Patent Number: 
Section: description

FIG. 1 shows diagrammatically an example of an X-ray examination apparatus 1 in which the present invention can be implemented. The X-ray source 2 emits an X-ray beam 15 for irradiating an object 16. Due to differences in X-ray absorption within the object 16, for example a patient to be radiologically examined, an X-ray image is formed on an X-ray sensitive surface 17 of the X-ray detector 3, which is arranged opposite the X-ray source. The X-ray detector 3 of the present embodiment is formed by an image intensifier pick-up chain which includes an X-ray image intensifier 18 for converting the X-ray image into an optical image on an exit window 19 and a video camera 23 for picking up the optical image. The entrance screen 20 acts as the X-ray sensitive surface of the X-ray image intensifier which converts X-rays into an electron beam which is imaged on the exit window by means of an electron optical system 21. The incident electrons generate the optical image on a phosphor layer 22 of the exit window 19. The video camera 23 is coupled to the X-ray image intensifier 18 by way of an optical coupling 24, for example a lens system or a fiber-optical coupling. The video camera 23 extracts an electronic image signal from the optical image, which signal is applied to a monitor 25 for the display of the image information in the X-ray image. The electronic image signal may also be applied to an image processing unit 26 for further processing. Between the X-ray source 2 and the object 16 there is arranged the X-ray filter 4 for local attenuation of the X-ray beam. The X-ray filter 4 comprises a large number of filter elements 5 in the form of capillary tubes whose X-ray absorptivity can be adjusted by application of an electric voltage, referred to hereinafter as adjusting voltage, to the inner side of the capillary tubes by means of the adjusting unit 7. FIG. 2 is a side elevation of an X-ray filter 4 of the X-ray examination apparatus of FIG. 1. The Figure shows seven capillary tubes by way of example, but a practical embodiment of an X-ray filter 4 of an X-ray examination apparatus in accordance with the invention may comprise a large number of capillary tubes, for example 16384 tubes in a 128-128 matrix arrangement. Each of the capillary tubes 5 communicates with the X-ray absorbing liquid 6 via an end 31. The inner side of the capillary tubes is covered by an electrically conductive layer 37, for example of gold or platinum, which layer 37 is coupled to a voltage line 35. The adhesion of the X-ray absorbing liquid to the inner side of the capillary tubes can be adjusted by means of an electric voltage applied to an electrically conductive layer 37 on the inner side of the capillary tubes 5. One end of the capillary tubes communicates with a reservoir 30 for an X-ray absorbing liquid. The capillary tubes are filled with a given quantity of X-ray absorbing liquid as a function of the electric voltage applied to the individual tubes. Because the capillary tubes extend approximately parallel to the X-ray beam, the X-ray absorptivity of the individual capillary tubes is dependent on the relative quantity of X-ray absorbing liquid in such a capillary tube. The electric adjusting voltage applied to the individual filter elements is adjusted by means of the adjusting unit 7, for example on the basis of brightness values in the X-ray image and/or the setting of the X-ray source 2. To this end, the adjusting unit is coupled to the output terminal 40 of the video camera and to the power supply 11 of the X-ray source 2. The construction of an X-ray filter 4 of this kind and the composition of the X-ray absorbing liquid are described in detail in the International Patent Application No. IB 95/00874 and in U.S. Pat. No. 5,666,396, which are both incorporated herein by reference. The height of the fluid level inside the capillary tubes is influenced by the electrocapillary pressure, also called electrowetting. The electrocapillary pressure p behaves as p=constxc2x7V2, with V the electrical potential applied between an in-capillary electrode (37 in FIG. 2) and the conducting liquid (6 in FIG. 2). The height of the fluid level inside the capillary tubes is further determined by the repelling force of the capillary tube walls and the externally applied hydrostatic pressure. In this respect it is noted that use is made of watery solutions and hydrophobic materials. The fluid level in the capillary tubes is a result of the balance between said three forces of which the electrocapillary pressure p is actively used to set the fluid level at a desired height. Dynamic measurements show that the switching takes place in 0.1-1 second (speed 1-10 cm/s, electrode length 1 cm). In an X-ray filter the liquid level in every capillary has to be individually controllable. If every capillary is connected to an individual wire, the number of required electronic control elements scales with N2. A well-know method to reduce the number of control elements to a number of the order N, is by matrix-addressing. Matrix addressing means that rows (indexed i, ixcex5{1 . . . N}, voltage Vi) are activated one-by-one while the programming signals are placed on column wires (indexed j, jxcex5{1 . . . N}, voltage Vj). In order to apply matrix addressing in an X-ray filter an electrical matrix structure is needed in every capillary tube, i.e. every capillary tube (i, j) needs to be connected to voltages Vi and Vj. Hereinafter three different examples of capillary tubes according to the invention are shown. FIG. 3 shows a cross sectional view of a first embodiment of capillary tube 45 of a device according to the invention. Capillary tube 45 comprises an electrode formed by a conducting layer, an insulator 46 and a hydrophobic coating 47. The electrode is divided into three segments 42, 43 and 44 in longitudinal direction of the tube. The segments are mutually electrically insulated and different voltages are applied thereto. A voltage of Vmem is applied to the lowest segment 42 by means of the row connection. A voltage of Vj is applied to the middle segment 43 by means of the column connection. A voltage of Vmem is applied to the highest segment 44. A voltage Vi is applied to the liquid which acts as an electrode. This embodiment is therefore also referred to as the liquid/in-capillary electrode embodiment. The segmentation of the data-electrode introduces