Patent Publication Number: US-6671348-B2

Title: X-ray image detecting apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an X-ray image detecting apparatus for detecting an X-ray image of a subject, such as a person to be examined for diagnosis or the like. 
     2. Description of the Related Art 
     Recently, X-ray image acquisition systems for taking X-ray images of subjects being examined for diagnosis using semiconductor sensors have been developed. 
     When compared with conventional X-ray radiographic systems employing ordinary silver halide photography, these X-ray image acquisition systems have such advantages in practical use that images can be recorded which have a very wide dynamic range corresponding to a very wide range in the amount of radiation, to which the sensor is exposed. That is, X-ray images can be obtained which are unlikely to be affected by variations in the amount of exposure of radiation; after X-rays with a very wide dynamic range are read with a detector including a photoelectric transducer and converted into an electric signal, the electric signal is processed so as to output X-ray images on recording materials such as a photosensitive material, and the like and on display units such as a CRT, and the like, as visible images. In this radiography, an X-ray grid, which removes scattered X-rays generated in subjects, are used in many cases in order to improve contrast in a radiographic image. 
     FIG. 1 is a sectional view of an X-ray grid and a detector used in a conventional radiographic apparatus. An X-ray image detector  1  is arranged such that a plurality of photoelectric conversion elements  3  are two-dimensionally disposed on an insulation substrate  2 , and further, fluorescent substance  4  is laminated on the photoelectric conversion elements  3 . In addition, a grid  5  is disposed above the X-ray image detector  1  with a predetermined space therebetween. The grid  5  is arranged such that foils  7 , which are composed of lead or the like, having a high X-ray absorption ratio, and intermediate materials  8 , which are composed of aluminum or the like, having a low X-ray absorption ratio, are held by a cover member  6 . Using the grid  5  arranged as described above permits primary X-rays L 1 , which have passed through a subject without being scattered thereby, to pass through the grid  5  and to reach the fluorescent substance  4  of the X-ray image detector  1 . When X-rays L 1  are irradiated onto the fluorescent substance  4 , the optical materials (light emitting materials) in the fluorescent substance  4  are excited and emit fluorescence L 2  having a wavelength within the spectral sensitivity wavelength range of the photoelectric conversion elements  3 . Further, X-rays which are incident on the grid  5  with a large angle with respect to the primary X-rays L 1 , such as a scattered X-ray component L 3  generated by the subject, are absorbed by the foils  7 . 
     During exposure of radiation, the grid  5  is moved in a direction B or C by a drive unit (not shown). With this operation, an excellent image can be obtained by the X-ray image detector  1  which has no image component of stripes of the grid  5  as well as no moires or aliasing caused by a difference between the pitch of the foils  7  and the pitch of the pixels of the X-ray image detector  1 . 
     Radiography is required to satisfy contradictory conditions (1) that an excellent image with a high contrast is to be obtained while (2) reducing the dosage of the subjects as much as possible by reducing the amount of X-rays with which they are irradiated. However, the grid  5  shown in FIG. 1 may act as a factor for deteriorating an image by reducing the intensity of X-rays on the X-ray image detector  1 . 
     One reason for this reducing of the intensity of X-rays is that the X-rays L 1 , which reach the X-ray image detector  1 , must pass through the intermediate materials  8 . While the intermediate materials  8  are composed of aluminum or the like having a high X-ray transmission ratio as described above, that transmittance is not 100% as a matter of fact. When, for example, the thickness Δ1 of the foils  7  is set to 43 μm at a time a grid density is 40 lines/cm and a grid ratio is 10:1, the intermediate materials  8  have a thickness Δ2 of 207 μm (=1 cm/40−Δ1) and a height Δ3 of 2070 μm (=Δ2×10). 
