Hanig Richard
Westin Edward P.
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