Patent Publication Number: US-10779784-B2

Title: Mammary gland amount acquisition apparatus, method, and program

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. 2017-065097, filed on Mar. 29, 2017. Each of the above application(s) is hereby expressly incorporated by reference, in its entirety, into the present application. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a mammary gland amount acquisition apparatus, method, and a non-transitory computer readable recording medium storing program for acquiring the amount of mammary gland contained in the breast based on two radiographic images acquired by radiations transmitted through the breast that have different energy distributions. 
     2. Description of the Related Art 
     In the related art, a radiographic image capturing apparatus (called a mammography apparatus) for imaging the breast is used. In mammography, the breast is placed on the imaging table, and imaging is performed in a state in which the breast is compressed by a compression plate. The breast is mainly formed of mammary gland tissues and adipose tissues, and it is important for diagnosis to find a lesion hidden in the mammary gland tissue. Therefore, by generating an energy subtraction image in which the mammary gland tissue in the breast has been extracted, it is possible to find a lesion hidden in the mammary gland tissue. 
     Energy subtraction processing in mammography is performed as follows. That is, by using the fact that mammary gland and fat forming the breast have different radiation absorbances for radiations having different energies, radiations having different energies are emitted to the breast to obtain two radiographic images. Then, an energy subtraction image in which adipose tissue has been removed and mammary gland tissue has been extracted is acquired by multiplying the two radiographic images by a weighting coefficient and then performing subtraction therebetween. 
     Here, the proportion of mammary gland in the breast is called a mammary gland content rate. The mammary gland content rate is very useful for accurately ascertaining the characteristics of the breast, and this is medically indispensable information. For example, research suggests that there is a correlation between the mammary gland content rate and the risk of carcinogenesis. For this reason, various methods for calculating the mammary gland content rate have been proposed. For example, JP2010-253245A has proposed a method in which a fat image, which is an image having a pixel value in a case where all mammary gland tissues of the breast are replaced with adipose tissues, is generated from a breast image acquired by mammography and the mammary gland content rate is calculated based on the relationship between the breast image and the fat image. In a case where the mammary gland content rate can be calculated, the mammary gland amount can also be calculated by multiplying the thickness of the breast by the mammary gland content rate. 
     The attenuation of the radiation transmitted through the breast changes depending on the amount of mammary gland contained in the breast. Therefore, a method of calculating the mammary gland amount based on the attenuation amount of radiation transmitted through the breast in the case of performing pre-shot imaging, in which imaging is performed with a low dose of radiation, in order to perform the positioning of the subject or the like has been proposed (refer to JP2016-535622A). 
     SUMMARY OF THE INVENTION 
     However, in the method of calculating the mammary gland amount from only one radiographic image as disclosed in JP2010-253245A and the method of calculating the mammary gland amount based on the attenuation of radiation as disclosed in JP2016-535622A, there is an influence of errors due to deflection and inclination of the compression plate used at the time of imaging. For this reason, in the methods disclosed in JP2010-253245A and JP2016-535622A, the calculation accuracy of the mammary gland amount is low. 
     The invention has been made in view of the above circumstances, and it is an object of the invention to accurately calculate the mammary gland amount of the breast. 
     A mammary gland amount acquisition apparatus according to the invention comprises: an image acquisition unit that acquires two radiographic images acquired by radiations transmitted through a breast that have different energy distributions; and a mammary gland amount calculation unit that calculates an amount of mammary gland contained in the breast based on the two radiographic images. 
     In the mammary gland amount acquisition apparatus according to the invention, the mammary gland amount calculation unit may comprise: a subject information acquisition unit that acquires a temporary thickness and a temporary mammary gland amount of the breast; an absorption coefficient acquisition unit that acquires a fat absorption coefficient corresponding to the temporary thickness and the temporary mammary gland amount of the breast and a mammary gland absorption coefficient corresponding to the temporary thickness and the temporary mammary gland amount of the breast with reference to a relationship between a thickness of the breast and a fat absorption coefficient and a relationship between the thickness of the breast and a mammary gland absorption coefficient, which are calculated in advance according to fat forming the breast and mammary gland forming the breast, and the energy distributions; and a calculation unit that calculates a new thickness and a new mammary gland amount of the breast based on the two radiographic images, a tube voltage at a time of imaging, the fat absorption coefficient, and the mammary gland absorption coefficient. 
