Patent Publication Number: US-7708462-B2

Title: Transmission image capturing system and transmission image capturing method

Description:
This application is based on application No. 2007-178396 filed in Japan, the contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a transmission image capturing technique. 
     2. Description of the Background Art 
     In medical fields, transmission images of a human body are captured by using X ray or the like. By reading the transmission images, diagnosis is conducted. 
     An X-ray diagnosing apparatus for so-called tomosynthesis is proposed, capable of observing a slice plane of a specimen at an arbitrary depth by synthesizing (reconstructing) a plurality of pieces of image data obtained by irradiating the specimen with X ray in different directions by image capturing using X ray. 
     At the time of capturing a plurality of projection images to generate an image of a slice plane, the position and angle of a scan system such as a part that emits radiation (for example, an X-ray tube) tend to be deviated from settings. Due to the deviation, so-called artifact such as distortion occurs in an image of a slice plane reconstructed. It is consequently very important to accurately grasp the position and angle of a part for generating radiation (radiation generating part) at the time of capturing a projection image. 
     To address such a problem, a technique of disposing a chart for calibration made of two microspheres, performing image capturing, and calibrating a scan system on the basis of the position of the chart for calibration in a projection image, thereby grasping the position and angle of a radiation generator more accurately and performing more accurate reconstruction has been proposed (for example, Japanese Unexamined Patent Application Publication No. 2003-61944). 
     However, in the technique proposed in Japanese Unexamined Patent Application Publication No. 2003-61944, calibration is performed in advance using the chart for calibration before image capturing. In the case where a different deviation occurs in the scan system at the time of image capturing, the position and angle of a radiation generator at the time of image capturing cannot be accurately grasped, and it is difficult to perform accurate reconstruction. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a transmission image capturing system. 
     According to the invention, the transmission image capturing system includes: a generator for generating radiation; a predetermined member for specifying a radiation path of the radiation by its inner-edge shape; a detector for detecting radiation emitted from the generator and passing through a specimen via the predetermined member; an obtaining unit for obtaining a plurality of transmission images of the specimen by detecting the radiation for a plurality of times by the detector while changing a relative position relation and a relative angle relation of the generator to the detector; and a computing unit for obtaining the relative position and angle relations on the basis of an outer-edge shape of a radiation area which is irradiated with the radiation emitted from the generator to a detection surface of the detector and the inner-edge shape of the predetermined member. 
     With the configuration, the position and the angle of the radiation generator at the time of image capturing can be grasped accurately. 
     The present invention is also directed to a transmission image capturing method of obtaining a plurality of transmission images by detecting radiation emitted from a generator and passing through a specimen via a predetermined member for a plurality of times by a detector while changing a relative position relation and a relative angle relation of the generator to the detector. 
     Therefore, an object of the present invention is to provide a technique capable of accurately grasping position and angle of a radiation generator at the time of image capturing. 
     These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a schematic configuration of an image capturing system of a first preferred embodiment; 
         FIGS. 2A and 2B  are diagrams for explaining the configuration of a emitting generator in the first preferred embodiment; 
         FIG. 3  is a diagram illustrating the functional configuration of a control unit in the first preferred embodiment; 
         FIG. 4  is a diagram for explaining the principle of the method of recognizing an outer-edge shape of a radiation area; 
         FIG. 5  is a diagram for explaining the principle of the method of recognizing the outer-edge shape of the radiation area; 
         FIGS. 6A and 6B  are diagrams for explaining the principle of deriving the position relation and the angle relation between the emitting generator and a detector; 
         FIGS. 7A and 7B  are diagrams for explaining the principle of deriving the position relation and the angle relation between the emitting generator and the detector; 
         FIG. 8  is a diagram for explaining the principle of deriving the position relation and the angle relation between the emitting generator and the detector; 
         FIG. 9  is a diagram for explaining the principle of deriving the position relation and the angle relation between the emitting generator and the detector; 
         FIG. 10  is a flowchart showing an image capturing operation flow of the first preferred embodiment; 
         FIG. 11  is a schematic diagram showing the principle of tomosynthesis; 
         FIGS. 12A and 12B  are schematic diagrams showing the principle of tomosynthesis; 
         FIG. 13  is a diagram showing a schematic configuration of an image capturing system of a second preferred embodiment; 
         FIG. 14  is a diagram for explaining the configuration of a emitting generator in the second preferred embodiment; 
         FIG. 15  is a diagram for explaining the principle of a method of recognizing the outer-edge shape of the radiation area; 
         FIG. 16  is a diagram illustrating the functional configuration of a control unit in the second preferred embodiment; and 
         FIG. 17  is a flowchart showing an image capturing operation flow in the second preferred embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will be described below with reference to the drawings. 
     First Preferred Embodiment 
     Schematic Configuration of Image Capturing System 
       FIG. 1  is a diagram showing a schematic configuration of an image capturing system  1  of a first preferred embodiment of the present invention. The image capturing system  1  detects a distribution of radiation passing through a specimen  120  by using radiation (typically, X ray), and obtains a distribution of pixel values (transmission image), so that various information processes using the transmission image can be performed. That is, the image capturing system  1  functions as a system for capturing a transmission image (transmission image capturing system). 
     The image capturing system  1  includes an image capturing apparatus  100  and an image capture control processing apparatus  200 . It is assumed that the specimen  120  as an object to image capturing is the body of a person to take a test. An oval in the diagram schematically expresses the body of the specimen. 
