Patent Number: 
Section: description

FIG. 1 shows diametrically an X-ray apparatus which includes a filter in accordance with the invention. The X-ray source 1 emits an X-ray beam 2 which 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 and X-ray image is formed on the X-ray detector 4 which in this case an image intensifier 6 and is converted into a light image on the exit window 7; this 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 various 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 which can be derived from the signal present 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 thereof are described in greater detail in U.S. Pat. No. 5,625,665 (PHN 15.044). FIG. 2a is a diagrammatic sectional view of the 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 formed by one electrically conductive and X-ray absorbing liquid. For each filter element the longitudinal direction z and the internal volume 21 are defined, the latter being bounded by the walls 28 of the filter element. Each filter element includes the first electrode 23 in the form of an electrically conductive layer which is electrically isolated from the liquid filling in the internal volume 21 by means of an isolator layer 34, an inert cover layer 24 which is provided on an inner side of the walls 28, and a second electrode 29 for applying an electric potential to the liquid filling. The electrically conductive layer 23 of the filter element 13 is coupled to a switching element which, in the present embodiment, is formed by a drain contact 30 of a field effect transistor 25 whose source contact 31 is coupled to a power supply circuit 26. The field effect transistor 25 is turned on, i.e. the switching element is closed, by means of a control voltage which is applied, via the control line 27, to a gate contact 32 of the field effect transistor 25. The electric voltage of the voltage line 26 is applied to the electrically conductive layer 23 by closing the switching element. When the voltage line is adjusted to the value of the xe2x80x9cfillingxe2x80x9d voltage, the contact angle xcex8 between the liquid filling 22 and 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 in case the filter element is filled with a liquid filling composed of an electrically conductive liquid component 122 and an X-ray absorbing liquid component 124 which is not miscible therewith. The liquid components are supplied via respective supply ducts 120 and 121. The other functional parts of the filter element 113 are substantially identical to those of the filter element 13, so that the control chart for the electrically conductive liquid component can be executed in a similar manner. This control chart determines 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. 3a is a diagrammatic cross-sectional view of a first embodiment of the tubular filter element 13. In this embodiment the filter element 13 has a circular cross-section whereas, generally speaking, the cross-section of the filter element may be a polygon. The filter element contains the liquid filling 22 which is in contact with the inert cover layer 24. The liquid filling is electrically isolated from the first electrode 23 by means of the isolator layer 34; this involves a capacitance per unit of surface area of the filter element. The electrode 23 is provided on a substrate 38. FIG. 3b is a 360xc2x0 view of the projection of the electrode 23 on the substrate 38. In order to enable local variation of the capacitance per unit of surface area in the longitudinal direction z of the filter element, the electrode 23 in this embodiment is subdivided into successive first electrode segments 37 and second electrode segments 39 of different surface area. The voltage line 27 enables application of the electric voltage to the electrode 23. FIG. 4 is a diagrammatic sectional view of a second embodiment of the filter element 13. The filter element in this embodiment is provided with an isolator layer which is composed of different isolator segments. The isolator layer 134 is subdivided into first isolator segments 136 and second isolator segments 138 which succeed one another in the longitudinal direction z of the filter element. The first isolator segment 136 has a dielectric constant which is higher than that of the second isolator segment 138, thus enabling a local variation of the capacitance per unit of surface area in the longitudinal direction of the filter element. This step enables the step-wise filling of the filter element 13 with the liquid filling 22. FIG. 5 is a diagrammatic sectional view of the third embodiment of the filter element 13 which is provided with an isolator layer composed of different isolator layer segments. The isolator layer 234 is subdivided into first isolator layer segments 236 and second isolator layer segments 238 which succeed one another in the longitudinal direction of the filter element. The thickness of the first isolator layer segment 236 is greater than that of the second isolator segment 238, thus enabling a local variation of the capacitance per unit of surface area in the longitudinal direction of the filter element. This step enables the step-wise filling of the filter element 13 with the liquid filling 22. In order to realize an optimum effect of this embodiment it is advantageous when the diameter of the tubular filter element 13 is reduced only slightly at the area of the first isolator layer segment 236.