Abstract:
With a view to providing a beam diaphragm having a large maximum value of aperture opening under a limited profile dimension, the beam diaphragm comprises a pair of control rings having coaxial apertures for the passage of X-rays therethrough and being opposed to each other axially through a spacing and coaxially rotatable independently of each other, a blade positioned between the pair of control rings, and position adjusting means which, in accordance with a relative rotation of the pair of control rings, causes the blade to move toward or away from a common axis of the apertures so as to describe a sectorial plane whose radius increases or decreases continuously.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a beam diaphragm and an X-ray imaging apparatus. Particularly, the present invention is concerned with a beam diaphragm for radiating X-rays emitted from an X-ray source to a subject through apertures, as well as an X-ray imaging apparatus provided with the beam diaphragm.  
         [0002]     In an X-ray imaging apparatus, X-rays emitted from an X-ray source are radiated to a subject through apertures of a beam diaphragm. The beam diaphragm includes two control rings having apertures of a common axis and being opposed to each other axially through a spacing and rotatable independently of each other, a blade positioned between the two control rings, and position adjusting means which causes the blade to move toward or away from the axis of the apertures in accordance with a difference in rotation between the two control rings (see, for example, Patent Literature 1).  
         [0003]     [Patent Literature 1] 
         [0004]     U.S. Pat. No. 5,689,544 (Columns 3 to 5, FIGS. 1 and 2)  
         [0005]     In the above beam diaphragm, the degree of opening of each aperture decreases when the blade is moved toward the axis of the apertures, and increases when the blade is moved away from the aperture axis.  
         [0006]     Therefore, the degree of opening of each aperture becomes maximum when the blade is moved remotest from the axis of the apertures. It is desirable that a maximum value of aperture opening be as large as possible under a limited profile dimension of the beam diaphragm.  
       SUMMARY OF THE INVENTION  
       [0007]     Therefore, it is an object of the present invention to provide a beam diaphragm wherein a maximum value of aperture opening is large under a limited profile dimension, as well as an X-ray imaging apparatus provided with such a beam diaphragm.  
         [0008]     (1) In one aspect of the present invention for solving the above-mentioned problem there is provided a beam diaphragm comprising a pair of control rings having coaxial apertures for the passage of X-rays therethrough, the pair of control rings being opposed to each other axially through a spacing and coaxially rotatable independently of each other, a blade positioned between the pair of control rings, and position adjusting means which, in accordance with a relative rotation of the pair of control rings, causes the blade to move toward or away from a common axis of the apertures so as to describe a sectorial plane whose radius increases or decreases continuously.  
         [0009]     (2) In another aspect of the present invention for solving the above-mentioned problem there is provided an X-ray imaging apparatus for photographing a radioscopic image by radiating X-rays emitted from an X-ray source to a subject through apertures of a beam diaphragm, the beam diaphragm comprising a pair of control rings having coaxial apertures for the passage of X-rays therethrough, the pair of control rings being opposed to each other axially through a spacing and coaxially rotatable independently of each other, a blade positioned between the pair of control rings, and position adjusting means which, in accordance with a relative rotation of the pair of control rings, causes the blade to move toward or away from a common axis of the apertures so as to describe a sectorial plane whose radius increases or decreases continuously.  
         [0010]     For adjusting the position of the blade appropriately, it is preferable for the position adjusting means to comprise a first groove radially formed outside the aperture in one of the pair of control rings, a second groove arcuately formed outside the aperture in the other control ring, a first pin for bringing one end portion of the blade into engagement with the first and second grooves simultaneously, and a second pin provided on the other control ring, an opposite end portion of the blade being brought into engagement with the second pin through a longitudinal slot formed in the opposite end portion.  
         [0011]     It is preferable that a planar shape of the blade be symmetric right and left with respect to a center line. This is because a maximum value of aperture opening can be made larger easily.  
         [0012]     It is preferable that the thickness of the blade decrease gradually toward both right and left sides. This is because the amount of X-rays absorbed decreases gradually.  
         [0013]     It is preferable that the decrease in thickness of the blade be asymmetric right and left. This is because the absorption of X-rays becomes asymmetric right and left.  