     The image capturing apparatus  100  has, mainly, a emitting generator  101 , a guide  102 , a mounting part  104 , a coupling part  105 , and a detector  108 . 
     The emitting generator  101  generates and emits radiation as a kind of electromagnetic waves. It is assumed here that the emitting generator  101  generates and emits X rays. In  FIG. 1  and subsequent diagrams, alternate long and short dash lines are drawn at the outer edges of the path of emission of radiation. 
       FIGS. 2A and 2B  are diagrams illustrating the configuration of the emitting generator  101 . In  FIGS. 2A and 2B  and subsequent diagrams, to clarify the azimuth relation, three axes of X, Y, and Z which cross orthogonal to each other are properly shown. 
       FIG. 2A  is a cross section diagram schematically showing the configuration of the emitting generator  101 . The emitting generator  101  is constructed by, for example, an X-ray tube and has a generating unit  101   a  and a diaphragm  101   c . In  FIG. 2A , a radiation passage area is shaded. 
     The generating unit  101   a  is a part for generating radiation, and the diaphragm  101   c  is provided for the generating unit  101   a  and functions as a predetermined member that specifies the path of radiation (radiation path) emitted from the generating unit  101   a  toward the specimen  120 , that is, the shape of the radiation path. By disposing the generating unit  101   a  and the diaphragm  101   c , the emitting generator  101  forms a focal point Fp of radiation (for example, focal point of an X-ray tube). 
     As shown in  FIG. 2B , the diaphragm  101   c  (a hatched portion in the diagram) has, for example, an opening  101   h  of a square shape whose length of one side has a predetermined value, that is, an inner-edge shape. The distance from the focal point Fp to the opening  101   h , specifically, the distance from the focal point Fp to the center point of the opening  101   h  is set to be a predetermined distance. 
     By the shape of the opening  101   h , the outer edges of the radiation path of the radiation form a square shape. Further, the size of the square as the outer-edge shape of the radiation path increases in proportion to distance from the generating unit  101   a.    
     Referring again to  FIG. 1 , the explanation will be continued. 
     The guide  102  extends almost an arc shape and can change the position and posture of the emitting generator  101 . Concretely, the emitting generator  101  is movably coupled to the guide  102  in the extending direction and moves along the extending direction on the guide  102  according to an image capture control processing apparatus  200 . 
     The mounting unit  104  is a part on which the specimen  120  is left at rest. The mounting unit  104  is disposed so as to satisfy a predetermined relative disposing condition with respect to the emitting generator  101  coupled to the guide  102  via the coupling unit  105 . The specimen  120  is mounted within a radiation range of an X-ray emitted from the emitting generator  101 . More specifically, the mounting unit  104  is fixed in a predetermined position on the side where the focal point of the arc-shaped part specified by the guide  102  is located. 
     The mounting unit  104  is made of a material substantially transmitting an X-ray by having small absorption of an X-ray, and X-ray attenuation coefficient (absorption coefficient) is known. In a state where the specimen  102  is left at rest on the mounting unit  104 , the emitting generator  101  emits an X-ray while being properly moved along the guide  102 , thereby irradiating the specimen  120  with the X-ray from desired directions. 
     The detector  108  detects the radiation (X-ray in this case) emitted from the emitting generator  101  and passed through the specimen  120  mounted on the mounting unit  104  and through the mounting unit  104 . The detector  108  detects, for example, both of the X ray passed through the specimen  120  and the X-ray passed through the space around the specimen  120 . 
     A surface on the side of emitting generator  101 , of the detector  108 , that is, an X-ray detecting surface (detection surface)  108   s  has, for example, a rectangular outer shape and has an almost flat surface in which a number of sensors for detecting X rays are arranged two-dimensionally (for example, in a lattice shape). Therefore, radiation passed through the specimen  120  and the mounting unit  104  in the radiation emitted from the emitting generator  101  is detected by the detector  108 , and a distribution of detection values of the radiation (in this case, a two-dimensional distribution having the lattice shape) is obtained. 
     The emitting generator  101 , the guide  102 , the mounting unit  104 , and the detector  108  satisfy positional relations described below. Specifically, since the radiation range of the X ray emitted from the emitting generator  101  covers a wide range of the mounting unit  104 , even when the emitting generator  101  moves to any of positions on the guide  102 , the X ray emitted from any of the positions on the guide  102  is detected by the detector  108 . 
     In  FIG. 1 , the guide  102  is formed almost in an arc shape, and the emitting generator  101  emits X ray toward the center point of the arc. The invention is not limited thereto. For example, the guide  102  may be extended almost linearly and the emitting generator  101  emits X ray in a direction almost perpendicular to the extending direction of the guide  102  even when the emitting generator  101  moves to any position on the guide  102 . In any of the cases, radiation is sequentially emitted in a plurality of directions to a predetermined side (upper side in  FIG. 1 ) of the specimen  120 , and a distribution of detection values of the radiation (hereinafter, also called “radiation detection value distribution”) is obtained. 
     On the other hand, the image capture control processing apparatus  200  has a configuration similar to that of a general personal computer and includes, mainly, a control unit  210 , a display unit  220 , an operating unit  230 , and a storing unit  240 . 
     The control unit  210  has a CPU  210   a , a RAM  210   b , and a ROM  210   c , and controls the operations of the image capturing system  1  in a centralized manner. The control unit  210  realizes various functions and operations by reading a program PG stored in the storing unit  240  and executing the program PG. 