     The “fat absorption coefficient” is a radiation absorption coefficient of fat, and the “mammary gland absorption coefficient” is a radiation absorption coefficient of mammary gland. 
     In the mammary gland amount acquisition apparatus according to the invention, the absorption coefficient acquisition unit may acquire a new fat absorption coefficient and a new mammary gland absorption coefficient based on the new thickness and the new mammary gland amount of the breast, and the absorption coefficient acquisition unit and the calculation unit may calculate the thickness and the mammary gland amount of the breast by repeating i) calculation of a newer thickness and a newer mammary gland amount of the breast based on the new fat absorption coefficient and the new mammary gland absorption coefficient and ii) acquisition of a newer fat absorption coefficient and a newer mammary gland absorption coefficient based on the newer thickness and the newer mammary gland amount of the breast. 
     In the mammary gland amount acquisition apparatus according to the invention, the mammary gland amount calculation unit may calculate the mammary gland amount by performing registration between the two radiographic images. 
     In the mammary gland amount acquisition apparatus according to the invention, the mammary gland amount calculation unit may further calculate a mammary gland content rate. 
     The mammary gland amount acquisition apparatus according to the invention may further comprise a classification unit that classifies a state of the breast based on the mammary gland content rate. 
     The mammary gland amount acquisition apparatus according to the invention may further comprise an image processing unit that performs image processing according to the mammary gland amount on at least one of the two radiographic images or another radiographic image obtained from the two radiographic images. 
     “Another radiographic image obtained from two radiographic images” means one radiographic image obtained by performing calculation on the two radiographic images. For example, an energy subtraction image obtained by performing weighting and subtraction processing on the two radiographic images and an addition image obtained by performing weighting and addition processing on the two radiographic images can be mentioned. 
     The mammary gland amount acquisition apparatus according to the invention may further comprise an index generation unit that generates an index indicating a possibility of disease occurrence according to the mammary gland amount. 
     In the mammary gland amount acquisition apparatus according to the invention, an exposure dose to the breast at a time of acquiring a radiographic image having a higher energy distribution between the two radiographic images may be smaller than an exposure dose to the breast at a time of acquiring a radiographic image having a lower energy distribution. 
     In the mammary gland amount acquisition apparatus according to the invention, a radiographic image having a higher energy distribution between the two radiographic images may be a pre-shot image. 
     At the time of capturing a radiographic image, in order to perform the positioning of the subject or the like, pre-shot imaging may be performed in which imaging is performed with a low dose of radiation. The “pre-shot image” is an image acquired by such pre-shot imaging. In a case where a radiographic image having a high energy distribution is assumed to be a pre-shot image, a radiographic image having a low energy distribution may be acquired by main imaging performed after pre-shot imaging. 
     In the mammary gland amount acquisition apparatus according to the invention, a radiographic image having the high energy distribution between the two radiographic images may be one of a plurality of projection images acquired by tomosynthesis imaging of the breast. 
     In the mammary gland amount acquisition apparatus according to the invention, a radiographic image having the high energy distribution between the two radiographic images may be a composite image that is generated by combining a plurality of tomographic images which are reconstructed from a plurality of projection images acquired by tomosynthesis imaging of the breast. 
     A mammary gland amount acquisition method according to the invention comprises: acquiring two radiographic images acquired by radiations transmitted through a breast that have different energy distributions; and calculating an amount of mammary gland contained in the breast based on the two radiographic images. 
     In addition, a non-transitory computer readable recording medium storing a program causing a computer to execute the mammary gland amount acquisition method according to the invention may be provided. 
     Another mammary gland amount acquisition apparatus according to the invention comprises: a memory that stores commands to be executed by a computer; and a processor that is configured to execute the stored commands and that performs processing for acquiring two radiographic images acquired by radiations transmitted through a breast that have different energy distributions and calculating an amount of mammary gland contained in the breast based on the two radiographic images. 
