Abstract:
A collimator capable of being reduced in its external size without sacrificing an aperture is to be provided. To this end, the collimator comprises a pair of first plate members which defines a radiation passing aperture by a spacing between respective opposed end faces, a pair of second plate members which respectively overlap the first plate members at least partially so as to block any other radiation than the radiation passing through the aperture, a pair of third plate members which respectively overlap the second plate members at least partially so as to block any other radiation than the radiation passing through the aperture, an adjusting mechanism which adjusts the aperture by moving the pair of first plate members, and a follow-up mechanism which causes the pair of second plate members to move following the pair of first plate members with movement of the first plate members.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims benefit of PCT/CN2003/000985 filed Nov. 20, 2003. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to a collimator and a radiation irradiator. Particularly, the invention is concerned with a collimator for a radiation, e.g., X-ray, and a radiation irradiator provided with the collimator. 
   In a radiation irradiator there is used a collimator for controlling the irradiation range of a radiation. The collimator has an aperture which permits a radiation to pass therethrough, and the radiation cannot pass through other than the aperture. Thus, the irradiation range of the radiation is controlled by the aperture. The degree of opening of the aperture can be changed to adjust the irradiation range of the radiation. 
   As shown in  FIG. 10 , the aperture-adjustable collimator has a pair of movable plate members having a radiation shielding property, i.e., blades  901  and  901 ′. The blades  901  and  901 ′ are disposed so that respective end faces are opposed to each other, and are movable in directions opposite to each other in a plane parallel to their surfaces. For widening the aperture, the pair of blades  901  and  901 ′ are moved away from each other, while for narrowing the aperture, the both blades are moved to close to each other. Thus, the aperture becomes maximum when the pair of blades are remotest from each other, and becomes minimum when both blades are closest to each other. 
   In a collimator having a large aperture adjusting range, there are used blades of a large area. If the blade area is large, an external form of the collimator comes to have a large size corresponding to a maximum moving distance of blade outer edges, as shown in  FIG. 11 . 
   In a mammography apparatus which makes fluoroscopy of the breast with X-ray, a collimator assumes a position confronting the face of a subject and so it is preferable that its external form be as small as possible. However, in the case of a mammography apparatus capable of making tomography, an increase in external form of the collimator has heretofore been unavoidable because a large aperture is needed. 
   SUMMARY OF THE INVENTION 
   Therefore, it is an object of the present invention to provide a collimator capable of being reduced in its external form without sacrificing an aperture, as well as a radiation irradiator provided with such a collimator. 
   (1) In one aspect of the present invention for achieving the above-mentioned object there is provided a collimator comprising a pair of first plate members having a shielding property against a radiation and movable in a direction parallel to surfaces thereof, the pair of first plate members defining a radiation passing apertures by a spacing between respective opposed end faces, a pair of second plate members having a shielding property against a radiation and parallel to the pair of first plate members and movable in a direction parallel to surfaces thereof, the pair of second plate members having end faces opposed to each other, the pair of second plate members overlapping the pair of first plate members at least partially so as to block any other radiation than the radiation passing through the aperture, a pair of third plate members having a shielding property against a radiation and parallel to the pair of second plate members, the pair of third plate members having respective end faces opposed to each other with a predetermined spacing, the pair of third plate members overlapping the pair of second plate members at least partially so as to block any other radiation than the radiation passing through the aperture, an adjusting mechanism which adjusts the aperture by moving the pair of first plate members, and a follow-up mechanism which causes the pair of second plate members to move following the pair of first plate members with movement of the first plate members. 
   (2) In another aspect of the present invention for achieving the above-mentioned object there is provided a radiation irradiator having a radiation source and a collimator for applying a radiation from the radiation source to an object through an aperture, the collimator comprising a pair of first plate members having a shielding property against a radiation and movable in a direction parallel to surfaces thereof, the pair of first plate members defining a radiation passing aperture by a spacing between respective opposed end faces, a pair of second plate members having a shielding property against a radiation and parallel to the pair of first plate members and movable in a direction parallel to surfaces thereof, the pair of second plate members having end faces opposed to each other, the pair of second plate members overlapping the pair of first plate members at least partially so as to block any other radiation than the radiation passing through the aperture, a pair of third plate members having a shielding property against a radiation and parallel to the pair of second plate members, the pair of third plate members having respective end faces opposed to each other with a predetermined spacing, the pair of third plate members overlapping the pair of second plate members at least partially so as to block any other radiation than the radiation passing through the aperture, an adjusting mechanism which adjusts the aperture by moving the pair of first plate members, and a follow-up mechanism which causes the pair of second plate members to move following the pair of first plate members with movement of the first plate members. 