     The display unit  220  is constructed by, for example, a liquid crystal display and the like. Under control of the control unit  210 , various images are visibly output. For example, a transmission image obtained by image capturing of the image capturing apparatus  100  or the like is visibly output. 
     More specifically, a planar image (plane image) and a stereoscopic image viewed from a specific direction are visibly output. Concretely, not only a plane image expressed by data of a transmission image (transmission image data) stored in the RAM  210   b  and the like but also a stereoscopic image expressed by stereoscopic image data generated by an image generating unit  216  (to be described later), other various image information, numerical information, and character information are displayed. Display of a stereoscopic image viewed from a specific direction as a two-dimensional image will be called “display of a stereoscopic image” hereinafter. 
     The operating unit  230  includes a keyboard and a mouse, accepts various inputs of the user, and transmits signals according to the inputs to the control unit  210 . 
     The storing unit  240  includes a hard disk and the like, and stores, for example, the program PG for controlling various operations of the image capturing system  1 , various data, and the like. 
     Functional Configuration in Control Unit 
       FIG. 3  is a diagram illustrating the functional configuration realized when the program PG is executed by the control unit  210 . 
     As shown in  FIG. 3 , the control unit  210  has, as functions, an image capturing control unit  211 , a detection value obtaining unit  212 , a value converting unit  213 , a radiation area recognizing unit  214 , a position/angle computing unit  215 , and the image generating unit  216 . 
     The image capturing control unit  211  controls operation of the image capturing apparatus  100 . For example, the image capturing control unit  211  controls the position of the emitting generator  101  on the guide  102 , thereby controlling the positional relation of the mounting unit  104 , that is, the specimen  120  to the emitting generator  101  and the guide  102 . By this, the spatial relation between the emitting generator  101  and the mounting unit  104  varies relatively. At this time, the distance between the emitting generator  101  and the detector  108  and the angle relation between the emitting generator  101  and the detector  108  are properly changed. 
     The “angle relation” includes the relation of the angle formed by the center line of radiation emitted from the emitting generator  101 , that is, the radiation travel direction and the surface (detection surface)  108   s  in which a number of sensors are arranged in the detector  108 . 
     The detection value obtaining unit  212  accepts and obtains a distribution of detection values of radiation detected by the detector  108 . In the embodiment, a distribution of detection values detected by the sensors disposed two-dimensionally in the detection surface  108   s , that is, a two-dimensional detection value distribution (two-dimensional distribution of the detection values) is obtained. The distribution of detection values obtained by the detection value obtaining unit  212  is temporarily stored in the RAM  210   b  or the storing unit  240 . 
     The value converting unit  213  converts the distribution of the detection values obtained by the detection value obtaining unit  212  to a distribution of pixel values corresponding to a visible image (hereinafter, also called “pixel value distribution”), that is, image data. For example, a relatively large X-ray detection value is converted to a pixel value of low luminance (low tone), and a relatively small X-ray detection value is converted to a pixel value of high luminance (high tone). Image data (transmission image data, also called “transmission image”) is a two-dimensional distribution of pixel values and temporarily stored in the RAM  210   b  or the storing unit  240 . 
     By detecting radiation for a plurality of times by the detector  108  while changing the relative position and angle relations of the emitting generator  101  to the detector  108  by the image capturing control unit  211 , a plurality of transmission images of the specimen  120  are obtained by the value converting unit  213 . 
     The radiation area recognizing unit  214  recognizes the shape of an area irradiated with radiation (hereinafter also called “radiation area” or “radiation field”) on the detection surface  108   s  from the emitting generator  101 , concretely, the shape of the outer edges (hereinafter, also called “outer-edge shape”) of the radiation area. A method of recognizing the outer-edge shape of the radiation area will be described later. 
     The position/angle computing unit  215  obtains the relative position relation and the relative angle relation between the emitting generator  101  and the detector  108  by computation on the basis of the outer-edge shape of the radiation area recognized by the radiation area recognizing unit  215  and the inner-edge shape of the diaphragm  101   c . A method of computing the relative position relation and the relative angle relation will be described later. 
     The image generating unit  216  generates various images (for example, an image of a slice plane) by using the transmission images obtained by the value converting unit  213 . 
     For example, when a distribution of a plurality of pixel values, that is, a plurality of transmission images are obtained while changing the position of the emitting generator  101  along the guide  102 , the image generating unit  216  generates data of an image showing a slice plane (slice plane image) of the specimen  120  on the basis of the plurality of transmission images and the relative position relation and the relative angle relation of the emitting generator  101  to the detector  108  when the radiation of each of the transmission images is detected. The image generating unit  216  also generates data of a stereoscopic image of the specimen  120  having a three-dimensional structure on the basis of the data of the slice plane image. 
     Concretely, for example, the image generating unit  216  generates data of the slice plane image while temporarily storing the data of the transmission image into the RAM  210   b  in cooperation with the RAM  210   b  for temporarily storing data. Further, the image generating unit  216  generates data of the stereoscopic image while temporarily storing the data of the slice plane image into the RAM  210   b . A method of generating the data of the slice plane image will be described later. 
     Method of Recognizing Outer-Edge Shape of Radiation Area 
       FIGS. 4 and 5  are diagrams for explaining the principle of the method of recognizing the outer-edge shape of the radiation area in the radiation area recognizing unit  214 . 
