Patent Publication Number: US-2011075791-A1

Title: Radiographic image capturing apparatus, radiographic image capturing method, and position calculating method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2009-223138 filed on Sep. 28, 2009, of which the contents are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a radiographic image capturing apparatus and a radiographic image capturing method for applying radiation from a radiation source with respect to a reference point disposed between the radiation source and a radiation detector, and for detecting radiation by the radiation detector and converting the same into a radiographic image, as well as to a position calculating method for calculating a three dimensional position of the reference point based on the radiographic image. 
     2. Description of the Related Art 
     There have heretofore been developed biopsy apparatus for sampling tissue of a biopsy region (e.g., a lesion region in a subject&#39;s breast) contained within an object to be examined of the subject, and thoroughly examining the sampled tissue to perform a disease diagnosis. In this case, in order to reliably sample such tissue, it is essential that the three dimensional position of the biopsy region be accurately specified beforehand. 
     Consequently, a stereographic image capturing process is carried out in which, using the radiographic image capturing apparatus, radiation is applied respectively with respect to the examination object from a radiation source which is positioned at two different angles, radiation that has passed through the examination object is detected by a radiation detector, and two radiographic images are obtained. Based on the two radiographic images thus obtained, a calculation is performed to determine the three dimensional position of the biopsy region. 
     In this case, by rotating the radiation source about a central position of rotation, the radiation source can be moved to the aforementioned two angles. Further, the positional relationship between the focal position of the radiation source at the two angles and the radiation detector is regulated beforehand. However, in the case that the aforementioned angle (stereoscopic angle) becomes shifted, as a result of variances in assembly or manufacturing variances of the radiographic image capturing apparatus, the accuracy of the positional relationship becomes deteriorated, such that the three dimensional position of the biopsy region cannot be calculated with good and sufficient precision. 
     In response to this type of problem, in. Japanese Laid Open Patent Publication No. 2002-528220 (PCT), and in Japanese Laid-Open Patent Publication No. 2003-024321, there have been proposed stereographic image capturing processes, in which a reference point (marker) is disposed between a radiation source and a radiation detector, and radiation is applied from the radiation source with respect to the reference point as well as the examination object, whereby two radiographic images are obtained containing both the reference point and the examination object. In this case, based on the two radiographic images, the three dimensional position of the reference point and the three dimensional position of the biopsy region are calculated, such that the three dimensional position of the biopsy region can be corrected taking as a standard the three dimensional position of the reference point. 
     In this manner, in accordance with the disclosures of Japanese Laid-Open Patent Publication No. 2002-528220 (PCT) and Japanese Laid-Open Patent Publication No. 2003-024321, by simultaneously capturing stereographic images with respect to the reference point and the examination object, although the accuracy of the three dimensional position of the biopsy region may become deteriorated due to shifting in the stereoscopic angle as a result of variances in assembly or manufacturing variances of the radiographic image capturing apparatus, by correcting the three dimensional position of the biopsy region utilizing the three dimensional position of the reference point, degradation of the calculation result of the three dimensional position of the biopsy region can be suppressed. 
     Incidentally, while observing the two radiographic images obtained by stereographic image capturing, a doctor or radiological technician positions a biopsy device at the biopsy region from which tissue is to be sampled, or performs positioning to move the examination object to an appropriate position. Notwithstanding, if the two radiographic images are images in which the reference point is captured and included within the examination object, the existence of the reference point causes an obstacle to carrying out the aforementioned positioning procedure. 
     Although it might be considered to dispose the reference point at a position such that the reference paint would not be captured within the examination object, and to carry out stereographic image capturing in such a manner, in this case, the applied area (radiation field) of the radiation becomes enlarged (widened), and thus the examinee is exposed needlessly and excessively to radiation. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to enable correction of errors in the three dimensional position of a biopsy region caused by shifting of the stereoscopic angle, even though stereographic image capturing is not carried out simultaneously with respect to both a reference point and an object to be examined. 
     To accomplish the aforementioned object, a radiographic image capturing apparatus according to the present invention comprises a radiation source that outputs radiation, a radiation detector for detecting radiation and converting such radiation into a radiographic image, and a reference point, which is disposed in a removable manner between the radiation source and the radiation detector, wherein the reference point is disposed at a position of a center of rotation of the radiation source, which is set between the radiation source and the radiation detector, and wherein, by rotating the radiation source about the position of the center of rotation, the radiation source applies radiation with respect to the reference point from at least two different angles. 
     Further, a radiographic image capturing method according to the present invention comprises the steps of disposing a reference point in a removable manner on a position of an axis of rotation of a radiation source, which is set between the radiation source and a radiation detector, applying radiation by the radiation source with respect to the reference point from at least two different angles by rotating the radiation source about the position of the center of rotation, and detecting radiation by the radiation detector and converting such radiation into two radiographic images. 
     Furthermore, a position calculating method comprises the steps of disposing a reference point in a removable manner on a position of an axis of rotation of a radiation source, which is set between the radiation source and an image capturing base, applying radiation by the radiation source with respect to the reference point from at least two different angles by rotating the radiation source about the position of the center of rotation, detecting radiation by the radiation detector and converting such radiation into two radiographic images, and calculating a three dimensional position of the reference point by a position calculating unit based on the two radiographic images. 
     According to the present invention, because the reference point is disposed removably at the position of the center of rotation of the radiation source, when a stereographic image is taken with respect to the reference point, the reference point is disposed at the position of the center of rotation and stereographic image capturing may be performed, whereas, when a stereographic image is taken with respect to the biopsy region, the biopsy region is positioned and stereographic image capturing may be carried out after the reference point has been removed. 
