Abstract:
In this breast thickness measurement device and breast thickness measurement method, radiation is radiated from a plurality of different angles to a breast that is in a compressed state, a plurality of image data are generated by means of a radiation detector, and a plurality of tomographic images are generated by reconfiguring on the basis of each image datum after same has been generated. In each tomographic image, the thickness of the compressed breast is calculated on the basis of a tomographic image in which the focal point matches a first marker and a tomographic image in which the focal point matches a second marker.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of International Application No. PCT/JP2014/064610 filed on Jun. 2, 2014, which is based upon and claims the benefit of priority from Japanese Patent Application No. 2013-203536 filed on Sep. 30, 2013, the contents all of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a breast thickness measuring device and a breast thickness measuring method for measuring the thickness of a breast of a subject that has been compressed by a support table and a compression plate. 
     BACKGROUND ART 
     Heretofore, for capturing a radiographic image of a breast of a subject using a radiographic image capturing apparatus, it has been customary to place the breast on a support table, displace a compression plate toward the support table to compress the breast, and thereafter, irradiate the breast with radiation emitted from a radiation source. Then, the radiation that has passed through the breast is converted into image data with a radiation detector. 
     In the above sequence, it is desirable to accurately grasp the thickness of the breast in the compressed state. In Japanese Laid-Open Patent Publication No. 6-261896, Japanese Laid-Open Patent Publication No. 2004-208752, and Japanese Laid-Open Patent Publication No. 2009-22536, it is disclosed that a position detecting sensor is provided on a proximal end portion of the compression plate, which is spaced from the chest wall of the subject, and the vertical position of the compression plate is detected by the position detecting sensor in order to measure the distance between the compression plate and the support table, i.e., to measure the thickness of the compressed breast. 
     SUMMARY OF INVENTION 
     Heretofore, as described above, for capturing a radiographic image of a breast, the breast is compressed at a distal end portion of the compression plate while at the same time the thickness of the breast is measured at a proximal end portion of the compression plate that is spaced from the chest wall (breast) of the subject. In order to acquire good image data, the radiological technician touches and spreads the breast, which has been placed on the support table, into a thin state, and then causes the compression plate to compress the breast. In the event that the compressed breast is of a distorted shape, the thickness of the breast undergoes local variations, which tend to increase measurement errors of the position detecting sensor, and hence it becomes difficult to suitably measure the thickness of the breast. Furthermore, since the thickness of the breast is measured at the proximal end portion of the compression plate, which is spaced from the compressed breast, measurement errors are likely to increase further. 
     The present invention has been made in order to solve the above problems. An object of the present invention is to provide a breast thickness measuring device and a breast thickness measuring method, which are capable of accurately grasping the thickness of a breast that is in a compressed state. 
     A breast thickness measuring device according to the present invention basically includes a support table on which a breast of a subject is placed, a compression plate that is displaced toward the support table in order to compress the breast, a radiation source configured to apply radiation from a plurality of different angles to the breast, which has been compressed, a radiation detector configured to generate a plurality of image data based on the radiation that has been transmitted through the breast, and a reconstruction processor configured to reconstruct the image data in order to generate a plurality of tomographic images. 
     In order to achieve the above object, the breast thickness measuring device according to the present invention further includes a first marker provided on the compression plate on a side of a chest wall of the subject, a second marker provided on the support table on the side of the chest wall of the subject, a marker detector configured to detect tomographic images that have captured the first marker and tomographic images that have captured the second marker, from among the tomographic images, a marker selector configured to select a tomographic image that is focused on the first marker from among the tomographic images that have captured the first marker, and selecting a tomographic image that is focused on the second marker from among the tomographic images that have captured the second marker, and a thickness calculator configured to calculate the thickness of the breast, which has been compressed, based on the tomographic image that is focused on the first marker and the tomographic image that is focused on the second marker. 
     In order to achieve the above object, a breast thickness measuring method according to the present invention includes the following first through sixth steps. 
     In a first step, a compression plate with a first marker provided thereon on the side of a chest wall of a subject is displaced toward a support table with a second marker provided thereon on the side of the chest wall of the subject, for thereby compressing a breast of the subject that has been placed on the support table. In a second step, a radiation source applies radiation from a plurality of different angles to the breast, which has been compressed, and a radiation detector generates a plurality of image data based on the radiation that has been transmitted through the breast. In a third step, a reconstruction processor reconstructs the image data in order to generate a plurality of tomographic images. In a fourth step, a marker detector detects tomographic images that have captured the first marker and tomographic images that have captured the second marker, from among the tomographic images. In a fifth step, a marker selector selects a tomographic image that is focused on the first marker from among the tomographic images that have captured the first marker, and selects a tomographic image that is focused on the second marker from among the tomographic images that have captured the second marker. In a sixth step, a thickness calculator calculates a thickness of the breast, which has been compressed, based on the tomographic image that is focused on the first marker and the tomographic image that is focused on the second marker. 
     According to the present invention, there is performed a tomosynthesis image capturing process for applying radiation from a plurality of different angles to a breast in the compressed state, and a plurality of tomographic images are generated by reconstructing image data obtained by the tomosynthesis image capturing process. The first marker is provided on the compression plate, whereas the second marker is provided on the support table. Therefore, the tomographic image that is focused on the first marker is a tomographic image of an upper end of the breast along the thickness-wise direction of the breast. The tomographic image that is focused on the second marker is a tomographic image of a lower end of the breast along the thickness-wise direction. Consequently, using the tomographic image that is focused on the first marker and the tomographic image that is focused on the second marker, it is possible to directly calculate the thickness of the breast in the compressed state. 
     In addition, the first marker and the second marker are disposed on the side of the chest wall of the subject. Therefore, the present invention allows the compressed thickness to be calculated more accurately than with the technologies disclosed in the publications referred to above. 
     Therefore, according to the present invention, it is possible to accurately grasp the compressed thickness of the breast. 
     The tomographic images represent images at discrete cross sections spaced at predetermined slice intervals. Consequently, depending on the slice intervals or the slicing method, it is conceivable that a tomographic image may not necessarily be obtained at the vertical position of the first marker or the second marker. According to the present invention, a tomographic image in which the first marker is clearly visible is selected as the tomographic image that is focused on the first marker, from among the tomographic images that have captured the first marker. Similarly, a tomographic image in which the second marker is clearly visible is selected as the tomographic image that is focused on the second marker, from among the tomographic images that have captured the second marker. By selecting the tomographic image of the first marker and the tomographic image of the second marker in this manner, the accuracy in calculating the thickness of the breast is prevented from becoming lowered on account of the slice intervals and the slicing method. 
     The thickness calculator may calculate the thickness of the breast based on slice intervals of the tomographic images and the number of tomographic images from the tomographic image that is focused on the first marker to the tomographic image that is focused on the second marker. Therefore, the thickness calculator can reliably calculate the actual thickness of the breast. 
     The slice intervals are pre-adjusted using a commercially available calibration phantom (geometric calibration phantom), not shown, and are stored in a memory provided in the breast thickness measuring device, for example. 
     The reconstruction processor may reconstruct the image data to generate tomographic images such that the tomographic images are tomographic images sliced parallel to the support table. In this manner, it is easy to detect the tomographic images that have captured the first marker or the second marker. 
     The first marker and the second marker may be disposed in the manner indicated by items [1] through [3] below. 
     [1] The first marker and the second marker preferably are disposed in superposed relation as viewed as a planar view. Therefore, the burden imposed by a correcting process for generating a two-dimensional image from the tomographic images is reduced. 
     The radiation source preferably is supported for angular movement about a rotational shaft, and is angularly movable through a predetermined angle about a vertical axis perpendicular to the rotational shaft. The support table, the compression plate, and the radiation detector preferably are disposed on the vertical axis, and the first marker and the second marker preferably are disposed such that the vertical axis extends through the first marker and the second marker. 
     [2] The first marker preferably is disposed at one corner and another corner, along the chest wall, of the compression plate on the side of the chest wall. More specifically, as long as the first marker is disposed on left and right corners (one corner and another corner) of the compression plate on the side of the chest wall as viewed from the perspective of the subject, then even in a case that the compression plate compresses the breast while being tilted with respect to the support table, it is possible to calculate the thickness of the breast in the compressed state based on the average of the vertical positions of the two first markers. 
