Patent Publication Number: US-6707884-B1

Title: X-ray scatter reducing grid and fabrication method thereof

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an X-ray scatter reducing grid and a fabrication method thereof which are used in an apparatus for X-ray imaging. 
     2. Description of the Related Art 
     In the radiation-transmitted image of a subject (such as human body or the like) by radiation transmitted through the subject, it is known that an X-ray scatter reducing grid, for absorbing rays scattered when radiation is transmitted through the subject, is employed in order to obtain a high quality transmitted image in which scattered radiations are reduced. 
     For the general configuration of the above-mentioned X-ray scatter reducing grid, radiation-absorbing portions and radiation-transmitting portions, which have width in the direction in which radiation travels, are alternately disposed in parallel and are formed into the shape of a flat plate as a whole. When radiation is transmitted through the subject, the scattered radiation travel obliquely and are absorbed and reduced by the radiation-absorbing portions, and only the primary radiation are transmitted through the subject and travel substantially linearly. The primary radiation, transmitted through the radiation-transmitting portions, reach a detector and form a radiation-transmitted image. The radiation-transmitting portions are formed from wood, aluminum or the like, while the radiation-absorbing portions are formed from lead or the like. These portions are alternately and closely disposed and maintain structural strength as a whole. It is desirable that the radiation-transmitting portions have a high transmittance so as not to reduce the transmission of the primary radiation. 
     As an example of an X-ray scatter reducing grid with its radiation-transmitting portion being air (i.e., a so-called air grid), an X-ray scatter reducing grid disclosed in Japanese Unexamined Patent Publication No. 10(1998)-5207 is known. This X-ray scatter reducing grid is provided with two support members  202   a ,  202   b  curved in the form of a circular arc with respect to focal point F, as shown by reference numeral  200  in FIG. 17. A plurality of paired grooves  204 ,  206  extending along a Z-axis are formed in the inner surfaces of the support members  202   a ,  202   b  and are directed toward the focal point F (radiation source). Collimator plates  210 , which are composed of metal such as tungsten whose radiation (X-rays) absorption is great, are inserted in the paired grooves  204 ,  206  along the Z-axis through the upper ends of the support members  202   a ,  202   b  and are fixed between the support members  202   a ,  202   b , as shown in FIG.  17 A. 
     When fabricating the X-ray scatter reducing grid  200  which supports strips (collimator plates  210 ) as radiation-absorbing members between the two support members  202   a  and  202   b , the support grooves  204 ,  206  are first formed at predetermined intervals in the two support members  202   a ,  202   b . Then, the two support members  202   a ,  202   b  are fixed with a constant space to form the frame of the X-ray scatter reducing grid  200 . Next, the collimator plates  210  are inserted in the grooves  204 ,  206  through the end of the grid frame. 
     However, because of deflection in the support members  202   a ,  202   b , deflection in the collimator plate  200 , friction between the collimator plate  210  and the grooves  204 ,  206  developed in inserting the collimator plate  210 , etc., the aforementioned method has the disadvantage that the collimator plates  210  are easily bent when they are being inserted over a long distance and the number of fabrication steps is increased. If the width of the grooves  204 ,  206  is widened to make insertion easy, play will occur between the collimator plate  210  and the groove  204  (or  206 ) and therefore accurate positioning will become difficult. As a result, focusing accuracy of the collimator plates  210  is reduced. Also, if another set of collimator plates extending in a direction perpendicular to the collimator plates  210  are used to make a cross grid, as shown at  12  in FIG. 1 of the aforementioned Publication No. 10(1998)-5207, the collimator plates  210  have to curved. As a result, the step of inserting the collimator plates  210  along the grooves curved over an even longer length becomes necessary and the fabrication becomes even more difficult. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the aforementioned disadvantages found in the prior art. Accordingly, the primary object of the invention is to provide an X-ray scatter reducing grid which can be reliably and easily fabricated with a high degree of accuracy. 
     To achieve this end, there is provided a self-supporting grid comprising: 
     a plurality of radiation-absorbing plates disposed in parallel at predetermined intervals over an entire area to which radiation is exposed, each radiation-absorbing plate consisting of a radiation-absorbing substance and having width in a direction in which the radiation travels; and 
     at least two support members for supporting the opposite end portions of each of the radiation-absorbing plates; 
     wherein the support members are provided with plate-receiving means which receives the plurality of radiation-absorbing plates, the radiation-absorbing plates being inserted in the plate-receiving means and being supported by the support members. 
     The expression “the radiation-absorbing plates are inserted in the plate-receiving means and are supported by the support members” includes fixing the radiation-absorbing plates by firm attaching means, such as adhesion, fusing and the like, as well as supporting the radiation-absorbing plates by friction. 
     In the X-ray scatter reducing grid according to the present invention, the radiation-absorbing plates do not need to be inserted over a long distance, because the radiation-absorbing plates are inserted and supported at the opposite ends thereof with respect to the two support members. In addition, there is only a slight possibility that the radiation-absorbing plates will bend during insertion, since the frictional resistance at the time of insertion is low. Thus, the X-ray scatter reducing grid can be fabricated reliably and easily with a high degree of accuracy. 