     According to the invention, two radiographic images acquired by radiations transmitted through the breast that have different energy distributions are acquired, and the amount of mammary gland contained in the breast is calculated based on the two radiographic images. Therefore, in the invention, since two images having different energy distributions are used, an error from the actual mammary gland amount can be reduced compared with a case where only one radiographic image is used. As a result, it is possible to accurately calculate the mammary gland amount. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing the schematic configuration of a radiographic image capturing apparatus to which a mammary gland amount acquisition apparatus according to a first embodiment of the invention is applied. 
         FIG. 2  is a diagram of the radiographic image capturing apparatus as viewed from the direction of arrow A in  FIG. 1 . 
         FIG. 3  is a diagram showing the schematic configuration of the mammary gland amount acquisition apparatus according to the first embodiment. 
         FIG. 4  is a diagram illustrating the detection of a breast region. 
         FIG. 5  is a diagram showing the breast interposed between a compression plate and an imaging table. 
         FIG. 6  is a diagram showing the relationship between the breast thickness and an X-ray absorption coefficient calculated in advance according to the substance forming the breast (low energy). 
         FIG. 7  is a diagram showing the relationship between the breast thickness and an X-ray absorption coefficient calculated in advance according to the substance forming the breast (high energy). 
         FIG. 8  is a diagram showing a cross section of a compressed breast. 
         FIG. 9  is a diagram showing an examination result displayed on a display unit. 
         FIG. 10  is a flowchart showing the process performed in the first embodiment. 
         FIG. 11  is a flowchart showing the process performed in a second embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, embodiments of the invention will be described with reference to the accompanying diagrams.  FIG. 1  is a schematic configuration diagram of a radiographic image capturing apparatus to which a mammary gland amount acquisition apparatus according to a first embodiment of the invention is applied, and  FIG. 2  is a diagram of the radiographic image capturing apparatus as viewed from the direction of arrow A in  FIG. 1 . A radiographic image capturing apparatus  1  is a mammography apparatus for imaging a breast M that is a subject. In the present embodiment, similarly to a case of performing energy subtraction imaging of the breast M, radiations having different energy distributions are emitted to the breast M, which is a subject, to acquire a low voltage image based on low energy radiation and a high voltage image based on high energy radiation. As shown in  FIG. 1 , the radiographic image capturing apparatus  1  includes an imaging unit  10 , a computer  2  connected to the imaging unit  10 , and a display unit  3  and an input unit  4  connected to the computer  2 . 
     The imaging unit  10  includes an arm unit  12  connected to a base (not shown) by a rotary shaft  11 . An imaging table  13  is attached to one end portion of the arm unit  12 , and an irradiation unit  14  is attached to the other end portion so as to face the imaging table  13 . The arm unit  12  is configured to be able to rotate only the end portion to which the irradiation unit  14  is attached. Therefore, it is possible to rotate only the irradiation unit  14  with the imaging table  13  fixed. The rotation of the arm unit  12  is controlled by the computer  2 . 
     By rotating the arm unit  12  in this manner, the radiographic image capturing apparatus  1  according to the present embodiment can acquire a tomographic image of the breast M by performing tomosynthesis imaging. The tomosynthesis imaging is a method in which, in order to observe an affected part in more detail, a radiation source is moved to irradiate a subject with radiation from a plurality of radiation source positions to perform imaging and a tomographic image emphasizing a desired tomographic plane is generated from a plurality of projection images acquired as described above. In tomosynthesis imaging, a plurality of projection images are acquired by imaging the subject at a plurality of radiation source positions by moving the radiation source in parallel with a radiation detector or so as to draw a circular or elliptical arc according to the characteristics of the imaging apparatus or required tomographic images and the projection images are reconstructed using a reconstruction method, such as a shift addition method, a simple back projection method, or a filtered back projection method (FBP method), to generate a tomographic image. In the present embodiment, explanation will be given on the assumption that the radiographic image capturing apparatus  1  acquires a low voltage image based on low energy radiation and a high voltage image based on high energy radiation instead of tomosynthesis imaging. 
     A radiation detector  15 , such as a flat panel detector, is provided inside the imaging table  13 . In addition, a circuit board on which a charge amplifier for converting a charge signal read from the radiation detector  15  into a voltage signal, a correlated double sampling circuit for sampling a voltage signal output from the charge amplifier, an AD conversion unit for converting a voltage signal into a digital signal, and the like are provided is provided inside the imaging table  13 . 
     The radiation detector  15  can perform recording and reading of a radiographic image repeatedly. A so-called direct type radiation detector that generates an electric charge by direct reception of radiation may be used, or a so-called indirect type radiation detector that converts radiation into visible light and then converts the visible light into a charge signal may be used. As a method of reading a radiographic image signal, it is desirable to use a so-called TFT reading method in which a radiographic image signal is read by ON and OFF of a thin film transistor (TFT) switch or a so-called optical reading method in which a radiographic image signal is read by emission of reading light. However, other methods may also be used without being limited to the above methods. 