     [3] In [2] above, the first marker preferably is further disposed in a central area of the compression plate on the side of the chest wall. Therefore, based on the vertical positions of the three first markers, it is possible to grasp whether or not the compression plate has become distorted. 
     According to the present invention, the breast thickness measuring device may further comprise a two-dimensional image generator that generates a two-dimensional image of the breast by performing an addition process on the tomographic images. However, in the event that a simple addition process is performed on all of the tomographic images in order to generate a two-dimensional image of the breast from the tomographic images, then a two-dimensional image is generated in which the first marker and the second marker is captured. In this case, in a case where a doctor interprets and diagnoses the two-dimensional image which has been generated in this manner, the doctor may mistake the first marker and the second marker for a calcified region, spicula, mass, or the like, which is formed in the breast, and thus there is a possibility that the burden on the doctor could increase. 
     According to the present invention, as indicated in paragraphs (1) through (3) below, the two-dimensional image generator performs a predetermined correcting process for excluding the first marker and the second marker, to thereby generate a two-dimensional image. 
     (1) The two-dimensional image generator simply adds tomographic images, which have not captured the first marker or the second marker from among the tomographic images, in order to generate the two-dimensional image. 
     (2) The two-dimensional image generator performs a first addition process for simply adding tomographic images for image areas in which the first marker or the second marker does not exist, from among the tomographic images, performs a second addition process for simply adding, for image areas in which the first marker or the second marker exists, tomographic images from which the tomographic images that have captured the first marker or the second marker are excluded, from among the tomographic images, and combines two new images obtained by the first addition process and the second addition process, thereby generating the two-dimensional image. 
     (3) The two-dimensional image generator performs a correcting process for removing the first marker or the second marker on tomographic images in which the first marker or the second marker has been captured, from among the tomographic images, and simply adds tomographic images that have not captured the first marker or the second marker and the tomographic images on which the correcting process has been performed, thereby generating the two-dimensional image. 
     The breast thickness measuring device may further comprise an average glandular dose calculator that calculates an average glandular dose based on the thickness of the breast, which has been calculated by the thickness calculator. In this manner, it is possible to grasp an accurate radiation dose to which the breast is exposed. Consequently, provided that the thickness of the breast in the compressed state is calculated accurately from the tomographic images obtained by the tomosynthesis image capturing process, and provided that the average glandular dose is calculated accurately based on the calculated thickness of the breast, then in a case where a normal image capturing process is carried out on the breast after the tomosynthesis image capturing process, it is possible to accurately calculate a dose of radiation that is required to be applied in the normal image capturing process, based on the average glandular dose. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a side elevational view of a radiographic image capturing apparatus to which a breast thickness measuring device according to an embodiment of the present invention is applied; 
         FIG. 2  is a front elevational view of the radiographic image capturing apparatus shown in  FIG. 1 ; 
         FIG. 3  is a planar view illustrating chest-wall sides of a compression plate and an image capturing table; 
         FIG. 4  is a block diagram of the breast thickness measuring device according to the embodiment; 
         FIG. 5  is a flowchart of an operation sequence of the breast thickness measuring device shown in  FIG. 4 ; 
         FIG. 6  is a view schematically illustrating slice intervals of tomographic images; 
         FIG. 7  is a view illustrating the tomographic images; 
         FIG. 8  is a side elevational view illustrating another arrangement (first modification) of the radiographic image capturing apparatus shown in  FIGS. 1 and 2 ; 
         FIG. 9  is a view schematically illustrating slice intervals of tomographic images; 
         FIG. 10  is a side elevational view illustrating still another arrangement (second modification) of the radiographic image capturing apparatus shown in  FIGS. 1 and 2 ; 
         FIG. 11  is a front elevational view of the radiographic image capturing apparatus shown in  FIG. 10 ; 
         FIG. 12  is a planar view illustrating chest-wall sides of a compression plate and an image capturing table; 
         FIG. 13  is a view schematically illustrating slice intervals of tomographic images; 
         FIG. 14  is a side elevational view illustrating yet another arrangement (third modification) of the radiographic image capturing apparatus shown in  FIGS. 1 and 2 ; 
         FIG. 15  is a view schematically illustrating slice intervals of tomographic images; 
         FIG. 16  is a view schematically illustrating a manner in which a 2D image is generated from tomographic images (fourth modification); 
         FIG. 17  is a view illustrating 2D images; 
         FIG. 18  is a planar view illustrating another arrangement (fifth modification) of the compression plate; and 
         FIG. 19  is a planar view illustrating still another arrangement (sixth modification) of the compression plate. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Breast thickness measuring devices according to preferred embodiments of the present invention in relation to a breast thickness measuring method will be described in detail below with reference to the accompanying drawings. 
     [Arrangement of Breast Thickness Measuring Device] 
     As shown in  FIGS. 1 through 4 , a breast thickness measuring device  10  according to an embodiment of the present invention is applied to a radiographic image capturing system  20 , which comprises a radiographic image capturing apparatus  16  for capturing a radiographic image of a breast  14  of a subject  12 , and a console  18  for controlling the radiographic image capturing apparatus  16 , in a radiological department of a medical organization. 
     As shown in  FIGS. 1 and 2 , the radiographic image capturing apparatus  16  includes an upstanding base  22  and a rotational shaft  24  that extends in the direction of the arrow Y as a horizontal direction from an upper portion of a side surface of the base  22  which faces toward the subject  12 . An arm  26  is fixed to the rotational shaft  24 . 
     The arm  26 , which is supported on the rotational shaft  24 , includes a distal end portion constructed as a radiation source housing  30  that houses a radiation source  28  therein. A vertical axis  32  extends perpendicularly to the rotational shaft  24  in a vertical direction (the direction of the arrow Z). Upon rotation of the rotational shaft  24  about its own axis, the arm  26 , the radiation source  28 , and the radiation source housing  30  rotate together within a predetermined angular range (−θ1 to +θ1) with the vertical axis  32  being located at a central angle (θ=0°). In the description that follows, the position of the radiation source  28  at θ=0° will be referred to as position A, the position of the radiation source  28  at θ=+θ1° will be referred to as position B, and the position of the radiation source  28  at θ=−θ1° will be referred to as position C. 
     A holder  34  is coupled to the distal end of the rotational shaft  24 . An image capturing table (support table)  36  on which the breast  14  of the subject  12  is placed is mounted on a lower end of the holder  34 . At least a portion of the image capturing table  36  on the side of a placement surface  38  on which the breast  14  is placed is made of a material permeable to radiation  40  (see  FIG. 4 ). The image capturing table  36  houses therein a radiation detector  42 , which generates a radiographic image (an example of image data) based on radiation  40  that is emitted from the radiation source  28 . 
     A compression plate  44 , which is made of a material permeable to radiation  40 , is mounted on the holder  34 . The compression plate  44  is displaceable along the direction of the arrow Z by a compression plate moving mechanism  46  such as a rail or the like that is disposed in the holder  34 . The holder  34 , the image capturing table  36 , the radiation detector  42 , and the compression plate  44  are disposed bilaterally and symmetrically along the direction of the arrow X with respect to the vertical axis  32 . 
     In a case where the compression plate  44  is lowered toward the image capturing table  36  by the compression plate moving mechanism  46 , with a chest wall  48  of the subject  12  being held in contact with a side surface  50  of the image capturing table  36  on the side of the subject  12 , and also with the breast  14  being placed on the placement surface  38 , it is possible to compress the breast  14  between the placement surface  38  of the image capturing table  36  and a compression surface  52  defined by the bottom surface of the compression plate  44 . 
     Respective gears, not shown, are provided on the rotational shaft  24  and the holder  34 . By adjusting the intermeshing state of the gears, it is possible to switch between a state in which the holder  34  is coupled to the rotational shaft  24  for rotation therewith, and a state in which the holder  34  is separated from the rotational shaft  24  for idle rotation. Hereinbelow, a description will be given in which the placement surface  38  of the image capturing table  36  lies along the horizontal direction (the direction of the arrow X and the direction of the arrow Y) in a case where the holder  34  is rotatable in an idle state with respect to the rotational shaft  24 . 
     According to the present embodiment, a first marker  56  is provided on the compression plate  44  proximate a side surface  54  thereof on the side of the chest wall  48  of the subject  12 , and a second marker  58  is provided on the image capturing table  36  proximate a side surface  50  thereof on the side of the chest wall  48 . 
     More specifically, the first marker  56  is embedded in the compression plate  44  so as to lie substantially flush with the compression surface  52  proximate the side surface  54 , at a position on a central line  60  that is perpendicular to the vertical axis  32  and coaxial with the rotational shaft  24 , i.e., at a central position proximate the side surface  54 . The second marker  58  is embedded in the image capturing table  36  so as to lie substantially flush with the placement surface  38  proximate the side surface  50 , at a position through which the vertical axis  32  and the central line  60  extend, i.e., at a central position proximate the side surface  50 . 