a gap between the segments 42 and 43. In order for the liquid to rise over the above mentioned gap a threshold voltage should be applied to the data-electrode. Only for a large enough magnitude of Vixe2x88x92Vj the liquid will jump across the gap and rise above it. The gap size (i.e. the distance between the segments of the data-electrode) as well as the gap geometry determine the threshold-behavior. An example of a gap geometry that reduces the required threshold voltage is shown schematically in FIG. 6, where two electrode segments are indented in the direction of the liquid rise. In a similar fashion a local discontinuity of geometry (such as a local shape change, local opening-up and/or constriction of the capillary), of insulator properties (such as thickness or dielectric constant) or of hydrophobicity introduces a nonlinear behavior of liquid level versus applied voltage. The structuring of the metallic electrodes, insulator thickness or hydrophobic coating can be achieved in a filter that is composed of semi-planar plates or of foils. FIG. 4 shows a cross sectional view of a second embodiment of a capillary tube 55 of a device according to the invention. This embodiment is referred to as the in-capillary/in-capillary electrode embodiment. Capillary tube 55 comprises an electrode formed by a conducting layer, an insulator 56 and a hydrophobic coating 57. The electrode is divided into three segments 52, 53 and 54 in longitudinal direction of the tube. The segments are mutually electrically insulated and different voltages are applied thereto. A voltage of Vi is applied to the lowest segment 52 by means of the row connection. A voltage of Vj is applied to the middle segment 53 by means of the column connection. A voltage of Vmem is applied to the highest segment 54. A voltage Vliq with approximately zero value is applied to the liquid. At the mouth 51 of the capillary the electrodes 52 and 53 act as a valve between the supply channel and the bulk of the capillary thus introducing a threshold-like behavior. Only when a voltage with a predetermined value is applied to electrode 52 as well as to electrode 53 the liquid fills the capillary. The liquid does not rise if only one of the electrodes is activated. Due to the limited velocity of liquid movement (speed 1-10 cm/s), the meniscus senses a time-averaged electrocapillary pressure. To avoid that programmed rows loose their pattern in the time that other rows are being programmed, a memory function has to be added to every capillary tube. Such a memory function can be achieved with an extra electrode in the capillaries. This electrode can be shared by more than one capillary as is the case with Vmem in FIG. 3. The value of Vmem is chosen such that the capillary tube 55 remains filled once the liquid has risen, independent of the value of voltages Vi and Vj. This is referred to as the xe2x80x98memory effectxe2x80x99. The memory effect is advantageous because it allows for the sequential addressing of several rows. In order to empty the filled capillaries, a zero voltage should be applied to electrodes 52, 53 and 54 resulting in a reset of the capillaries. After resetting the capillaries can be reprogrammed. Preferably every row of capillaries has a separate Vmem connection allowing the resetting to occur in a row-wise fashion. FIG. 5 shows a cross sectional view of a third embodiment of a capillary tube of a device according to the invention. This embodiment is also referred to as the improved in-capillary/in-capillary electrode embodiment. FIG. 5 shows a preferred embodiment of an electrode architecture that allows filling and emptying of a single capillary tube. Every capillary tube is provided with an electrode sequence 62, 63, 64, 68, 69, whereby a voltage Vi is applied to electrode segments 62 and 69, a voltage Vj is applied to electrode segments 63 and 68 and a voltage Vmem is applied to electrode segment 64 so that the voltage sequence is Vi/Vj/Vmem/Vi/Vj. Vmem indicates the voltage applied to the memory electrode, electrode segment 64 in this case. The conducting fluid is supplied from below. In the examples given below, V=0 means that no potential is applied, so that the electrode is being de-wetted. V=1 means that a high potential is applied (e.g. V greater than 200 V), so that the electrode is being wetted. Suppose that all capillary tubes are empty and we want to fill only one capillary tube (i=n, j=m). We can do this by applying for example: Vi=1 for all i, Vj=0 for all j, and then Vmem=1, Vi=1 for all i, Vj=0 for all j, and then Vmem=1, Vi=0 for all i, Vj=0 for all j, and then Vmem=1, Vi=n=1, Vixe2x89xa0n=0, Vj=m=1, Vjxe2x89xa0m=0, (the capillary fills) and then Vmem=1, Vi=0 for all i, Vj=0 for all j. Suppose that all capillary tubes are full and we want to empty only one capillary tube (i=n, j=m). We can do this by applying for example: Vi=1 for all i, Vj=0 for all j, and then Vmem=0, Vi=1 for all i, Vj=0 for all j, and then Vmem=0, Vi=1 for all i, Vj=1 for all j, and then Vmem=0, Vi=n=0, Vixe2x89xa0n=1, Vj=1 for all j, and then Vmem=0, Vi=n=0, Vixe2x89xa0n=1, Vj=m=0, Vjxe2x89xa0m=1, (the capillary empties) and then Vmem=0, Vi=1 for all i, Vj=0 for all j, and then Vmem=1, Vi=1 for all i, Vj=0 for all j, and then Vmem=1, Vi=0 for all i, Vj=0 for all j. It is noted that grey-scale programming is possible when the architecture of FIG. 5 is repeated several times in a capillary tube. FIG. 6 schematically shows an embodiment of a capillary tube 71 according to the invention comprising two electrode segments 72 and 73, which are indented in the direction of the liquid rise. This gap geometry reduces the required threshold voltage. It will be understood that the indented parts can have a variety of shapes apart from the rectangular shape that is shown. It will be clear for a person skilled in the art that in the matrix structures described above the functions of rows and columns are interchangeable. Summarizing the invention provides the insight that in a device as described above the construction of the fluid elements can be designed such that it induces the desired threshold like behavior of the fluid rise without the necessity of extra components. The invention is of course not limited to the described or shown embodiment(s), but generally extends to any embodiment, which falls within the scope of the appended claims as seen in light of the foregoing description and drawings.