   In the above aspects of the present invention, since movable blades are constituted by two sets of plate members so that corresponding ones overlap each other, it is possible to reduce the external form of the collimator without sacrificing the aperture. 
   For changing the aperture symmetrically with respect to the center of the collimator, it is preferable that the adjusting mechanism be able to move the pair of first plate members so as to be closed to and away from each other. 
   For permitting an appropriate follow-up motion of the pair of second plate members with respect to the pair of first plate members, it is preferable that the follow-up mechanism comprise a rack provided in the first plate member, a gear provided in the second plate member rotatably and engaging with the rack, and a fixed rack provided in the moving direction of the second plate member and engaging with the gear. 
   For the simplification of construction, it is preferable that the follow-up mechanism comprise an arm member mounted at an intermediate portion thereof to the second plate member and rotatable about the mounting portion in a plane parallel to the plate surface, a groove formed in the first plate member and with which one end of the arm member is engaged, the groove permitting movement of the end of the arm member in a direction perpendicular to the moving direction of the first plate member, and a groove formed in the third plate member and with which an opposite end of the arm member is engaged, the groove permitting movement of the opposite end of the arm member in a direction perpendicular to the moving direction of the second plate member. 
   For facilitating the adjustment of dose, it is preferable that the radiation be X-ray. 
   According to the present invention, it is possible to realize a collimator capable of being reduced in its external size without sacrificing the aperture, as well as a radiation irradiator provided with such a collimator. 

   
     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 
       FIG. 1  illustrates a schematic construction of a radiation irradiator; 
       FIG. 2  illustrates the construction of a collimator; 
       FIG. 3  illustrates the construction of the collimator; 
       FIG. 4  illustrates the construction of the collimator; 
       FIG. 5  illustrates a blade drive mechanism; 
       FIG. 6  illustrates a partial construction of a blade; 
       FIG. 7  illustrates the construction of a follow-up mechanism; 
       FIG. 8  illustrates the construction of a follow-up mechanism; 
       FIG. 9  illustrates a fully open condition of an aperture; 
       FIG. 10  illustrates a conventional collimator; and 
       FIG. 11  illustrates a fully open condition of an aperture in the conventional collimator. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Embodiments of the present invention will be described in detail hereinunder with reference to the drawings.  FIG. 1  illustrates a schematic construction of a radiation irradiator according to an embodiment of the present invention. The construction of this radiation irradiator shows an example of how to carry out the present invention. 
   As shown in the same figure, the radiation irradiator has a radiation source  1 . As the radiation source  1  there is used an X-ray tube for example. The radiation source  1  is not limited to the X-ray tube, but may be any other radiation source capable of emitting a suitable radiation such as β ray or γ ray. The radiation source  1  is an example of the radiation source used in the present invention. 
   A radiation  3  emitted from the radiation source  1  passes through an aperture of a collimator  5  which embodies the present invention and is applied to an object  7 . The object  7  is an object for fluoroscopy using the radiation  3  or an object for therapy using the radiation  3 . In fluoroscopy, the radiation which has passed through the object  7  is received by a suitable light receiving means, e.g., a photosensitive plate. 
     FIG. 2  schematically illustrates the collimator  5 , which embodies the present invention. Through the constitution of this, one embodiment of the present invention is described. 
   As shown in the same figure, the collimator  5  has three pairs of blades  501 ,  501 ′,  503 ,  503 ′, and  505 ,  505 ′. Each blade is constituted by a quadrangular plate member. 
   As the material of the plate members there is used a material of a high radiation absorbance such as, for example, lead (Pb) or tungsten (W), whereby each blade comes to have a radiation shielding property. 
   The blades  501  and  501 ′ are an example of the pair of first plate members in the present invention. The blades  503  and  503 ′ are an example of the pair of second plate members in the present invention. The blades  505  and  505 ′ are an example of the pair of third plate members in the present invention. 
   In  FIG. 2 , three directions perpendicular to one another assumed to be x, y, and z. The x direction is a direction of one side of each blade, and the direction will hereinafter be referred to also as the width direction. The y direction is a direction of another side of each blade, and the direction will hereinafter be referred to also as the length direction. The z direction is the thickness direction of each blade. The radiation source  1  lies in the z direction. 
   The three pairs of blades  501 ,  501 ′,  503 ,  503 ′, and  505 ,  505 ′ overlap one another in the thickness direction. The blades  501  and  501 ′ are upper blades, the blades  503  and  503 ′ are intermediate blades, and the blades  505  and  505 ′ are lower blades. 