       FIG. 4  is a diagram paying attention to the emitting generator  101 , the detector  108 , and the specimen  120  at the time of imaging the specimen  120 . In  FIG. 4 , the focal point Fp of the emitting generator  101  is shown by a filled circle, the outer edges of radiation emitted from the emitting generator  101  to the specimen  120  are shown by alternate long and short dash lines, and center line (center axis) Lc of the radiation emitted from the emitting generator  101  to the specimen  120  is shown by a broken line. It is assumed that each of the outer edges of the radiation emitted from the emitting generator  101  makes a predetermined angle α to the center axis Lc. 
     In  FIG. 4 , the direction of emitting radiation from the emitting generator  101  to the specimen  120 , that is, the travel direction of radiation is inclined with respect to the normal line to the detection surface  108   s . Image capturing performed by irradiating the detection surface  108   s  with radiation emitted obliquely will be also called “oblique image capturing”. 
       FIG. 5  is a diagram illustrating a transmission image G obtained at the time of oblique image capturing as shown in  FIG. 4 . In the transmission image G, a pattern (trapezoidal image area which is shaded in the diagram) PA of the radiation emitted from the emitting generator  101  to the detection surface  108   s  appears. The pattern PA corresponds to the area irradiated with the radiation from the emitting generator  101  (radiation area) in the detection surface  108   s.    
     In  FIG. 5 , the outer edges of the pattern PA form a trapezoidal shape. However, the invention is not limited thereto. For example, at the time of image capturing (facing-position image capturing) in which radiation is emitted from the emitting generator  101  in a state where the irradiation direction of the radiation from the emitting generator  101  toward the specimen  120  is almost perpendicular to the detection surface  108   s  and the detector  108  detects the radiation, thereby obtaining a distribution of detection values, the outer shape of the pattern PA is a square shape similar to the shape of the opening  101   h.    
     For example, the radiation area recognizing unit  214  recognizes the pattern PA of the radiation from the transmission image G obtained by the value converter  213  and the outer-edge shape of the pattern PA, thereby recognizing the outer-edge shape of the radiation area of the radiation from the emitting generator  101  to the detection surface  108   s . Concretely, the radiation area recognizing unit  214  recognizes the length of the upper base, the length of the lower base, and the height of the trapezoid as the outer-edge shape of the radiation area from the size of the detection surface  108   s , the size of the projection image G, and the size of the pattern PA. That is, in the specification, the words “the outer-edge shape of the radiation area” are used for meaning including the size of the outer-edge shape, and the words “the inner-edge shape” are used for meaning including the size of the inner-edge shape. 
     Although the outer-edge shape of the radiation area is recognized from the transmission image G obtained by the value converting unit  213  in the embodiment, the invention is not limited thereto. For example, the outer-edge shape of the radiation area may be recognized from a distribution of detection values obtained by the detection value obtaining unit  212 . 
     Principle of Deriving Position Relation and Angle Relation 
       FIGS. 6A and 6B  to  FIG. 9  are diagrams for explaining the principle of deriving the relative position relation and the relative angle relation between the emitting generator  101  and the detector  108  in the position/angle computing unit  215 . 
       FIG. 6A  is a sectional schematic view of the path of radiation seen from a side paying attention to the position relation between the emitting generator  101  and the detector  108 .  FIG. 6B  is a diagram showing the outer-edge shape of the radiation area in the state of  FIG. 6A . 
     The position/angle computing unit  215  computes the length of a perpendicular from the focal point Fp of the emitting generator  101  to the detection surface  108   s , that is, distance “h” between the focal point Fp and the detection surface  108   s , and angle θ formed by the center line Lc of the radiation emitted from the emitting generator  101  and a perpendicular from the focal point Fp to the detection surface  108   s.    
       FIG. 7A  is a diagram paying attention to the radiation path shown in  FIG. 6A .  FIG. 7B  is a diagram paying attention to the outer-edge shape of the radiation area shown in  FIG. 6B . 
     As shown in  FIGS. 7A and 7B , the distance from the focal point Fp to the opening  101   h  is expressed as “d”, the distance from the focal point Fp to the center point of the upper base of the trapezoid as the outer edges of the radiation area is expressed as x1, the distance from the focal point Fp to the center point of the lower base of the trapezoid as the outer edges of the radiation area is expressed as x2, the length of the upper base of the trapezoid as the outer edges of the radiation area is expressed as “m”, the length of the lower base is expressed as “n”, and the height is expressed as “l”. The length of one side of the opening  101   h  is set as a predetermined value “a”. Further, a square surface (virtual surface) Sf using the upper base of the trapezoid as the outer edges of the radiation area as one side and perpendicular to the center line Lc of the path of radiation is virtually set, and the angle formed between the detection surface  108   s  and the virtual surface Sf is expressed as φ. 
     Since the angle φ is the same as the angle θ, when the angle φ is obtained, the angle θ is obtained. 
     A method of calculating the angle φ (that is, the angle θ) and the distance “h” will be described concretely below. 
     The predetermined value “a” and the distance “d” are known in designing of the emitting generator  101 . The length “m” of the upper base, the length “n” of the lower base, and the height “l” of the trapezoid as the outer-edge shape of the radiation area are recognized by the radiation area recognizing unit  214 . The distance from the center point of one side on the lower base side of the trapezoid as the radiation area in the vertical surface Sf to the center point of the lower base of the trapezoid as the radiation area is calculated by “x2−x1”. 