         [0014]     In each of the above aspects the beam diaphragm comprises a pair of control rings having coaxial apertures for the passage of X-rays therethrough, the pair of control rings being opposed to each other axially through a spacing and coaxially rotatable independently of each other, a blade positioned between the pair of control rings, and position adjusting means which, in accordance with a relative rotation of the pair of control rings, causes the blade to move toward or away from the axis of the apertures so as to describe a sectorial plane whose radius increases or decreases continuously. Thus, it is possible to provide a beam diaphragm wherein a maximum value of aperture opening is large under a limited profile dimension, as well as an X-ray imaging apparatus provided with such a beam diaphragm.  
         [0015]     Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a diagram showing the construction of an X-ray imaging apparatus according to an example of the best mode for carrying out the present invention.  
         [0017]      FIG. 2  is a diagram showing the construction of a beam diaphragm according to another example of the best mode for carrying out the present invention.  
         [0018]      FIG. 3  is a diagram showing a cross section of a blade;  
         [0019]      FIG. 4  is a diagram showing the geometry of a beam diaphragm.  
         [0020]      FIG. 5  is a diagram showing a partial construction of the beam diaphragm.  
         [0021]      FIG. 6  is a diagram showing a sweep position of the blade.  
         [0022]      FIG. 7  is a diagram showing a sweep position of the blade.  
         [0023]      FIG. 8  is a diagram showing a sweep position of the blade.  
         [0024]      FIG. 9  is a diagram showing a sweep position of the blade.  
         [0025]      FIG. 10  is a diagram showing a sweep position of the diagram.  
         [0026]      FIG. 11  is a diagram showing how radius changes with sweep of the blade.  
         [0027]      FIG. 12  is a diagram showing an effective aperture.  
         [0028]      FIG. 13  is a diagram showing a maximum value of an effective aperture. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]     A best mode for carrying out the present invention will be described hereinunder with reference to the drawings. The present invention is not limited to the best mode for carrying out the invention.  FIG. 1  shows the construction of an X-ray imaging apparatus. This apparatus is an example of the best mode for carrying out the invention. By the construction of this apparatus there is shown an example of the best mode for carrying out the present invention related to the X-ray imaging apparatus.  
         [0030]     In this apparatus, as shown in the same figure, X-rays  6  emitted from an X-ray source  2  are radiated to a subject  8  through apertures of a beam diaphragm  4 . Then, transmitted X-rays are received by an X-ray receiving panel  10  and a received light signal is processed in a photographing console  12  to reconstruct a radioscopic image of the subject  8 . This radioscopic image thus reconstructed is displayed on a display  14  of the photographing console  12 . The console  12  also functions to control the X-ray source  2  and the beam diaphragm  4 .  
         [0031]      FIG. 2  is an exploded diagram showing the construction of a main portion of the beam diaphragm  4 . This beam diaphragm is an example of the best mode for carrying out the present invention. By the construction of this beam diaphragm there is shown an example of the best mode for carrying out the present invention related to the beam diaphragm.  
         [0032]     As shown in the same figure, the beam diaphragm  4  includes a first control ring  500  and a second control ring  600  which are opposed in parallel to each other spacedly along an axis  400 . The axis  400  coincides with the axis of an X-ray beam. The first control ring  500  and the second control ring  600  are constructed of an X-ray absorbing material such as, for example, tungsten (W), molybdenum (Mo), or lead (Pb).  
         [0033]     The first control ring  500  and the second control ring  600  are discs having a first aperture  502  and a second aperture  602 , respectively, which are circular in shape. The first and second apertures  502 ,  602  are concentric circles in the first and second control rings  500 ,  600 , respectively. The first and second apertures  502 ,  602  are of the same radius and have the axis  400  in common.  
         [0034]     The first control ring  500  and the second control ring  600  have a first groove  504  and a second groove  604 , respectively. The first groove  504  perpendicularly pass through the plate surface, in an outside of the first aperture  502 . A longitudinal direction of the first groove  504  corresponds to the radial direction of the first control ring  500 .  
