Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Patent
1992-01-13
1994-08-09
Nelms, David C.
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
378207, H01J 4014
Patent
active
053368800
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
An object of the present invention is a process for the correction of the distortion of radiological images acquired with a luminance intensifier tube. It can be applied more particularly to the medical field. It can be implemented either in direct radioscopy or in radiology with digitized processing of the signal representing the image. It relates more particularly to future-generation tomodensitometers in which the detection element will be a luminance intensifier device such as this. Its object is to resolve the problems of morphometry raised by the use of such tubes.
An intensifier tube of radiological images is designed to receive a low-power X-radiation and to convert this X-radiation into a more powerful light radiation that can be more easily detected by a display means, especially by a camera. The reason for the weakness of the X-radiation received must be sought in the need to provide protection, especially in medicine, for patients subjected to examinations with radiation of this kind. This is so especially when such examinations are lengthy, as is the case with tomodensitometry processing operations or processing with digitization of image information elements.
An image intensifier tube essentially has a conversion panel to convert a received X-radiation into a light radiation that is capable of striking a photocathode placed in a position where it faces this panel. The conversion of X-radiation into light radiation is obtained in a known way by providing the panel with caesium iodide crystals. Under the effect of the X-ray illumination, photoelectrons are liberated from the photocathode and move towards the screen. This movement towards the screen is subjected to the effects of an electronic optical system. This electronic optical system tends towards an effect where the impacts of the photo-electrons on the screen correspond to the places on the photo-cathode from which they have been emitted.
The screen is itself of a special type: it re-emits a light image representing the electronic image conveyed by the electrons, and this image itself represents the X-ray image. This light image can then be displayed by any display means, especially a standard camera, so as to be displayed on a display device, especially a device of the television monitor type.
A display system such as this has a major drawback: the revealed image is an image that is geometrically distorted in relation to the X-ray image from which it has originated. This distortion occurs essentially between the photo-cathode, excited by the photons emerging from the conversion panel, and the screen that receives the electron radiation emitted by this photo-cathode. Indeed, during their journey, the photo-electrons are subjected to disturbing effects, notably magnetic effects, due to the earth's magnetic field. If all the photo-electrons were to be affected, during this journey, by one and the same type of disturbance, then correcting the effect of these disturbances at any part of the sequence of images to would be enough to avert problems. Unfortunately, these photo-electrons are highly sensitive to disturbances. And the inhomogeneity of the magnetic field in the places through which they pass is then such as to result in a distortion in the electronic image projected on the screen.
To give a more concrete explanation of the effects of a distortion such as this, it may be said that the image of a straight line interposed between an X-ray tube and an image intensifier such as this will be a straight line in the X-ray image that excites the panel, it will be a straight line in the photon image that strikes the photo-cathode, and it will be a straight line in the electron image that leaves this photo-cathode, but it will no longer be a straight line in the electronic image that gets displayed on the screen. Consequently, it can no longer be a straight line in the light image produced by this screen. The display device placed downline then reveals, so to speak, the result of the distortion due to the non-homog
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Proceedings of 1986 IEEE International Conference on Robotics and Automation, Apr. 7-10, 1986, San Francisco (Calif., US), vol. 1, IEEE, N. Yokobori et al.: "Sub-pixel geometric correction of pictures by calibration and decalibration" pp. 448-453.
Lavayssiere Blandine
Leclerc Vincent
Picard Catherine
General Electric CGR S.A.
Nelms David C.
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