     An X-ray source  16 , which is a radiation source, is housed inside the irradiation unit  14 . The timing of emission of X-rays, which are radiations from the X-ray source  16 , and X-ray generation conditions in the X-ray source  16 , that is, imaging conditions such as a tube voltage and irradiation time, are controlled by the computer  2 . 
     A compression plate  17  disposed above the imaging table  13  in order to compress the breast M, a support unit  18  for supporting the compression plate  17 , and a moving mechanism  19  for moving the support unit  18  in the vertical direction in  FIGS. 1 and 2  are provided at the arm unit  12 . Information of the distance between the compression plate  17  and the imaging table  13 , that is, information of the height of the compression plate  17 , is input to the computer  2 . 
     The display unit  3  is a display device, such as a cathode ray tube (CRT) or a liquid crystal monitor, and displays a subtraction image obtained as will be described later, a message required for the operation, and the like. The display unit  3  may include a speaker to output sound. 
     The input unit  4  is a keyboard, a mouse, or a touch panel type input device, and receives an operation on the radiographic image capturing apparatus  1  by the operator. In addition, the input unit  4  receives an input of various kinds of information, such as imaging conditions, and an instruction to modify the information, which are required to perform energy subtraction imaging. In the present embodiment, each unit of the radiographic image capturing apparatus  1  operates according to the information input from the input unit  4  by the operator. 
     A mammary gland amount acquisition program is installed on the computer  2 . In the present embodiment, the computer may be a workstation or a personal computer that is directly operated by the operator, or may be a server computer connected to these through a network. The mammary gland amount acquisition program is distributed by being recorded on a recording medium, such as a digital versatile disc (DVD) and a compact disc read only memory (CD-ROM), and is installed on the computer from the recording medium. Alternatively, the mammary gland amount acquisition program is stored in a storage device of a server computer connected to the network or in a network storage so as to be accessible from the outside, and is downloaded and installed on the computer as necessary. 
       FIG. 3  is a diagram showing the schematic configuration of a mammary gland amount acquisition apparatus realized by installing the mammary gland amount acquisition program on the computer  2 . As shown in  FIG. 3 , the mammary gland amount acquisition apparatus includes a central processing unit (CPU)  21 , a memory  22 , and a storage  23  as the configuration of a standard computer. 
     The storage  23  is a storage device, such as a hard disk or a solid state drive (SSD), and stores various kinds of information including a program for driving each unit of the radiographic image capturing apparatus  1  and a mammary gland amount acquisition program. A low voltage image and a high voltage image acquired by imaging are also stored. Various tables to be described later are also stored in the storage  23 . 
     The memory  22  temporarily stores programs and the like stored in the storage  23  so that the CPU  21  execute various kinds of processing. As processing to be executed by the CPU  21 , the mammary gland amount acquisition program defines: image acquisition processing for acquiring two radiographic images of a low voltage image GL and a high voltage image GH by causing the radiographic image capturing apparatus  1  to perform imaging; mammary gland amount calculation processing for calculating the amount of mammary gland contained in the breast M based on the low voltage image GL and the high voltage image GH; image processing performed on at least one of the low voltage image GL or the high voltage image GH according to the mammary gland amount; classification processing for classifying the state of the breast M based on the mammary gland content rate; index generation processing for generating an index indicating the possibility of disease occurrence according to the mammary gland amount; and display control processing for displaying an image subjected to the image processing, a classification result, an index, and the like on the display unit  3 . 
     In addition, as processing to be executed by the CPU  21 , the mammary gland amount acquisition program defines: subject information acquisition processing for acquiring the temporary thickness and the temporary mammary gland amount of the breast M; absorption coefficient acquisition processing for acquiring a fat absorption coefficient corresponding to the temporary thickness and the temporary mammary gland amount of the breast M and a mammary gland absorption coefficient corresponding to the temporary thickness and the temporary mammary gland amount of the breast M; and calculation processing for calculating the new thickness and the new mammary gland amount of the breast M based on the low voltage image, the high voltage image, the tube voltage at the time of imaging, the fat absorption coefficient, and the mammary gland absorption coefficient. The absorption coefficient acquisition processing is a process of acquiring the fat absorption coefficient corresponding to the temporary thickness and the temporary mammary gland amount of the breast M and the mammary gland absorption coefficient corresponding to the temporary thickness and the temporary mammary gland amount of the breast M with reference to the relationship between the thickness of the breast M and the fat absorption coefficient and the relationship between the thickness of the breast M and the mammary gland absorption coefficient, which are calculated in advance according to the fat forming the breast M and the mammary gland forming the breast M, and the energy distribution. 