     As shown in  FIG. 3 , given that the horizontal width of the image capturing table  36  along the direction of the arrow X is represented by Xs, the horizontal width of the radiation detector  42  is represented by Xd, and the horizontal width of the compression plate  44  is represented by Xp, the first marker  56  and the second marker  58  are located on the central line  60 , which is a central position with respect to the direction of the arrow X of the image capturing table  36 , the radiation detector  42 , and the compression plate  44 . Further, the first marker  56  and the second marker  58  are superposed one on the other, at a position that is spaced Ym back from the side surfaces  50 ,  54  in the direction of the rotational shaft  24  as viewed as a planar view. 
     Since the position that is spaced Ym back from the side surfaces  50 ,  54  is a position on the radiation detector  42  as viewed as a planar view, the first marker  56  and the second marker  58  are disposed within the irradiation range of the radiation  40 . The first marker  56  and the second marker  58  are provided within the irradiation range of the radiation  40  and in the vicinity of the side surfaces  50 ,  54 , such that Ym is allowed to be a distance that does not approach the central area of the compression plate  44  (e.g., only about few mm from the side surfaces  50 ,  54 ). 
     Since the image capturing table  36  is harder (of greater mechanical strength) than the compression plate  44 , the second marker  58  may be provided in an arbitrary location proximate the side surface  50 . In  FIGS. 2 and 3 , by way of example, the second marker  58  is illustrated as being disposed at a position through which the vertical axis  32  and the central line  60  extend. 
     The first marker  56  and the second marker  58  preferably are made of a material that is capable of absorbing radiation  40  (i.e., a material that is impermeable to radiation  40 ), such as copper, lead, platinum, gold, tantalum alloy, alumina, or the like. The first marker  56  and the second marker  58  may be of a shape that can be distinguished from a calcified region, spicula, mass, or the like, which may be formed in the breast  14 . For example, the first marker  56  and the second marker  58  may be of a circular shape, a ring shape, a crisscross shape, or a heart shape as viewed as a planar view. 
     As shown in  FIG. 4 , the radiographic image capturing system  20  to which the breast thickness measuring device  10  according to the present embodiment is applied comprises the radiographic image capturing apparatus  16  and the console  18 . 
     The radiographic image capturing apparatus  16  includes, in addition to the components shown in  FIGS. 1 through 3 , a radiation source controller  62 , a detector controller  64 , a display control panel  66 , and a transceiver  68 . The radiation source controller  62  controls the radiation source  28  according to image capturing conditions transmitted from the console  18  through the transceiver  68 . The detector controller  64  controls the radiation detector  42  according to the image capturing conditions in order to acquire a radiographic image that is generated by the radiation detector  42 , and transmits the acquired radiographic image to the console  18  through the transceiver  68 . The display control panel  66  displays image capturing information, such as an imaged region, an imaging direction, etc. of the subject  12 , ID information of the subject  12 , etc., and allows setting of such items, as necessary. The transceiver  68  sends signals to and receives signals from the console  18 . 
     The image capturing conditions refer to conditions representing a tube voltage, an mAs value, etc., which define the dose of radiation  40  that is applied to the breast  14 . In order to perform a radiographic image capturing process on the breast  14 , such image capturing conditions are set in the radiation source controller  62 . 
     The console  18  is installed in a treatment room adjacent to an image capturing room of the radiological department and controls the radiographic image capturing apparatus  16 . The console  18  is connected to a hospital information system (HIS) for managing medical clerical work in the hospital, a radiological information system (RIS) for managing a process of capturing radiographic images in the radiological department under the management of the HIS, and a viewer that is used by the doctor in order to interpret and diagnose radiographic images. 
     More specifically, the console  18  includes a transceiver  70  for sending signals to and receiving signals from the radiographic image capturing apparatus  16 , as well as sending signals to and receiving signals from the viewer, the HIS, and the RIS through an in-hospital network, and a controller  72  for controlling various components of the radiographic image capturing apparatus  16  and the console  18 . In the console  18 , an image capturing condition memory  74 , a projected image memory  76 , a tomographic image memory  78 , a 2D image memory  80 , an input operation panel  82 , a display unit  84 , a reconstruction processor  86 , a marker detector  88 , a marker determiner (selector)  90 , a compressed thickness calculator (thickness calculator)  92 , an AGD calculator (average glandular dose calculator)  94 , and a 2D image generator (two-dimensional image generator)  96  are connected to the controller  72 . 
     The image capturing condition memory  74  stores therein image capturing conditions, which have been set by the radiological technician operating the input operation panel  82 . For performing a radiographic image capturing process on the breast  14 , the controller  72  is capable of setting the image capturing conditions in the radiation source controller  62  through the transceivers  68 ,  70 . 
     The projected image memory  76  stores a radiographic image acquired from the radiographic image capturing apparatus  16 . More specifically, in a tomosynthesis image capturing process, which is performed on the breast  14  by the radiographic image capturing system  20 , in a case where image capturing conditions depending on the tomosynthesis image capturing process are set in the radiation source controller  62 , the radiation source controller  62  rotates the rotational shaft  24  according to the image capturing conditions, so as to turn the arm  26  within the angular range from −θ1 to +θ1 (i.e., a range between position B and position C) (see  FIGS. 1 and 2 ). 
     At this time, the radiation source controller  62  controls the radiation source  28  to apply radiation  40  to the breast  14 , which has been compressed by the image capturing table  36  and the compression plate  44 , successively from positions at a plurality of different angles θ within the above angular range. Each time that radiation  40 , which has passed through the breast  14 , is projected onto the radiation detector  42 , the radiation detector  42  converts the projected radiation  40  into a radiographic image. The detector controller  64  controls the radiation detector  42  in order to acquire the converted radiographic image. Therefore, the detector controller  64  is capable of acquiring from the radiation detector  42  a plurality of radiographic images in the tomosynthesis image capturing process that is performed on the breast  14 . The acquired radiographic images are transmitted from the detector controller  64  to the console  18  through the transceivers  68 ,  70 . 
     The projected image memory  76  stores the radiographic images that are represented by image data generated in a case where radiation  40  passes through the breast  14  and is projected onto the radiation detector  42  at respective different angles θ. Since the first marker  56  and the second marker  58 , which are capable of absorbing radiation  40 , are positioned within the irradiation range of the radiation  40 , each of the radiographic images is represented by image data in which the first marker  56  and the second marker  58  are captured. The projected image memory  76  also stores various items of information assigned to the radiographic images, such as numbers, file names, headers, or the like, together with the radiographic images. 
     The reconstruction processor  86  reads the radiographic images that are stored in the projected image memory  76 , and generates tomographic images (reconstructed images) of the breast  14  at arbitrary sectional positions (slice height) along the direction of the arrow Z, by applying a known image reconstructing process such as FBP (Filtered Back Projection) to each of the radiographic images that have been read. The tomographic images, which are generated by the reconstruction processor  86 , are reconstructed images sliced parallel to the placement surface  38  of the image capturing table  36 . Each of the reconstructed tomographic images is stored in the tomographic image memory  78 . The tomographic image memory  78  also stores various items of information assigned to the tomographic images, such as numbers, file names, headers, or the like, together with the tomographic images. 
     The marker detector  88  detects, from among the tomographic images that are stored in the tomographic image memory  78 , tomographic images that have captured the first marker  56  and tomographic images that have captured the second marker  58 . The marker determiner  90  selects, from among the tomographic images detected by the marker detector  88 , a tomographic image that is focused on the first marker  56 , and a tomographic image that is focused on the second marker  58 . 
     The tomographic images represent images at discrete cross sections, which are spaced at predetermined slice intervals. Consequently, depending on the slice intervals or the slicing method, it is conceivable that a tomographic image at the vertical position of the first marker  56  or at the vertical position of the second marker  58  may not necessarily be obtained. Therefore, the marker determiner  90  selects, from among the tomographic images that have captured the first marker  56 , the tomographic image in which the first marker  56  is most clearly visible as the tomographic image that is focused on the first marker  56 . The marker determiner  90  also selects, from among the tomographic images that have captured the second marker  58 , the tomographic image in which the second marker  58  is most clearly visible as the tomographic image that is focused on the second marker  58 . 
     In addition to the tomographic images themselves, the tomographic image memory  78  stores various items of information concerning the tomographic images. Therefore, during the process of detecting tomographic images, which is carried out by the marker detector  88 , and the process of selecting tomographic images, which is carried out by the marker determiner  90 , various processes may be performed using various items of information that are assigned to the tomographic images. 
     More specifically, from among the various items of information assigned to the tomographic images, the marker detector  88  may specify various items of information assigned to the tomographic images that have captured the first marker  56  and the tomographic images that have captured the second marker  58 , to thereby enable detection of the tomographic images that have captured the first marker  56  and the tomographic images that have captured the second marker  58 . 