         [0035]     The second groove  604  is formed outside the second aperture  602 , describing a circular arc having a curvature larger than the circumference of the second aperture. The second groove  604  is formed in an arcuate rail  606  provided on a surface (hereinafter referred to as the “inner surface”) opposed to the first control ring  500 .  
         [0036]     A blade  700  is provided between the first control ring  500  and the second control ring  600 . The blade  700  is also constructed of an X-ray absorbing material. The blade  700  is a generally rectangular plate. A planar shape of the blade  700  is symmetric right and left with respect to a center line. The thickness of the blade  700 , as shown in a cross-sectional view of  FIG. 3 , decreases gradually toward both right and left sides. Inclination of the decrease is asymmetrical.  
         [0037]     The blade  700  has at both ends thereof extending portions  710  and  720  of different lengths. The extending portion  710 , which is the shorter, has a hole  712  formed perpendicularly through the plate surface thereof. The extending portion  720 , which is the longer, has a slot  722  formed through the plate surface thereof. A longitudinal direction of the slot  722  corresponds to the longitudinal direction of the extending portion  720 .  
         [0038]     A first pin  714  is fitted through the hole  712 . Both ends of the first pin  714  are fitted in the first groove  504  of the first control ring  500  and the second groove  604  of the second control ring  600 , respectively.  
         [0039]     A second pin  724  is fitted through the slot  722 . The second pin  724  is provided on the inner surface of the second control ring  600  on the side opposite to the second groove  604  with respect to the axis  400 .  
         [0040]     Such a geometrical relation is shown in  FIG. 4 . In the same figure, the reference mark A denotes the axis  400 , B denotes the center of curvature, and C denotes the center of the second pin  724 . A central position of the second pin  724  lies on an extension line of line AB to B side.  
         [0041]      FIG. 5  shows in what state the blade  700  is secured to the second control ring  600 . As shown in the same figure, the blade  700  is secured to the second control ring  600  while one end side thereof utilizes the engagement between the first pin  714  and the second groove  604  and the opposite end side thereof utilizes the engagement between the slot  722  and the second pin  724 . The first control ring  500  with the first groove  504  engaged to the first pin  714  is applied over the second control ring  600  with the blade  700  mounted thereon.  
         [0042]     The first control ring  500  and the second control ring  600  are rotatable independently of each other about the common axis  400  by means of a first motor  800  and a second motor  900 , respectively.  
         [0043]     When there is a difference between the rotational speed of the first control ring  500  and that of the second control ring  600 , both rotate in a relative manner. It can be said that the first control ring  500  rotates with respect to the second control ring  600 . Alternatively, it can be said that the second control ring  600  rotates with respect to the first control ring  500 .  
         [0044]     Assuming that the first control ring  500  rotates with respect to the second control ring  600 , the first pin  714  is displaced along the second groove  604  with rotation of the first control ring  500 , whereby the blade  700  rotates around the second pin  724 . With this rotation, the blade  700  sweeps so as to describe a sectorial plane with the second pin  724  as the pivot.  
         [0045]     The portion including the first groove  504 , extending portion  710 , hole  712 , first pin  714 , second groove  604 , extending portion  720 , slot  722  and second pin  724  is an example of the position adjusting means in the present invention.  
         [0046]     Sweeping states of the blade  700  are shown in FIGS.  6  to  10 . These figures show successive movements of the blade  700  with clockwise rotation of the first control ring  500 , provided the first control ring  500  is omitted.  
         [0047]     As shown in these figures, the blade  700  sweeps from left to right so as to describe a sectorial plane with the second pin  724  as the pivot along the second groove  604 .  
         [0048]     At this time, since the rotational center of the blade  700 , i.e., the center C of the second pin  724  lies farther than the curvature center B of the second groove  604  with respect to the axis  400 , as shown in  FIG. 4 , so that the distance from the rotational center C to the second groove  604  varies depending on the sweep position. This change of distance appears as a positional change of the second pin  724  in the slot  722  of the blade  700 .  