     Owing thereto, radiographic images, which are obtained in each of the aforementioned stereographic image capturing processes, are made up of an image that contains the reference point therein, or alternatively, an image that contains only the biopsy region therein. Therefore, within each of these radiographic images, the reference point is not reflected in the object to be examined. Accordingly, a doctor or radiological technician while observing two radiographic images in which the object to be examined is imaged, a biopsy apparatus can easily be positioned on a biopsy region from which tissue is to be sampled, or positioning can easily be performed to move the object to be examined to an appropriate position. 
     Further, since the reference point is not present when a stereographic image is taken with respect to the examination object, the applied area (radiation field) of the radiation can be limited to within a minimum necessary range, and thus needless and excessive exposure of the examinee to such radiation can be avoided. 
     Further, a three dimensional position of the reference point is calculated based on two radiographic images obtained by carrying out stereographic image capturing with respect to the reference point, while on the other hand, a three dimensional position of the biopsy region inside the examination object is calculated based on two radiographic images obtained by carrying out stereographic image capturing with respect to the examination object. 
     Accordingly, even if there are errors in the three dimensional position of the biopsy region caused by shifting of the stereoscopic angle, the three dimensional position of the biopsy region can be corrected using the three dimensional position of the reference point. 
     In this manner, compared to the disclosures of Japanese Laid-Open Patent Publication No. 2002-528220 (PCT) and Japanese Laid-Open Patent Publication No. 2003-024321, with the present invention, the number of times that stereographic image capturing is carried out is increased by one. However, even though such stereographic image capturing is not performed simultaneously with respect to the reference point and the examination object, any errors in the three dimensional position of the biopsy region caused by shifting of the stereoscopic angle can easily be corrected. 
     In addition, the radiographic image capturing apparatus further comprises an image capturing base accommodating the radiation detector therein, wherein the reference point is disposed on a jig, which is arranged removably on the image capturing base. 
     Owing thereto, setting of the reference point at the position of the center of rotation can easily be carried out. 
     In this case, the radiographic image capturing apparatus further comprises a compression plate, which is displaceable toward the image capturing base to compress and secure an object to be examined of a subject on the image capturing base in case that the object to be examined is positioned on the image capturing base. In a state in which the jig is arranged on the image capturing base, and after the radiation source has applied radiation with respect to the reference point from the two angles, the jig is removed from the image capturing base. Then, in a state in which the jig has been removed from the image capturing base and the object to be examined has been compressed and secured between the compression plate and the image capturing base, the radiation source applies radiation with respect to the object to be examined from the two angles. 
     Owing thereto, assuming that the object to be examined is the breast of a subject and the biopsy region is a lesion within the breast, errors in the three dimensional position of the lesion area can be corrected effectively. 
     Additionally, assuming that, in front view, the position of the center of rotation and the position of the reference point are disposed at a predetermined position within the object to be examined, which is compressed and secured between the compression plate and the image capturing base, errors in the three dimensional position of a biopsy region caused by shifting of the stereoscopic angle can accurately be corrected. 
     Further, even in the case that, in plan view, the reference point is disposed at a center position inside the object to be examined, which is compressed and secured between the compression plate and the image capturing base, errors in the three dimensional position of a biopsy region caused by shifting of the stereoscopic angle can be corrected accurately. 
     Preferably, the radiographic image capturing apparatus further comprises a memory unit for storing two radiographic images obtained by the radiation detector by applying radiation with respect to the reference point, and also storing two radiographic images obtained by the radiation detector by applying radiation with respect to the object to be examined, which has been compressed and secured, and a position calculating unit for calculating a three dimensional position of the reference point and a three dimensional position of a biopsy region inside the object to be examined based on the radiographic images stored in the memory unit. The position calculating unit corrects the three dimensional position of the biopsy region inside the object to be examined using the three dimensional position of the reference point. 
     Owing thereto, the three dimensional position of the biopsy region can be corrected with better efficiency. 
     Further, preferably, in case that the jig is arranged on the image capturing base, an auxiliary reference point is disposed on the jig, wherein the auxiliary reference point is arranged on a vertical axis, which is orthogonal to the radiation detector and passes through the reference point and the position of the center of rotation. The radiation source applies radiation with respect to the reference point and the auxiliary reference point from the two angles in case that the jig is arranged on the image capturing base. Then, the memory unit stores two radiographic images obtained by the radiation detector by applying radiation with respect to the reference point and the auxiliary reference point. The position calculating unit calculates a three dimensional position of the reference point, a three dimensional position of the auxiliary reference point, and a three dimensional position of the biopsy region based on the radiographic images stored in the memory unit, and concerning the three dimensional position of the biopsy region, the position calculating unit corrects a position in a direction along the radiation detector based on the calculated three dimensional position of the reference point and the calculated three dimensional position of the auxiliary reference point. 
     Owing thereto, the three dimensional position of the biopsy region can be corrected with even greater precision. 