     The plate-receiving means provided in the support member can be constructed by a plurality of grooves which receive and support the opposite edges of the radiation-absorbing plate, or by a plurality of slots which receive and support the opposite end portions of the radiation-absorbing plate, or by a plurality of elongated holes which receive and support the opposite end portions of the radiation-absorbing plate. In the case where the plate-receiving means is constructed by the grooves, the structural strength of the support members can be kept because there is no slot in the support members. In the case where the plate-receiving means is constructed by the slots, the structural strength of the grid after fabrication can be increased because the radiation-absorbing plates are firmly supported by the support members. In the case where the plate-receiving means is constructed by the elongated holes, vertical positioning can be performed even more accurately, because there is no possibility that the radiation-absorbing plates will shift vertically, i.e., in the direction perpendicular to the longitudinal direction of the support members, after the insertion of the radiation-absorbing plates into the elongated holes. 
     The radiation-absorbing plates may be pulled so that they are stretched in the longitudinal direction of the radiation-absorbing plates and may be fixed to the support members under the pulled condition. Even if deflection occurs in the radiation-absorbing plates, in the case where the radiation-absorbing plates are stretched in the longitudinal direction and fixed to the support members and/or the ceiling plate (or the bottom plate), focusing accuracy is enhanced because the deflection can be reduced. 
     The X-ray scatter reducing grid may further include a ceiling plate and/or a bottom plate, and the radiation-absorbing plates may be fixed to at least one among the plate-receiving means, the ceiling plate, and the bottom plate. 
     In the X-ray scatter reducing grid, the support members may be constructed by two first support members which support the opposite end portions of each of the radiation-absorbing plates and two second support members which connect to the two first support members so that the four support members constitute a rectangular frame. In such a case, the rigidity of the support members increases the radiation-absorbing plates are easily positioned with accuracy and the structural strength of the grid can be made greater. 
     The plate-receiving means can be provided so that it extends in a direction converging toward a radiation source being operated. More specifically, a focusing grid with a higher transmittance can be constructed by inserting the radiation-absorbing plates into the plate-receiving means provided so as to incline in the direction that focuses toward the radiation source. In the case where support members (plates) consisting of a radiation-absorbing substance incline in the direction which focuses toward the radiation source, the transmittance of the radiation, which is transmitted through a subject from the radiation source and travels substantially linearly, becomes high. Since cutoff in the circumferential portion of the X-ray scatter reducing grid is eliminated, a variation in the transmittance radiation in a transmitted image is eliminated and high image quality is obtainable. Similarly, in the case where the radiation-absorbing plates are inclined in the directions that focuses toward the radiation source by inserting the plates into the plate-receiving means provided so as to incline in the direction that focuses toward the radiation source, a variation in the transmitted-radiation amount is eliminated and high image quality is obtainable. 
     In addition to the support members, a plurality of radiation-absorbing support members, which are perpendicular to the radiation-absorbing plates and consist of a radiation-absorbing substance, may be provided over an entire area, to which radiation is exposed, in a direction parallel to the support members. In this case the radiation-absorbing plates and the radiation-absorbing support members form a cross grid as a whole. In such a case, even higher image quality is obtainable over the entire transmitted image. 
     Furthermore, in the case where slots are formed in both the support members and the radiation-absorbing plates, the grid has advantages in that resistance to insertion can be further reduced, fabrication becomes easy, and mutual positioning is performed with reliability. 
     Elastic bodies may be interposed between the two support members so that the two support members are urged in a direction in which the radiation-absorbing plates are stretched. The elastic bodies are intended to mean spring material. For example, a compression coil spring can be employed. In this case, flatness in the radiation-absorbing plates is always maintained, because the radiation-absorbing plates are kept stretched. 
     In accordance with the present invention, there is provided a method of fabricating an X-ray scatter reducing grid, comprising the steps of: 
     inserting a plurality of radiation-absorbing plates into plate-receiving means formed in at least two support members, the radiation-absorbing plates being disposed in parallel at predetermined intervals over an entire area to which radiation is exposed, and each radiation-absorbing plate consisting of a radiation-absorbing substance and having width in a direction in which the radiation travels; and 
     supporting the opposite end portions of each of the radiation-absorbing plates by the plate-receiving means and thereby constituting the X-ray scatter reducing grid. 
     In the fabrication method according to the present invention, the radiation-absorbing plates do not need to be inserted over a long distance, because the radiation-absorbing plates are inserted and supported at the opposite ends thereof with respect to the two support members. In addition, there is a little possibility that the radiation-absorbing plates will bend during insertion, since the frictional resistance at the insertion is low. Thus, the X-ray scatter reducing grid can be fabricated reliably and easily with a high degree of accuracy. 
