Patent Number: 
Section: description

FIG. 1 shows diagrammatically an X-ray examination apparatus that includes a filter in accordance with the invention. The X-ray source 1 emits an X-ray beam 2 that irradiates an object 3, for example a patient, to be examined. As a result of local differences in the absorption of X-rays in the object 3 an X-ray image is formed on the X-ray detector 4, in this case being an image intensifier pick-up chain. The X-ray image is formed on the entrance screen 5 of the X-ray image intensifier 6 and is converted into a light image on the exit window 7, which light image is imaged on a video camera 9 by means of a lens system 8. The video camera 9 forms an electronic image signal from the light image. The electronic image signal is applied, for example for further processing, to an image processing unit 10 or to a monitor 11 on which the image information in the X-ray image is displayed. Between the X-ray source 1 and the object 3 there is arranged a filter 12 for local attenuation of the X-ray beam 2. The filter 12 includes several tubular filter elements 13 whose X-ray absorptivity can be adjusted by application of electric voltages to the wall of the filter elements by means of an adjusting circuit 14. The electric voltages are adjusted, for example, on the basis of the setting of the X-ray source 1 by means of the power supply 15 of the X-ray source and/or on the basis of, for example, brightness values of the X-ray image that can be derived from the signal on the output terminal 16 of the video camera 9. The general construction of a filter 12 of this kind and the composition of the liquid filling are described in greater detail in United States patent U.S. Pat. No. 5,625,665 (PHN 15.044). FIG. 2a is a diagrammatic sectional view of a tubular filter element 13 of a filter as shown in FIG. 1. The filter element 13 is filled, via the supply duct 20, with the liquid filling 22 which is electrically conductive and X-ray absorbing. For each filter element there is defined the longitudinal direction z as well as the internal volume 21 that is bounded by the walls 28 of the filter element. Each filter element includes a first electrode 23 in the form of an electrically conductive layer which is electrically isolated from the liquid filling 22 present in the internal volume 21, said isolation being realized by way of an isolator layer 34 and an inert cover layer 24 that is provided on an inner side of the walls 28, and also includes a second electrode 29 for applying an electric potential to the liquid filling. The first electrode 23 of the filter element 13 is coupled to a switching element which forms part of the first means for applying an electric voltage to an individual filter element. In the present example the switching element consists of a drain contact 30 of a field effect transistor 25 whose source contact 31 is coupled to a voltage line 26 that forms part of an electrical control device (not shown). The field effect transistor 25 is turned on, that is, the switching element is closed, by means of a control voltage that is applied to a gate contact 32 of the field effect transistor 25 via the control line 27. The electric voltage on the voltage line 26 is applied to the first electrode 23 by closing the switching element. When the voltage line is set to the value of the xe2x80x9cfillingxe2x80x9d voltage, the contact angle O enclosed by the liquid filling 22 relative to the inert cover layer 24 decreases and the relevant filter element is filled with the liquid filling. FIG. 2b is a diagrammatic sectional view of the tubular filter element 113 of a filter as shown in FIG. 1 when the filter element is filled with the liquid filling that consists of an electrically conductive liquid component 122 and an X-ray absorbing liquid component 124. In this case the liquid components are not miscible. The liquid components are applied via respective supply ducts 120 and 121. The other functional parts of the filter element 113 are substantially the same as those of the filter element 13, so that the electric control circuits for the electrically conductive liquid component can be constructed in a similar manner. The control circuits determine the level of the electrically conductive liquid component 122 in the internal volume 21 of the filter element 113 which in its turn determines the level of the X-ray absorbing liquid component 124 in the filter element 113, because the respective components constitute one common liquid column with an interface 130. The degree of X-ray absorption is in this case determined by the degree of filling of the filter element 113 with the X-ray absorbing component 124. FIG. 3 is a diagrammatic representation of a filter 12 in accordance with the invention in which the liquid filling comprises two liquid components 222, 224 that are not miscible, each liquid component being applied to the filter 22 from a respective liquid reservoir 126, 128. The filter 12 is provided with a hydrostatic pressure control system in the form of two liquid reservoirs. The positions of the liquid reservoirs 126, 128 relative to one another and to the filter 12 can be varied. The resultant hydrostatic pressure in the filter is thus determined. Each liquid component 222, 224 is applied to the matrix of filter elements via a flexible duct 127, 129 and a corresponding common supply duct 220, 221. In the present example the liquid reservoirs are shown as reservoirs that are isolated from one another (path 126, 13, 128). It is also possible to interconnect the liquid reservoirs 126, 128 by way of a tube 125 that is denoted by a dashed line. The function of the tube 125 is to create a system that is completely closed relative to the environment, so as to counteract evaporation of liquid. The assembly can be mounted in the head of an X-ray apparatus which is not shown in FIG. 3. A hydrostatic pressure in the system of filter elements is determined by the densities of the liquid components 222, 224 and by the heights of the liquid reservoirs 