   The lower blades  505  and  505 ′ are supported by a pair of cross beams  507  and  507 ′. The cross beams  507  and  507 ′ are also constituted by a material of a high radiation absorbance. The cross beams  507  and  507 ′ constitute a picture frame-like frame together with the blades  505  and  505 ′. 
   The upper blades  501  and  501 ′ are made movable in the x direction by means of a drive mechanism which will be described later. The blades  501  and  501 ′ are movable so as to be close to and away from each other. 
   The intermediate blades  503  and  503 ′ are made movable following the upper blades  501  and  501 ′ by means of a follow-up mechanism to be described later. The movement of the blades  503  and  503 ′ is done while maintaining their overlap with the blades  501 ,  501 ′ and the blades  505 ,  505 ′ constantly. 
   The size of the aperture through which the radiation passes is determined in the x direction by the spacing between opposed end faces of the blades  501  and  501 ′ and in the y direction by the spacing between opposed end faces of the cross beams  507  and  507 ′. 
   The aperture size in the x direction varies with movement of the blades  501  and  501 ′, while the aperture size in the y direction is fixed. That is, the collimator  5  has an aperture whose size in the x direction can be changed. 
   A fully closed state of the aperture is shown in  FIG. 3  and a fully open state thereof is shown in  FIG. 4 . Throughout the whole process of aperture changes, the blades  503  and  503 ′ are kept overlapped with the blades  501 ,  501 ′ and the blades  505 ,  505 ′. Consequently, the passage of any other radiation than the radiation passing through the aperture is blocked. 
     FIG. 5  schematically illustrates the construction of a drive mechanism for the blades  501  and  501 ′. This drive mechanism is an example of the adjusting mechanism in the present invention. As shown in the same figure, the blades  501  and  501 ′ have arms  601  and  601 ′, respectively, which extend in the y direction. End portions of the arms  601  and  601 ′ are in engagement with shafts  603  and  603 ′, respectively. 
   The shafts  603  and  603 ′ are parallel shafts extending in the x direction. Both shafts are spaced a predetermined distance in the z direction. The arm  601 ′ is bent to equalize the height in the z direction of the blade  501 ′ to that of the blade  501 . 
   The shafts  603  and  603 ′ are threaded throughout the overall lengths thereof. The arms  601  and  601 ′ are internally threaded at their portions engaged with the shafts  603  and  603 ′. Gears  605  and  605 ′ are provided coaxially at one ends of the shafts  603  and  603 ′ respectively. The gears  605  and  605 ′ are in mesh with each other at a gear ratio of 1:1. 
   The gear  605  is rotated by means of a motor  607 . The motor  607 , which is a reversible motor, is controlled by a control means (not shown). The control means controls both rotational direction and rotational quantity of the motor  607 . 
   Since the gears  605  and  605 ′ are in mesh with each other, the shafts  603  and  603 ′ rotate in directions opposite to each other. Consequently, the arms  601  and  601 ′ engaged with the shafts  603  and  603 ′ move reverse to each other in the x direction. That is, as the motor  607  rotates in one direction, both arms move to be close to each other, while as the motor  607  rotates in the opposite direction, both arms move away from each other. Their movement quantity is determined by the amount of rotation of the motor  607 . By such movements of the arms  601  and  601 ′ there is adjusted the spacing between the blades  501  and  501 ′, i.e., the degree of opening of the aperture. In this way the degree of opening of the aperture can be changed symmetrically with respect to the center of the collimator. 
   There is provided a follow-up mechanism for allowing the blade  503  and  503 ′ to follow such movements of the blades  501  and  501 ′. The follow-up mechanism is constituted so as to span the three pairs of blades and the cross beams. As a part of the follow-up mechanism, the blade  503  is constituted as shown in  FIG. 6 , in which (a) is a plan view and (b) is a sectional view taken on line A-A in (a). 
   As shown in the same figure, the blade  503  has a gear  701 . The gear  701  is mounted rotatably on a shaft  703  which is provided in the blade  503 . To be more specific, the shaft  703  is disposed within a cutout portion  705  formed in one end portion in the y direction of the blade  503 . The cutout portion  705  is formed in the x direction and the shaft  703  is mounted so as to cross the cutout portion  705  in the y direction. The cutout portion  705  is formed on one side in the x direction of the blade  503 . The side where the cutout portion  705  is formed confronts the blade  503 ′ which makes a pair with the blade  503 . 