     The above and other objects, features and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a mammography apparatus according to an embodiment of the present invention; 
         FIG. 2  is a partial side view of the mammography apparatus shown in  FIG. 1 ; 
         FIG. 3  is a perspective view showing a state in which a jig used for calibration is arranged on an image capturing base; 
         FIG. 4  is a side view showing application of radiation under a condition in which the jig of  FIG. 3  is arranged on the image capturing base; 
         FIG. 5  is a frontal view showing stereographic image capturing under a condition in which the jig of  FIG. 3  is arranged on the image capturing base; 
         FIG. 6  is a plan view showing a condition in which the jig of  FIG. 3  is arranged on the image capturing base; 
         FIG. 7  is an explanatory drawing concerning positioning of a reference point; 
         FIG. 8  is an explanatory drawing concerning positioning of a reference point; 
         FIG. 9  is a perspective view showing a case in which another jig is arranged on the image capturing base; 
         FIG. 10  is a side view showing a condition in which the jig of  FIG. 9  is arranged on the image capturing base; 
         FIG. 11  is a block diagram of the mammography apparatus shown in  FIG. 1 ; and 
         FIG. 12  is a flowchart for explaining an operation sequence of the mammography apparatus. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A radiographic image capturing apparatus according to a preferred embodiment of the present invention will be described below with reference to  FIGS. 1 to 12 , in relation to a radiographic image capturing method and position calculating method carried out by the radiographic image capturing apparatus. 
     The basic structure of a mammography apparatus (radiographic image capturing apparatus, mammography image capturing apparatus)  12  according to an embodiment of the present invention, which incorporates a biopsy apparatus  10  therein, will be described below with reference to  FIGS. 1 and 2 . 
     The mammography apparatus  12  essentially includes an upstanding base  14 , a vertical arm  18  fixed to the distal end of a swing shaft  16  disposed substantially centrally on the base  14 , a radiation source housing unit  28  fixed to an upper end of the arm  18  and housing therein a radiation source  26  for applying radiation  24  (see  FIG. 4 ) to a breast  22 , which defines a mass to be examined of an examinee (subject)  20 , an image capturing base  32  mounted on a lower end of the arm  18  and housing therein a solid-state detector (radiation detector)  30  for detecting radiation  24  that has passed through the breast  22 , a compression plate  34  for compressing and holding the breast  22  against the image capturing base  32 , and a biopsy hand assembly  38  mounted on the compression plate  34  for removing a tissue sample from a biopsy region  36  of the breast  22 . 
     As shown in  FIGS. 1 and 2 , the mammography apparatus  12  applies radiation  24  to the breast  22  of the examinee  20  and a sample tissue is removed from the biopsy region  36 , while the breast  22  of the examinee  20 , who is in a sitting position, is compressed and secured between the compression plate  34  and the image capturing base  32 . Further, a display control panel  40  is connected to the base  14  for displaying image capturing conditions representing an image capturing region of the examinee  20 , ID information of the examinee  20 , etc., and for enabling setting of such items of information, as necessary. 
     When the arm  18 , to which the radiation source housing unit  28  and the image capturing base  32  are secured, is angularly moved about the swing shaft  16 , the direction of the radiation source housing unit  28  and the image capturing base  32  with respect to the breast  22  of the examinee  20  can be adjusted. The radiation source housing unit  28  is operatively coupled to the arm  18  by a hinge  42 , and can be turned about the hinge  42  in directions indicated by the arrow θ independently of the image capturing base  32 . 
     The arm  18  has a groove  44  defined vertically in a side (front side) thereof, which faces toward the examinee  20  in the direction indicated by the arrow X. The groove  44  extends along the direction indicated by the arrow Z. Handles  43  are mounted on respective sides of the arm  18 , which face away from each other along the direction indicated by the arrow Y. The handles  43  may be gripped by the examinee  20 . The compression plate  34  has a proximal end inserted into the groove  44  and held in interfitting engagement with a non-illustrated mount. The compression plate  34 , which is thus coupled to the arm  18 , is disposed between the radiation source housing unit  28  and the image capturing base  32 . The compression plate  34  is vertically displaceable in unison with the mount along the arm  18  in directions indicated by the arrow Z, when the mount similarly is displaced in directions indicated by the arrow Z along the groove  44 . 
     The compression plate  34  has an opening  48  defined therein near a chest wall  46  of the examinee  20 , for allowing the biopsy hand assembly  38  to remove a tissue sample. The biopsy hand assembly  38  comprises a post  50  fixedly mounted on the compression plate  34 , a first arm  52  having one end thereof pivotally supported on the post  50 , and which is angularly movable about the post  50  along the surface of the compression plate  34 , and a second arm  54  having one end thereof pivotally supported on another end of the first arm  52 , and which is angularly movable about the other end of the first arm  52  along the surface of the compression plate  34 . A biopsy needle  56  is mounted on the other end of the second arm  54  for movement in directions indicated by the arrow Z. 
     The biopsy needle  56  has a sampler  58  disposed near the lower end thereof, for sampling under suction a tissue (e.g., calcified tissue) from the biopsy region  36 , which forms a lesion area (e.g., calcified area) of the breast  22 . The sampler  58  of the biopsy needle  56  can be moved to a position in the vicinity of the biopsy region  36  when the first arm  52  and the second arm  54  of the biopsy hand assembly  38  are moved in an X-Y plane parallel to the surface of the compression plate  34 , and when the biopsy needle  56  is moved in directions indicated by the arrow Z. 
     Additionally, with the mammography apparatus  12 , the radiation source housing unit  28  is turned about the hinge  42 , such that the radiation source  26  is placed at two positions (e.g., two angles from among the three rotational angles  0 °, +θ 1  and −θ 1  shown in  FIG. 5 ) and image capturing is performed at such positions, whereby a stereographic image capturing process can be performed for obtaining a radiographic image (two radiographic images) of the breast at two respective imaging angles (stereoscopic angles). 