     The marker determiner  90  may specify various items of information assigned to the tomographic image that is focused on the first marker  56  and the tomographic image that is focused on the second marker  58 , from among various items of information assigned to the tomographic images that have captured the first marker  56  and the tomographic images that have captured the second marker  58 , thereby selecting a tomographic image that is focused on the first marker  56  and a tomographic image that is focused on the second marker  58 . 
     Therefore, the process of detecting tomographic images, which is carried out by the marker detector  88 , and the process of selecting tomographic images, which is carried out by the marker determiner  90 , include various processes with respect to various items of information assigned to the tomographic images. 
     The compressed thickness calculator  92  calculates the thickness of the breast  14  in a compressed state (compressed thickness), using the tomographic image that is focused on the first marker  56  and the tomographic image that is focused on the second marker  58 , which have been selected by the marker determiner  90 . More specifically, the tomographic image that is focused on the first marker  56  and the tomographic image that is focused on the second marker  58  can be regarded as tomographic images sliced at vertical positions of the first marker  56  and the second marker  58 . Consequently, the compressed thickness calculator  92  calculates the compressed thickness on the basis of the two tomographic images, depending on the vertical positions of the first marker  56  and the second marker  58 . 
     The compressed thickness calculator  92  may calculate the compressed thickness using various items of information assigned to the tomographic image that is focused on the first marker  56 , and the tomographic image that is focused on the second marker  58 . Therefore, the calculating process carried out by the compressed thickness calculator  92  covers the concept of calculating the compressed thickness using various items of information assigned to the tomographic images. 
     The AGD calculator  94  calculates an average glandular dose (AGD) of the breast  14 , on the basis of the compressed thickness calculated by the compressed thickness calculator  92 . The 2D image generator  96  performs a predetermined addition process on the tomographic images that are stored in the tomographic image memory  78 , thereby generating a 2D image (two-dimensional image) that is displayed on the viewer for the doctor to interpret and diagnose the radiographic images. The generated 2D image is stored in 2D image memory  80 . 
     The display unit  84  displays the image capturing conditions, which are stored in the image capturing condition memory  74 , the radiographic images and the various items of information assigned thereto, which are stored in the projected image memory  76 , and the tomographic images and the various items of information assigned thereto, which are stored in the tomographic image memory  78 . The display unit  84  also displays the 2D image stored in the 2D image memory  80 , the compressed thickness calculated by the compressed thickness calculator  92 , and/or the AGD calculated by the AGD calculator  94 . 
     The 2D image memory  80 , the AGD calculator  94 , and the 2D image generator  96  are included in the console  18  only as necessary, and are not considered indispensable components. The radiographic image capturing apparatus  16  may include angle sensors  98 ,  99  that detect a tilt angle of the compression plate  44  with respect to the direction of the arrow X or the direction of the arrow Y (horizontal plane). Similarly, the angle sensors  98 ,  99  are included in the radiographic image capturing apparatus  16  only as necessary, and are not considered indispensable components. 
     The image data will be described in greater detail below. As has been described above, the radiation detector  42  generates radiographic images as an example of image data, the detector controller  64  reads the radiographic images and transmits the radiographic images to the console  18 , and the reconstruction processor  86  of the console  18  reconstructs the radiographic images in order to generate a plurality of tomographic images. 
     In the present embodiment, the term “image data” is a general term that covers image data generated by the radiation detector  42 , image data sent from the detector controller  64  to the console  18  through the transceivers  68 ,  70 , and image data used in the reconstructing process that is carried out by the reconstruction processor  86 . 
     In other words, the term “image data” refers to images generated on the basis of radiation  40  that passes through the breast  14  and irradiates the radiation detector  42 . Consequently, the term “image data” denotes a concept that includes digital data (analog data) representing electric signals converted from the radiation  40  by the radiation detector  42 , digital data obtained by converting analog data with an A/D converter, not shown, and image data obtained by a predetermined signal processing technique performed on the digital data by a signal processor, not shown. Therefore, tomographic images are images generated by the reconstructing process that is carried out by the reconstruction processor  86 , on the basis of image data generated by the radiation detector  42 . 
     In the present embodiment, the term “radiographic images” refers to image data obtained by the signal processing technique described above. The above description of the arrangement of the breast thickness measuring device  10  signifies a case in which the radiation detector  42  performs a predetermined signal processing technique for generating radiographic images. In particular, the radiation detector  42  includes an A/D converter and a signal processor. The aforementioned signal processing technique is a signal processing technique, which is necessary to obtain images of the breast  14  that can be interpreted and diagnosed by the doctor. 
     However, the breast thickness measuring device  10  according to the present embodiment is not limited to the arrangement described above. The radiation detector  42  may include at least a function to detect radiation  40  and convert the detected radiation into analog data. Therefore, the A/D converter and the signal processor may be included in a component or components apart from the radiation detector  42 . 
     More specifically, the detector controller  64  may include the signal processor, or may include the A/D converter and the signal processor. In this case, the radiation detector  42  outputs analog data or digital data to the detector controller  64 , and the detector controller  64  generates radiographic images. The detector controller  64  transmits the radiographic images to the console  18 , and the radiographic images are stored in the projected image memory  76 . The detector controller  64  may transmit the digital data in addition to the radiographic images to the console  18 , and the digital data and the radiographic images may both be stored in the projected image memory  76 . 
     Alternatively, the A/D converter may be included in the radiation detector  42  or the detector controller  64 , and the signal processor may be included in the controller  72  or the reconstruction processor  86  of the console  18 . In this case, the detector controller  64  transmits digital data to the console  18 , and the digital data are stored in the projected image memory  76 . Further, the controller  72  or the reconstruction processor  86  performs a predetermined signal processing technique on the digital data that are stored in the projected image memory  76  in order to generate radiographic images, and the generated radiographic images are stored in the projected image memory  76 . 
     Hereinbelow, unless otherwise noted, a description will be given in which the radiation detector  42  generates the radiographic images. 
     [Operations of Breast Thickness Measuring Device (Breast Thickness Measuring Method)] 
     The breast thickness measuring device  10  according to the present embodiment is arranged as described above. Operations of the breast thickness measuring device  10  (breast thickness measuring method) will be described below with reference to  FIGS. 5 through 7 . As necessary, in describing such operations,  FIGS. 1 through 4  may also be referred to. A tomosynthesis image capturing process, in which the radiation source  28  is moved within a range between position B and position C and radiation  40  is applied from different angles θ to the breast  14  in the compressed state, will be described below. 
     In step S 1 , as shown in  FIG. 5 , the radiological technician operates the input operation panel  82  (see  FIG. 4 ) of the console  18  in order to set ID information of the subject  12 , in addition to an image capturing method and image capturing conditions for the breast  14 . The ID information is information that identifies the subject  12 , such as the name, age, etc., of the subject  12 . 
     The controller  72  controls the display unit  84  to display the ID information, the image capturing method, and the image capturing conditions that have been set, and temporarily stores the image capturing conditions in the image capturing condition memory  74 . The radiological technician confirms the information that is displayed on the display unit  84  and, as necessary, may add to or change such information using the input operation panel  82 . The determined image capturing conditions are transmitted from the transceiver  70  to the transceiver  68  of the radiographic image capturing apparatus  16 , and the image capturing conditions are set in the radiation source controller  62 . 
     After the image capturing conditions have been set, the radiographic image capturing apparatus  16  initiates a tomosynthesis image capturing process. At this time, the console  18  sends various items of information to the display control panel  66  of the radiographic image capturing apparatus  16 , thus enabling the radiological technician to make adjustments to the radiographic image capturing apparatus  16  while confirming the displayed information. 
     First, the radiological technician positions the breast  14  of the subject  12  with respect to the radiographic image capturing apparatus  16 . More specifically, the radiological technician places the breast  14  on the placement surface  38  of the image capturing table  36 , such that the chest wall  48  of the subject  12  is held in contact with the side surface  50  of the image capturing table  36 , and the breast  14  to be imaged is disposed bilaterally and symmetrically with respect to the central line  60 . Then, the compression plate moving mechanism  46  moves the compression plate  44  gradually toward the image capturing table  36 , thereby positioning and holding the breast  14  in a predetermined position between the image capturing table  36  and the compression plate  44 . 
     Next, in accordance with the position of the breast  14 , which is compressed and fixed between the image capturing table  36  and the compression plate  44 , the rotational shaft  24  is rotated to turn the arm  26 , thereby moving the radiation source housing  30  to a predetermined position (image capturing start position) between position B and position C. 