     Since the regular square pyramid using the focal point Fp as an apex and using the opening  101   h  as a bottom face and the regular square pyramid using the focal point Fp as an apex and using the virtual surface Sf as a bottom surface are similar figures, the distance x1 is obtained by the following equation (1). 
     
       
         
           
             
               
                 
                   
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       FIG. 8  is a perspective view of the path of radiation. As shown in  FIG. 8 , an isosceles triangle using the focal point Fp as an apex and the upper base of the trapezoid as the base, and an isosceles triangle using the focal point Fp as an apex and the lower base of the trapezoid as the base are similar figures, and the ratio of the sizes is m:n. 
     Since the relation of x1:x2=m:n is satisfied, the distance x2 is obtained by the following equation (2). 
     
       
         
           
             
               
                 
                   
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     Therefore, the lengths (l, m, and x2−x1) of the three sides of a triangle Tr drawn by the thick lines in  FIG. 7A  are obtained. From the lengths of the three sides forming the triangle Tr, the three internal angles of the triangle Tr are unconditionally obtained. That is, the angle φ (that is, the angle θ) is obtained. 
       FIG. 9  is a diagram for explaining concrete calculation of the distance “h”. In  FIG. 9 , a shape similar to that of  FIG. 7A  is drawn, and the triangle Tr drawn by the thick lines in  FIG. 7A  is similarly drawn by thick lines. 
     As described above, when the lengths (l, m, and x2−x1) of the three sides of the triangle Tr are obtained, the internal angle ρ of the upper base side of the trapezoid as the radiation area, as one of the internal angles of the triangle Tr, is also unconditionally obtained. 
     The distance “h” is obtained by the following equation (3).
 
 h=x 2·sin ρ  (Equation 3)
 
     When the distance “h” and the angle θ are obtained as described above, the relative position and angle relations between the emitting generator  101  and the detector  108  are obtained unconditionally. For example, the position (x, y, z) of the emitting generator  101  using a predetermined point (for example, center point) of the detection surface  108   s  as an original point is obtained. 
     Image Capturing Operation Flow 
       FIG. 10  is a flowchart showing an image capturing operation flow of continuously capturing transmission images of a plurality of frames while changing the radiation angle of X ray from the emitting generator  101  to the specimen  120  in multiple stages in the image capturing system  1 . The operation flow is realized under control of, mainly, the image capturing control unit  211  when the control unit  210  executes the program PG. The operation flow starts when the specimen  120  is mounted on the mounting unit  104  and a predetermined input is entered from the operating unit  230 . 
     First, in step S 1 , by the control of the image capturing control unit  211 , the emitting generator  101  is set in an initial position. 
     The initial position of the emitting generator  101  on the guide  102  is preset. It is assumed here that the initial position is set so that the irradiation angle of X ray from the emitting generator  101  to the specimen  120  is the smallest. Concretely, the emitting generator  101  is disposed, for example, at one end in the extending direction of the guide  102  (the right end in  FIG. 1 ). 
     In step S 2 , by the control of the image capturing control unit  211 , image capturing process is performed. The image capturing process is performed here by emitting radiation from the emitting generator  101  to the specimen  120  and detecting the radiation by the detector  108 . 
     In step S 3 , on the basis of detection values of the radiation obtained by the sensors in the detector  108  in step S 2 , the detection value obtaining unit  212  obtains a two-dimensional distribution of the detection values. 
     In step S 4 , the value converting unit  213  converts the two-dimensional distribution of the detection values obtained in step S 3  to a two-dimensional distribution of pixel values, thereby generating a transmission image. 
     In step S 5 , from the transmission image obtained in step S 4 , the radiation area recognizing unit  214  recognizes the outer-edge shape (concretely, the size) of the radiation area irradiated with the radiation in the detection surface  108   s . For example, the pattern PA corresponding to the radiation area is recognized from the transmission image G as shown in  FIG. 5 , and the shape and size of the outer edges are recognized. 
     In step S 6 , on the basis of the outer-edge shape of the radiation area recognized in step S 5  and the inner-edge shape of the diaphragm  101   c , the position/angle computing unit  215  calculates the relative position relation and the relative angle relation between the emitting generator  101  and the detector  108 . For example, by a method as described with reference to  FIGS. 6A to 9 , the distance “h” and the angle θ are calculated, and the relative position and angle relations between the emitting generator  101  and the detector  108  are unconditionally calculated from the distance “h” and the angle θ. 
     In step S 7 , whether the image capturing is finished or not is determined. For example, when a predetermined parameter reaches a predetermined value, it is determined to finish the image capturing. 
     Concretely, until the predetermined parameter reaches the predetermined value, the emitting generator  101  is moved along the guide  102  in step S 8 , and the program returns to step S 2 . On the other hand, when the predetermined parameter reaches the predetermined value, the operation flow is finished. 
     At this time, a predetermined number of transmission images are sequentially obtained, and the relative position relation and the relative angle relation between the emitting generator  101  and the detector  108  in each of image capturing processes corresponding to the transmission images are calculated. Information indicative of the relative position relation and the relative angle relation between the emitting generator  101  and the detector  108  is associated with the transmission image data and stored in the RAM  210   b  or the storing unit  240 . 
     The predetermined parameters are the number of image capturing times, the travel distance of the emitting generator  101 , the travel angle of the emitting generator  101 , and the like. For example, until the number of image capturing times reaches a predetermined number (for example, 19), the processes in steps S 2  to S 8  are repeated. After the number of image capturing times reaches the predetermined number (for example, 19), the operation flow is finished. 