     More specifically, by rotating the radiation source housing unit  28  about the hinge  42 , the radiation source  26  is disposed at a substantially central portion in the height direction of the breast  22  as viewed from the side in  FIG. 4 , as well as at a substantially central portion of the breast  22  in front view (as viewed frontally) in  FIG. 5 . Moreover, the radiation source  26  is rotated in the direction of the angle θ about the axis of the center of rotation (position of the center of rotation)  70 , which is disposed roughly in a center portion in the lateral direction of the breast  22  in plan view (as viewed in plan) in  FIG. 6 . 
     Accordingly, when a straight line interconnecting the position of the radiation source  26  at  0 ° and the central axis of rotation  70  is taken as a central axis  72 , a straight line interconnecting the position A of the radiation source  26  at +θ 1  and the central axis of rotation  70  is taken as a central axis  74 , and a straight line interconnecting the position B of the radiation source  26  at −θ 1  and the central axis of rotation  70  is taken as a central axis  76 , and under a condition in which the radiation source  26  is arranged at two angles from among the three stereoscopic angles ( 0 °, +θ 1 , −θ 1 ) shown in  FIG. 5 , by applying radiation  24  along the central axes  72 ,  74 ,  76  and toward the central axis of rotation  70  (the breast  22 ), radiation  24  that passes through the breast  22  can be detected as radiation images by the solid-state detector  30  of the image capturing base  32 . 
     Incidentally, with the mammography apparatus  12 , prior to taking a stereographic image of the breast  22 , a jig  60  shown in  FIGS. 3 to 6  is arranged on the image capturing base  32  and a stereographic image of the jig  60  is taken. 
     The jig  60  is constructed from a base plate  62  capable of being arranged on the image capturing base  32 , a rod  64  which is attached to an upper surface of the base plate  62  along the Z-direction, and needle members  66 ,  68  that are affixed respectively at different heights on the rod  64 . 
     The base plate  62  is made of a material that is permeable to radiation  24 , whereas the rod  64  and the needle members  66 ,  68  are made from a material that is impervious to radiation  24 , or from a material capable of absorbing radiation  24 . More preferably, among the components that make up the jig  60 , at least the needle members  66 ,  68  are made from a material impervious to radiation  24 , or from a material capable of absorbing radiation  24 . 
     Further, the end of the needle member  66 , which is affixed at an intermediate portion of the rod  64 , forms a reference point  66   t , and the end of the needle member  66 , which is affixed at the upper end of the rod  64 , forms an auxiliary reference point  68   t.    
     Concerning placement of the jig  60 , the compression plate  34  is moved upwardly along the groove  44 , or the compression plate  34  is taken out from the groove  44 , and in a state in which the breast  22  is not arranged thereon, the jig  60  is arranged on the image capturing base  32 . 
     At this time, the base plate  62  is arranged at a predetermined location on the image capturing base  32 , such that the reference point  66   t  and the auxiliary reference point  68   t  are positioned within an area where the breast  22  will be compressed and secured between the compression plate  34  and the image capturing base  32 . Stated otherwise, the jig  60  is arranged on the image capturing base  32  so that the reference point  66   t  and the auxiliary reference point  68   t  enter into the application range (radiation field) of the radiation  24 . 
     At this time, the needle member  66  is positioned on the central axis of rotation  70 , and together therewith, the reference point  66   t  is arranged at a center position where the breast  22  is compressed and secured. On the other hand, the needle member  68  is arranged at a position slid upwardly from the reference point  66   t  along the central axis  72 , i.e., further upwardly than the needle member  66  that lies on the central axis  72  as viewed from the front in  FIG. 5 , and at a position on the central axis of rotation  70  as viewed in plan in  FIG. 6 . 
     Next, the significance of disposing the reference point  66   t  and the auxiliary reference point  68   t  in the aforementioned manner shall be explained with reference to  FIGS. 7 and 8 . 
       FIG. 7  is a design drawing that illustrates schematically a stereographic image capturing process, in a case where a focal point  78  of the radiation source  26 , which was intended to be arranged at the position A of +θ 1 , actually is arranged at a position shifted Δθ from +θ 1 , as a result of variances in assembly or manufacturing variances of the mammography apparatus  12 . 
     In this case, in terms of design, it would be acceptable if radiation  24  were applied from the focal point  78  of the radiation source  26  toward the central axis of rotation  70  along the central axes  72 ,  74 ,  76  as shown by the one-dot-dash lines. However, in actuality, because the focal point  78  at the position A is shifted by Δθ from +θ 1 , the radiation source  26  causes the radiation  24  to be applied along the straight lines (central axes  72   a ,  76   a ,  78   a ) shown by the dashed lines. 
     More specifically, as a result of shifting of the focal point  78  at the position A by Δθ from +θ 1 , the position of the central axis of rotation  70  also becomes shifted to a position (central axis of rotation  70   a ) along the direction of the arrow Y. 
     A distance c is defined along the Z-direction between the central axis of rotation  70  and the solid-state detector  30 , and a resultant angle θ is defined by the central axis  72  and the central axis  74 . When radiation  24  is applied respectively from the  0 ° position and from the position A along the central axes  72 ,  74  toward the central axis of rotation  70 , then assuming that an interval between projection images of the central axis of rotation  70  projected in the two radiation images is defined by a distance b, then the relation b=c×tan θ results. 
     On the other hand, when radiation  24  is applied respectively from the  0 ° position and from the position A along the central axes  72   a ,  74   a  toward the central axis of rotation  70   a , the interval between projection images of the central axis of rotation  70   a  projected in the two radiation images also is defined by the distance b. 