     In the method, it is preferable that the radiation-absorbing plates be fixed to-the plate-receiving means. Also, the X-ray scatter reducing grid may include a ceiling plate and/or a bottom plate. It is preferable that the radiation-absorbing plates be fixed to at least one among the plate-receiving means, the ceiling plate, and the bottom plate. In addition, it is preferable that the radiation-absorbing plates be fixed to the support member under the condition in which the radiation-absorbing plates are pulled in the longitudinal direction of the radiation-absorbing plates. Furthermore, the X-ray scatter reducing grid may include support members, which have the plate-receiving means, a ceiling plate, and/or a bottom plate, and the support members may be removed after the radiation-absorbing plates have been fixed to either the ceiling plate or the bottom plate, or both of them. In the case where the support members are removed after the radiation-absorbing plates have been fixed, the grid can be reduced in size and becomes easy to handle, because the number of components can be reduced. 
     At the positions where the radiation-absorbing plates are supported by the support members, the radiation-absorbing plates may be provided with a second set of slots (plate-receiving means) which engage a first set of slots (plate-receiving means) provided in the support members, and an X-ray scatter reducing grid may be constructed by the engagement between the first and second sets of slots. In this case, if the height of the support members is made the same as that of the radiation-absorbing plates, and if each slot is formed by approximately half of the height of the support members or the radiation-absorbing plates, the upper and lower ends of the plates become substantially coplanar with those of the support members when they are assembled. As a result, the grid is capable of having a well-ordered configuration as a whole. 
     The opposite end portions of the radiation-absorbing plate may be formed with holes and stretched in the opposite directions by metal wires, or rods, passed through the holes. Also, the opposite end portions of the radiation-absorbing plate may be provided with cutouts and stretched in the opposite directions by metal wires or the like wound around the cutouts. In these cases, the other end of the metal wire or the rod may be fixed to a jig disposed to surround the circumference of the X-ray scatter reducing grid, and a stretch in the radiation-absorbing plate may be temporarily maintained until the radiation-absorbing plate is fixed to the support members and/or the ceiling plate (or the bottom plate). Furthermore, the opposite end portions of the radiation-absorbing plate may be clamped by a tool such as cutting pliers and stretched in the opposite directions. 
     The above and many other objects, features and advantages of the present invention will become manifest to those skilled in the art upon making reference to the following detailed description and accompanying drawings in which preferred embodiments incorporating the principle of the present invention are shown by way of illustrative example. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a plan view showing an X-ray scatter reducing grid constructed according to a first embodiment of the present invention; 
     FIG. 1B is a front view of the support member used in the grid of FIG. 1A; 
     FIG. 1C is aside view of the radiation-absorbing plate used in the grid of FIG. 1A; 
     FIG. 2A is a plan view showing an X-ray scatter reducing grid constructed according to a second embodiment of the present invention; 
     FIG. 2B is a front view of the support member used in the grid of FIG. 2A; 
     FIG. 2C is a side view of the radiation-absorbing plate used in the grid of FIG. 2A; 
     FIG. 3 is a perspective view of the X-ray scatter reducing grid constructed according to the second embodiment of the present invention; 
     FIG. 4A is a plan view showing an X-ray scatter reducing grid constructed according to a third embodiment of the present invention; 
     FIG. 4B is a front view of the support member used in the grid of FIG. 4A; 
     FIG. 4C is a side view of the radiation-absorbing plate used in the grid of FIG. 4A; 
     FIG. 5 is a perspective view of the X-ray scatter reducing grid constructed according to the third embodiment of the present invention; 
     FIG. 6 is a perspective view of an X-ray scatter reducing grid constructed according to a fourth embodiment of the present invention; 
     FIG. 7A is a front view of the support member used in the grid of FIG. 6; 
     FIG. 7B is a side view of the radiation-absorbing plate used in the grid of FIG. 6; 
     FIG. 8A is a plan view showing an X-ray scatter reducing grid constructed according to a fifth embodiment of the present invention; 
     FIG. 8B is a front view of the support member used in the grid of FIG. 8A; 
     FIG. 8C is a front view of another thin support member used in the grid of FIG. 8A; 
     FIG. 8D is a side view of the radiation-absorbing plate used in the grid of FIG. 8A; 
     FIG. 9 is a perspective view of an X-ray scatter reducing grid constructed according to a sixth embodiment of the present invention; 
     FIG. 10 shows front and side views of the support member and radiation-absorbing plate used in the grid of FIG. 9, along with a radiation source; 
     FIG. 11A is a plan view showing an X-ray scatter reducing grid constructed according to a seventh embodiment of the present invention; 
     FIG. 11B is a front view of the support member used in the grid of FIG. 11A; 
     FIG. 11C is a side view of the radiation-absorbing plate used in the grid of FIG. 11A; 
     FIG. 12A is a diagram showing an embodiment of the method of stretching the radiation-absorbing plate shown in FIG. 11C; 
     FIG. 12B is a diagram showing another embodiment of the stretching method; 
     FIG. 12C is a diagram showing still another embodiment of the stretching method; 
     FIG. 13A a plan view showing a grid that is capable of keeping radiation-absorbing plates stretched; 
     FIG. 13B a plan view showing another grid that is capable of keeping radiation-absorbing plates stretched; 
     FIG. 14 is a perspective view showing a grid constructed according to an eighth embodiment of the present invention; 
     FIG. 15 is a schematic view showing another embodiment of the grid shown in FIG. 14; 
     FIG. 16 is a schematic view showing still another embodiment of the grid shown in FIG. 14; 
     FIG. 17A is a perspective view showing a conventional X-ray scatter reducing grid; and 
     FIG. 17B is an enlarged plan view of the part enclosed by a two-dotted line in FIG.  17 A. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will hereinafter be described in detail with reference to the drawings. Note that in FIGS. 1 to  16 , the thickness of each component, the width of each slot, the number of radiation-absorbing plates, the ratio of the dimensions of each component, etc., do not always agree with reality. 
