Patent Number: 
Section: description

FIG. 1 shows diagrammatically an X-ray apparatus with a filter according to the invention. The X-ray source 1 emits an X-ray beam 2 whereto an object 3, for example a patient to be examined, is exposed. Due to local differences in the absorption of X-rays in the object 3, an X-ray image is formed on the X-ray detector 4, being an image intensifier pick-up chain in the present example. The X-ray image is formed on the entrance screen 5 of the X-ray image intensifier 6 and is converted into an optical image on the exit window 7 which is imaged on a video camera 9 by means of a system of lenses 8. The video camera 9 forms an electronic image signal from the optical image. For example, for the purpose of further processing the electronic image signal is applied to an image processing unit 10 or to a monitor 11 on which the image information contained in the X-ray image is displayed. A filter 12 for locally attenuating the X-ray beam 2 is arranged between the X-ray source 1 and the object 3. The filter 12 includes a plurality of tubular filter elements 13, the X-ray absorptivity of which is adjustable by application, by way of an adjusting circuit 14, of electric voltages to the wall of the filter elements. The electric voltages are adjusted, for example, on the basis of the setting of the X-ray source 1 with 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 at the output terminal 16 of the video camera 9. The general construction of such a filter 12 and the composition of the absorption liquid are described in 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. Via the supply channel 20 the filter element 13 is filled with the liquid filling 22 which is electrically conductive and X-ray absorbing. For each filter element there are defined the longitudinal direction z and the internal volume 21, 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 insulated, by means of an insulation layer 34, from the liquid filling present in the internal volume 21, 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 is in this case formed by a drain contact 30 of a field effect transistor 25, its source contact 31 being coupled to a voltage line 26. The field effect transistor 25 is turned on, that is, the switching element is closed by means of a control voltage which is applied to a gate contact 32 of the field effect transistor 25 via the control line 27. 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 of 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, the filter element now being filled with the liquid filling consisting of an electrically conductive liquid component 122 and an X-ray absorbing liquid component 124. The liquid components are supplied via respective supply channels 120 and 121. The further 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 similar. This control chart determines the level of the electrically conductive liquid component 122 in the internal volume 21 of the filter element 113 which itself 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 360xc2x0 view of the electrode segments 23 on a substrate 38 in a first embodiment of the filter element 13 according to the invention, the electrode segments bearing even sequence numbers 40 in the series and the electrode segments bearing odd sequence numbers 41 in the series constituting respective sub-groups and the electrical device being provided with switches 25 for controlling said sub-groups. The field effect transistors in this embodiment again act as switching elements. It is an advantage of the present embodiment that the electrical wiring is simplified. For optimum transport of the liquid filling in the longitudinal direction z of the filter element, in this embodiment a given time overlap is desired between the pulses of the electric xe2x80x9cfillingxe2x80x9d voltages applied to both sub-groups. As soon as the meniscus of the liquid filling is present in the Nth electrode segment to be filled, the voltage applied to the electrode segments of the other sub-group must be adjusted to the value of the xe2x80x9cdrainingxe2x80x9d voltage, unless transport to the N+1th electrode segment is required. In order to make the latter switching superfluous, a second embodiment of the filter element 13 is presented in which each electrode segment is connected to the electrical device via a respective connection. FIG. 4 is a 360xc2x0 view of the projection of the electrode segments 23 on a substrate 38 in this embodiment. For optimum transport of the liquid filling in the longitudinal direction z of the filter element, a given time overlap is desired between the pulses of the electric xe2x80x9cfillingxe2x80x9d voltages applied to both sub-groups in this embodiment. FIG. 5 is a diagrammatic 360xc2x0 view of the electrode segments of the filter element 13 in a fourth embodiment according to the invention, wherein the facing edges of directly successive electrode segments are provided with meshing teeth. This step enhances the reliability of the transport of the liquid filling in the longitudinal direction z of the filter element. FIG. 5a shows an example of the crenellation-like teeth 37 and 39 of the electrode segments of the type 123. FIG. 5b shows an example of the sawtooth-like teeth 50 of the electrode segment of the type 223.