Patent Number: 061887498
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows diagrammatically an X-ray examination apparatus 1 according to the invention. The X-ray source 2 emits an X-ray beam 3 in order to irradiate an object 4. As a result of differences in the X-ray absorption in the object 4, for example a patient to be radiologically examined, an X-ray image is formed on an X-ray-sensitive surface 15 of the X-ray detector 5 which is arranged opposite the X-ray source. A highvoltage power supply unit 51 supplies the X-ray source 2 with an electric high voltage. The X-ray detector 5 of the present embodiment is an image intensifier pick-up chain which includes an X-ray image intensifier 16 for converting the X-ray image into a light image on an exit window 17, and also includes a video camera 18 for picking up the light image. The entrance screen 19 acts as an X-ray-sensitive surface of the X-ray image intensifier which converts incident X-rays into an electron beam which is imaged onto the exit window by means of an electron optical system 20. The incident electrons generate the light image on a phosphor layer 45 of the exit window 17. The video camera 18 is coupled to the X-ray image intensifier 16 by means of an optical coupling 22, for example a lens system or an optical fiber coupling. The video camera 18 derives an electronic image signal from the light image, said image signal being applied to a monitor 23 in order to visualize image information in the X-ray image. The electronic image signal may also be applied to an image processing unit 24 for further processing. Between the X-ray source 2 and the object 4 there is arranged the X-ray filter 6 for local attenuation of the X-ray beam. The X-ray absorptivity of individual filter elements 7 of the X-ray filter 6 is adjusted by means of an adjusting unit 50. The adjusting unit 50 is coupled to the high-voltage power supply unit 51 so that the X-ray filter 6 can be adjusted on the basis of the intensity of the X-ray beam 3 emitted by the X-ray source. FIG. 2 is a diagrammatic representation of the X-ray filter of the X-ray examination apparatus according to the invention. The X-ray filter includes a system of approximately parallel row ducts 11 which are filled with an X-ray transparent liquid 12. The X-ray filter also includes a system of approximately parallel column ducts 13 which are filled with an X-ray absorbing liquid 14. The row ducts extend approximately perpendicularly to the column ducts in the example shown. A suitable X-ray absorbing liquid is, for example a solution of a lead salt, for example lead nitrate, lead dithionate or lead perchlorate in demineralized water, or liquid mercury. A suitable X-ray transparent liquid is, for example an oil which mixes only poorly with water. The filter elements 7 in the form of capillary tubes are provided between the row ducts 11 and the column ducts 13 in such a manner that each time a filter element is connected to a row duct 11 by way of an end 30 and to a column duct 13 by way of its other end 31. More specifically, an individual capillary tube is connected, by way of a first valve 32 and via the relevant row duct 11, to a first pump 41 and, by way of a second valve 33 and the relevant column duct 13, to a second pump 42. Each of the capillary tubes is provided with a piston 34 which keeps the X-ray absorbing liquid separated from the X-ray transparent liquid. The capillary tubes have a cross-section with a dimension of approximately 1 mm. The pistons in the example shown in FIG. 2 are formed by small balls, but other bodies can also be used as pistons. The pistons accurately fit in the relevant capillary tubes so that leakage of X-ray transparent and X-ray absorbing liquid between the piston and the wall of the capillary is avoided. The pistons are made, for example of an X-ray transparent material such as glass, anorganic oxides such as aluminium oxide (Al.sub.2 O.sub.3) and silicon dioxide SiO.sub.2 or polymers such as polycarbonate. In order to achieve suitable sealing for the liquids and/or a suitable degree of friction, it is advantageous to provide the pistons with a coating layer of, for example aluminium oxide (Al.sub.2 O.sub.3) or polyimide. The row ducts 11 and the column ducts are connected to a pressure control system. The pressure control system includes the first pump 41, the row valves 32, via which the first pump 41 is connected to the individual row ducts 11, and the column valves 33, via which the second pump 42 is connected to the individual column ducts 13. Preferably, electronically controllable row and column valves are used. The pumps 41, 42 and the row and column valves 31, 32 are controlled by means of a control unit 43. To this end, the control unit 43 is connected, via bus connections 44, 45, to control inputs of the row and column valves. Furthermore, the control unit is connected to control inputs of the pumps 41 and 42. It is to be noted, however, that use can be made of a single pump instead of two separate pumps, but in that case the control unit 43 must ensure that the row valves 32 are closed when only the column ducts 13 are to be pressurized, and that the column valves 33 are closed when only the row ducts 11 are to be pressurized. The X-ray absorbing liquid and the X-ray transparent liquid in the individual row and column ducts can be pressurized by means of the pump(s), the control unit 43 and the row and column valves. The amount of X-ray absorbing liquid in the capillary tubes can be adjusted on the basis of the liquid pressure in the row and column ducts whereto the relevant capillary tube is connected. The pumps 41, 42 and the control unit 43 form part of the adjusting unit 50. Only a small amount of time is required to open the valves and to displace the pistons under the influence of the liquid pressure so as to adjust the X-ray filter. It has been found that the X-ray filter can be adjusted within 40-50 ms, or even within 10 ms, depending on the liquid pressure. The adjustment of the X-ray filter can be readily canceled by opening all valves of the ducts containing the X-ray transparent liquid, being the row ducts 11 in the example shown in FIG. 2. The capillary tubes extend approximately parallel to the X-ray beam. Using a 5 molar lead salt solution and capillary tubes having a length of from approximately 5 to 6 mm, a 100-fold attenuation of the X-ray beam can be achieved and the X-ray absorption of individual capillary tubes may deviate by a factor of 20. Cylindrical pistons can also be used instead of balls. Such cylindrical pistons offer slightly more friction with respect to the wall of the capillary tubes. Because of this friction, the pistons can remain in their respective positions until liquid pressure is applied. The row and column ducts can be comparatively simply formed in a plate of glass, quartz, silicon or a polymer by chemical etching. All references cited herein are incorporated herein, as well as the priority document European Patent Application 98200179.4 filed Jan. 23, 1998, by reference in their entirety and for all purposes to the same extent as if each individual publication or patent or patent application was specifically and individually indicated to be incorporated by reference in its entirety for all purposes.