Patent Number: 061852791
Section: description

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 represents the X-ray emission part of an X-ray source of the prior art as well as the part of the diaphragm from this source. Conventionally, the X-ray source comprises a cathode 10 and a rotating anode 11 which are contained in a casing 12 that is transparent to X-rays. The assembly is itself closed in a casing 13 provided with a skirt 14 made of material that is opaque to X-rays, for example made of lead, with the exception of a region located facing the anode 11, which consists of a window 15 made of a material that is transparent to X-rays. The gap between the casing 12 and the protective casing 13 and the window 15 is filled with oil which ensures cooling and high-voltage electrical insulation of the X-ray source. Consequently, the casing 13 must provide sealing with respect to the oil contained in the gap between this casing 13 and the casing 12 and also ensure that the X-radiation beam will only be emitted through the window 15. As is well known, the cathode 10 emits an electron beam which strikes the focal track of the rotating anode 11 which in turn emits an X-ray beam towards the window 15. As shown by FIG. 1, conventionally, the X-ray beam emitted by the anode 11 is shaped by means of a diaphragm 30 made of a material that is opaque to X-rays, the opening 31 of which is dimensioned in order to obtain an X-radiation beam output by the focus of the anode and to intercept the X-radiation emitted by the source outside the focus, or extrafocal radiation. The parasitic extrafocal radiation should be eliminated because it impairs the quality of the image subsequently obtained. It is furthermore desirable to be capable of varying the dimension of the focal surface of the anode and consequently the geometry of the X-ray beam. Therefore, in order to eliminate the extrafocal radiation, it is necessary to modify the dimension of the opening of the diaphragm 30 as a function of the dimension of the focal surface as well as the collimation of the X-ray beam. Conventionally, in order to adapt the opening of the diaphragm 30 to the X-ray beam and to be able to eliminate the extrafocal radiation, moving fingers 32, made of a material that is opaque to X-rays, are provided and are manipulated by means of a complex rod system 33 in order to dimension the opening 31 as a function of the focal area and the desired area for the final image. This arrangement of the prior art, which has just been described, has several drawbacks. Firstly, since the diaphragm 30 is a separate part from the window 15, it must be placed externally to this window and it is therefore not possible to eliminate the extrafocal radiation as close as possible to the focal surface for emitting the X-ray beam. With regard to the quality of the final image obtained, it is particularly beneficial to be able to eliminate the extrafocal radiation as close as possible to the focal surface of the source. Furthermore, the use of opaque fingers 32 requires a complex mechanical rod system 33 for dimensioning the opening 31 of the X-ray beam diaphragm. In an embodiment of the invention, these drawbacks are overcome by providing a window which is also used as a diaphragm for eliminating the extrafocal radiation. By virtue of the fact that the window itself fulfills the function of a diaphragm, it is possible to place this diaphragm closer to the focal surface of the X-ray beam. Furthermore, the window of one embodiment of the invention permits simple dimensioning of the passage opening for the X-ray beam without requiring a complex mechanical rod system. FIG. 2 represents a window 15 in a casing for an X-ray source in an embodiment of the invention. This window 15 has the general shape of a frustoconical dome and has an end wall 17 and a sidewall 18 ending in a flange 19 for mounting it in the protective casing 13. This window 15 consists of a material that is transparent to X-rays. In an embodiment of the invention, this window 15 comprises an internal chamber designed to permit the displacement of an opaque material which is introduced from outside the chamber in such a way that the area of the X-ray beam passage region varies as a function of the volume of opaque material in the chamber, in order thus to eliminate the extrafocal parasitic radiation from the beam. In a recommended embodiment, the chamber comprises a first channel wound on itself in the shape of a spiral. Preferably, the chamber comprises a second channel in the shape of a spiral, juxtaposed with the first channel in a direction perpendicular to the planes on which the openings of the first and second channels are located, this first and this second channel being offset relative to each other in such a way that the turns of one spiral cover the space between the turns of the other spiral, so as to ensure complete opacity to radiation in the part of the spirals through which the opaque material passes. Preferably, the electromagnetic radiation beam is an X-ray beam and the source is an X-ray source. In the embodiment represented, this chamber comprises an annular cavity 20 arranged in the sidewall 18 and connected at its lower end to a means 25 for controlling the displacement of the material that is opaque to X-rays. This annular chamber 20 is connected at its upper end to the outermost turn of each of two spiral channels 22, 23 arranged in the end wall 17 of the window by means of tubes 21. The spiral channels 22 and 23 are arranged in the end wall 17 in juxtaposed planes parallel to the end wall 17. As represented, the turns of the two channels 22, 23 are offset relative to one another in such a way as jointly to cover virtually the entire area of the end wall 17. Clearly, a single spiral channel could be used, preferably a spiral channel with adjoining turns, as represented in FIG. 3. Use could also be made of more than two superposed spiral channels. The innermost turn of each of these spiral channels 22 and 23 is also connected, via a conduit 24 arranged in the window, to the device for controlling the opaque material. Clearly, it is, if so desired, possible to do without the annular cavity 20 and replace it by two conduits. The material that is opaque to X-rays, for example mercury, is introduced by means of the control device 25 into the annular chamber 20 then into the outer turns of the spiral channels 22 and 23 towards the center of these spiral channels, so as to leave a central passage of suitable dimension for the X-ray beam. The turns of the spiral channels 22 and 23, forming the central passage for the X-ray beam, are filled, also by means of the control device 25, within a liquid that is transparent to X-rays, for example alcohol. The turns of the spiral channels 22 and 23, which are filled with opaque material and surround the central turns filled with material that is transparent to X-rays, therefore form a diaphragm that is opaque to X-rays, making it possible to eliminate the extrafocal radiation. Depending on the quantity of opaque material introduced into the turns of the channels 22 and 23, it is therefore possible to dimension the central passage that is transparent to X-rays and easily to eliminate the extrafocal radiation as a function of the dimension of the focal surface of the X-ray source. Furthermore, since the diaphragm function is fulfilled in the window of the outer casing of the X-ray source itself, it is possible to achieve this elimination of the extrafocal radiation very close to the focal surface of the source. FIG. 4 schematically represents a spiral channel connected to a device for controlling the displacement of the opaque material 25, which is particularly recommended for the present invention. This control device 25 comprises an enclosure in which a piston 26 is arranged, this piston being movable in translation and dividing the enclosure into a first chamber and a second chamber 28. The first chamber 27 is connected by a conduit 21 to the outer turn, for example, of the spiral channel 22. The second chamber 28 is itself connected by a conduit 24 to the innermost turn, for example, of the spiral channel 22. The chamber 27 is filled with material that is opaque to X-rays, for example mercury, whereas the chamber 28 is filled with a material that is transparent to X-rays, for example alcohol. The piston 26 may, for example, be a piston that is movable by means of a magnet 29. As represented, the piston 26 includes two movable magnetic plates, separated from each other, to define a space that is generally filled with air for the purpose of taking up thermal expansion. As is immediately clear, when the magnet is moved either to the right or to the left, the piston 26 is also moved, either to the right or the left, thus introducing into the spiral channel 22 more or less of the product that is opaque to X-rays. It is thus easily possible to dimension the central passage for the X-ray beam and eliminate the extrafocal parasitic radiation in accordance with the purpose intended for the X-ray source. It is also possible to use, instead of the device described above, a peristaltic pump as the device for controlling displacement of the material that is opaque to X-rays. Various modifications in structure and/or function and/or steps may be made by one skilled in the art without departing from the scope and extent of the invention.