Patent Number: 
Section: description

FIG. 1 presents a schematic view of an X-ray apparatus comprising a filter in accordance with 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. 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 which is in this case an image intensifier pick-up chain. The X-ray image is formed on the entrance screen 5 of the X-ray 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 by means of various filter elements 13 in the form of tube-like structures in line with the X-ray beam, whose X-ray absorptivity can be adjusted by application of electric voltages to an electrode located in a wall of each filter element. The electric voltages are adjusted by voltage supply means 14 on the basis of, for example brightness values of the X-ray image and/or on the basis of the setting of the X-ray source; to this end, the adjusting circuit is coupled to the power supply 15 of the X-ray source and to the output terminal 16 of the video camera 9. Part of the light of the exit window is guided, by way of a splitting prism 19, to an exposure control system 20 which derives a control signal from the light image in order to control the high voltage supply on the basis of image information of the image on the exit window. In order to receive image information of the image on the exit window 7, the voltage supply means 14 of the filter 12 are coupled to the exposure control system 20, so that the filter 12 can be adjusted on the basis of the image on the exit window 7. The filter 12 is constructed and oriented in such a way that the filter elements 13 extend approximately in the propagation direction of the X-ray beam 2; a uniform spatial resolution of the spatial X-ray absorption pattern is thus achieved across the cross-section of the X-ray beam. Alternatively, the filter 12 can also be constructed in such a manner that the filter elements 13 extend approximately parallel to one another; when the X-ray beam 2 diverges, it is thus achieved that substantially all X-rays pass at least partly through a filter element, so that the X-ray cannot pass between two filter elements without being attenuated. The voltage supply means apply electric voltages to the electrodes located in the walls of the filter elements 13 so as to influence the adhesion of the X-ray absorption fluid to the inner surface of filter elements. In order to adjust a filter element to a high X-ray absorptivity, an electric voltage of the first value is applied to the wall of the relevant filter element by the voltage supply means 14, the relevant filter element then being filled with the X-ray absorbing fluid from the reservoir 17 via supply channels 18 by strong adhesion of the X-ray absorbing fluid to the inner side. In order to adjust a filter element to a low X-ray absorptivity, the voltage supply means 14 apply an electric voltage of the second value, for example equal to the potential of a reference electrode in the inner volume of the filter element, to the wall of the filter element, the X-ray absorbing fluid the exhibiting poor adhesion to the inner surface of the filter element, so that this filter element is not filled with the X-ray absorbing fluid from the reservoir 17. The general construction of the filter 12 and the composition of the X-ray absorbing fluid is described in more detail in American U.S. Pat. No. 5,625,665 (PHN 15.044). The construction of a filter element is described in more detail with reference to FIG. 2. FIG. 2a presents a schematic sectional view of a filter element 13 of the filter 12 of FIG. 1. The filter element 13 is filled by the X-ray absorption fluid 32 via a supply channel 30. In this case the X-ray absorption fluid 32 forms one homogeneous liquid which combines electrically conducting properties and X-ray absorbing properties. For each filter element in the filter 12 one defines a length direction z and an inner volume 31, which volume is limited by the walls 38 of the corresponding filter element. Each filter element comprises a first electrode 33 located in the wall 38 in the form of an electrically conducting layer being electrically isolated form the inner volume 31 by means of en isolator layer 44, an inert coating layer 35 being located on the inner surface of the walls 38 and a second electrode 39 to supply an electrical potential to the X-ray absorption fluid 32. The electrically conducting layer 33 of the filter element 13 is coupled to a switch, which is this example is a drain contact 40 of a field effect transistor 45, which source contact 41 is connected to voltage supply means 36. The field effect transistor 45 is put in a conduction mode, in other words the switching element is closed by a control voltage from a control voltage supply line 37 supplied to a gate contact 34 of the field effect transistor 45. By doing this, one applies an electric voltage to the first electrode 33 from the voltage supply means 36. When the voltage supply means 36 supply a voltage of the xe2x80x98fillxe2x80x99-voltage value, the contact angle xcex8 formed by the meniscus of the X-ray absorption fluid 32 with the inert coating layer 35 decreases and the filter element 13 is filled with the X-ray absorption fluid. FIG. 2b shows a schematic section of a filter element 113 of the filter 12 of FIG. 1 in case the X-ray absorption fluid comprises two mutually not mixable first and second fluid components, wherein the