     When the intermediate materials  8  are composed of aluminum, the aluminum in the above case has a thickness of about 2 mm, and the primary X-rays L 1  have a transmittance of about 70%. Accordingly, about 30% of the intensity of the X-rays will be lost. Further, when viewed from the direction from which the X-rays are incident, 17% (=Δ1/(Δ2+Δ1)) of the grid  5  is composed of lead through which X-rays do not pass. Accordingly, the total X-ray transmittance of the grid  5  is about 60% (0.7×(1−0.17)) when the loss of the intermediate materials  8  is also taken into consideration, which means that the reduction of the intensity of X-rays caused by the grid  5  is large and cannot be ignored. 
     Further, the fluorescence L 2  generated in the fluorescent substance  4  by the primary X-rays which have passed through the grid  5 , radiates in various directions because the fluorescent substance  4  is formed in a continuous flat shape so as to entirely cover the photoelectric conversion elements  3 . Accordingly, this fluorescence L 2  reaches not only a photoelectric conversion element  3   a  located just below a position where it emits but also other photoelectric conversion elements, for example,  3   b , and the like adjacent to the photoelectric conversion element  3   a.    
     Therefore, as described below, the grid  5  reduces the intensity of X-rays, while it does remove the incident scattered X-ray component L 3 . Further, the continuous flat-shaped fluorescent substance  4  may deteriorate the MTF (modulation transfer function) of the X-ray image detector because the fluorescence L 2  generated in the fluorescent substance  4  reaches a plurality of adjacent photoelectric conversion elements. Furthermore, when the intensity of the emitting fluorescence L 2  is increased by increasing the thickness of the fluorescent substance  4  to improve the intensity of signals outputted from the photoelectric conversion elements  3 , the above tendency becomes stronger, and improvement of the sensitivity of X-ray image detectors may be impeded. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention, which was made based on the above recognition of the problem, to provide an excellent X-ray image detecting apparatus capable of obtaining a good image having a high contrast while reducing the dosage received by a subject. 
     Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of an X-ray grid and a detector of a conventional X-ray image detecting apparatus; 
     FIG. 2 is a schematic view of a radiographic system; 
     FIG. 3 is a sectional view of a first embodiment of an X-ray image detecting apparatus of the present invention; 
     FIG. 4 is a plan view of the first embodiment the X-ray image detecting apparatus of the present invention; 
     FIG. 5 is a sectional view of a second embodiment of the X-ray image detecting apparatus of the present invention; 
     FIG. 6 is a sectional view of a third embodiment of the X-ray image detecting apparatus of the present invention; and 
     FIG. 7 is a sectional view of a fourth embodiment of the X-ray image detecting apparatus of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will be described below in detail with reference to the embodiments shown in to FIGS. 2 to  7 . 
     FIG. 2 shows an overall schematic view of a radiographic system. 
     A radiographic apparatus  11  includes an X-ray image detecting apparatus  12  having a detecting surface on which a plurality of photoelectric conversion elements are disposed two-dimensionally. As will be described below, the X-ray image detecting apparatus  12  includes an X-ray image detector in which X-ray grids, fluorescent substances (which serve, as is well known, to convert incident X-rays to light of a predetermined wave-length; such substances, and any and all arrangements that can transform X-rays to light, are herein referred to sometimes as an X-ray converter or conversion member) and the photoelectric conversion elements are constituted together as a unit, i.e., integrated in one united body. X-rays irradiated from an X-ray generator  13  having an X-ray tube are applied to a person S as a subject being examined for diagnosis, and X-rays that have passed through the person S are detected by the X-ray image detecting apparatus  12 . Thus-obtained image data is digitally processed by an image processing apparatus  14  including a computer, and the image data that has been processed is stored in the computer as well as displayed on a display unit  15  as an X-ray image of the person being examined. 
     FIG. 3 is shows a sectional view of an X-ray image detector  21  built in the X-ray image detecting apparatus  12 . The X-ray image detector  21  is arranged such that a plurality of photoelectric conversion elements  23  are disposed two-dimensionally on the plane of an insulation substrate  22 , the plane extending in the direction perpendicular to the sheet surface of FIG. 3, and further, a grid unit  24  is disposed on the photoelectric conversion elements  23 , that is, at a side toward incident X-rays. In addition, the spaces between the photoelectric conversion elements  23  are arranged as insensitive regions  25 , which have no sensitivity to fluorescence. 