     Referring to FIG. 1, there is shown an x-ray scatter reducing grid (hereinafter referred simply to as a grid)  1  in accordance with a first embodiment of the present invention. The grid  1  has support members (first support members)  2 ,  2  consisting of radiation-transmitting material (radiation non-absorbing material) such as wood, aluminum and the like. The support members  2  are formed thick and connected at the opposite ends of each member to two connecting members (second support members)  6 . That is, the support members  2  and the connecting members  6  as a whole constitute a rectangular frame  8 ,thereby giving rigidity to the grid  1 . The connecting members  6  and the support members  2  may be coupled by means of adhesion, or they may be formed integrally with one another. While this first embodiment is provided with the connecting members  6 , structure without the connecting members  6  is also possible. Similarly, in other embodiments to be described later, structure without the connecting members  6  is possible. The grid  1  further has radiation-absorbing plates  4 . Each radiation-absorbing plate  4  consists of a plate containing a substance, which absorbs radiation relatively well, such as lead, tantalum, tungsten and the like. Note that in other embodiments to be described later, radiation-absorbing plates also consist of the same material. 
     In the support members  2  of the first embodiment, from the upper edge  2   a  thereof toward the lower edge  2   b  a plurality of plate-receiving means (in this embodiment, slots  14 ) are formed in parallel at predetermined intervals at approximately half (½ h) of the height h of the support member  2 , as shown in FIG.  1 B. The slots  14  extend in a direction going substantially toward the side of a radiation source (not shown), i.e., in a direction perpendicular to the paper surface of FIG.  1 A. On the other hand, the radiation-absorbing plate  4  is formed with two parallel slots  16  (which extend in the direction opposite from the slots  14  of the support member  2 ), at positions corresponding to the two opposite support members  2 , i.e., positions crossing the opposite support members  2  perpendicularly. That is, each slot  16  of the radiation-absorbing plate  4  is formed from the lower edge  4   b  thereof toward the upper edge  4   a  at approximately half (½ h) of the height h of the radiation-absorbing plate  4 . 
     If the slots  16  of the radiation-absorbing plates  4  are positioned with respect to the slots  14  of the support members  2  and engage with the slots  14 , a linear grid, i.e., a grid with the radiation-absorbing plates  4  disposed in parallel at predetermined intervals, is constructed as shown in FIG.  1 A. In this construction, the radiation-absorbing plates  4  are disposed in parallel to one another and form a parallel grid and are also disposed at right angles to the support members  2 . In this way, the support members  2  are capable of supporting and holding the radiation-absorbing plates  4  at predetermined positions. 
     Since the slots  14  and  16  each have a dimension of half the height h of the respective members, the upper edge  4   a  of the radiation-absorbing plate  4  becomes substantially coplanar with the upper edge  2   a  of the support member  2  after fabrication. The height dimension h of the radiation-absorbing plate  4  is, for example, 1 to 3 cm, while the thickness is 0.1 mm. In addition, the spacing between adjacent slots  14  of the support member  2 , i.e., the intervals at which the radiation-absorbing plates  4  are disposed, is approximately 1 mm. 
     In fabricating the radiation-absorbing plates  4  and the support members  2 , the radiation-absorbing plates  4  are inserted in the support members  2  through the respective lower edges  16  and upper edges  14 . In this case, the height h of the support member  2  is short compared with the longitudinal direction thereof, and consequently, the resistance during the insertion becomes low. Furthermore, the insertion up to half of the height h is very easy because the resistance between the slot  16  of the radiation-absorbing plate  4  and the slot  14  of the support member  2  is much lower. The same may be said of the following embodiments in which the slot length is approximately half of the height h. Of course, the same is also true of the case where the length of one slot is one-third of h and the other slot length is two-thirds of h. After fabrication, the radiation-absorbing plates  4  and the support members  2  support one another without having solid matter as a member intervening between adjacent radiation-absorbing plates  4 , and consequently, the radiation-absorbing plates  4  and the support members  2 , as they are, can hold the fabricated form and result in a so-called self-supporting grid. The fixation between the radiation-absorbing plates  4  and the support members  2  may remain inserted, or the fixation may be reinforced by an adhesive agent, fusing, etc. Reinforcing the structure by an adhesive agent, fusing or the like is likewise possible for other embodiments that are to be described later. 