     Furthermore, the shifted amount (distance Δb) of the distance b as a result of shifting of the focal point  78  at the position A by Δθ from +θ 1 , and from the resultant angle Δθ defined by the central axis  74   a  and the central axis  78   a , becomes Δb=c×tan Δθ. 
     In this case, the angle θ or the distance b can easily be determined assuming that the positional relationship between the focal point  78 , the central axis of rotation  70 , and the solid-state detector  30  correspond to the design values thereof. On the other hand, the distance Δb can be determined from the actual radiation images, which are obtained by the stereographic image capturing process. 
     Accordingly, by disposing a reference point at a position (a position having the same height as the central axis of rotation  70 ) upwardly from the solid-state detector  30  by the distance c, and carrying out stereographic image capturing, the angle Δθ can be calculated using the angle θ and the distances b and Δb. As a result, a shift in the stereoscopic angle is capable of being corrected. 
     Consequently, with the present invention, by applying this concept, by disposing the reference point  66   t  on the central axis of rotation  70 , and together therewith, by disposing the auxiliary reference point  68   t  upwardly (in the Z-direction) from the central axis of rotation  70  and the reference point  66   t  by a given distance d, and then applying radiation  24  to the reference point  66   t  and the auxiliary reference point  68   t  and carrying out stereographic image capturing to obtain two radiographic images, even if shifting of the stereoscopic angle is generated caused by variances in assembly or manufacturing variances of the mammography apparatus  12 , the three dimensional position of the biopsy region  36  can be determined with good precision. In  FIG. 8 , the SID (source to image distance) is defined by a distance between the focal point  78  of the radiation source  26  and the solid-state detector  30  along the Z-direction. 
     When radiation  24  is applied with respect to the reference point  66   t  and the auxiliary reference point  68   t  from the radiation source  26 , which is disposed at the position A, the reference point  66   t  and the auxiliary reference point  68   t  are projected in the radiation image converted from the radiation  24 , and are imaged therein as projected images separated by a given distance e 1 . Further, when radiation  24  is applied with respect to the reference point  66   t  and the auxiliary reference point  68   t  from the radiation source  26 , which is positioned at the position B, the reference point  66   t  and the auxiliary reference point  68   t  are projected in the radiation image converted from the radiation  24 , and are imaged therein as projected images separated by a given distance e 2 . 
     Accordingly, if the position of the radiation source  26  is arranged at position A and position B shown in  FIG. 5 , when radiation  24  is applied with respect to the reference point  66   t  and the auxiliary reference point  68   t  from position A and position B, then e 1 =e 2 . On the other hand, in the case that the radiation source  26  becomes shifted from position A and position B, or if the central axis of rotation  70  becomes shifted from the central axis  72 , then e 1 ≠e 2 . Owing thereto, with the present embodiment, based on the ratio of the distances e 1  and e 2 , amounts by which the positions and/or the angles are shifted are determined, and based on such determined amounts, among the three dimensional coordinate positions of the biopsy region  36 , a positional shift amount thereof in the X-Y plane is corrected. 
     Instead of the construction shown in  FIGS. 3 through 6 , the jig  60  may be constructed as shown in  FIGS. 9 and 10 . 
     The jig  60  shown in  FIGS. 9 and 10  is constituted from a rod shaped member  80  that extends along the Y-direction, an upstanding portion  82  that extends upwardly from a central region of the rod shaped member  80 , a bridge  84  that extends in the X-direction from the upstanding portion  82 , a mount  86  that is suspended downward in the Z-direction from an end of the bridge  84 , and needle members  66 ,  68  that are affixed to a side portion on the X-direction side of the mount  86 . 
     In this case, the upstanding portion  82  is formed so as to expand toward the groove  44  from the rod shaped member  80 . Further, a width in the Y-direction of the upstanding portion  82  is shaped to substantially coincide with the width of the groove  44 . Accordingly, by fitting a part of the upstanding portion  82  into the groove  44  such that the rod shaped member  80  is brought into abutment against the arm  18 , the needle members  66 ,  68  can be arranged within a region where the breast  22  is to be compressed and secured, while the needle member  66  and the reference point  66   t  can be arranged on the central axis of rotation  70 . 
       FIG. 11  shows a block diagram of the mammography apparatus  12 . 
     As shown in  FIG. 11 , the mammography apparatus  12  includes an image capturing condition setting section  90 , a radiation source energization controller  92 , a biopsy needle controller  94 , a biopsy needle position information calculator  96 , a compression plate controller  98 , a compression plate position information calculator  100 , a detector controller  102 , an image information storage unit (memory unit)  104 , a CAD (Computer Aided Diagnosis) processor  106 , a display unit  108 , a biopsy region selector  110 , a biopsy region position information calculator (position calculating unit)  112 , and a traveled distance calculator  114 . 
     Within the mammography apparatus  12 , the biopsy hand assembly  38 , the biopsy needle  56 , the opening  48 , the biopsy needle controller  94 , the biopsy needle position information calculator  96 , the biopsy region selector  110 , and the traveled distance calculator  114  collectively make up the biopsy apparatus  10 . More specifically, by assembling the biopsy apparatus  10  including these structural components, which are incorporated together in the mammography apparatus  12 , a tissue of the biopsy region  36  is capable of being sampled. 