     A glass sheet is used as the insulation substrate  22  because it does not chemically act on semiconductor devices that form the photoelectric conversion elements  23  and the like, endures the high temperatures involved in semiconductor manufacturing processes, is stable dimensionally, and is able to have a high degree of flatness. 
     The grid unit  24  has grids which are formed of foils  26  composed of lead having a large x-ray absorption ratio, and the spaces between the respective grids are filled with fluorescent substances  27  and intermediate substances  28 , sequentially in this order, in a direction opposite to the direction from which X-rays are incident. A photoelectric conversion element  23  is disposed just under a corresponding fluorescent substance  27 , which is located between each pair of grids, and as well, each photoelectric conversion elements  23  is disposed so as to avoid a portion shaded by a foil  26 , and the shaded portion is arranged as an insensitive region  25 . The thickness of each foil  26  is in approximate agreement with the width of the insensitive region  25 . 
     The fluorescent substances  27  located between the respective grids are spatially separated from each other by the foils  26  in order to prevent crosstalk in which fluorescence L 2  generated by the fluorescent substances  27  on the respective photoelectric conversion elements  23  is incident on adjacent photoelectric conversion elements  23 . The intermediate substances  28  are disposed to reinforce the foils  26  having low rigidity and composed of aluminum, paper, wood, synthetic resin or carbon-fiber-reinforced resin, or the like, having a small X-ray absorption ratio. The fluorescent substances  27  are partitioned by the foils  26 , and the intermediate substances  28  are laminated or layered on the fluorescent substances  27 . While the foils  26  are mainly composed of lead, when the surfaces thereof are arranged as reflecting surfaces for reflecting fluorescence, fluorescence generated by the fluorescent substances  27  is reflected on the foils  26 , which increases the amount of fluorescence incident on corresponding photoelectric conversion elements  23 , thereby improving the S/N of a detection signal. 
     FIG. 4 is a sectional view of the X-ray image detector  21  shown in FIG. 3 when it is viewed from a direction D (an x-ray incident direction). Each photoelectric conversion element  23  is formed in an approximate square shape, and each grid formed by the foils  26  have a slit or strip shape. The grids are filled with intermediate substances  28  having a slit shape formed in accordance with the shape of the grids. The photoelectric conversion elements  23  formed just under the intermediate substances  28 , each having the approximately square shape, are distributed two-dimensionally. Insensitive regions  29  are formed between the respective photoelectric conversion elements  23 . Note that it is not always necessary that the girds formed by an X-ray grid be formed in a shape of stripes, and they may instead be formed in a matrix shape. In this case, the respective grids may be formed in a quadrangular shape (a square shape or a rectangular shape) or may be formed in a polygonal shape other than a quadrangular shape, for example, in a hexagonal honeycomb shape. 
     Since primary X-rays L 1  are incident on the grid unit  24  in approximately parallel to the foils  26 , they pass through the intermediate substances  28  and reach the fluorescent substances  27  and make it emit the fluorescence L 2 , to which the photoelectric conversion elements  23  have sensitivity, in the fluorescent substances  27 . While the fluorescence L 2  is emitted at various angles, it does not reach other adjacent photoelectric conversion elements  23  because it does not pass through the foils  26 . In contrast, since scattered X-rays L 3  are incident on the grid unit  24  obliquely to (i.e., not parallel to) the foils  26  but at a certain angle with respect to it, most of the scattered X-rays L 3  are absorbed by the foils  26 , and the ratio of them that reach the fluorescent substances  27  or the photoelectric conversion elements  23  is small. 