     The CPU  21  executes these processes according to the mammary gland amount acquisition program, so that the computer  2  functions as an image acquisition unit  31 , a mammary gland amount calculation unit  32 , an image processing unit  33 , a classification unit  34 , an index generation unit  35 , a display control unit  36 , a subject information acquisition unit  37 , an absorption coefficient acquisition unit  38 , and a calculation unit  39 . The computer  2  may include a processor or a processing circuit that performs image acquisition processing, mammary gland amount calculation processing, image processing, classification processing, index generation processing, display control processing, and subject information acquisition processing, absorption coefficient acquisition processing, and calculation processing in the mammary gland amount calculation processing. 
     The image acquisition unit  31  controls the irradiation unit  14  to emit two types of radiations having different energy distributions, thereby imaging the breast M in the same manner as in a so-called two-shot method in energy subtraction imaging. Specifically, low voltage imaging using X-rays of relatively low energy and high voltage imaging using X-rays of relatively high energy are performed. In the low voltage imaging, the X-ray source  16  is driven under the imaging conditions in which X-rays of relatively low energy are emitted (for example, a tube voltage of the X-ray source  16  is set to 26 kV) so that the X-rays are emitted to the breast M, and X-rays transmitted through the breast M are detected by the radiation detector  15 , thereby acquiring the low voltage image GL. In the high voltage imaging, the X-ray source  16  is driven under the imaging conditions in which X-rays of relatively high energy are emitted (for example, a tube voltage of the X-ray source  16  is set to 45 kV) and the exposure dose to the breast M is lower than that in the low voltage imaging so that the X-rays are emitted to the breast M, and X-rays transmitted through the breast M are detected by the radiation detector  15 , thereby acquiring the high voltage image GH. Alternatively, the low voltage image GL and the high voltage image GH may be acquired by using a one-shot method in which two radiation detectors  15  are disposed so as to overlap each other inside the imaging table  13  and X-rays transmitted through the breast M are simultaneously emitted to the two radiation detectors  15  overlapping each other so that X-rays having different energy distributions are emitted to the two radiation detectors  15 . 
     The subject information acquisition unit  37  of the mammary gland amount calculation unit  32  acquires the temporary thickness and the temporary mammary gland amount of the breast M. For the temporary thickness of the breast M, in the present embodiment, the temporary thickness of the breast M is acquired based on at least one of the low voltage image GL or the high voltage image GH. In the present embodiment, the temporary thickness of the breast M is acquired based on the low voltage image GL. The subject information acquisition unit  37  detects a region of the breast M from the low voltage image GL first.  FIG. 4  is a diagram illustrating the detection of the region of the breast M. As shown in  FIG. 4 , the low voltage image GL includes a region A 1  of the breast M and a blank region A 2  which is a region where X-rays are directly emitted to the radiation detector  15 . Here, the blank region A 2  has a higher density than the region A 1  of the breast. Therefore, by performing threshold processing on the low voltage image GL, it is possible to detect the region A 1  of the breast M. 
     As shown in  FIG. 5 , the breast M is interposed between the imaging table  13  and the compression plate  17  at the time of imaging. Therefore, in the vicinity of a boundary between the region A 1  of the breast and the blank region A 2  (hereinafter, the boundary is referred to as a skin line) in the image acquired by imaging, the breast M is in contact with neither the imaging table  13  nor the compression plate  17 . The subject information acquisition unit  37  acquires information of the height of the compression plate  17  from the imaging unit  10 . Here, it is assumed that the cross-sectional shape of the breast M in the vicinity of the skin line of the breast M is semicircular. Assuming that the height of the compression plate  17  is h, in a region exceeding h/2 from the skin line in the region A 1  of the breast M acquired from the low voltage image GL, the height h of the compression plate  17  is acquired as the thickness of the breast M. A position at the distance h/2 from the skin line is shown by a broken line. On the other hand, in a region from the skin line to the distance h/2, the temporary thickness of the breast M is acquired by calculating a thickness R according to the following Equation (1) with the distance from the skin line being x. 
     