     The image capturing condition setting section  90  sets image capturing conditions including a tube current and a tube voltage, an application dose and an application time of the radiation  24 , and the stereoscopic angle, etc. The radiation source energization controller  92  controls energization of the radiation source  26  according to the image capturing conditions. The biopsy needle controller  94  controls the biopsy hand assembly  38  (see  FIGS. 1 and 2 ) in order to move the biopsy needle  56  to a desired position. The compression plate controller  98  moves the compression plate  34  in the directions indicated by the arrow Z. The detector controller  102  controls the solid-state detector  30  in order to store radiographic images, which are converted by the solid-state detector  30  from the radiation  24 , in the image information storage unit  104 . 
     As discussed above, because the mammography apparatus  12  performs stereographic image capturing, two radiographic images of the breast  22  taken at two stereoscopic angles, and two radiographic images of the jig  60  taken at two stereoscopic angles are stored in the image information storage unit  104 . In this case, the detector controller  102  reads out the two stereoscopic angles corresponding to the two radiographic images from the image capturing condition setting section  90 , and the two stereoscopic angles are stored together with the two radiographic images in the image information storage unit  104 . 
     The CAD processor  106  processes the two radiographic images stored in the image information storage unit  104 , and displays the processed radiographic images on the display unit  108  and the display control panel  40 . 
     The biopsy region selector  110  comprises a pointing device such as a mouse or the like. Using the pointing device as the biopsy region selector  110 , a doctor or radiological technician in charge, who has seen the displayed contents, i.e., the two radiographic images, on the display unit  108  and/or the display control panel  40 , can select one out of a plurality of biopsy regions  36  displayed in the two radiographic images from which a tissue is to be removed. More specifically, the doctor or radiological technician selects a biopsy region  36  in one of the two radiographic images, and also selects the corresponding biopsy region  36  in the other of the two radiographic images. 
     First, the biopsy region position information calculator  112  reads out the two radiographic images in which the jig  60  is projected, calculates distances e 1 , e 2  between the reference point  66   t  and the auxiliary reference point  68   t  within the two radiographic images, and from the ratio of the calculated distances e 1 , e 2 , calculates an amount of angular shift from the set value of the stereoscopic angle, and an amount of shift in position from the set value of the central axis of rotation  70 . Next, The biopsy region position information calculator  112  calculates the three-dimensional position of the selected biopsy region  36  based on positions of the selected biopsy region  36  in the two radiographic images, and using the angular shift amount and the positional shift amount, corrects the positional component thereof in the X-Y plane from among the three dimensional coordinate positions. The three-dimensional position of the selected biopsy region  36  can be calculated according to a known three-dimensional position calculating scheme implemented in the stereographic image capturing process. 
     The biopsy needle position information calculator  96  calculates position information of the tip end of the biopsy needle  56 . When a tissue is sampled from the biopsy region  36 , the biopsy needle position information calculator  96  calculates the position of the tip end of the biopsy needle  56  before tissue is sampled from the biopsy region  36 . Stated otherwise, the position of the tip end of the biopsy needle  56  is calculated before the biopsy needle  56  pierces the breast  22 . 
     The compression plate position information calculator  100  calculates position information of the compression plate  34 , which has been moved with respect to the image capturing base  32  by the compression plate controller  98 . Since the compression plate  34  presses against the breast  22  with respect to the image capturing base  32  and holds the breast  22  in a pressed state, the position information of the compression plate  34  represents thickness information of the breast  22  as the breast  22  is being pressed. 
     The traveled distance calculator  114  calculates the distance by which the biopsy needle  56  is moved with respect to the biopsy region  36 , based on the three-dimensional position of the biopsy region  36 , which has been calculated and corrected by the biopsy region position information calculator  112 , the position of the tip end of the biopsy needle  56 , which has been calculated by the biopsy needle position information calculator  96 , and the position of the compression plate  34  (i.e., the thickness of the breast  22 ), which has been calculated by the compression plate position information calculator  100 . Based on the distance calculated by the traveled distance calculator  114  that the biopsy needle  56  has moved with respect to the biopsy region  36 , the biopsy needle controller  94  moves the biopsy needle  56  in order to enable a tissue to be sampled from the biopsy region  36 . 
     The mammography apparatus  12  according to the present invention is constructed basically as described above. Next, operations of the mammography apparatus  12  (radiographic image capturing method, position calculating method) shall be described below with reference to the flowchart shown in  FIG. 12 . 
     Herein, a case shall be explained in which the compression plate  34  first is removed from the groove  44  when a stereographic image is captured with respect to the jig  60 , and then after a stereographic image thereof has been captured, the compression plate  34  is installed in the groove  44  and a stereographic image capturing process is carried out with respect to the breast  22  of a subject. 
     In step S 1 , initially, the image capturing condition setting section  90  (see  FIG. 11 ) sets image capturing conditions, including a tube current and a tube voltage of the radiation source  26 , an application dose and an application time for the radiation  24 , a stereoscopic angle of the radiation source  26 , etc., depending on conditions of the breast  22 . The image capturing conditions are set in this manner in the radiation source energization controller  92 . 
     Next, in step S 2 , the jig  60  is disposed on the image capturing base  32  in the position shown in  FIGS. 4 to 6 , such that the reference point  66   t  is arranged on the central axis of rotation  70 , while both the reference point  66   t  and the auxiliary reference point  68   t  are arranged on the central axis  72 . 
     In step S 3 , the mammography apparatus  12  drives the radiation source  26  and carries out stereographic image capturing with respect to the jig  60 . In this case, the radiation source housing unit  28  is rotated about the hinge  42  (see  FIG. 1 ) so that, for example, by positioning the radiation source  26  respectively at the positions A and B shown in  FIG. 5 , and applying radiation  24  therefrom, radiation  24  that has permeated the jig  60  is detected as radiographic images by the solid-state detector  30  provided in the image capturing base  32 . 
