Patent Number: 054835720
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a schematic diagram of an x-ray apparatus according to the invention. A carrier 1 in the form of a C-shaped arm carrier supports an x-ray source unit 2 and an x-ray detector 3 in the form of an x-ray image intensifier facing the x-ray source unit. The carrier is moveable through a sleeve 4 and by means of a bearing 5. The carrier is attached by way of the bearing 5 to a predominantly vertical stand 6 which is rotatably mounted to a ceiling of a room in which the x-ray apparatus is set up. Thus, the x-ray source unit and the x-ray detector are moveable so as to orient a beam-path 7 of x-radiation emitted by the x-ray source unit so that various projections can be applied for making an x-ray image. A falter-arrangement 10 comprising absorption filters 11 and filter-drive-means 12 is incorporated in the x-ray source unit 2, the filter-arrangement 10 being positioned between the x-ray source 2a and the x-ray detector 3. When an x-ray image is being made, overexposure in certain areas of the x-ray image is to be avoided. Overexposure is liable to occur when regions of very low x-ray absorption and adjacent regions of high x-ray absorption are imaged in the same x-ray image. For instance, when imaging a patient's heart having a high x-radiation absorption, the surrounding lung-tissue having a low x-ray absorption will be overexposed. By covering regions of high x-ray transmittance by positioning absorption filters 11 in the x-ray beam path 7 overexposure by regions of high transmittance are avoided. Correct positioning of the absorption filters depends on the orientation of the x-ray beam-path with respect to an object that is being examined, because the object is imaged according to a projection corresponding to the orientation of the beam-path. Filter-drive-means 12 are provided for positioning the absorption filters 11. The filter-drive-means are connected to drive-control-means 13 to which also carrier-drive-means 14 are coupled. For orientating the beam-path 7 and adjusting the absorption filters accordingly, a selection-means 15 is provided for supplying a positioning signal to a geometry controller 19. Furthermore, a posture sensor 18 is further provided to produce a posture signal pertaining to the position of the carrier. The posture signal is supplied to the geometry-controller. The geometry-controller 19 supplies a positioning signal to the drive-control-means 13, for positioning the carrier 1 in correspondence with a required beam-path orientation. The carrier-drive means 14 is controlled by the drive-control-means 13 for displacing the carrier 1. The geometry-controller 19 also applies said positioning signal to a memory-device 16 for selecting a filter-position. To this end, one or a plurality of filter-position curves is stored in the memory-device 16; each of the filter-position curves represents filter-adjustments as a function of the position of the carrier 1. When a positioning signal is supplied to the memory-device 16, a filter-adjustment signal is supplied by the memory-means to the drive-control-means 13. In this manner, filter-control-means 17 is constituted by the drive-control-means in combination with the memory device 16. Subsequently, the drive-control-means controls the filter-drive-means 12 so as to adequately position the absorption filters 11. The positioning of the absorption filters will be described in more detail with reference to FIGS. 2 and 3. FIG. 2 shows a side elevation of a filter-arrangement for use in an x-ray apparatus according to the invention. The absorption filters have a wedge-shaped extremity so that a gradual transition between a covered region and an uncovered region is obtained and consequently steep gradients in the x-ray image are avoided. The absorption filters are mounted to filter-drive-means 12 in such a way that the filters are moveable into and out of the x-my beam path 7 as indicated by arrows 20. Furthermore the absorption filters are rotatable about axes 22 as indicated by arrows 21. FIG. 3 shows a top view of an absorption filter incorporated in a filter-arrangement shown in FIG. 2. The absorption filter has an x-ray absorbing part 23, e.g. containing lead, and an x-ray transmitting part 24 which has the shape of a section of a substantially circular disk. Thus, when rotating either one of the absorption filters 11 about its axis 22 the part of the x-ray beam that impinges on the absorption filter is varied and consequently, the part of the x-ray image being covered by the absorption filter is varied. FIG. 4a shows a graph of a first adjustment curve, representing a rotation angle of the absorption filter as a function of the beam-path orientation. The rotation angle .phi. according to which the absorption filter is positioned is given as a function of the angle .theta., which determines the orientation of the beam path with respect to a patient to be examined. For further illustration, a few particular orientations of the absorption filter have been presented as small diagrams, along the abscissa. When the carrier is positioned in accordance with .theta.=-80.degree., then the absorption filter is placed in an orientation of .phi.=-50.degree.. When the carrier is moved so as to change the beam-path orientation to .theta.=10.degree., is increased to .phi.=86.degree.. Finally, when the beam-path orientation angle is increased further, the angle of orientation of the absorption filter is slightly decreased. FIG. 4b shows a graph of a second adjustment curve, representing translation of the absorption filter with respect to the beam-path as a function of beam-path orientation. As the curve shows, the absorption filter is moved further into the beam-path as the beam path is inclined from perpendicular incidence, viz. .DELTA. has a minimum when .theta.=0.degree. and .DELTA. increases as .vertline..theta..vertline. increases. The features of the adjustment curves are determined by anatomical properties, notably the location of a heart of a patient with respect to surrounding lungtissue. In practice, these curves can be determined empirically, viz. by manually positioning absorption filters, for a multitude of carder orientations, so as to obtain optimum image quality. It appears that a single set of adjustment curves is already quite satisfactory for a multitude of patients. Further improvement can, however, be achieved by determining sets of adjustment curves, each set pertaining to a class of patients, such as corpulent or slender patients, adults or infants, etc.