Patent Number: 062263552
Section: description

FIG. 1 shows an embodiment of an X-ray examination apparatus. The X-ray source 2 emits an X-ray beam 15 in order to irradiate an object 16. Due to differences in the 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 so as to face the X-ray source. The X-ray detector is provided, for example with an image intensifier pick-up chain which includes an X-ray image intensifier 18 for converting an X-ray image into a light image on an exit window 19, and a video camera 23 for picking up the light image. An entrance screen 20 acts as an X-ray-sensitive surface which converts incident X-rays into an electron beam which is imaged onto an exit window 19 by means of an electron optical system 21. The incident electrons generate the light image by way of a phosphor layer 22 of the exit window. The video camera 23 is optically coupled to the X-ray image intensifier 18 by way of an optical coupling. The optical coupling includes, for example a lens system or an optical fiber coupling 24. The video camera derives an electronic image signal from the light image and applies it to a monitor 25 in order to visualize the image information of the X-ray image. The electronic image signal can also be applied, for example to an image processing unit 26 so as to be processed further. In order to realize local attenuation of the X-ray beam so as to adjust a two-dimensional intensity profile, an X-ray filter 4 is arranged in the X-ray beam 15 between the X-ray source 2 and the object 16. The X-ray filter includes a large number of filter elements 5. Furthermore, a filter element 5 preferably includes a capillary tube whose inner side is covered with an electrically conductive layer. The capillary tubes communicate, via a first opening, with a reservoir (not shown in the Figure) containing an X-ray absorbing liquid. The construction of such an X-ray filter and the composition of the X-ray-absorbing liquid are known from the cited European patent application EP-A-740839. The X-ray absorptivity of X-ray filters of this kind can be adjusted by application, using an adjusting unit 7, of electric voltages across the inner side of the capillary tubes 5 and the X-ray-absorbing liquid. This is because the adhesion of the X-ray-absorbing liquid to the inner side of the capillary tubes is dependent on the electric voltage applied across the inner side of the capillary tubes and the X-ray-absorbing liquid. The capillary tubes are filled with a given quantity of X-ray-absorbing liquid in dependence on the electric voltage across the individual tubes 5 and the X-ray-absorbing liquid. Because the capillary tubes extend approximately parallel to the lines from the X-ray source 2 via the X-ray filter 4 to the X-ray detector 3, the X-ray absorptivity of the individual capillary tubes is dependent on the relative amount of X-ray absorbing liquid present in such a capillary tube. The electric adjusting voltages for the individual filter elements are adjusted by means of the adjusting unit 7 while taking into account the brightness values in the X-ray image and/or the adjustment of the X-ray source 2. To this end, the adjusting unit 7 is coupled to an output terminal 40 of the video camera 23 and to the power supply 11 for the X-ray source 2. FIG. 2 is a sectional view of an X-ray filter in a first embodiment of the X-ray examination apparatus according to the invention. In FIG. 2 the cross-sections of capillary tubes 5 are oriented in a longitudinal direction towards a center F.sub.2 of, for example a curved surface. A first cross-section 52 of the X-ray filter preferably comprises in a first direction a first segment of a first circle having a first radius CR.sub.1 whereas a second cross-section of the X-ray filter comprises, in a second direction which is perpendicular to the first direction, a second segment of a second circle having a second radius CR.sub.2. A spherical entrance surface of the X-ray filter 5 is obtained, for example when the first radius CR.sub.1 is chosen so as to be equal to the second radius CR.sub.2. For such a spherical surface the radii CR.sub.1 and CR.sub.2 amount to, for example 10 cm. The resolution uniformity of the X-ray filter 4 is enhanced by arranging the tubes 5 in the X-ray filter in this manner. Furthermore, in practice the cross-sections 53 of the capillary tubes 5 amount to, for example approximately 300 .mu.m and the wall thickness 50 of the capillary tubes amounts to, for example 10 .mu.m. The number of capillary tubes 5 in the X-ray filter 4 amounts to, for example 256.sup.2, said number being arranged in a square matrix of 256.times.256 tubes. The positioning of the X-ray filter 4 in the first embodiment of the X-ray examination apparatus 1 according to the invention will be described with reference to FIG. 3. FIG. 3 shows an X-ray source 2 which has a focus with a first focus radius R.sub.1 and a center F.sub.1. FIG. 3 also shows an embodiment of an X-ray filter 4 whose tubes 5 are arranged on a spherical surface as shown, for example in FIG. 2. A center 60 of the X-ray filter, being coincident with a center of the spherical surface, a cross-section 52 of which is shown in FIG. 3, contains a center of