     In step S 2 , while the rotational shaft  24  is rotated to turn the arm  26 , the radiation source controller  62  controls the radiation source  28  to apply radiation  40  from different angles θ to the compressed breast  14  according to the image capturing conditions. 
     The first marker  56  and the second marker  58  are disposed within the irradiation range of the radiation  40 . Therefore, among the radiation  40  that is emitted from the radiation source  28  and applied to the compression plate  44 , a portion of the radiation  40 , which is applied to the first marker  56 , is absorbed by the first marker  56 , and the remainder of such radiation  40  is applied to the breast  14 . 
     Among the radiation  40  that is transmitted through the breast  14  and reaches the placement surface  38  of the image capturing table  36 , a portion of the radiation  40 , which is applied to the second marker  58 , is absorbed by the second marker  58 , and the remainder of such radiation  40  reaches the radiation detector  42 . Therefore, the radiation detector  42  detects the radiation  40  that has reached the radiation detector  42 , and converts the detected radiation  40  into a radiographic image. The detector controller  64  acquires the radiographic image from the radiation detector  42 . 
     As described above, the radiation source  28  applies radiation  40  from different angles θ to the breast  14  while moving between position B and position C. Consequently, each time that the radiation source  28  applies radiation  40  to the breast  14 , the detector controller  64  acquires a radiographic image, which is converted from the radiation  40  by the radiation detector  42 . Therefore, upon completion of the tomosynthesis image capturing process in step S 2 , the detector controller  64  acquires a plurality of radiographic images of the breast  14 . 
     In step S 3 , the detector controller  64  transmits the acquired radiographic images to the console  18  through the transceivers  68 ,  70 . In a case where the controller  72  receives the radiographic images, the received radiographic images are stored in the projected image memory  76 . At this time, the projected image memory  76  also stores various items of information assigned to the radiographic images, such as numbers, file names, headers, or the like. 
     In step S 4 , the reconstruction processor  86  reads the radiographic images that are stored in the projected image memory  76 , reconstructs the read radiographic images in order to generate a plurality of tomographic images, and stores the generated tomographic images in the tomographic image memory  78 . At this time, the tomographic image memory  78  also stores various items of information assigned to the tomographic images, such as numbers, file names, headers, or the like. 
       FIG. 6  is a view in which slice intervals of tomographic images, which are generated at the time that the reconstruction processor  86  (see  FIG. 4 ) performs the reconstructing process, are illustrated.  FIG. 7  is a view illustrating the tomographic images generated by the reconstructing process. 
     The reconstruction processor  86  generates a plurality of tomographic images at given slice intervals t along the direction of the arrow Z. Each of the tomographic images is an image sliced parallel to the placement surface  38  of the image capturing table  36 . The slice intervals t are pre-adjusted using a commercially available calibration phantom (geometric calibration phantom), not shown, and are stored in the image capturing condition memory  74 , for example. 
     In  FIG. 6 , on the assumption that the number of tomographic images sliced between the compression surface  52  of the compression plate  44  and the placement surface  38  of the image capturing table  36  is n, and the tomographic images are given numbers 1, 2, 3, . . . , (n−2), (n−1), and n, respectively, from the compression surface  52  to the placement surface  38 , then the first through nth tomographic images are as shown in  FIG. 7 . 
     In  FIG. 7 , each of the tomographic images is denoted by  100  and includes a breast image  102  representing the breast  14 . The first and second tomographic images  100  include a marker image  104  representing the first marker  56 . The areas shown by the broken lines in the first and second tomographic images  100  are referred to as marker image display areas  106  in which the marker image  104  can be included. 
     The first tomographic image  100  includes a clearly visible circle representing the first marker  56  as the marker image  104  in the marker image display area  106 . The second tomographic image  100  includes a plurality of blurred circles representing the first marker  56  as the marker image  104  in the marker image display area  106 . Therefore, the first tomographic image  100  can be interpreted as a tomographic image that is focused on the first marker  56 , whereas the second tomographic image  100  can be interpreted as a tomographic image that is not focused on the first marker  56 . 
     The (n−1)th and nth tomographic images  100  include a marker image  108  representing the second marker  58 . The areas shown by the broken lines in the (n−1)th and nth tomographic images  100  are referred to as marker image display areas  110  in which the marker image  108  can be included. 
     The nth tomographic image  100  includes a clearly visible circle representing the second marker  58  as the marker image  108  in the marker image display area  110 . The (n−1)th tomographic image  100  includes a plurality of blurred circles representing the second marker  58  as the marker image  108  in the marker image display area  110 . Therefore, the nth tomographic image  100  can be interpreted as a tomographic image that is focused on the second marker  58 , whereas the (n−1)th tomographic image  100  can be interpreted as a tomographic image that is not focused on the second marker  58 . 
     The third through (n−2)th tomographic images  100  include the breast image  102  only, with no marker images  104 ,  108  included in the marker image display areas  106 ,  110 . Therefore, the third through (n−2)th tomographic images  100  can be interpreted as tomographic images that are sliced at vertical positions, which differ from the positions of the first marker  56  and the second marker  58 . 
     In  FIG. 6 , the size of the first marker  56  and the size of the second marker  58  along the horizontal direction (the direction of the arrow X and the direction of the arrow Y) is illustrated as a size that depends on a single pixel  112 , which is part of the radiation detector  42  for converting radiation  40  into electric signals. According to the present embodiment, the size of the first marker  56  and the size of the second marker  58  may be of a size such that tomographic images  100  are produced that are focused on the first marker  56  and the second marker  58 , e.g., may be a diameter ranging from 100 μm to several mm. Therefore, the size of the first marker  56  and the size of the second marker  58  may be of a size as large as a single pixel or a size as large as several pixels. 
     According to the present embodiment, at least two tomographic images  100  may be acquired, including respective marker images  104 ,  108  that are focused on the first marker  56  and the second marker  58 . Consequently, instead of the tomographic images  100  shown in  FIG. 7 , the reconstruction processor  86  may generate the first tomographic image  100  and the nth tomographic image  100 , which include the focused marker images  104 ,  108 , as well as the second through (n−1)th tomographic images  100 , which do not include the marker images  104 ,  108 . 
     In step S 5 , the marker detector  88  detects the tomographic images  100 , which include the marker images  104  and  108 , from among the tomographic images  100  that are stored in the tomographic image memory  78 . For example, in a case where the tomographic images  100  shown in  FIG. 7  are stored in the tomographic image memory  78 , the marker detector  88  detects the first and second tomographic images  100 , which include the marker image  104 , and the (n−1)th and nth tomographic images  100 , which include the marker image  108 . 
     In step S 5 , rather than performing the process described above, the marker detector  88  may detect the first and second tomographic images  100 , which include the marker image  104 , and the (n−1)th and nth tomographic images  100 , which include the marker image  108 , by specifying various items of information that are assigned to the first and second tomographic images  100 , which include the marker image  104  (the numbers “1” and “2” are indicative of the tomographic images  100 , the file names, or headers or the like of the tomographic images  100 ), and various items of information that are assigned to the (n−1)th and nth tomographic images  100 , which include the marker image  108  (the numbers “n−1” and “n” are indicative of the tomographic images  100 , the file names, or headers or the like of the tomographic images  100 ). 
     In step S 6 , from among the tomographic images  100  detected by the marker detector  88 , the marker determiner  90  selects the tomographic image  100  that is focused on the first marker  56  and the tomographic image  100  that is focused on the second marker  58 . More specifically, the marker determiner  90  selects the first tomographic image  100  from among the first and second tomographic images  100 , and further selects the nth tomographic image  100  from among the (n−1)th and the nth tomographic images  100 . 
     In step S 6 , rather than performing the process described above, the marker determiner  90  may select the tomographic image  100 , which is focused on the first marker  56 , and the tomographic image  100 , which is focused on the second marker  58 , by specifying various items of information assigned to the tomographic image  100  that is focused on the first marker  56  and the tomographic image  100  that is focused on the second marker  58 . 
     In step S 7 , the display unit  84  displays all of the tomographic images  100  that are stored in the tomographic image memory  78 . 
     In step S 8 , the compressed thickness calculator  92  calculates the compressed thickness T of the breast  14  using the tomographic image  100  (first tomographic image  100 ) that is focused on the first marker  56  and the tomographic image  100  (nth tomographic image  100 ) that is focused on the second marker  58 , which have been selected by the marker determiner  90  in step S 6 . 
     The first tomographic image  100 , which is focused on the first marker  56 , can be regarded as a tomographic image sliced at the vertical position of the first marker  56 . The nth tomographic image  100 , which is focused on the second marker  58 , can be regarded as a tomographic image sliced at the vertical position of the second marker  58 . The first marker  56  is a marker that is embedded in the compression plate  44 , so as to lie substantially flush with the compression surface  52 . The second marker  58  is a marker that is embedded in the image capturing table  36 , so as to lie substantially flush with the placement surface  38 . Therefore, the compressed thickness calculator  92  calculates the compressed thickness T according to the following equation (1).
 
