Patent Number: 052009866
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to an x-ray examination apparatus comprising a frame to which are connected an x-ray source for emission of an x-ray beam and an x-ray detector that is placed opposite the x-ray source, and filter means that are placed between the x-ray source and the x-ray detector, the filter means comprising an x-ray absorbing filter body and drive means for positioning the filter body in the x-ray beam. PA1 two holding members having a central opening positioned around a central ray connecting the x-ray source and the x-ray detector, the filter body being placed between the holding members and being movably connected thereto via a guiding means, PA1 a motor coupled to at least one holding member for rotation of the holding member around the central ray and PA1 coupling means for coupling and uncoupling the holding members joint rotation of the holding members around the central ray causing the filter body to be fixed relative to the holding members, rotation of one of the holding members with respect to the other holding member causing the filter body to move relative to the holding members via the guiding means. The invention also relates to a filter means for use in such an x-ray examination apparatus. 2. Description of the Prior Art An x-ray examination apparatus of the above-mentioned kind is known from the European Patent EP-B1-157 688 which corresponds to U.S. Pat. No. 4,670,896. From the above-mentioned patent it is known that when objects having a mutually different x-ray absorption appear in the same x-ray image, such as heart and lungs, the x-ray image shows very bright areas next to very dark areas. Structures in the x-ray image having only a small difference in contrast falling within these areas are difficult to observe. By placing a wedge-shaped filter body in the x-ray beam such that the x-ray beam is attenuated in the areas of high transmissivity, the average contrast in the picture is reduced, such that the dynamic range of the x-ray image can be adjusted to cover the range of absorption values of the details that are of interest. Thereto first an x-ray image is made that is optically projected onto the patient. By placing wedge-shaped filter bodies, consisting for instance of plastic and lead, having an optical absorption corresponding with the x-ray absorption of the filter body in the light path, a visual indication of the filter position is obtained. After the filter bodies are in the desired position the final x-ray picture is taken. In an x-ray examination apparatus of the above-mentioned kind, the x-ray beam is collimated by two pairs of x-ray absorbing shutters the projection of which onto the patient determines the field of view. These shutters are movably mounted within a housing that is close to the x-ray source. In the same housing a mirror is present to project light onto the patient from a position corresponding to the position of the x-ray source such as to obtain an optical indication of the field of view. An extra filter that can be placed into the x-ray beam is preferably mounted in the same housing with the mirror and the collimating lead shutters and should therefor take up little space. SUMMARY OF THE INVENTION It is therefor an object of the invention to provide for an x-ray examination apparatus of the above-mentioned kind, in which filter means are provided that take up as little space as possible. It is also an object of the invention to provide for a filter means of simple, relatively cheap and failsafe design. Thereto an x-ray examination apparatus in accordance with the invention is characterized in that the drive means comprise Because for both rotation and translation of the filter body in the x-ray beam one single motor is used, the filter means take up less space than would have been the case if two separate motors were used. Because by simple mechanical means both rotation and translation of the filter body are performed, the number of components of the filter means is kept small thus, reducing the price of the filter means. An embodiment of an x-ray examination apparatus in accordance with the invention is characterized in that the holding members and the filter body are plateshaped, the filter body being connected to a first holding member in a pivot-point such as to be pivotable in a plane parallel to the holding members, the guiding means comprising a pawl which is provided on the filter body or the second holding member and a groove in the second holding member or in the filter body in which groove the pawl is guided. When the holding members are rotated together, the filter body is clamped between the holding members and rotates with the holding members around the central ray. When the holding members rotate with respect to one another, the pawl and the groove force the filter body to pivot around the pivot point and move into the central opening toward or away from central ray depending on the direction of rotation of the holding members. The pawl can be provided on the filter body, in which case the groove is provided in the second holding member that faces the side of the filter body having the pawl. The groove is curved and extends in a radial direction. The pawl can also be provided on the second holding member in which case the groove runs in the filter body. Another embodiment of an x-ray examination apparatus in accordance with the invention is characterized in that the holding members have a toothed circular circumference. The holding members can be coupled to a gearwheel of the motor either directly or via a transmission gear. Since the connection requires no intermediate transmission means such as pulleys, chains or drive belts, the number of components of the filter means can remain relatively small. Another embodiment of an x-ray examination apparatus in accordance with the invention is characterized in that the coupling means comprise a friction member for transmission of rotation of one holding member to the other holding member and a brake for locking one of the holding members. Both holding members are for example each connected to a gearwheel, one of which is driven by the motor. Via a friction member the gearwheels of both the holding members are coupled, such that rotation of the gearwheel that is directly driven by the motor is transferred to the second gearwheel and both holding members rotate together. By actuation of the brake, that can be an electromagnetic clutch, the gearwheel of the holding member that is not directly driven by the motor can be blocked. Since the force exerted by the friction member is not large enough to cause rotation of said blocked holding member, only the holding member that is directly driven by the motor is able to rotate. Another embodiment of an x-ray examination apparatus in accordance with the invention is characterized in that two filter means are provided as one integral unit which is placed near the x-ray source. By using two filter means in the same housing, a large number of areas of different geometries can be covered in an x-ray image by the two filter bodies.