the first circle as well as a center of the second circle. FIG. 3 also shows an X-ray sensitive surface 17 of the X-ray detector 18. In order to ensure that the lines trough all the longitudinal axes of the tubes intersects the lines from the X-ray source via the X-ray filter to different points of the X-ray detector, the distance from a first point F.sub.2 of a center 60 of the spherical surface of the X-ray filter 5 to a midpoint F.sub.1 of a focus of the X-ray source 2 is preferably chosen to be at least equal to a focus radius R.sub.1 of the focus. In practice the focus radius R.sub.1 amounts to, for example 150 .mu.m and said distance thus amounts to, for example 150 .mu.m. X rays starting from the X-ray source travelling to different points of the X-ray detector then traverses a substantial part of the absorbing liquid of at least one tube of the X-ray filter so that efficient X-ray absorption is achieved. The absorption efficiency in the X-ray-absorbing liquid is thus enhanced, and hence also the dynamic range of the X-ray filter. As a result of this step the theoretically feasible dynamic range is approached A cylindrically symmetrical surface can be used as an alternative for a spherical surface of the X-ray filter 5. The center 60 of the cylindrically symmetrical surface then contains a line. FIG. 4 shows an example of the positioning of the X-ray filter 4 in a second embodiment of the X-ray examination apparatus according to the invention. The second embodiment of the X-ray examination apparatus includes an X-ray source 2 with a focus having a first focus radius R.sub.1 and a center F.sub.1, an X-ray filter 4 in which the capillary tubes 5 are arranged so as to extend parallel to one another, and an X-ray detector 18. FIG. 4 also shows an X-ray-sensitive surface 17 of the X-ray detector 18. The X-ray filter 4 comprises, for example 256.sup.2 capillary tubes. The length of such capillary tubes 5 amounts to, for example 25 mm and their cross-section is, for example 200 .mu.m. Furthermore, the entrance surface 54 of the X-ray filter extends parallel to the image plane of the X-ray-sensitive surface 17 of the X-ray detector 18. In order to enhance the dynamic range of the X-ray filter 5, an angle .alpha. enclosed by a longitudinal direction of a tube 5 of the X-ray filter 4 with respect to a second normal l.sub.2 to the X-ray-sensitive surface 17 of the X-ray detector 18 is chosen to be equal to the arc tan of the ratio of a first distance D.sub.1 between a center of an entrance surface 54 of an X-ray filter 4 and a tube 55 near an edge of the X-ray filter 5 to a second distance D.sub.2 between the center of the entrance surface 54 and a center F.sub.1 of the focus of the X-ray source 2. It is thus achieved that the lines trough all the longitudinal axes of the tubes intersects the lines from the X-ray source 2 via the X-ray filter 4 to different points of the X-ray detector 18 and that X rays starting from the X-ray source travelling to different points of the X-ray detector traverses a substantial part of the absorbing liquid of at least one tube of the X-ray filter so that efficient X-ray absorption is achieved. The distance D.sub.1 in practice amounts to, for example approximately 25 mm whereas the distance D.sub.2 amounts to approximately 100 mm. The angle .alpha. then amounts to approximately 14 degrees. In order to prevent the appearance of an undesirable pattern in the X-ray image due to a difference in absorption between the X-ray absorbing liquid and the glued joints between various walls at the side of the capillary tubes which communicates with the X-ray absorbing liquid and a first plate of the X-ray filter, the adhesive contains X-ray absorbing particles. This step will be described in detail with reference to FIG. 5. FIG. 5 is a sectional view of an X-ray filter 4 which includes capillary tubes 5. The X-ray filter also includes a first plate 70 and a second plate 73, wherebetween the capillary tubes 5 are arranged. Preferably, the first and second plates 70,73 are positioned perpendicular to the capillary tubes 5. The tubes also have walls 50 which contain, for example a synthetic material. The tubes contain an X-ray-absorbing liquid 72. Different walls 50 of different tubes 5 are connected to the first plate 70 by way of a first glued joint 71 and to the second plate 73 of the X-ray filter by way of a second glued joint 74. The first and the second glued joint 71, 74 comprise an adhesive as customarily used by those skilled in the art, for example an epoxy-type adhesive or an adhesive of the two-component type. In order to prevent the adhesive in the first glued joint 71 between the walls 50 at the sides of the tubes which communicate with a reservoir containing the X-ray-absorbing liquid and the first plate 70 from causing an undesirable pattern in the X-ray image, an X-ray absorbing material, for example molybdenum (Mo), lead (Pb) or tungsten (W) is added to the adhesive of the first glued joints 71, so that the X-ray absorption of the glued joint approximately equals that of the X-ray absorbing liquid 72. An advantage of the use of molybdenum and tungsten over lead is that their effects on the environment are less severe.