     In step S 8 , by the control of the image capturing control unit  211 , the emitting generator  101  is moved along the guide  102 . The position of the emitting generator  101  on the guide  102  is changed from the position in the image capturing process of last time to the next position. For example, in the case where the travel range along the extending direction of the guide  102  is divided in 18 parts and the emitting generator  101  moves in multiple stages, in step S 8 , the emitting generator  101  travels in the distance of 1/18 of the travel range. 
     Principle of Generation of Slice Plane Image Data 
     As described above, in the case where the position of the emitting generator  101  is varied along the guide  102  and a plurality of transmission images are obtained sequentially, for example, by the image generating unit  216 , data of slice plane images (slice plane image data) of the specimen  120  is properly generated. 
     The principle of generating the slice plane image data, that is, the principle of tomosynthesis in the image generating unit  216  will be described. 
       FIGS. 11 and 12  are schematic diagrams showing the principle of tomosynthesis. 
     In the tomosynthesis, radiation, concretely, X ray passing through the specimen  120  is emitted at different angles on the side of one direction of the specimen  120  to the specimen  120  and data of a plurality of transmission images is obtained and synthesized, thereby obtaining an image of a slice plane. The case where a star-shaped element  121  and a round-shaped element  122  schematically showing internal structures (concretely, a human organ, a lesioned part, and the like) of the specimen  120  are arranged in the direction perpendicular to the detection surface  108   s  as shown in  FIG. 11  will be described as an example. 
     As shown in  FIG. 11 , by emitting radiation to the specimen  120  at different angles, data of a plurality of transmission images is obtained. In transmission images  41 ,  42 , and  43  expressed by the data of the plurality of transmission images obtained in such a manner, the positions of images of the elements vary according to the distance (height) from the detection surface  108   s . While utilizing the phenomenon, arbitrary slice plane image data is generated by using a known method of synthesizing a plurality of images. One of the known methods of synthesizing a plurality of images in tomosynthesis is shift-and-add algorithm. 
     In shift-and-add algorithm, on the basis of the plurality of transmission images  41  to  43  and the positions (x, y, z) and angles of the emitting generator  101  on detection of radiations corresponding to the transmission images  41  to  43  (that is, in the image capturing process), a process of sequentially adding the transmission images while shifting the relative positions of the transmission images  41  to  43  is performed. 
     For example, as shown in  FIG. 12A , an image  51  in which the star-shaped element  121  which is vague in the transmission images  41  to  43  is emphasized is obtained. As shown in  FIG. 12B , an image  52  in which the round-shaped element  122  which is vague in the transmission images  41  to  43  is emphasized is obtained. The image  51  is a slice plane image obtained by emphasizing a cross section at height where the star-shaped element  121  exists in the internal structure of the specimen  120 . The image  52  is a slice plane image obtained by emphasizing a cross section at height where the round-shaped element  122  exists in the internal structure of the specimen  120 . 
     The example of generating the images  51  and  52  by synthesizing the three transmission images  41  to  43  by addition has been described to simplify the explanation. In practice, a number of transmission images are obtained and synthesized. 
     As described above, in the image capturing system  1  of the first preferred embodiment of the invention, on the basis of the outer-edge shape of the radiation area irradiated with radiation on the detection surface  108   s  and the inner-edge shape of a predetermined member (for example, the diaphragm  101   c ) specifying the path of radiation, the relative position relation and the relative angle relation of the emitting generator  101  to the detector  108  are obtained. That is, at the time of actual image capturing, the relative position and angle relations of the emitting generator  101  with respect to the detector  108  are obtained. Consequently, the position and angle of the emitting generator  101  at the time of image capturing can be grasped accurately. 
     On the basis of the outer-edge shape of the radiation area irradiated with ration on the detection surface  108   s  and the inner-edge shape of the diaphragm  101   c  generally used, the relative position and angle relations of the emitting generator  101  with respect to the detector  108  are obtained. Consequently, without adding a special configuration that specifies the path of radiation, the position and angle of the emitting generator  101  at the time of image capturing can be grasped accurately. 
     The outer-edge shape of the radiation area irradiated with radiation on the detection surface  108   s  is recognized from the transmission images. Therefore, without adding a special configuration, the outer-edge shape of the radiation area is recognized. 
     Using the information indicative of the accurately grasped position and angle of the emitting generator  101  in the image capturing, a slice plane image is generated on the basis of a plurality of transmission images. Consequently, a high-quality slice plane image in which occurrence of so-called artifact is suppressed is obtained. 
     Second Preferred Embodiment 
     In the image capturing system  1  of the first preferred embodiment, the outer-edge shape of the radiation area is recognized from the transmission images. On the other hand, in an image capturing system  1 A of a second preferred embodiment, an irradiation area on the detection surface  108   s  is illuminated by lighting and is photographed by a camera. The outer-edge shape of the radiation area is recognized from obtained photographed images. 
     The image capturing system  1 A of the second preferred embodiment will be described below. The image capturing system  1 A of the second preferred embodiment has a configuration similar to that of the image capturing system  1  of the first preferred embodiment except for the configuration of recognizing the outer-edge shape of the radiation area. Consequently, the same reference numerals are designated to the similar components and their description will not be repeated. Mainly, the different configuration will be described. 