     FIGS. 2 and 3 show a grid  20  similar to the grid  1  of the first embodiment, constructed according to a second embodiment of the present invention. Notice that in FIG. 3, the thickness of each component and the connecting members  26  shown in FIG. 2 are omitted for a clear understanding of the present invention. 
     As illustrated in FIGS. 2 and 3, the essential difference between the grid  20  of the second embodiment and the grid  1  of the first embodiment is that a radiation-absorbing plate  24  has no slot and the slots  34  of a support member  22  extend from its upper edge  22   a  to the vicinity of its lower edge  22   b.    
     The manufacture of the radiation-absorbing plate  24  is easy because it has no slot. When fabricating the grid  20 , all that is required is to insert the radiation-absorbing plates  24  into the slots  34  of the support members  22 . As the slots  34  of the two support members  22  are aligned with one another and formed in parallel, the radiation-absorbing plates  24  are disposed in parallel and constitute a parallel grid, as with the first embodiment. In FIGS. 2 and 3, although the number of radiation-absorbing plates  24  is omitted for convenience, a large number of radiation-absorbing plates  24  are actually disposed in the slots  34  of the support members  22 . It is preferable that the radiation-absorbing plates  24  be bonded to the slots  34  of the support members  22  so that the plates  24  do not to move within the slots  34 . Alternatively, protrusions (FIG. 3) may be formed on the radiation-absorbing plate  24  to interpose the support member  22  therebetween in order to prevent positional misalignment. In this case, the fixation between the protrusions  25  and the support member  22  can also be reinforced by adhesion. 
     FIGS. 4 and 5 show a grid  40  constructed according to a third embodiment of the present invention. In the third embodiment, the plate-receiving means for receiving and supporting radiation-absorbing plates is constructed by grooves  54  formed in support members  42 . Note that in FIG. 5, the connecting members  46  shown in FIG. 4 are omitted for a clear understanding of the present invention. 
     As illustrated in FIGS. 4 and 5, the grid  40  of the third embodiment, as with the aforementioned two embodiments, is a linear grid, but differs in that the plate-receiving means is constructed by the grooves  54  of the support members  42 . Radiation-absorbing plates  44  have no slot, as in the second embodiment. In the inner surfaces of the opposite support members  42 , a plurality of grooves  54  are formed in parallel from the upper edge  42   a  of the support member  42  to the lower edge  42   b . Therefore, the opposite edges  44   c  of each radiation-absorbing plate  44  are inserted and supported in the corresponding grooves  54  of the support members  42  through the upper edges  42   a  of the support members  42 , and the parallel grid  40  is formed. 
     The width of the groove  54  of the support member  42  is of such a dimension that the edge  44   c  of the radiation-absorbing plate  44  is press-fitted and supported. However, since the insertion is performed over a short distance, the frictional resistance at the time of insertion is low even if the groove  54  is not formed wide, and there is only a slight possibility that the radiation-absorbing plate  44  will bend. Because the structure of the radiation-absorbing plate  44  in the third embodiment is also simple, it can be easily manufactured and is inexpensive. In addition, as the groove  54  is formed over the overall length from the upper edge  42   a  of the support member  42  to the lower edge  42   b , the two support members  42  can be made the same. In the third embodiment, the support member  42  is very strong because the groove  54  is not an opening penetrating the plate thickness of the support member  42 . Therefore, the rigidity of the grid  40  is significantly increased and positioning accuracy of the radiation-absorbing plate  44  is enhanced. 
     FIGS. 6 and 7 show a grid  60  constructed according to a fourth embodiment of the present invention. This fourth embodiment, as with the aforementioned embodiments, is a linear grid, but is different in that a focusing grid in which radiation-absorbing plates  64  incline toward a radiation source X (FIG. 7) is located at a predetermined position. As illustrated in FIGS. 6 and 7A, the plate-receiving means in the fourth embodiment is constructed by a plurality of slots  74 , which extend by approximately half of the height h of a support member  62  in the directions that focus toward the radiation source X. Note that some of the slots  74  shown in FIGS. 6 and 7 are omitted in order to make understanding of the present invention easy, but there are actually a large number of slots  74 . Since the radiation source X is usually positioned above the central portion of the grid  60 , the opposite slots  74   d  of the support member  62  incline most so that they are directed toward the radiation source X. As shown in FIG. 7A, the slots  74  inside the opposite slots  74   d  gradually sequentially approach a right angle with respect to the upper edge  62   a  of the support member  62 , and only the central slot  74   c  crosses the upper edge  62   a  at a right angle. 