         [0049]      FIG. 11  is a conceptual diagram of such sweep. In the same figure, p 1  to p 5  represent positions of the first pin  714  in the second groove  604  in the sweep process. More specifically, p 1  and p 5  represent left- and right-end positions, respectively, of the second groove  604 , p 3  represents a central position, and p 2  and p 4  represent intermediate positions respectively from the left and right ends up to the central position.  
         [0050]     The distance from the rotational center C to the second groove  604  is the shortest when the first pin  714  lies at the left end p 1  or the right end p 5  of the second groove  604 , is the longest when the first pin  714  lies at the center p 3  of the second groove  604 , and is a distance intermediate between both distances when the first pin  714  lies at the intermediate positions p 2  and p 4 .  
         [0051]     That is, from both right and left ends to the center of the second groove  604 , the blade  700  sweeps so as to describe a sectorial plane whose radius increases gradually, while from the center to both right and left ends of the second groove  604  the blade  700  sweeps so as to describe a sectorial plane whose radius decreases gradually.  
         [0052]     The direction from both ends to the center of the second groove  604  is the direction of approaching the axis  400 , while the direction from the center to both ends of the second groove  604  is the direction of leaving the axis  400 . Thus, the blade  700  moves in the direction of approaching or leaving the axis  400  of the apertures so as to describe a sectorial plane whose radius increases or decreases continuously.  
         [0053]     When the sweep position of the blade  700  has reached a desired position, the rotational speed of the first control ring  500  and that of the second control ring  600  are made equal to each other. As a result, the relative rotation of the first control ring  500  stops and the blade  700  stays in its position shown in  FIG. 12  for example.  
         [0054]     In this state both first and second control rings  500 ,  600  rotate at an equal speed, so that the blade  700  rotates around the axis  400 . Consequently, the area through which X-rays can pass without being obstructed by the blade  700  is a circular area indicated with a broken line in the same figure. This circular area corresponds to an effective aperture  1000  of the beam diaphragm.  
         [0055]     The radius of the effective aperture  1000  is given in terms of the length of a perpendicular dropped from the axis  400  to an edge of the blade  700 . This length varies depending on the sweep stop position of the blade  700 . Thus, the beam diaphragm  4  is a variable aperture type beam diaphragm.  
         [0056]     The closer to the edge of the blade  700 , the thinner the blade, and the amount of absorbed X-rays decreases (the amount of transmitted X-rays increases) accordingly. Therefore, outside the effective aperture  1000 , a gradation is given such that the closer to the effective aperture  1000 , the higher the intensity of transmitted X-rays. Since the thickness decreasing inclination is made different between the right and the left portion of the blade  700 , the gradation can be used properly according to the purpose.  
         [0057]     The effective aperture  1000  becomes maximum when the blade  700  is retracted to a maximum extent. This state is shown in  FIG. 13 . As shown in the same figure, the blade  700  lies in a position in which the greater part of the blade is retracted from the second aperture  602 , with only a part of its edge overlapping the second aperture  602 . At this time, the effective aperture  1000  becomes a circular area of a diameter R, as indicated with a broken line.  
         [0058]     The diameter R, i.e., a maximum value, of the effective aperture  1000  can be made as large as possible by increasing the amount of retraction of the blade  700  to diminish the portion overlapping the second aperture  602 .  
         [0059]     However, trade-off is needed because protruding of the blade  700  to the outside of the second control ring  600  must be avoided.  
         [0060]     On this regard, as shown in  FIG. 11 , since the beam diaphragm  4  is constructed so that the radius of a sectorial plane formed by sweeping of the blade  700  is the shortest at both ends, the portion overlapping the second aperture  602  can be diminished while preventing protrusion of the blade  700  to the outside of the second control ring  600 . Thus, the maximum value of aperture opening can be made larger under a limited profile dimension.  
         [0061]     That a planar shape of the blade  700  is symmetric right and left with respect to a center line also facilitates enlarging the maximum value of aperture opening under a limited profile dimension. This is because if the planar shape of the blade is made asymmetric right and left, the wider side overlapping the second aperture  602  becomes larger and eventually the maximum value of opening of the effective aperture  1000  decreases.  
         [0062]     Many widely different embodiments of the invention may be configured without departing from the spirit and the scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in the specification, except as defined in the appended claims.