       
         
           
             
               
                 
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     Next, acquisition of the temporary mammary gland amount will be described. In the present embodiment, the subject information acquisition unit  37  calculates the mammary gland content rate from the low voltage image GL using the method disclosed in JP2010-253245A, for example. In the method disclosed in JP2010-253245A, the temporary mammary gland content rate is calculated from the low voltage image GL as follows. First, in the low voltage image GL, a fat image that is an image having a pixel value in a case where all mammary gland tissues of the breast M are replaced with adipose tissues is generated. Then, based on the low voltage image GL, the fat image, the pixel value of the blank region A 2  in the low voltage image GL, and a ratio between the average attenuation coefficient of the mammary gland and the average attenuation coefficient of fat stored in the storage  23 , the mammary gland content rate is calculated for each pixel of the low voltage image GL. Then, the subject information acquisition unit  37  calculates the temporary mammary gland amount by multiplying the thickness of the breast M by the mammary gland content rate. The mammary gland content rate is not limited to that calculated as described above. For example, the mammary gland content rate may be set to a predetermined value (for example, 50%). 
     The absorption coefficient acquisition unit  38  acquires an absorption coefficient corresponding to the subject information with reference to the relationship between the thickness of the breast M and the X-ray absorption coefficient, which is calculated in advance according to the energy distribution and the substance forming the breast M. In the present embodiment, the relationship between the thickness of the breast M and the X-ray absorption coefficient, which is calculated in advance according to the energy distribution and the substance forming the breast M, is stored in the storage  23 .  FIGS. 6 and 7  are diagrams showing the relationship between the thickness of the breast M and the X-ray absorption coefficient, which is calculated in advance according to the substance forming the breast M.  FIG. 6  shows an absorption coefficient with respect to low energy X-rays as the energy distribution, and  FIG. 7  shows an absorption coefficient with respect to high energy X-rays as the energy distribution. As shown in  FIGS. 6 and 7 , the relationship between the thickness of the breast M and the absorption coefficient is stored in the storage  23  for each of fat and mammary gland forming the breast M. Here, the absorption coefficient of the substance is determined depending on the energy of X-rays. In a process in which X-rays are transmitted through the subject, the low energy component of the X-rays is absorbed by the subject. As a result, beam hardening that causes increasing ratio of high energy X-rays occurs. Therefore, as shown in  FIGS. 6 and 7 , the absorption coefficient decreases as the thickness of the breast M increases. The beam hardening differs depending on the composition of the subject. In a case where the subject is the breast M, as shown in  FIGS. 6 and 7 , the absorption coefficient decreases as the mammary gland content rate increases. In addition, the absorption coefficient decreases as the energy of X-rays increases. 
     Based on the relationship shown in  FIGS. 6 and 7  and the mammary gland content rate used to calculate the temporary thickness and the temporary mammary gland amount of the breast M acquired by the subject information acquisition unit  37 , the absorption coefficient acquisition unit  38  acquires the absorption coefficients of fat and mammary gland corresponding to the energy of X-rays and the temporary thickness of the breast M and the mammary gland content rate. Specifically, a mammary gland absorption coefficient μ g   L  and a fat absorption coefficient μ a   L  for low energy X-rays and a mammary gland absorption coefficient μ g   H  and a fat absorption coefficient μ a   H  for high energy X-rays are acquired. 
       FIGS. 6 and 7  show the relationship between the thickness of the breast M and the absorption coefficient in a case where the mammary gland content rate is 1%, 50%, and 99%. However, in a case where the mammary gland content rate is an intermediate value between these values, the absorption coefficient may be calculated by interpolation calculation. 
     The calculation unit  39  calculates the new thickness and the new mammary gland amount of the breast M based on the low voltage image GL, the high voltage image GH, the tube voltage at the time of imaging, the fat absorption coefficient, and the mammary gland absorption coefficient. 
       FIG. 8  is a conceptual diagram illustrating the processing performed by the calculation unit.  FIG. 8  is a conceptual diagram showing a cross section of the breast M in a state in which the breast M is compressed by the compression plate  17 . In addition,  FIG. 8  schematically shows a state in which fat Ma and mammary gland Mg are contained in the breast M. R in  FIG. 8  indicates the thickness of the breast M. In order to accurately perform the processing shown below, the calculation unit  39  performs registration between the low voltage image GL and the high voltage image GH so that the corresponding pixel positions of the low voltage image GL and the high voltage image GH match each other. As shown in  FIG. 8 , in the breast M containing the fat Ma and the mammary gland Mg, the dose reaching the radiation detector  15  is expressed by the following Equation (2).
 