     The radiation-absorbing plate  64  has two slots  76  similar to those of the radiation-absorbing plate  4  of the first embodiment shown in FIG.  1 . If the support members  62  and the radiation-absorbing plates  64  are assembled, the grid  60  is obtained as shown in FIG.  6 . Since the radiation-absorbing plates  64  are disposed in the directions that focus at the radiation source X, some of the rays, transmitted through a subject (not shown) positioned between the radiation source X and the grid  60 , are linearly incident on the grid  60  without being intercepted by the radiation-absorbing plates  64 . These rays then reach a radiation detector (not shown) positioned under the grid  60 , and form a transmitted image. As a result, so-called cutoff, which is normally caused by interception of the transmitted radiation performed by the radiation-absorbing plates  64 , will not occur, and a variation in the transmittance is eliminated and an image of high image quality is obtained. As with the aforementioned embodiments, the two support members  62  can be made the same. 
     FIG. 8 shows a cross grid  80  constructed according to a fifth embodiment of the present invention The difference between the grid  80  of the fifth embodiment and the linear grids  1 ,  20 ,  40  and  60  of the aforementioned four embodiments is that radiation-absorbing plates  84  are each provided with a plurality of slots  96  disposed in parallel at predetermined intervals. Also, a plurality of thin support members (plates)  82 , which are composed of the same material as the radiation-absorbing plate  84 , i.e., a radiation-absorbing substance such as lead, tantalum and the like, are disposed in parallel in the slots  96  of the radiation-absorbing plates  84 . With this disposition, the radiation-absorbing support members  82  and the radiation-absorbing plates  84  as a whole constitute the cross grid  80 . The opposite ends of each radiation-absorbing support member  82  are connected to the opposite connecting members  86  through the opposite slots  96  of the radiation-absorbing support member  84 . In addition, since the radiation-absorbing support members  82  and the radiation-absorbing plates  84  engage with one another, the self-supporting grid  80  with great structural strength is obtained. 
     In cooperation with the radiation-absorbing plates  84 , the radiation-absorbing support members  82  in the cross grid  80  absorb more scattered radiation than the linear grid, and consequently, the cross grid  80  achieves high image quality. However, cutoff will occur in the circumferential portion of the grid  80 , because the radiation-absorbing support members  82  and the radiation-absorbing plates  84  in the fifth embodiment of FIG. 8 do not incline in the directions that focus at the radiation source X (FIG.  7 ). For this reason, radiation, transmitted through the subject and traveling linearly, is absorbed to some degree in the circumferential portion of the grid  80 , so there is a possibility that the image quality will degrade. 
     A grid  100  of a sixth embodiment improving the above disadvantage is shown in FIGS. 9 and 10. FIG. 10 shows a support member  102  and a radiation-absorbing plate  104  used in the grid  100 . In the grid  100  of the sixth embodiment, slots  114  and  116 , inclining in the directions that focus at a radiation source X (FIG.  10 ), are formed in the support member  102  and the radiation-absorbing plate  104 , respectively. The slot  116  of the radiation-absorbing plate  104  is formed from one edge  104   b  of the radiation-absorbing plate  104  toward the other edge  104   a  by approximately half of the height h of the radiation-absorbing plate  104 . With this construction, the support members  102  and the radiation-absorbing plates  104  engage with one another, whereby the cross grid  100  is formed as shown in FIG.  9 . As with the fifth embodiment, it is desirable that the support members  102  intervening between the opposite support members  102  be thin. 
     The height of the slot  114  of the support member  102  is approximately half of the height h of the support member  102 , as in FIG.  7 A. Since the intervening support members  102 , as with the fifth embodiment, consist of a radiation-absorbing substance, rays scattered at the subject (not shown) are absorbed by the cross grid  100 . In addition, the rays, transmitted through the subject and traveling linearly, arrive at a detector (not shown) without being intercepted by the cross grid  100 , i.e., without giving rise to cutoff. Therefore, in the cross grid  100  of this sixth embodiment, the transmittance is enhanced and the scattered radiation are effectively reduced. Thus, a high quality transmitted image is obtained over the entire surface of the grid  100 . 
     FIG. 11 shows a grid  120  of a seventh embodiment of the present invention. The seventh embodiment differs from the aforementioned embodiments in that the plate-receiving means provided in the support members  122  are constructed by elongated holes  134 . The support members  122  are connected at the opposite ends to the connecting members  126  and are formed into the shape of a frame as a whole, as with the first embodiment. In each support member  122 , a plurality of vertical elongated holes  134  (i.e., plate-receiving means) are formed at predetermined intervals along the longitudinal direction of the support member  122 . Rectangular radiation-absorbing plates  124  are inserted into these elongated holes  134 , and the end portions  125  of each radiation-absorbing plate  124  penetrate the elongated holes  134  and project from the holes  134 . After the radiation-absorbing plates  124  have been inserted into the elongated holes  134 , movement of the radiation-absorbing plates  124  in the vertical direction perpendicular to the longitudinal direction is regulated and therefore there is no possibility that the radiation-absorbing plates  124  will slide in the vertical direction. In this way, the radiation-absorbing plates  124  are supported in parallel by the support members  122 , whereby the grid  120  is constructed. In this condition the radiation-absorbing plates  124  may be fixed to the support members  122  by adhesion or the like. However, in the case where there is deformation, such as deflection, wrinkles and the like, in the radiation-absorbing plates  124 , there is a need to correct the plate deformation before fixation and make the radiation-absorbing plates  124  flat. 