 T=t ×( n− 1)  (1)
 
     In step S 8 , rather than performing the process described above, the compressed thickness calculator  92  may calculate the compressed thickness T by using various items of information assigned to the tomographic image  100  that is focused on the first marker  56  and the tomographic image  100  that is focused on the second marker  58 . More specifically, in a case where the number given to the tomographic image  100  that is focused on the first marker  56  is n1, and the number given to the tomographic image  100  that is focused on the second marker  58  is n2, the compressed thickness calculator  92  may calculate the compressed thickness T according to the following equation (2).
 
 T=t ×( n 2− n 1)  (2)
 
     In the example shown in  FIGS. 6 and 7 , since n1=1 (first tomographic image  100 ) and n2=n (nth tomographic image), the compressed thickness calculator  92  can easily calculate the compressed thickness T according to equation (2). In equation (2), (n2−n1) represents the number of tomographic images  100  from the n1th tomographic image  100  to the n2th tomographic image  100 . 
     Assuming that the numbers 1, 2, 3, . . . , 100 are assigned to a plurality of tomographic images  100 , the tomographic image  100  in which the first marker  56  is clearest is the 5th tomographic image, and the tomographic image  100  in which the second marker  58  is clearest is the 95th tomographic image, the compressed thickness calculator  92  is capable of determining the compressed thickness T according to equation (2), where n1=5 and n2=95, i.e., T=t×(95−5). 
     In step S 9 , the display unit  84  displays the compressed thickness T that was calculated by the compressed thickness calculator  92 . Thus, the radiological technician is capable of accurately grasping the thickness (compressed thickness) T of the breast  14  in the compressed state. 
     According to the present embodiment, steps S 10  through S 13  may be carried out after step S 9 , as necessary. 
     In step S 10 , the AGD calculator  94  calculates the AGD of the breast  14  on the basis of the compressed thickness T that was calculated by the compressed thickness calculator  92 . In step S 11 , the display unit  84  displays the AGD that was calculated by the AGD calculator  94 . Thus, the radiological technician is capable of grasping the accurate AGD of the breast  14  in the compressed state. 
     In step S 12 , the 2D image generator  96  performs a predetermined addition process on the tomographic images  100  that are stored in the tomographic image memory  78  in order to generate a 2D image, and the generated 2D image is stored in the 2D image memory  80 . In step S 13 , the display unit  84  displays the 2D image stored in the 2D image memory  80 . Thus, the radiological technician is capable of observing the 2D image of the breast  14  in the compressed state. In a case where the console  18  transmits the 2D image to the viewer through the in-hospital network, the doctor is able to interpret and diagnose the 2D image that is displayed on the viewer. 
     Details of the process for generating the 2D image in step S 12  will be described later. 
     Advantages of the Present Embodiment 
     As described above, the breast thickness measuring device  10  and the breast thickness measuring method according to the present embodiment perform a tomosynthesis image capturing process for applying radiation  40  from a plurality of different angles θ to the breast  14  in the compressed state, and a plurality of tomographic images  100  are generated by reconstructing radiographic images obtained by the tomosynthesis image capturing process. 
     The first marker  56  is provided on the compression plate  44 , whereas the second marker  58  is provided on the image capturing table  36 . Therefore, the tomographic image  100  that is focused on the first marker  56  is a tomographic image of an upper end of the breast  14  along the thickness-wise direction (the direction of the arrow Z) of the breast  14 . The tomographic image  100  that is focused on the second marker  58  is a tomographic image of a lower end of the breast  14  along the direction of the arrow Z. 
     Consequently, using the tomographic image  100  that is focused on the first marker  56  and the tomographic image  100  that is focused on the second marker  58 , it is possible to directly calculate the thickness (compressed thickness) T of the breast  14  in the compressed state. 
     In addition, the first marker  56  and the second marker  58  are disposed on the side of the chest wall  48  of the subject  12 . Therefore, the present embodiment allows the compressed thickness T to be calculated more accurately than with the technologies disclosed in the publications referred to above. 
     Therefore, according to the present embodiment, it is possible to accurately grasp the compressed thickness T of the breast. 
     More specifically, from among the tomographic images  100 , the marker determiner  90  selects the tomographic image  100  that is focused on (the marker image  104  representing) the first marker  56 , and also, from among the tomographic images  100 , selects the tomographic image  100  that is focused on (the marker image  108  representing) the second marker  58 . The compressed thickness calculator  92  then calculates the compressed thickness T based on the two tomographic images  100  that have been selected by the marker determiner  90 . The tomographic images can be regarded as tomographic images sliced at vertical positions of the first marker  56  and the second marker  58 . The compressed thickness T can be calculated highly accurately by selecting the two tomographic images  100  that are focused on the first marker  56  and the second marker  58 . 
     The tomographic images  100  represent images at discrete cross sections, which are spaced at predetermined slice intervals t. Consequently, depending on the slice intervals t or the slicing method, it is conceivable that a tomographic image  100  may not necessarily be obtained at the vertical position of the first marker  56  or the second marker  58 . According to the present embodiment, therefore, the marker determiner  90  selects, from among the tomographic images  100  that have captured (the marker image  104  representing) the first marker  56 , a tomographic image  100  in which the first marker  56  is clearly visible as the tomographic image  100  that is focused on the first marker  56 . Similarly, the marker determiner  90  also selects, from among the tomographic images  100  that have captured (the marker image  108  representing) the second marker  58 , a tomographic image  100  in which the second marker  58  is clearly visible as the tomographic image  100  that is focused on the second marker  58 . 
     By selecting the tomographic image  100  of the first marker  56  and the tomographic image  100  of the second marker  58  in this manner, the accuracy with which the compressed thickness T is calculated is prevented from becoming lowered on account of the slice intervals t and the slicing method. Assuming that the tomographic image  100  at the vertical position of the first marker  56  and the tomographic image  100  at the vertical position of the second marker  58  are obtained, then naturally, by using such tomographic images  100 , it is possible to calculate the compressed thickness T with high accuracy. 
     Various items of information with respect to the tomographic images  100 , such as numbers, file names, headers, or the like of the tomographic images  100 , are assigned to the tomographic images  100 , and such items are stored in the tomographic image memory  78 . According to the present embodiment, therefore, the process of detecting tomographic images  100  by the marker detector  88 , the process of selecting tomographic images  100  by the marker determiner  90 , and the process of calculating the compressed thickness T by the compressed thickness calculator  92  may be carried out using the various items of information that are assigned to the tomographic images  100 . 
     More specifically, in a case where the tomographic images  100  and the various items information are stored in the tomographic image memory  78 , the marker detector  88  may detect the tomographic images  100 , which have captured the first marker  56 , and the tomographic images  100 , which have captured the second marker  58 , by specifying the various items of information assigned to the tomographic images  100  that have captured the first marker  56  and the tomographic images  100  that have captured the second marker  58 . 
     From among the tomographic images  100  that have captured the first marker  56  and the tomographic images  100  that have captured the second marker  58 , the marker determiner  90  may select the tomographic image  100  that is focused on the first marker  56  and the tomographic image  100  that is focused on the second marker  58 , by specifying various items of information that are assigned to the tomographic image  100  that is focused on the first marker  56  and the tomographic image  100  that is focused on the second marker  58 . 
     Furthermore, the compressed thickness calculator  92  may calculate the compressed thickness T using the information that is assigned to the tomographic image  100  that is focused on the first marker  56  and the tomographic image  100  that is focused on the second marker  58 . 
     Consequently, the process of detecting the tomographic images  100  by the marker detector  88 , the process of selecting the tomographic images  100  by the marker determiner  90 , and the process of calculating the compressed thickness T by the compressed thickness calculator  92  include processes involving various items of information assigned to the tomographic images  100 . For example, the phrase “calculating the compressed thickness T based on the tomographic image  100  that is focused on the first marker  56  and the tomographic image  100  that is focused on the second marker  58 ” implies a concept that includes calculation of the compressed thickness T using the various items of information assigned to the tomographic images  100 . 
     In as much as the compressed thickness calculator  92  calculates the compressed thickness T according to equation (1) or (2), the compressed thickness calculator  92  can reliably calculate the actual thickness of the breast  14  in the compressed state. 
     The reconstruction processor  86  reconstructs the radiographic images in order to generate tomographic images  100 , such that the tomographic images  100  become tomographic images sliced parallel to the image capturing table  36 . Therefore, the marker detector  88  is capable of easily detecting tomographic images  100 , which include the first marker  56  or the second marker  58 . 
     Since the first marker  56  and the second marker  58  are disposed in superposed relation as viewed as a planar view, the burden imposed by the correcting process for generating a 2D image from the tomographic images  100  is reduced. 
     The burden imposed by the aforementioned correcting process is further lessened due to the fact that the image capturing table  36 , the radiation detector  42 , the compression plate  44 , the first marker  56 , and the second marker  58  are disposed on the vertical axis  32  at the central angle (θ=0°) of the radiation source  28 . 
     Further, since the AGD calculator  94  calculates the AGD based on the compressed thickness T, which is calculated by the compressed thickness calculator  92 , it is possible to grasp an accurate radiation dose to which the breast  14  is exposed. Consequently, provided that the compressed thickness T is calculated accurately from the tomographic images  100  obtained by the tomosynthesis image capturing process, and the AGD is calculated accurately on the basis of the calculated compressed thickness T, in a case where a normal image capturing process is carried out on the breast  14  after the tomosynthesis image capturing process, on the basis of the AGD, it is possible to accurately calculate the required dose of the radiation  40  to be applied in the normal image capturing process. 
     Modifications of the Present Embodiment 
     Modifications (first through sixth modifications) of the breast thickness measuring device  10  and the breast thickness measuring method according to the present embodiment will be described below with reference to  FIGS. 8 through 19 . 
     First Modification 
     According to the first modification, as shown in  FIGS. 8 and 9 , a shaft  114  is provided on a proximal end portion of the compression plate  44 , and the compression plate  44  is arranged so as to be capable of moving angularly about the shaft  114 . Therefore, in a case where the compression plate  44  is lowered toward the image capturing table  36  by the compression plate moving mechanism  46 , and is brought into contact with the breast  14 , the side of the compression plate  44  proximate the side surface  54  turns counterclockwise as shown in  FIGS. 8 and 9  (toward the radiation source  28 ) about the shaft  114 . Thus, the compression plate  44  compresses the breast  14  while being tilted along the breast  14 . The angle sensor  98  is mounted on the shaft  114  for detecting the angle φ of tilt of the compression plate  44  with respect to the horizontal plane (i.e., the angle of the compression plate  44  before the compression plate  44  contacts the breast  14 ). 
     According to the first modification, the first marker  56  also is provided on the compression plate  44  proximate the side surface  54  thereof, and the second marker  58  also is provided on the image capturing table  36  proximate the side surface  50  thereof. Therefore, the compressed thickness T is represented by the distance between the compression surface  52  of the tilted compression plate  44  at the side surface  54 , and the placement surface  38  of the image capturing table  36 . In addition, according to the first modification, the reconstruction processor  86  reconstructs the radiographic images in order to generate tomographic images  100  in such a manner that the tomographic images  100  are sliced parallel to the placement surface  38 . 
     In a case where the compression plate  44  compresses the breast  14  in a tilted state, the technologies disclosed in the above publications detect the position of the compression plate  44  at a proximal end portion thereof. Therefore, in the event that the compressed thickness T is estimated from the detected position of the compression plate  44  at the proximal end portion, a large estimation error occurs, making it impossible to acquire an accurate estimation of the compressed thickness T. 
     According to the first modification, the first marker  56  and the second marker  58  are disposed on the side of the chest wall  48  of the subject  12 , and the compressed thickness T is calculated using tomographic images  100  that have captured the first marker  56  and the second marker  58 . In other words, according to the first modification, the thickness (compressed thickness) T of the breast  14 , which is actually compressed, is directly measured from the tomographic images  100 . Therefore, according to the first modification, it is possible to measure the compressed thickness T more accurately than with the technologies disclosed in the above publications. 
     Second Modification 
     According to the second modification, as shown in  FIGS. 10 through 13 , the compression plate  44  is mounted on the holder  34  by a rotational shaft  116 , and the compression plate  44  is arranged so as to be angularly movable about the rotational shaft  116 . The angle α of tilt of the compression plate  44  with respect to the horizontal direction (the direction of the arrow X and the direction of the arrow Y) is detected by the angle sensor  99 . 
     According to the second modification, first markers  56  are disposed on respective left and right corners (angular portions) along the direction of the arrow X proximate the side surface  54  of the compression plate  44 . More specifically, according to the second modification, as shown in  FIG. 12 , the two first markers  56  are embedded in the compression plate  44  so as to lie substantially flush with the compression surface  52  at positions spaced Xm to the left and right from the central line  60 , and also spaced Ym back from the side surface  54  toward the rotational shaft  24 . According to the second modification, therefore, a first marker  56  is not disposed on the central line  60 . Furthermore, according to the second modification, the two first markers  56  are positioned over the radiation detector  42  as viewed as a planar view, and are disposed within the irradiation range of the radiation  40 . 
     According to the second modification, the second marker  58  is disposed in the same position as the second marker  58  shown in  FIGS. 1 through 3  (i.e., at a position located on the central line  60  and spaced Ym back from the side surface  50  toward the rotational shaft  24 ). As described above, since the image capturing table  36  is harder than the compression plate  44 , the second marker  58  is disposed only in one location on the image capturing table  36 . 
     As shown in  FIGS. 11 and 13 , according to the second modification, while the rotational shaft  116  is rotated to turn the compression plate  44  (i.e., to tilt the compression plate  44  laterally along the direction of the arrow X), the compression plate  44  is lowered toward the image capturing table  36 , thereby compressing and holding the breast  14  in a laterally tilted state. 
     According to the second modification, assuming that the distance between the central position of the compression surface  52  of the compression plate  44  on the vertical axis  32 , and the central position of the placement surface  38  of the image capturing table  36  is regarded as the compressed thickness T (average value) of the breast  14 , then the compressed thickness T is expressed by the following equation (3)
 