126, 128 relative to one another. For a given ratio of the densities of the liquid components a change in the hydrostatic pressure can be compensated by changing the heights of the liquid reservoirs. The filter is provided with measuring means in the form of a hydrostatic pressure meter 131 in order to measure the effect of the orientation of the filter 12 as a whole on the hydrostatic pressure in the filter elements 13 that is due to a rotation of the gantry of the X-ray apparatus. In the present example the hydrostatic pressure meter is arranged in the liquid supply duct 220, but it may also be arranged in a different location. It is also possible to provide two hydrostatic pressure meters, that is, one in the liquid supply duct 220 and the other in the liquid supply duct 221. The change of the hydrostatic pressure is thus measured across a meniscus that separates the liquid components 222 and 224 from one another. The filter 12 is calibrated for optimum operation in a reference position; a reference hydrostatic pressure corresponds thereto. As soon as the hydrostatic pressure meter 131 detects a deviation in the hydrostatic pressure, the height of a liquid reservoir is changed. In the present example this procedure involves the control of further control means (not shown) in the form of drive motors for the liquid reservoirs, the relative height of the liquid reservoirs thus being changed. In order to realize the desired rise of the liquid filling in a filter element, a given electric voltage is applied to the first electrode via an electrode 140. The degree of X-ray absorption is determined by the degree of filling of the filter element 13 with an X-ray absorbing liquid component. FIG. 4a is a diagrammatic sectional view of the filter 12 in accordance with the invention in which the liquid reservoir 150, 150xe2x80x2 includes filter elements 13xe2x80x2, 13xe2x80x3. In this case the filter elements 13xe2x80x2, 13xe2x80x3, belonging to the reservoir volume, are situated in the periphery of the overall construction. In addition to a compact construction, there is the advantage that the use of integrated liquid reservoirs 150, 150xe2x80x2 offers a reduction of the number of technological steps required for the manufacture of a filter of this type. In the case of an integrated liquid reservoir the filter is provided with the hydrostatic pressure control system in the form of an active pump 160 which keeps the hydrostatic pressure, measured by a hydrostatic pressure meter 131 at a given level as shown in FIG. 4b. FIG. 4b illustrates the case where the orientation of the filter as a whole (h) is moved through an angle (xcex2) relative to the vertical direction (g). The associated changes in the hydrostatic pressure are measured by the measuring means 131 and are compensated by the hydrostatic pressure control system in the form of the active pump 160. A desired height of the column of the liquid filling is in this case also determined by an electric voltage applied to the first electrode of a filter element 13xe2x80x2, 13xe2x80x3 and by the hydrostatic pressure. It is known that the absolute value of the filling voltage, or the voltage corresponding to the maximum height of the liquid column in a filter element, is dependent on a hydrostatic pressure in the system of filter elements 13. FIG. 5 shows diagrammatically a variation of the curve of the height of the liquid column as a function of the electric voltage applied to the first electrode, which curve is referred to hereinafter as the h/V curve. A further embodiment of the filter in accordance with the invention utilizes measuring means in the form of a calibrated reference filter element which is arranged, for example, in one of the liquid reservoirs 126,128. The reference filter element is calibrated in respect of the reference hydrostatic pressure in the filter. The calibration curve 300 represents the variation of the height of the column of the liquid filling in the internal volume of the filter element as a function of the applied electric voltage. It follows from FIG. 5 that in the reference condition the height of the column of the liquid filling increases when the value of the electric voltage becomes higher than the drain voltage Vleeg, the maximum height of the column of the liquid filling being reached at the value of the electric filling voltage Vvul. In a condition of the filter that deviates from the reference condition, the hydrostatic pressure assumes a value that deviates from the reference value. FIG. 5 shows a deviating variation of the h/V curve 301. The change in the variation of the h/V curve in the reference filter element, for example as represented by the curve 301, is decisive in respect of the change of the hydrostatic pressure. This change can again be compensated by means of a hydrostatic pressure control system in the form of, for example, the active pump 160 (FIG. 4b). As will be evident to those skilled in the relevant art, in the case of a large matrix of filter elements a local variation will occur in the hydrostatic pressure for a rotated position of the filter. This variation can influence the reproducibility of the height of the column of the liquid filling. In order to limit such a variation, FIG. 6 illustrates diagrammatically a further embodiment of the filter in accordance with the invention in which the matrix of filter elements is subdivided into a number of hydrostatically isolated sub-filters 212, 213, 214, 215. Each sub-filter is connected to a corresponding liquid sub-reservoir 250, 251, 252, 253, said liquid sub-reservoirs being integrated with the system of sub-reservoirs in the present example. When the filter 12 is thus subdivided into sub-filters, a distance between two filter elements 13 that are situated furthest apart in the matrix is reduced and hence the local variation of the hydrostatic pressure is also reduced. In this case each sub-filter is provided with its own pump and its own hydrostatic pressure meter in conformity with the principle shown in FIG. 4.