Patent Number: 062263552
Section: summary

The invention relates to an X-ray examination apparatus for forming X-ray images of an object, including an X-ray source for generating an X-ray beam, PA1 an X-ray filter which includes tubes containing an X-ray-absorbing liquid, the quantity of X-ray-absorbing liquid in each of the tubes being adjustable in order to adjust the intensity profile on the object, and PA1 an X-ray detector for detecting an X-ray image. A device of this kind is known from Netherlands patent application No. 8903110. The X-ray filter in the known device is arranged in the X-ray beam between the X-ray source and the object in order to enable adjustment of a two-dimensional intensity distribution of the beam, the tubes then being directed approximately parallel to the X-ray beam. In order to adjust the two-dimensional intensity distribution, the tubes are completely or partly filled with the X-ray-absorbing liquid. An important property of such an X-ray filter is the ratio of an X-ray transmittance in a first state with a maximum filling to the X-ray transmittance in a second state with a minimum filling. This property is referred to as the dynamic range. For a large dynamic range the X-ray transmittance in the second state with a minimum filling must be as high as possible, for example 80%. Furthermore, the X-ray transmittance in the filled, first state must be as low as possible. In order to achieve a high X-ray transmittance, the tubes of the X-ray filter contain compounds of elements having a low atomic number, for example synthetic materials. In order to achieve a low transmittance, the tubes also contain a liquid having a high X-ray absorptivity. Furthermore, the surface area of the tube walls is not negligibly small relative to the entrance surface area of the tube openings. The entrance surface area of the tube openings amounts to, for example 90% of the total area of both the area of the tube wall and the entrance surface area together. It is a drawback of the known device that the dynamic range of the two-dimensional intensity distribution of the X-ray beam to be adjusted is limited. It is an object of the invention to provide an X-ray examination apparatus in which the dynamic range of the two-dimensional intensity distribution to be adjusted for an X-ray beam to be generated is increased. To this end, an X-ray examination apparatus according to the invention is characterized in that it the tubes of the X-ray filter are arranged such that lines through all longitudinal axes of the tubes are intersecting lines extending through the X-ray source via the X-ray filter to the X-ray detector. Advantageous embodiments of an X-ray examination apparatus according to the invention are defined in the dependent claims. The invention also relates to an X-ray filter for use in such an X-ray examination apparatus. The invention is based on the recognition of the fact that, when the tubes of the X-ray filter are arranged approximately parallel to the lines extending through the X-ray source via the X-ray filter to the X-ray detector, leakage of the X-ray beam through the walls of the tubes occurs because some X-rays can pass the X-ray filter without travelling through the X-ray-absorbing liquid. Furthermore, the requirements to be satisfied by the walls of the tubes in the state of maximum filling and the state of minimum filling are contradictory for straight passage. In the first state the X-ray transmittance of the walls should be minimum whereas in the second state the X-ray transmittance of the walls should be maximum. Efficient X-ray absorption can be achieved by ensuring that X-rays to be generated extends through a substantial part of the X-ray-absorbing liquid of at least one tube. A special embodiment of the X-ray examination apparatus according to the invention is characterized in that the tubes of the X-ray filter are oriented towards a center, a distance from the center to a first normal to an image plane of the X-ray detector through a midpoint of a focus of the X-ray source being at least equal to a focus radius of the focus. An X-ray filter which is known from the European patent application EP-A-740839 includes tubes which are oriented substantially parallel to the lines extending from the X-ray source through the X-ray filter towards the X-ray detector. By placing the center of the X-ray filter outside the focus of the X-ray source, X-rays coming from the midpoint of the focus will intersect the tubes at an angle so that overlap occurs in the projection of neighbouring tubes. Furthermore, by orienting the tubes towards the center it is achieved that the distance which X-rays travel through the absorbing liquid is substantially equal for X-rays starting from the X-rays source and travelling via the X-ray filter to different points of the X-ray detector. So, the uniformity of resolution of the X-ray filter is enhanced. In the context of the present application the term uniformity of resolution is to be understood to mean the degree of equality between a resolution in a central region of the X-ray image and the resolution near the edges of the X-ray image. By arranging the center of the X-ray filter adjacent the focus of the X-ray beam to be generated it is achieved 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. A further embodiment of the X-ray examination apparatus according to the invention is characterized in that a first cross-section of the X-ray filter in a first direction comprises a first segment of a first circle having a first radius, and that 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. A practical X-ray filter will often have such a construction, because it is easy to manufacture such X-ray filters. Moreover, the first radius may be equal to the second radius, so that the tubes are oriented along normals to a spherical surface. Another embodiment of the X-ray examination apparatus according to the invention is characterized in that said means include the X-ray filter whose tubes are arranged in parallel, an angle enclosed by a longitudinal direction of the tubes relative to a second normal to an image plane of the X-ray detector being dependent on a first distance from a center of the X-ray filter to a tube near an edge of the X-ray filter and on a second distance from the center to the X-ray source. It is thus achieved that X rays starting from the X-ray source travelling through the X-ray filter 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 A further embodiment of the X-ray examination apparatus according to the invention is characterized in that the tubes are of capillary dimensions, each of the capillary tubes being provided with electrically conductive walls. The capillary dimensions of the tubes and the electrically conductive walls render the X-ray filter suitable for the transport of an X-ray-absorbing liquid in the capillary tubes on the basis of the so-called electro-wetting principle. X-ray filters operating according to the electro-wetting principle are known from the cited European patent application EP-A 740839. A further embodiment of the X-ray examination apparatus according to the invention is characterized in that the X-ray examination apparatus includes a first plate and a second plate both plates are positioned transverse to the longitudinal axes of the tubes, between which plates the tubes are arranged, and that a means for connecting various channels to the first plate of the X-ray filter contains an adhesive. This step enables simple assembly of X-ray filters. A further embodiment of the X-ray examination apparatus according to the invention is characterized in that the adhesive contains X-ray absorbing particles. This step prevents the appearance of an undesirable pattern in the X-ray image, which pattern is visible to the user and is due to the fact that the X-ray absorption of the adhesive is lower than that of the X-ray-absorbing liquid at a side of the tubes which communicates with the X-ray-absorbing liquid. The invention also relates to an X-ray filter for use in an X-ray examination apparatus as disclosed in claim 2.