       FIG. 13  is a diagram showing a schematic configuration of the image capturing system  1 A according to the second preferred embodiment of the present invention. 
     The image capturing system  1 A includes: an image capturing apparatus  100 A which has a emitting generator  101 A to which an illuminating mechanism  101   p  ( FIG. 14 ) is added in place of the emitting generator  101  of the first preferred embodiment and a camera unit  106 ; and an image capture control processing apparatus  200 A including a control unit  210 A having a functional configuration of recognizing the outer-edge shape of the radiation area different from that of the control unit  210  of the first preferred embodiment. A program PGA is stored in place of the program PG in the storing unit  240 . 
       FIG. 14  is a cross section diagram schematically showing the configuration of the emitting generator  101 A. The emitting generator  101 A is constructed by, for example, an X-ray tube and has the generating unit  101   a , the diaphragm  101   c , and the illuminating mechanism  101   p . That is, the emitting generator  101 A is obtained by adding the illuminating mechanism  101   p  to the emitting generator  101  of the first preferred embodiment. 
     The illuminating mechanism  101   p  is provided near the generating unit  101   a  and has a light source PR, a first reflection mirror M 1 , and a second reflection mirror M 2 . 
     The light source PR has an apparatus for generating a visible light ray and generates, for example, a laser beam of a predetermined color. The first reflection mirror M 1  reflects the light from the light source PR toward the second reflection mirror M 2 . The second reflection mirror M 2  reflects the light from the first reflection mirror M 1 , and light is emitted from the opening  101   h  in the diaphragm  101   c  to the detection surface  108   s . In  FIG. 14 , the path of light from the light source PR is shown by a thick broken-line arrow. 
     The path (optical path) of light generated by the light source PR and emitted via the diaphragm  101   c  (concretely, the opening  101   h ) is set almost the same as the path (radiation path) of radiation emitted from the generator  101   a  via the diaphragm  101   c  (concretely, the opening  101   h ). That is, the illuminating mechanism  101   p  is constructed so that light generated from the light source PR is applied to the detection surface  108   s  via the optical path which is almost the same as the radiation path via the diaphragm  101   c.    
     In the second preferred embodiment, the mounting unit  104  is made of a material which transmits a visible light ray such as transparent glass so that the light is not blocked by the mounting unit  104 . Further, since the second reflection mirror M 2  is disposed on a path of radiation extending from the generating unit  101   a  to the diaphragm  101   c  (concretely, the opening  101   h ), it is made of a material which easily transmits radiation. 
     The camera unit  106  is a sensor constructed by, for example, a digital camera including an image capturing device such as a CCD, and is mounted just above the mounting unit  104 . Concretely, the optical axis of a taking lens of the camera unit  106  is almost orthogonal to the detection surface  108   s  and passes almost the center of the detection surface  108   s . That is, the camera unit  106  is mounted so as to face the detection surface  108   s.    
     More specifically, as shown in  FIG. 13 , the camera unit  106  is disposed, for example, in a position around the center in the extending direction of the guide  102  (for example, in  FIG. 13 , near the emitting generator  101 A drawn by the solid line) so as not to disturb the travel of the emitting generator  101 A, the path of radiation emitted from the emitting generator  101 A, and the optical path of light emitted from the illuminating mechanism  101   p.    
     The detection surface  108   s  illuminated with the light from the illuminating mechanism  101   p  is photographed from just above by the camera unit  106  in the image capturing process using radiation, thereby obtaining an image. The image is transmitted to the control unit  210 A. 
       FIG. 15  is a diagram paying attention to the emitting generator  101 A, the detector  108 , the specimen  120 , and the camera unit  106  at the time of imaging the specimen  120 . As shown in  FIG. 15 , the position and the image capturing direction of the camera unit  106  are fixed. The camera unit  106  photographs the detection surface  108   s  from a predetermined position irrespective of the position of the emitting generator  101 A on the guide  102 . 
     The image obtained by the camera unit  106  has a pattern similar to that of the transmission image G shown in  FIG. 5  at the time of, for example, oblique imaging capturing as shown in  FIG. 15 . Specifically, the outer edges of the detection surface  108   s  corresponding to the outer edges of the transmission image G and the area (illuminated area) illuminated by light from the illuminating mechanism  101   p  corresponding to the pattern PA are captured. 
     The functional configuration of the control unit  210 A will now be described. 
       FIG. 16  is a diagram illustrating the functional configuration realized when the program PGA is executed by the control unit  210 A. 
     As shown in  FIG. 16 , the control unit  210 A has, as functions, the image capturing control unit  211 , the detection value obtaining unit  212 , the value converting unit  213 , a light-on control unit  214   a , a camera control unit  214   b , a photographed-image obtaining unit  214   c , a radiation area recognizing unit  214   d , the position/angle computing unit  215 , and the image generating unit  216 . 
     The image capturing control unit  211 , the detection value obtaining unit  212 , the value converting unit  213 , the position/angle computing unit  215 , and the image generating unit  216  are similar to those of the first preferred embodiment. 
     The light-on control unit  214   a  controls light-on of the illuminating mechanism  101   p , that is, emission of light from the light source PR. 
     The camera control unit  214   b  controls the operation of the camera unit  106 . For example, light from the illuminating mechanism  101   p  is emitted to the radiation area by the light-on control unit  214   a  in accordance with image capturing using radiation and the area is photographed by the camera unit  106  to obtain a photographed image. 