     In this case, because the needle members  66 ,  68  are made from a material impervious to radiation  24  or from a material that absorbs radiation  24 , as shown in  FIG. 8 , in each of the radiographic images at the positions A and B, the reference point  66   t  and the auxiliary reference point  68   t  are projected respectively, and distances between each of the projection images of the reference point  66   t  and the auxiliary reference point  68   t  are designated as e 1  and e 2 , respectively. 
     The detector controller  102  controls the solid-state detector  30  and two radiographic images are obtained. These two radiographic images are stored in the image information storage unit  104  together with the stereoscopic angles at each of the positions A and B as read out from the image capturing condition setting section  90 . Additionally, the CAD processor  106  carries out image processing with respect to the two stereographic images that are stored in the image information storage unit  104 , and after such image processing, displays the two radiographic images on the display unit  108  and the display control panel  40 . Consequently, a doctor or radiological technician can confirm easily that stereographic image capturing with respect to the jig  60  has been completed. 
     In step S 4 , a doctor or radiological technician removes the jig  60  from the image capturing base  32 , and next, in step S 5 , fits the compression plate  34  into the groove  44 . 
     Next, in step S 6 , the doctor or radiological technician positions the breast  22  of the examinee  20 . More specifically, the breast  22  is placed in a predetermined position (facing the opening  48 ), and thereafter, the compression plate controller  98  moves the compression plate  34  toward the image capturing base  32  in the direction indicated by the arrow Z, thereby compressing and positioning the breast  22  against the image capturing base  32 . 
     As a result, the breast is compressed and secured between the image capturing base  32  and the compression plate  34 . The compression plate position information calculator  100  calculates position information of the compression plate  34  with respect to the image capturing base  32 , and outputs the calculated position information to the traveled distance calculator  114 . 
     After the above preparatory process for stereographic image capturing of the breast  22  is completed, in step S 7 , the mammography apparatus  12  energizes the radiation source  26  again, this time in order to perform a stereographic image capturing process on the breast  22 . In this case, the radiation source housing unit  28  is turned about the hinge  42  (see  FIG. 1 ) to position the radiation source  26  respectively at the positions A and B, and to apply radiation  24  therefrom. Radiation  24  that passes through the breast  22  is applied to the solid-state detector  30  in the image capturing base  32 , and the radiation  24  is detected as radiographic images. 
     The detector controller  102  controls the solid-state detector  30 , whereby two radiographic images are obtained. These two radiographic images are stored in the image information storage unit  104  together with the stereoscopic angles at each of the positions A and B, which are read out from the image capturing condition setting section  90 . 
     In step S 8 , the CAD processor  106  processes the two radiographic images of the breast  22  that are stored in the image information storage unit  104 , and displays the two processed radiographic images on the display unit  108  and the display control panel  40 . 
     In step S 9 , using the biopsy region selector  110 , which is a pointing device such as a mouse or the like, a doctor or radiological technician selects, from among a plurality of biopsy regions  36  in the two radiographic images displayed on the display unit  108  and/or on the display control panel  40 , a biopsy region  36  from which a tissue sample will be removed. 
     In step S 10 , when the biopsy region  36  has been selected, first, the biopsy region position information calculator  112  reads out the two radiographic images in which the jig  60  has been captured and calculates distances e 1 , e 2  between the reference point  66   t  and the auxiliary reference point  68   t  within the two radiographic images, and from the ratio of the calculated distances e 1 , e 2 , calculates an amount of angular shift from the set value of the stereoscopic angle, and an amount of shift in position from the set value of the central axis of rotation  70 . Next, The biopsy region position information calculator  112  calculates the three-dimensional position of the biopsy region  36  based on positions of the biopsy region  36  in the two radiographic images of the breast  22 , and using the angular shift amount and the positional shift amount, corrects the positional component thereof in the X-Y plane from among the three dimensional coordinate positions. 
     On the other hand, the biopsy needle position information calculator  96  calculates the position of an end of the biopsy needle  56  before the biopsy needle  56  pierces the breast  22 . 
     Accordingly, the traveled distance calculator  114  calculates the distance by which the biopsy needle  56  is moved with respect to the biopsy region  36 , based on the three-dimensional position of the biopsy region  36 , which has been calculated and corrected by the biopsy region position information calculator  112 , the position of the tip end of the biopsy needle  56 , which has been calculated by the biopsy needle position information calculator  96 , and the position of the compression plate  34 , which has been calculated by the compression plate position information calculator  100 . 
     In step S 11 , based on the distance calculated from the traveled distance calculator  114 , the biopsy needle controller  94  moves the biopsy needle  56  in order to enable a tissue to be sampled from the biopsy region  36 . As a result, the biopsy hand assembly  38  moves the first arm  52  and the second arm  54  in the X-Y plane to position the biopsy needle  56  at a position confronting the biopsy region  36  (i.e., a predetermined position along the Z-direction with respect to the biopsy region  36 ). Next, in step S 12 , the biopsy needle  56  is moved in the Z-direction, and the biopsy needle  56  pierces the breast  22  through the opening  48  formed in the compression plate  34 . 
     In step S 13 , when the sampler  58  of the biopsy needle  56  has reached a position near the biopsy region  36 , the biopsy needle  56  starts to sample tissue from the biopsy region  36  under suction. Thereafter, in step S 14 , the biopsy needle  56  is moved in the Z-direction until the biopsy needle  56  is pulled out from the breast  22 , whereupon the tissue sampling operation is brought to an end. 