     The method of correcting plate deformation will be described with reference to FIG.  12 . As shown in FIG. 12A, the end portions of two metal wires  131  are passed through holes  126  formed in the end portions  125  of a radiation-absorbing plate  124   a  and are tied in loop form. Then, the radiation-absorbing plate  124   a  is pulled in the opposite directions by the two metal wires  131 , whereby deformation, such as wrinkles and the like, is corrected. This correcting operation is performed after the radiation-absorbing plates  124   a  have been inserted into the support members  122 , and the same applies to radiation-absorbing plates  124   b ,  124   c  to be described later. A frame-shaped jig  133  (only the part of which is shown in FIG. 12A) is disposed to surround the circumference of the grid  120 , and the other end of the metal wire  131  which stretches each radiation-absorbing plate  124   a  is wound and fixed to this jig  133 . Next, the radiation-absorbing plates  124   a  thus stretched are fixed to the support members  122  by adhesion or the like. In addition, instead of the metal wire  131 , a rod (not shown) may be inserted into the hole  125  and the other end of this rod fixed to the jig  133  by an appropriate method. 
     In the case of the radiation-absorbing plate  124   b  shown in FIG. 12B, cutouts  128  are formed in the opposite end portions  125  of the radiation-absorbing plate  124   b , respectively. The end portions of the aforementioned wires  131  are wound around these cutouts  128  and tied in the form of a loop. The operation thereafter is the same as the case of FIG.  12 A. 
     In the case where the metal wires  131  are not used, irregularities  130  on the surfaces of both end portions  125  of the radiation-absorbing plate  124   c  may be clamped by a tool  135  such as cutting pliers and pulled in the opposite directions, as shown in FIG.  12 C. The irregularities  130  are formed by embossing and prevent the tool  135  from slipping when clamped by the tool  135 . When the tool  135  is not used, the aforementioned jig  133  is not used. In addition, the irregularities  130  may be formed by notching. 
     Note that while the method of correcting plate deformation has been described in the case of the elongated holes  134 , plate deformation can also be corrected for the slots  14 ,  34  (FIGS. 1 and 2) and the grooves  54  (FIG. 4) in the same manner. For instance, for the slots  14  shown in FIG. 1, the radiation-absorbing plates  4  are inserted into the support members  2 , as in the elongated holes  134 . After insertion, the end portions of each radiation-absorbing plate  4  protruding from  64  the slots  14  are pulled, and after deformation in each radiation-absorbing plate  4  has been corrected, the radiation-absorbing plates  4  are glued to the support members  2 . This method can also be used in the cross grid  80  (FIG. 8) in which the radiation-absorbing support members  82  and the radiation-absorbing plates  84  are disposed in the form of a lattice. In this case, deformation in all the radiation-absorbing support members  82  and radiation-absorbing plates  84  can be corrected by pulling them vertically and horizontally, i.e., in 4 directions. Thereafter, they may likewise be fixed by adhesion. 
     In the grooves  54  shown in FIG. 4, each radiation-absorbing plate  44  is pulled to a length equal to the space between the support members  42  plus two groove depths, and then the radiation-absorbing plates  44  are connected to the grooves  54  by adhesion. When the radiation-absorbing plate  44  is longer than the aforementioned length, it may be cut to coincide with that length. Thereafter, the radiation-absorbing plates  44  are likewise glued to the support members  42 . 
     FIG. 13 shows a grid  140  that is capable of keeping radiation-absorbing plates  124  stretched, after the grid has been constructed. Note that a description is made by applying the same reference numerals to the same components. As illustrated in FIG. 13A, two compression coil springs (hereinafter referred to simply as springs) (elastic bodies)  144  are interposed between both end portions of two support members  142  supporting a large number of radiation-absorbing plates  124  in parallel. As the springs  144  pull support members  142  in the opposite directions, the radiation-absorbing plates  124  fixed to the support members  142  are stretched and their flatness is ensured. The springs  144  are inserted onto shafts (not shown) or into a cylindrical member (not shown), whereby the shape is maintained. Instead of the springs  144 , other elastic bodies, for example, synthetic resin material with elasticity, such as polyurethane, may be employed. 
     In a grid  160  shown in FIG. 13B, springs  164  for urging support members  162  are provided on both sides of a pair of fixed or unmovable portions  166 . The fixed portions  166  are disposed at the opposite end portions of the support members  162  and are coupled with a base  168 , which is part of the grid  160 , or are formed integrally with the base  168 . The fixed portions  166  are disposed approximately midway between the two support members  162 . This can make the length of the springs  164  shorter and prevent the springs  164  from being deflected horizontally. 