     The photographed-image obtaining unit  214   c  receives the photographed image obtained by the camera unit  106 . In the photographed-image obtaining unit  214   c , a necessary image process may be performed. 
     The radiation area recognizing unit  214   d  recognizes the outer-edge shape of the radiation area irradiated with radiation on the detection surface  108   s  emitted from the emitting generator  101 A in a manner similar to the radiation area recognizing unit  214  of the first preferred embodiment except for a recognizing method. 
     The radiation area recognizing unit  214   d  recognizes the outer-edge shape of the radiation area in the photographed image obtained by the photographed-image obtaining unit  214   c.    
     Specifically, the radiation area recognizing unit  214   d  recognizes an area irradiated with light in the detection surface  108   s  by detecting, for example, a predetermined color or a high-illuminance part in the photographed image obtained by the photographed-image obtaining unit  214   c . The radiation area recognizing unit  214   d  also recognizes the outer edges of the detection surface  108   s  by, for example, edge detection. From the size of the detection surface  108   s  and the size of the area irradiated with light, the length of the upper base, the length of the lower base, and the height of the trapezoid as the outer-edge shape of the radiation area are recognized. 
     In the position/angle computing unit  215  and the image generating unit  216 , processes similar to those of the first preferred embodiment are performed by using the recognition results of the radiation area recognizing unit  214   d.    
       FIG. 17  is a flowchart showing an image capturing operation flow of continuously capturing transmission images of a plurality of frames while changing the radiation angle of X ray from the emitting generator  101 A to the specimen  120  in multiple stages in the image capturing system  1 A. The operation flow is realized under control of, mainly, the image capturing control unit  211  when the control unit  210 A executes the program PGA. The operation flow starts when the specimen  120  is mounted on the mounting unit  104  and a predetermined input is entered from the operating unit  230 . 
     In steps SP 1  to SP 4 , processes similar to those in steps S 1  to S 4  in  FIG. 10  are performed. 
     In step SP 5 , under control of the light-on control unit  214   a , irradiation (illuminating) of the detection surface  108   s  with light by the illuminating mechanism  101   p  starts. 
     In step SP 6 , under control of the camera control unit  214   b , photographing by the camera unit  106  is performed. A photographed image of the detection surface  108   s  irradiated with light is obtained by the photographed-image obtaining unit  214   c.    
     In step SP 7 , under control of the light-on control unit  214   a , the irradiation (illuminating) of the detection surface  108   s  with light by the illuminating mechanism  101   p  is finished. 
     In step SP 8 , the radiation area recognizing unit  214   d  recognizes the outer-edge shape (concretely, the size) of the radiation area irradiated with radiation in the detection surface  108   s  from the photographed image obtained in step SP 6 . 
     In steps SP 9  to SP 11 , processes similar to those of steps S 6  to S 8  in  FIG. 10  are performed. 
     As described above, in the image capturing system  1 A of the second preferred embodiment of the invention, in a manner similar to the image capturing system  1  of the first preferred embodiment, the relative position relation and the relative angle relation of the emitting generator  101 A with respect to the detector  108  are obtained on the basis of the outer-edge shape of the radiation area irradiated with radiation on the detection surface  108   s  and the inner-edge shape of the predetermined member (for example, the diaphragm  101   c ) that specifies the path of radiation. Consequently, the position and angle of the emitting generator  101 A at the time of image capturing can be grasped accurately. 
     MODIFICATIONS 
     Although the embodiments of the present invention have been described above, the invention is not limited to the above description. 
     For example, in the foregoing embodiment, a slice plane image is generated by using the shift-and-add algorism. The invention is not limited thereto. For example, while changing the radiation angle of X ray from the emitting generator  101  to the specimen  120  in multiple stages, a plurality of transmission images obtained are regarded as a part of transmission images obtained by the CT (Computed Tomography) image capturing technique. A slice plane image may be generated by using a known filtered back projection method (FBPM) or the like as the technique of CT. 
     Although the inner-edge shape of a predetermined member (for example, the diaphragm  101   c ) is a square and the outer-edge shape of the radiation area is a trapezoid or square in the foregoing embodiments, the invention is not limited thereto. When each of the inner-edge shape of the predetermined member (for example, the diaphragm  101   c ) and the outer-edge shape of the radiation area is a shape having four or more vertexes such as a rectangular or trapezoid, the distance “h” and the angle θ can be obtained by a computing method similar to that of the embodiment. 
     Although the distance “d” from the focal point Fp to the opening  101   h  is determined in designing in the foregoing embodiments, the invention is not limited to the method. For example, when the distance “h” at the angle θ=0 is known, the distance “d” may be unconditionally calculated from the size of the radiation area (for example, square) at that time and the size of the inner-edge shape of the diaphragm  101   c.    
     In the second preferred embodiment, the detection surface  108   s  is irradiated with visible light from the illuminating mechanism  101   p  and photographed by the camera unit  106 , thereby obtaining a photographed image. The invention however is not limited to the embodiment. It is also possible to irradiate the detection surface  108   s  with other light such as infrared light and obtain a photographed image by using an infrared camera or the like. To reduce an error in calculation in the distance “h” an the angle θ, preferably, the linearity of light is high to some extent. For example, infrared light or light having a wavelength shorter than that of the infrared light is desirable. 
     While the invention has been shown and described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is therefore understood that numerous modifications and variations can be devised without departing from the scope of the invention.