     As described above, with the mammography apparatus  12  of the present embodiment, because the reference point  66   t  is positioned removably by the jig  60  on the central axis of rotation  70  of the radiation source  26 , when a stereographic image is captured with respect to the reference point  66   t , the reference point is disposed on the central axis of rotation  70  and stereographic image capturing is carried out. On the other hand, when a stereographic image is captured with respect to the breast  22 , after the jig  60  equipped with the reference point  66   t  has been removed, the breast  22  is positioned on the image capturing base  32  and stereographic image capturing of the breast  22  may be performed. 
     Consequently, in radiographic images obtained from each of the stereographic image capturing processes, because either an image in which the reference point  66   t  is reflected therein is produced, or alternatively, an image having the breast  22  is included therein is produced, the reference point  66   t  is not reflected within the breast  22 . Accordingly, a doctor or radiological technician, while observing the two radiographic images in which the breast  22  is imaged, can easily perform positioning of the biopsy apparatus  10  in the biopsy region  36  from which tissue is to be sampled, or alternatively, can easily carry out positioning in order to move the breast  22  to a suitable position. 
     Further, since the reference point  66   t  is not present when a stereographic image is taken with respect to the breast  22 , the applied area (radiation field) of the radiation  24  can be limited to within a minimum necessary range, and thus needless and excessive exposure of the examinee  20  to radiation can be avoided. 
     Further, a three dimensional position of the reference point  66   t  is calculated based on two radiographic images obtained by carrying out stereographic image capturing with respect to the reference point  66   t , while on the other hand, a three dimensional position of the biopsy region  36  inside the breast  22  is calculated based on two radiographic images obtained by carrying out stereographic image capturing with respect to the breast  22 . Accordingly, even if there are errors in the three dimensional position of the biopsy region  36  caused by shifting of the stereoscopic angle, the three dimensional position of the biopsy region  36  can be corrected using the three dimensional position of the reference point  66   t.    
     In this manner, compared to the disclosures of Japanese Laid-Open Patent Publication No. 2002-528220 (PCT) and Japanese Laid-Open Patent Publication No. 2003-024321, with the present invention, the number of times that stereographic image capturing is carried out is increased by one. However, even though stereographic image capturing is not performed simultaneously with respect to the reference point  66   t  and the breast  22 , any errors in the three dimensional position of the biopsy region  36  caused by shifting of the stereoscopic angle can easily be corrected. 
     In addition, in the mammography apparatus  12 , by arranging the jig  60 , which is equipped with the reference point  66   t , removably with respect to the image capturing base  32  that accommodates the solid-state detector  30 , setting of the reference point  66   t  on the central axis of rotation  70  can easily be carried out. 
     In this case, in a state in which the jig  60  is arranged on the image capturing base  32 , and after the radiation source  26  has applied radiation  24  with respect to the reference point  66   t  from two stereoscopic angles, the jig  60  is removed from the image capturing base  32 . Then, in a state in which the jig  60  has been removed from the image capturing base  32  and the breast  22  has been compressed and secured between the compression plate  34  and the image capturing base  32 , since the radiation source  26  applies radiation  24  from two stereoscopic angles with respect to the breast  22 , errors in the three dimensional position of the biopsy region  36  can be corrected effectively. 
     Additionally, as viewed from the front in  FIG. 5 , the central axis of rotation  70  and the reference point  66   t  are disposed at a central position within the image of the breast  22 , which is compressed and secured between the compression plate  34  and the image capturing base  32 . Further, as viewed in plan in  FIG. 6 , the central axis of rotation  70  and the reference point  66   t  are disposed at a central position within the image of the breast  22 , which is compressed and secured between the compression plate  34  and the image capturing base  32 . Owing thereto, errors in the three dimensional position of the biopsy region  36  caused by shifting of the stereoscopic angle can be corrected accurately. 
     Further, by additionally providing the auxiliary reference point  68   t  on the jig  60 , the three dimensional position of the biopsy region  36  is capable of being corrected with greater precision. 
     More specifically, in the image information storage unit  104 , on the one hand, two radiographic images obtained by stereographic image capturing with respect to the jig  60  are stored in the image information storage unit  104  together with two stereoscopic angles corresponding to such stereographic image capturing, while on the other hand, two radiographic images obtained by stereographic image capturing with respect to the breast  22  are stored in the image information storage unit  104  together with two stereoscopic angles corresponding to such stereographic image capturing. When the three dimensional position of the biopsy region  36  is calculated in the biopsy region position information calculator  112 , initially, the two radiographic images and the two stereoscopic angles of the jig  60  are read out from the image information storage unit  104 , the distances e 1 , e 2  in the two radiographic images are calculated, and the amount of angular shift from set values of the stereographic angles, as well as the amount of shift in position from the set value of the central axis of rotation  70 , are calculated from the ratio of the calculated distances e 1 , e 2 . Based thereon, among the three dimensional coordinate positions of the biopsy region  36 , which were obtained based on the position of the biopsy region  36  in the two radiographic images of the breast  22 , the position component in the X-Y plane (direction) is corrected using the angular shift amount and the amount of shift in position. As a result, the three dimensional position of the biopsy region  36  can be corrected with good efficiency. 
     Although certain preferred embodiments of the present invention have been shown and described in detail, it should be understood that various changes and modifications may be made to the embodiments without departing from the scope of the invention as set forth in the appended claims.