Patent Number: 052009866
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows an x-ray examination apparatus comprising a C-shaped frame 2 to which are connected an x-ray source 3 and an x-ray image intensifier tube 5. The x-ray source 3 emits a beam of x-rays 7 which after passing through a patient body 8 is detected on the entrance screen of the image intensifier tube 5. In the entrance screen, that comprises for instance CsI, a light image is formed which impinges on a photocathode and liberates electrons therefrom. The electrons are accelerated through a potential difference of for instance 20 kV and impinge on an output screen containing phosphorus to form a light image with increased intensity. The light image on the output screen of the image intensifier tube 5 is detected with a television camera 9 that forms a video signal which is displayed on a television monitor. The C-shaped frame 2 can move in a circumferential direction within a supporting member 11, the supporting member 11 being rotatable around an axis 13. By rotation of the frame 2 in the circumferential direction or by rotation of the supporting member 11 around the axis 13, a central ray 15 connecting the x-ray source 3 and the x-ray image intensifier tube 5 rotates around an isocenter 17. In an x-ray image the isocenter 11 always occupies the same position irrespective of the position of the frame 2. A table height of a patient table 19 is usually chosen such as to place an area of interest within a patient 8 in the isocenter 17. Close to the x-ray source 3 a collimating unit 21 is placed which contains lead shutters 23, FIG. 2, delimiting the x-ray beam 5. FIG. 2 schematically shows the collimating unit 21 which contains lead shutters 23 that can be moved away from and towards the central ray 15. In total there are four lead shutters 23 enclosing a rectangle. In order to get a visual indication of the collimating effect of the shutters 23 on the x-ray beam 7 an x-ray transparent mirror 25 is present in the collimating unit 21. A light source 27, that is in fact contained within the housing 22 of the collimating unit 21, is placed in a position corresponding to a focus 29 of the x-ray source 3. The projection of the lead shutters 23 onto the patient body 8 by the light source 27 corresponds to the field of view of the x-ray image. A filter 31, that comprises a wedge-shaped filter body 33 of for example copper or aluminum, is fixed in the housing 22 and can by means of a motor 34 be translated toward and away from the central ray 15 and be rotated around the central ray. The motor 34 can be activated by a user of the x-ray examination apparatus from a control panel 35, FIG. 2, connected to a side of the patient table 19. The filter 31 can also be placed below the lead shutters 23, or can be used in an x-ray examination apparatus in which no collimating unit 21 is present. FIG. 3 shows a top view of the filter 31 in which the filter body 33 is held between two overlying disk-shaped holding members 37 and 37' (only one of which is shown in FIG. 3) and is movable within a central opening 39 through which the central ray 15 passes, the central ray being perpendicular to the plane of drawing. In the embodiment shown in this figure, two filters 31 are placed on top of one another such that two filter bodies 33 and 33' and four holding member 37, 37', 38 and 38' are present as shown in FIG. 6. The four holding members 37, 37', 38 and 38' can be rotated around the central ray 15 by two motors 34 and 34', FIG. 3. Connected to the motors 34 and 34' are four potentiometers 41 and 41' to determine the angular position of each holding member 37, 37', 38 and 38'. The signal of the potentiometers 41, 41' is for example supplied to a control unit for automatic positioning of the filter bodies 33 and 33' in the x-ray beam. FIG. 4 shows the pair of holding members 37, 37' in a disassembled state. To the holding member 37 the filter body 33 is attached in a pivot-point 43 such as to be rotatable around an axis which runs through the pivot-point and which is perpendicular to the plane of drawing. The filter body 33 is provided with a pawl 45 that extends perpendicular to the plane of drawing and that fits in a curved radial groove 47 in the holding member 37'. The holding member 37' is to be placed on top of the holding member 37 such that the pawl 45 engages the groove 47. When, in the assembled state, the holding members are rotated with respect to one another, the groove 47 forces the filter body 33 to rotate around the pivot-point 43, whereby the filter body 33 covers or uncovers a part of the central opening 39. When both holding members rotate together, the straight edge 48 of the filter body 33 is rotated around the central ray 15 which extends perpendicular to the plane of drawing. FIG. 5 shows a schematic representation of the coupling means, comprising a magnetically energizable brake 42 and two spur gears 53 and 55 that are mutually coupled by means of a layer of friction material 54 such as "Ferodo 3701 F" is supplied by the company Ferodo Limited (GB). When the brake 42 is not energized, the rotation of the spur gear 53, that is driven by the motor 34 and spur gear 50, is transmitted to the spur gear 55 by means of the layer of friction material 54. The layer of friction material 55 is fixedly connected to one of the spur gears 53 or 54. In this way the holding members 37 and 37' are jointly rotated around the central ray 15 with equal angular velocity such that the filter body 33 is rotated around the central ray 15. Energizing the brake 42, results in the spur gear being blocked. Since the coefficient of friction of the layer of friction material 54 is not large enough to prevent rotation of the spur gear 53, only the holding member 37' is rotated around the central ray 15 resulting in translation of the filter body 33. FIG. 6 shows a sectional view of the filter 31. The spur gear 49 is via the spur gear 50 driven by the motor 34. Via the layer of friction material 54, the spur gear 51 is coupled to the spur gear 49. Via a transmission gear, not shown in this figure, the spur gears 49 and 51 cause the holding member 37 and 37' to rotate either jointly or relative to one another depending on whether the electromagnetic brake 42 has been energized or not. Rotation of the spur gear 51 is transmitted to a spindle 40 that is connected to a potentiometer 41 (not shown in this figure) for recording the angular position of the holding member 37. Likewise the spur gears 49, 53 and 55 are each connected to respective potentiometers 41 and 41'.