   The blade  503  has a similar gear also at its opposite end portion in the y direction. That is, the blade  503  has gears at both ends thereof in the y direction. The blade  503 ′ is also of the same construction, provided the blades  503  and  503 ′ are in a relation of specular symmetry. 
     FIG. 7  illustrates the construction of the follow-up mechanism schematically. This follow-up mechanism is an example of the follow-up mechanism defined in the present invention. As shown in the same figure, the follow-up mechanism is composed of the gear  701  provided in the blade  503 , a rack  707  provided in the blade  501  and meshing with the gear  701 , and a rack  709  provided in both blade  505  and cross beam  507  and meshing with the gear  701 . The racks  707  and  709  extend in the x direction in parallel with each other. 
   The gear  701  is an example of the gear defined in the present invention. The rack  707  is an example of the rack defined in the present invention. The rack  709  is an example of the fixed rack defined in the present invention. 
   As the blade  501  is moved in the x direction, the gear meshing with the rack  707  moves in the same direction while rotating on the rack  709 . The blade  503  also moves together with the gear  701 . As a result, the blade  503  moves following the blade  501 . The distance of the movement of the blade  503  is a half of that of the blade  501 . 
   Such a follow-up mechanism is provided at both end portions of the blades  501 ,  503 , and  505 . This is also the case with the mating blades  501 ′,  503 ′, and  505 ′. With the follow-up mechanism, the follow-up motion of the blades  503  and  503 ′ for the blades  501  and  501 ′ can be done appropriately. Consequently, it becomes possible to make such aperture adjustment as shown in  FIGS. 2 to 4 . 
     FIG. 8  schematically shows another constructional example of a follow-up mechanism. This follow-up mechanism is an example of the follow-up mechanism defined in the present invention. In the same figure, (a) is a plan view and (b) is a sectional view taken on line B-B in (a). As shown in the same figure, the blade  503  is provided with a shaft  803  at its center. The shaft  803  extends through the blade  503  perpendicularly to the plate surface. The shaft  803  is rotatable. 
   Arms  801  and  805  are fixed respectively to both ends of the shaft  803 . The arms  801  and  805  are perpendicular to the shaft  803  and extend in directions opposite to each other. The shaft  803  and the arms  801 ,  805  form a crank. The extending directions of the arms  801  and  805  in the crank are not coincident with the x direction. The arms  801  and  805  are an example of the arm member defined in the present invention. 
   The arms  801  and  805  are formed with lugs  811  and  851  at respective free ends. The lugs  811  and  851  extend in the z direction so as to face reverse to each other. The lug  811  is loosely fitted in a groove  511  formed in the blade  501 . The groove  511  is positioned on the blade  503  side of the blade  501  and extends in the y direction. The lug  851  is loosely fitted in a groove  551  formed in the blade  505 . The groove  551  is positioned on the blade  503  side of the blade  505  and extends in the y direction. The grooves  511  and  551  are an example of the grooves defined in the present invention. 
   In this construction, when the blade  501  is moved in the x direction, the lugs  811  and  851  move in directions opposite to each other along the grooves  511  and  551 , with the result that the crank rotates about the shaft  803 . 
   The position of the blade  505  is fixed, so with the rotation of the crank, the shaft  803  moves in the x direction following the blade  501  and the blade  503  moves in the same direction together with the shaft  803 . By setting the lengths of the arms  801  and  805  equal to each other, the distance of the movement of the blade  503  becomes half of that of the blade  501 . Thus, this follow-up mechanism becomes simple in construction. 
   Such a follow-up mechanism is provided also on the side of the mating blades  501 ′,  503 ′, and  505 ′. As a result, it becomes possible to make such aperture adjustment as shown in  FIGS. 2 to 4 . 
   In the fully open condition of the aperture shown in  FIG. 4 , corresponding ones in the three pairs of blades  501 ,  501 ′,  503 ,  503 ′,  505 , and  505 ′ overlap each other completely.  FIG. 9  shows this state in terms of a sectional view. As shown in the same figure, the movable blades  501 ,  501 ′,  503 , and  503 ′ overlap the fixed blades  505  and  505 ′ completely. In this state, outer edges of the movable blades are in alignment with outer edges of the fixed blades, not protruding therefrom. Consequently, an external form of the collimator becomes constant irrespective of the degree of opening of the aperture. 
   Therefore, if the maximum degree of opening of the aperture is set equal to that in the prior art shown in  FIG. 11 , it is possible to reduce the external form of the collimator to about three fourths. Alternatively, if the external form of the collimator is made about the same as in the prior art, it is possible to enlarge the maximum value of the aperture to approximately 4/3 time. 
   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.