first fluid component 132 is a liquid with electrically conducting properties and neglectable X-ray absorption properties and the second fluid component 134 is an electrically isolating liquid with high X-ray absorption properties, each fluid component being supplied to the filter element 113 from its own supply channel 120 and 121, respectively. The other functional parts of the filter element 113 are similar to those of the filter element of the FIG. 2a, resulting in a similar control of the level of the first component in the inner volume 31 of the filter element 113. The level of the second fluid component in the inner volume of the filter element is, thus, passively determined by the level of the first fluid component therein, due to the fact that these two fluids have a mutual separation plane 130. It is also possible to design a filter element, where the first and second fluid components do not have a mutual plane, but are separated by a layer of a gaseous phase, which is further not shown in the FIG. 2b. The degree of the X-ray absorption in these cases is determined by the length of the column of the second component in the inner volume of the filter element 113. FIG. 3 gives a schematic functional representation of a filter element of FIG. 2, where the first electrode is segmented in the length direction z of the filter element 213 and forms two electric subgroups 123 and 124, respectively. The function of the filter element will be described for the case that the X-ray absorption fluid 32 is a liquid solution between an electrically conducting fluid component and an X-ray absorbing fluid component and is supplied into the filter element 213 from the supply channel 30. FIG. 3a presents a schematic view on the subgroups 123 and 124 of the filter element that are controlled by the voltage supply means 136. An electrical switch 138 alters the voltage supply from one subgroup of electrode segments to another. Also, FIG. 3a gives a schematic representation of the fluid level detection circuit, comprising an AC source 50 in the form of a sinus wave generator and an AC detector 60. A switching control unit 70 performs a timing control of the voltage switching between subgroups 123 and 124 by the switch 138. FIG. 3b gives a temporal representation of the voltage pulses as supplied by the voltage supply means 136 to the corresponding subgroups of the first electrode. In order to transport the X-ray absorption fluid 32 from the supply channel 30 into the inner volume 31 of the filter element 213 and further on within the inner volume, the electrical switching between the subgroups of the first electrode has to be performed in a controlled way by the switching control unit 70, based on the results of the fluid level detection within each segment. FIG. 3c presents the corresponding course of the signal s from the AC detector 60 with time. From the FIG. 3c it follows that each rising part 90 of the signal s of the AC detector 60 corresponds to the fluid rising within a corresponding segment and each plateau 92 corresponds to the moment in time when the fluid has filled the corresponding segment. The insight of this interpretation is based on the equivalent electric circuit of the filter element of this type, which is given by FIG. 4. FIG. 4 presents an equivalent electrical circuit for a detection of the fluid level within a filter element, wherein the first electrode in segmented and two electrical subgroups based on resulting segments are formed. From FIG. 4a it follows that each subgroup of segments can be described by a variable electric capacitance 140, which is formed between each subgroup of segments and the fluid, the value of the capacitance being a function of the degree of filling of that subgroup by the fluid. The voltage supply means 136, initiating the filling of a subgroup of the filter elements, supply the voltage to these subgroups via switch 138. The measuring means comprise an AC source 50 and the electrical circuit of the detectors 100 and 102. In order to prevent the AC signal from being shorted by the voltage supply means 136, resistors 137 and 139 in the Mxcexa9 range are provided between the AC source 50 and the voltage supply means 136. For measuring the electric capacitance of a filter element 140, a capacitive voltage divider is provided, comprising the equivalent capacitance from each subgroup 140 and an additional electric capacitances 133 and 135, which are in this example in the order of 20 pF. FIG. 4b presents the resulting signals V1 and V2 as measured by the measuring means 100 and 102, respectively. In this example the detected signal is the voltage across the segment. As the fluid rises within the segment, the corresponding electric impedance of the segment decreases, leading to the decrease of the voltage across the segment. When the fluid has risen to the maximum level, the measured voltage reaches a constant value. FIG. 4c gives a further improvement of the detection method of the fluid level within the filter element. According to the FIG. 4a, the measuring means 100 and 102 can supply the signals from the detector to the difference amplifier 101, schematically sketched in the FIG. 4c, with the resulting signal 103 at the output of the difference amplifier as given in the graph of FIG. 4c. By detecting the plateau of this signal the switching between the corresponding subgroups can be enabled.