     FIG. 14 shows a grid  180  that is an eighth embodiment of the present invention, in which stretched radiation-absorbing plates  184  are fixed by use of surface plates consisting of carbon, i.e., a ceiling plate  186  and a bottom plate  188 . First, the radiation-absorbing plates  184  are fixed to the support members  182  by an adhesive agent  185 , or protrusions  187 , etc. Then, the ceiling plate  186  and the bottom plate  188  are disposed to interpose the radiation-absorbing plates  184  therebetween and are glued to the radiation-absorbing plates  184  by adhesion or the like. The ceiling plate  186  and the bottom plate  188  are slightly smaller in outside dimensions than a frame  192 , constructed by the support members  182  and connecting members  190 . The ceiling plate  186  and the bottom plate  188 , therefore, can easily be inserted into the frame  192  and glued to the radiation-absorbing plates  184 . In this way, fixing of the radiation-absorbing plates  184  can be performed even more reliably and therefore the rigidity of the entire grid and the structural strength of the frame  192  are enhanced. In this case, the support members  182  with slots are removable, since the ceiling plate  186 , the bottom plate  188 , and the radiation-absorbing plates  184  are fixed. In addition, in the case where the ceiling plate  186  and the bottom plate  188  are glued and fixed to the circumferential edges  194  of the frame  192  instead of being inserted into the frame  192 , the radiation-absorbing plates  184  are not glued to the ceiling plate  186  and the bottom plate  188 , but can maintain the entire rigidity. Furthermore, the radiation-absorbing plates  184  can be held in position, as they are protected from external influence. 
     In the case of using the ceiling plate  186  and the bottom plate  188  in this manner, the radiation-absorbing plates  184  can be fixed by various methods. For instance, another embodiment of the grid  180  is illustrated in FIG.  15 . In the case of this grid  180 , the bottom plate  188  is glued to the radiation-absorbing plates  184 , while the ceiling plate  186  is glued to the support members  182 , i.e., the upper edge of the frame  192 . In this case, the bottom plate  188  can also be glued to the frame  192 , because it is located inside the frame  192 . With this construction, straightness in the radiation-absorbing plates  184  is ensured and the rigidity of the frame  192  can be maintained. 
     Conversely, the ceiling plate  186  may be inserted into the frame  192  and glued to the radiation-absorbing plates  184 , and the bottom plate  188  may be glued to the lower edge  194  of the frame  192 , away from the radiation-absorbing plates  184 . Similarly, the same effect is obtainable. 
     In the former case, i.e., in the case where the ceiling plate  186  and the bottom plate  188  are glued to the radiation-absorbing plates  184 , grooves may be formed at positions on the inner surfaces of the ceiling plate and bottom plate  186  and  188  which correspond to the radiation-absorbing plates  184 . In this case, adhesion and positioning of the radiation-absorbing plates  184  can be performed reliably by inserting the radiation-absorbing plates  184  into the grooves. In addition, in the latter case, i.e., in the case where the ceiling plate  186  and the bottom plate  188  are not glued to the radiation-absorbing plates  184 , grooves or stepped portions may likewise be formed at positions on the ceiling plate and bottom plate  186  and  188  which correspond to the support members  182  and the connecting members  190 . In this case, positioning of the frame  192  can be formed reliably and these components become difficult to deform. 
     Illustrated in FIG. 16 is a grid  180   a  of still another embodiment. Although the radiation-absorbing plates used in this embodiment are the same as the aforementioned radiation-absorbing plates  184 , they are mounted on the bottom plate  188  so that they incline toward a source of radiation (not shown). For example, the radiation-absorbing plates  184  are inclined by use of support members  112   a  in which the elongated holes  134  shown in FIG. 11 are arranged to incline toward the radiation source. Then, the inclined radiation-absorbing plates  184  are glued and fixed to the bottom plate  188 . Notice that in FIG. 16, only one of the two support members  122   a  is shown. Thereafter, if the support members  122   a  are removed, the grid  180   a  is obtained as shown. In this case, the radiation-absorbing plates  184  are kept inclined by the bottom plate  188  alone, because they are not glued to the ceiling plate  186 . 
     conversely, as another variation, the radiation-absorbing plates  184  may be glued and fixed to the ceiling plate  184 , and the bottom plate  188  and the support members  122   a  may be removed. 
     In the case where the support members  122   a  are finally made unnecessary in this manner, the grid  180   a  can be reduced in size and becomes easy to handle. When the radiation-absorbing plates  184  are great in width, i.e., height, the effect of removing the support member  122   a  becomes much greater because the support members  122   a  becomes greater in height and weight. 
     While the present invention has been described with reference to the preferred embodiments thereof, the invention is not limited to the details given herein, but may be modified within the scope of the appended claims.