 T =( Za+Zb )/2  (3)
 
where Za represents the distance between the left first marker  56  shown in  FIGS. 11 and 13  and the placement surface  38 , and Zb represents the distance between the first marker  56  shown on the right in  FIGS. 11 and 13  and the placement surface  38 .
 
     More specifically, the two first markers  56  are disposed on the compression plate  44  and spaced equal distances (Xm) from the central line  60  to the left and right. Therefore, the compressed thickness T at an intermediate position between the two first markers  56  can easily be calculated from the two distances Za, Zb. 
     The distance Za can be calculated according to the following equation (4) using the above equation (1), or according to the following equation (5) using the above equation (2), on the basis of the first tomographic image  100  that has captured the left first marker  56  and the nth tomographic image  100  that has captured the second marker  58 .
 
 Za=t ×( n− 1)  (4)
 
 Za=t ×( n 2− n 1)  (5)
 
     The distance Zb can be calculated according to the following equation (6) using the above equation (1) or (2), as partially modified, on the basis of the ith tomographic image  100  that has captured the right first marker  56  and the nth tomographic image  100  that has captured the second marker  58 .
 
 Zb=t ×( n−i )  (6)
 
     Therefore, according to the second modification, the marker detector  88  detects a tomographic image  100  that has captured the left first marker  56  (first tomographic image  100 ), a tomographic image  100  that has captured the right first marker  56  (ith tomographic image  100 ), and a tomographic image  100  that has captured the second marker  58  (nth tomographic image  100 ). From among the tomographic images  100  detected by the marker detector  88 , the marker determiner  90  selects the tomographic images  100  that are focused on the left first marker  56 , the right first marker  56 , and the second marker  58 . Then, according to the above equations (3) through (6), the compressed thickness calculator  92  calculates the distances Za, Zb and the compressed thickness T. 
     According to the second modification, as described above, even though the breast  14  is compressed and held by the compression plate  44 , which is tilted laterally along the direction of the arrow X, the compressed thickness T can accurately be measured because the first markers  56  are provided on left and right sides of the compression plate  44 . 
     Furthermore, according to the second modification, since the distances Za, Zb, which are indicative of the compressed thickness, can be measured, the tilt of the compression plate  44  with respect to the placement surface  38  of the image capturing table  36  (the direction of the arrow X as a horizontal direction) can accurately be calculated from the distances Za, Zb. 
     Third Modification 
     As shown in  FIGS. 14 and 15 , the third modification differs from the second modification (see  FIGS. 10 through 13 ), in that a further first marker  56  is disposed at a central position (a position on the vertical axis  32 ) of the compression plate  44  proximate the side surface  54  thereof. Accordingly, the third modification is a combination of the arrangement of the embodiment shown in  FIGS. 1 through 3  and the arrangement of the second modification shown in  FIGS. 10 through 13 . 
     According to the third modification, as shown in  FIG. 15 , in the case that the compression plate  44  is curved (curved in an upwardly convex manner as shown in  FIG. 15 ), it is possible to grasp the degree to which the compression plate  44  is distorted. 
     More specifically, as shown in  FIG. 15 , assuming that the distance between the central first marker  56  and the placement surface  38  is represented by Zc, the central first marker  56  is captured in the first tomographic image  100 , and the left and right first markers  56  are captured in the third tomographic image  100 , the distances Za, Zb, Zc are expressed by the following equations (7) and (8).
 
 Za=Zb=t ×( n −3)  (7)
 
 Zc=t ×( n− 1)  (8)
 
     In the event that the distances Za, Zb, Zc satisfy the following inequality (9), it can easily be judged that the compression plate  44  is distorted.
 