     Since the foils  26  exist only on the insensitive regions  25  between the photoelectric conversion elements  23 , they do not block the X-rays to be intrinsically detected that are not scattered by the subject and incident toward the fluorescent substances  27  on the photoelectric conversion elements  23 . Therefore, the reduction of the transmittance of X-rays, which is determined by the ratio of the thicknesses of the intermediate substances  28  and the foils  26  (opening ratio), does not occur in this arrangement, while this reduction is a problem in the conventional art employing the moving grid. 
     When, for example, it is assumed in the conventional example shown in FIG. 1 that the foils  7  have a thickness of 43 μm and the intermediate substances  8  have a thickness of 207 μm, about 17% of the arrangement (43/(43+207)) blocks the transmission of X-rays, and thus their opening ratio is 83%. In contrast, in this embodiment, an opening ratio of 100% can be secured while having the grid unit  24 . This means that sensitivity can be improved about 20% while using the same photoelectric conversion elements  23 , which permits a reduction in dosage received by persons being examined. 
     Further, in the X-ray image detector of this embodiment, the foils  26  exhibit multiplied actions not only removing the scattered X-rays L 3  incident on the foils  26  but also solving the problem which is caused by the diverged component or the scattered component of the fluorescence L 2  by spatially separating the fluorescent substances  27 . That is, the foils  26  reduce the above-mentioned crosstalk between adjacent photoelectric conversion elements. By this arrangement, the MTF can be improved, and an excellent X-ray image can be taken. 
     Further, while the fluorescent substances  27  are formed continuously in the direction perpendicular to the sheet surface (the depth direction) of FIG. 3 in the above embodiment, the fluorescent substances  27  located on the insensitive regions  29  may be removed in correspondence to the approximately square shape of the photoelectric conversion elements  23 . In this case, the MTF also will be improved in this direction (the depth direction). The grid unit  24  arranged as described above, of which a grid ratio (i.e., height of the foil of the grid as shown in the Figures divided by spacing between adjacent vertical foils of the grid) is preferably set to at least 3:1, can achieve a large effect for removing the scattered X-rays L 3 . 
     FIG. 5 shows a sectional view of a second embodiment of the X-ray image detecting apparatus of the present invention, wherein the same components as used in the first embodiment are denoted by the same reference numerals. The second embodiment is different from the first embodiment as described below. That is, in the first embodiment, the grid unit  24  is arranged as a parallel grid all foils of which are disposed parallel to each other, whereas in the second embodiment, a grid unit  32  of an X-ray image detector  31  is arranged as a converging grid foils of which are tilted symmetrically with respect to a center line Z acting as a symmetrical axis. 
     Specifically, foils  33   a  in the vicinity of the center line Z are disposed perpendicular to the detecting surfaces of photoelectric conversion elements  23 , and foils  33   b  in the periphery of the grid unit  32  are tilted with respect to the direction of the center line Z. The angle θ of foils  33  with respect to the normal Y of the detecting surfaces of the photoelectric conversion elements  23  is 0 in the vicinity of the center line Z, and increases with distance of the foil  33  from the center line Z. Note that the extending lines of all the foils  33  (i.e., the planes of all the foils) intersect with each other at one point (focal point) on the center line Z. Ordinarily, a radiographic system is arranged such that this focal point is in approximate agreement with the emitting point of an X-ray source from which X-rays emit. 
     When an X-ray image is taken using the converging grid together with an X-ray tube having an emitting point located at the focal point of the converging grid, a still more excellent image can be obtained which does not have any vignetting caused by the foils  33  even in the periphery of the grid unit  32 , that is, in which the intensity of X-rays is not reduced even in the periphery thereof. 
     FIG. 6 is a sectional view of a third embodiment of the X-ray image detecting apparatus  41  of the present invention. In the previous embodiments, the flourescent substances in the grid unit of the X-ray image detector are partitioned by the grid foils. In the third embodiment, however, partitions  43 , which are different from grid foils  26 , are disposed only in the portions where flourescent substances  27  are partitioned so that adjacent flourescent substances can be spatially separated from each other by the partitions  43 . The partitions  43  have a property that they do not transmit the fluorescence L 2 , since they block it by reflecting or absorbing it, while they may absorb the X-rays in a small amount. 