 Zc &gt;( Za+Zb )/2  (9)
 
     The marker detector  88  detects the tomographic images  100  that have captured the first markers  56  (first and third tomographic images  100 ) and the tomographic image  100  that has captured the second marker  58  (nth tomographic image  100 ). From among the tomographic images  100  detected by the marker detector  88 , the marker determiner  90  selects tomographic images  100  that are focused on the first markers  56  and the second marker  58 . The compressed thickness calculator  92  detects whether or not the compression plate  44  is distorted according to the above equations (7) through (9). 
     According to the third modification, as described above, the first markers  56  are provided on left and right corners and on a central area of the compression plate  44  proximate the side surface  54  thereof, while in addition, the second marker  58  is provided on the image capturing table  36  proximate the side surface  50  thereof. Therefore, it is possible to make accurate measurements concerning whether or not the compression plate  44  is distorted. Even though only a single second marker  58  is disposed on the image capturing table  36 , because the image capturing table  36  is harder than the compression plate  44 , the occurrence of distortions in the compression plate  44  can be measured with high accuracy. 
     According to the third modification, the compression plate  44  compresses the breast  14  parallel to the placement surface  38  of the image capturing table  36  (i.e., in the direction of the arrow X as a horizontal direction). Even in a case where the compression plate  44  compresses the breast  14  while being tilted with respect to the placement surface  38 , as in the case of the second modification (see  FIGS. 10 through 13 ), equation (9) can still be applied. 
     Fourth Modification 
     Concerning the fourth modification, details of a process for generating a 2D image by the 2D image generator  96  shown in  FIG. 4  (step S 12  of  FIG. 5 ) will be described below with reference to  FIGS. 16 and 17 . 
     As described above, the 2D image generator  96  performs a predetermined addition process on a plurality of tomographic images  100  to thereby generate a 2D image of the breast  14 . 
     First, problems that are caused in a case where a simple addition process is performed on all of the tomographic images  100  in order to generate the 2D image will be described below. 
     As shown schematically in  FIG. 16 , on the assumption that a pixel depending on a particular location in the 2D image is treated as a pixel  112   d , then according to the simple addition process for the tomographic images  100 , the pixel values at locations  120   d  along a path of radiation  40   d  that is applied to the pixel  112   d  from the radiation source  28  are simply added together, thereby producing a pixel value at a particular location in the 2D image according to the pixel  112   d.    
     Therefore, in order to generate the 2D image according to the simple addition process, in the radiation detector  42 , the process of simply adding pixel values at the locations  120   d  along the path of the radiation  40   d  is carried out with respect to all of the pixels  112 . 
     However, with the simple addition process, the following problems occur. More specifically, according to the simple addition process, the pixel values at locations  120   d  along the path of the radiation  40   d  are simply added together. Therefore, in a case where radiation  40   e  is applied to the first marker  56  or the second marker  58 , the simple addition process is performed along the locations on the path of the radiation  40   e , at a location in a 2D image corresponding to the pixel  112   e  that is irradiated with the radiation  40   e . In a case where an abnormal location  122 , such as a calcified region, spicula, mass, or the like, is formed in the breast  14 , then at a location in a 2D image corresponding to the pixel  112   f  that is irradiated with radiation  40   f  transmitted through the abnormal location  122 , the simple addition process is performed on the locations on the path of the radiation  40   f.    
     Therefore, as shown in  FIG. 17 , the 2D image  130  (the left image according to a comparative example), which is obtained by the simple addition process, captures both a marker image  134  representing the first marker  56  and the second marker  58 , as well as an abnormal image  136  representing the abnormal location  122  in a breast image  132  representing the breast  14 . As a result, in a case where the doctor interprets and diagnoses the 2D image  130  that is displayed on the viewer, the doctor may mistake the first marker  56  and the second marker  58  for the abnormal location  122 , such as a calcified region, spicula, mass, or the like that is formed in the breast  14 . Thus, there is a possibility that the burden on the doctor will increase. 
     According to the fourth modification, as described in any of paragraphs (1) through (3) below, in step S 12  of  FIG. 5 , the 2D image generator  96  carries out a correcting process for excluding the marker image  134 , and generates a 2D image  140  in which only the abnormal image  136  is captured in the breast image  132 , as shown on the right in  FIG. 17 . 
     (1) From among the tomographic images  100 , the 2D image generator  96  simply adds the tomographic images  100  (e.g., the third through (n−2)th tomographic images  100  in  FIG. 7 ) that have not captured (the marker image  104  representing) the first marker  56  or the (marker image  108  representing) the second marker  58 , thereby generating the 2D image  140 . 
     (2) From among the tomographic images  100 , the 2D image generator  96  performs a first addition process for simply adding the tomographic images  100  for image areas in which the marker images  104 ,  108  do not exist (areas other than the marker image display areas  106 ). Then, from among the tomographic images  100 , the 2D image generator  96  performs a second addition process for simply adding the marker image display areas  106 ,  110  of the tomographic images  100  (e.g., the third through (n−2)th tomographic images  100  in  FIG. 7 ) from which the tomographic images  100  that have captured the marker images  104 ,  108  are excluded. Finally, the 2D image generator  96  combines the two new images, which were obtained by the first addition process and the second addition process, thereby generating the 2D image  140 . 
     (3) From among the tomographic images  100 , the 2D image generator  96  performs a correcting process for removing the marker images  104 ,  108  on the tomographic images  100  that have captured the marker images  104 ,  108 . Then, the 2D image generator  96  simply adds the tomographic images  100  that have not captured the marker images  104 ,  108 , and the tomographic images  100  on which the correcting process has been performed, thereby generating the 2D image  140 . 
     In step S 12 , after the above addition process has been carried out on the tomographic images  100 , the generated 2D image  140  is stored in the 2D image memory  80 . Therefore, in step S 13 , the display unit  84  is capable of displaying the 2D image  140  that is stored in the 2D image memory  80 . In a case where controller  72  transmits the 2D image  140  to the viewer through the in-hospital network, the doctor is able to interpret and diagnose the 2D image  140  that is displayed on the viewer. 
     According to the fourth modification, therefore, it is possible to generate a 2D image  140 , which has not captured the first marker  56  and the second marker  58 , by performing on the tomographic images  100  the correcting process described in any of the above paragraphs (1) through (3). As a result, the doctor can accurately interpret and diagnose the breast  14  by observing the 2D image  140 . 
     As described above, the first marker  56  and the second marker  58  are of a circular shape, a ring shape, a crisscross shape, or a heart shape, which is easily distinguishable from a calcified region, mass, spicula, or the like. Therefore, it is desirable for the 2D image generator  96  to distinguish the first marker  56  and the second marker  58  from a calcified region, mass, or spicula according to a known shape recognition process, before performing the addition process described above in any of paragraphs (1) through (3). 
     As described above, the addition process discussed in any of the above paragraphs (1) through (3) is performed on the tomographic images  100  on which the reconstructing process has been carried out, thereby generating a 2D image  140 . According to the present embodiment, the 2D image generator  96  may generate the 2D image  140  by performing an addition process on the radiographic images that are stored in the projected image memory  76 . 
     Fifth Modification 
     As shown in  FIG. 18 , the fifth modification differs from the second modification (see  FIGS. 10 through 13 ), in that two further first markers  56  are disposed on a proximal end portion of the compression plate  44 , such that the first markers  56  are provided on the four corners of the compression plate  44 . 
     According to the fifth modification, therefore, it is possible to accurately calculate the tilt of the compression plate  44  with respect to the placement surface  38  of the image capturing table  36  (a plane along the direction of the arrow X and the direction of the arrow Y), not only in a case where the compression plate  44  is tilted laterally along the direction of the arrow X, but also in a case where the compression plate  44  is tilted along the direction of the arrow Y. 
     Furthermore, since the first markers  56  are provided on the four corners of the compression plate  44 , it is possible to calculate the compressed thickness T at an arbitrary position in a two-dimensional plane along the direction of the arrow X and the direction of the arrow Y. Consequently, in a case where a normal image capturing process is carried out on the breast  14  after the tomosynthesis image capturing process, it is possible to accurately calculate a dose of radiation  40  to be applied in the normal image capturing process. 
     Sixth Modification 
     As shown in  FIG. 19 , the sixth modification differs from the fifth modification (see  FIG. 18 ), in that a further first marker  56  is disposed at a central position (the position on the vertical axis  32  and the central line  60 ) of the compression plate  44  proximate the side surface  54  thereof. Accordingly, the sixth modification is a combination of the arrangement of the fifth modification shown in  FIG. 18  and the arrangement of the third modification shown in  FIGS. 14 and 15 . 
     Therefore, the sixth modification offers the advantages of both the fifth modification and the third modification. More specifically, according to the sixth modification, it is possible to measure the tilt of the compression plate  44  with respect to the placement surface  38  of the image capturing table  36  (a plane along the direction of the arrow X and the direction of the arrow Y) as well as the distortion of the compression plate  44 . 
     According to the sixth embodiment, furthermore, since the compressed thickness T can be calculated at an arbitrary position in a two-dimensional plane along the direction of the arrow X and the direction of the arrow Y, in a case where a normal image capturing process is carried out on the breast  14  after the tomosynthesis image capturing process, it is possible to accurately calculate the dose of radiation  40  to be applied in the normal image capturing process. 
     The present invention is not limited to the embodiment described above, and changes may freely be made to the embodiment without departing from the scope of the invention.