     In a grid unit  42  arranged as described above, the foils  26  can remove scattered X-rays incident downward, and as well, the partitions  43  can prevent the diverged or scattered component of the fluorescence L 2  generated in the fluorescent substances from invading into adjacent regions, whereby occurrence of crosstalk can be prevented. Further, the portion of the grid unit  42  excluding the fluorescent substances  27  and the partitions  43  has a structure in which only the foils  26  and intermediate substances  28  are alternately disposed, and thus the portion of the grid unit  42  can be simply made by a conventional manufacturing method. 
     Note that, as a modification, a similar function also can be obtained in an arrangement in which the portions of the partitions  43  are composed of simple cavities, the fluorescent substances  27  are spatially separated for each grid, and a reflecting layer or a shading layer is formed on a side of each fluorescent substance  27 . 
     FIG. 7 shows a sectional view of a fourth embodiment of the X-ray image detecting apparatus of the present invention. Each of the foils  26  of a grid unit  52  of an X-ray image detector  51  is supported with its lower end inserted into one of a plurality of grooves  53   a  formed on the upper surface of a resin plate  53 . Further, a plurality of recesses  53   b  are formed on the lower surface of the resin plate  53  at the same pitch as the foils  26  and are filled with fluorescent substances  27 . That is, the fourth embodiment has a structure in which the fluorescent substances  27  are spatially separated in correspondence to the spaces between the respective foils. Note that the resin plate  53  has a property that it blocks fluorescence emitted from the fluorescent substances  27  by reflection, absorption or the like. Otherwise, the resin plate  53   b  is provided with this property. In contrast, the upper ends of the foils  26  are held by a resin plate  54  having grooves  54   a  formed thereon at the same pitch as the grooves  53   a . The grid unit  52  has sufficient rigidity because the foils  26  are held by the grooves  53   a  and  54   a , which permits the spaces between the foils  26  to be arranged as cavities  55  without being filled with intermediate substances. 
     This arrangement can avoid a loss caused when X-rays pass through the intermediate substances, in addition to being able to remove any scattered X-rays and crosstalk that is caused by fluorescence. For example, when the intermediate substances  28  in the first embodiment are composed of aluminum having a thickness of 2 mm, they have a transmittance for the X-rays L 1  of about 70%. That is, sensitivity can be improved about 40% (≈1/0.7−1) by the removal of the intermediate substances. As a result, compatibility can be established between a further reduction in the dosage received by the subject and improvement of image quality. 
     Note that rigidity may be further improved by providing cover portions or bonded layers on the surfaces of the grid foils, in the spaces between the grid foils and the fluorescent substances, or in the spaces between the fluorescent substances and the photoelectric conversion elements. 
     The employment of the grid unit  52  arranged as described above can remove a large amount of a scattered X-ray component, and can reduce crosstalk between the respective photoelectric conversion elements due to converged fluorescence, whereby image contrast can be improved, and as well, a decrease in the intensity of X-rays can be reduced when they transit the grid unit. That is, the reduction of the dosage received by subjects and the improvement of image quality, which are ordinarily inconsistent with each other, can be satisfied at the same time. 
     The X-ray image detecting apparatus using the X-ray image detectors of all of the embodiments described above has such advantages as high reliability, less expensive cost and easy maintenance because it can obtain an excellent image without the need for a mechanism for moving an X-ray grid. 
     Further, it is needless to say that the above-mentioned third and fourth embodiments may employ a converging grid, as in the second embodiment. 
     As described above, according to the X-ray image detecting apparatus of the present invention, an excellent image having high contrast can be obtained while reducing the dosage received by the subject. 
     While the present invention has been described with reference to what are presently considered to be the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, the invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.