 N≅ϕ   0 ( E   0 ) e   −μ     g     (E     0     )r     g     −μ     0     (E     0     )r     a     (2)
 
     Here, N is a dose at the time of acquiring a radiographic image, ϕ 0  is a function indicating the dose and the sensitivity of a radiation detector, E 0  is a tube voltage of the X-ray source  16 , r a  is a fat thickness, r g  is a mammary gland thickness, μ a  is an X-ray absorption coefficient of fat, and μ g  is an X-ray absorption coefficient of mammary gland. 
     Assuming that the tube voltages of the X-ray source  16  at the time of acquiring the low voltage image GL and the high voltage image GH are E L  and E H  and the doses reaching the radiation detector  15  at the time of acquiring the low voltage image GL and the high voltage image GH are N L  and N H , the doses N L  and N H  are calculated according to Equation (2) for the low voltage image GL and the high voltage image GH, respectively. By log converting the result, the following Equations (3) and (4) are obtained. C 1  and C 2  are constants.
 
LOG( N   H )=−μ g   H ( E   H ) r   g −μ u   H ( E   H ) r   a   +C   1   (3)
 
LOG( N   L )=−μ g   L ( E   L ) r   g −μ a   L ( E   L ) r   a   +C   2   (4)
 
     Here, N L  is a dose at the time of acquiring the low voltage image GL, N H  is a dose at the time of acquiring the high voltage image GH, E L  is a tube voltage of the X-ray source  16  at the time of low voltage imaging, E H  is a tube voltage of the X-ray source  16  at the time of high voltage imaging, μ a   L  is a fat absorption coefficient for low energy X-rays, μ g   L  is a mammary gland absorption coefficient for low energy X-rays, μ a   H  is a fat absorption coefficient for high energy X-rays, and μ g   H  is a mammary gland absorption coefficient for high energy X-rays. 
     In this case, since the thickness R of the breast M is R=r a +r g , the thickness R is calculated according to the following Equation (5). C 1 , C 2 , and C 3  are constants determined according to the imaging conditions and the sensitivity of the radiation detector  15 . 
     
       
         
           
             
               
                 
                   
                       
                   
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     In the present embodiment, the mammary gland absorption coefficient μ g   L  and the fat absorption coefficient μ a   L  for low energy X-rays and the mammary gland absorption coefficient μ g   H  and the fat absorption coefficient μ a   H  for high energy X-rays are acquired by the absorption coefficient acquisition unit  38 . Based on the signal values at the respective pixel positions of the low voltage image GL and the high voltage image GH, the calculation unit  39  acquires the dose N L  at the time of acquiring the low voltage image GL and the dose N H  at the time of acquiring the high voltage image GH. The dose is acquired for each pixel position. Then, the calculation unit  39  substitutes the acquired absorption coefficients and doses into Equations (3) and (4). Then, by solving Equations (3) and (4) for r a  and r g , the thickness r a  of fat and the thickness r g  of mammary gland are calculated for each pixel position. Then, by substituting the calculated thickness r a  of fat and the calculated thickness r g  of mammary gland into Equation (5), the thickness R of the breast M at each pixel position is calculated. The calculated thickness R becomes the new thickness of the breast M. 
     Here, since the thickness R of the breast M is R=r a +r g , r g =R−r a . Therefore, using the calculated thickness R of the breast M, the thickness r g  of the mammary gland can be calculated according to Equation (4) to the following Equation (6). The thickness r g  of the mammary gland becomes the new mammary gland amount. For this reason, r g  may be used as a reference numeral of the mammary gland amount. Accordingly, the mammary gland content rate can be calculated by r g /R. 
     
       
         
           
             
               
                 
                   
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     As described above, the calculation unit  39  calculates the new thickness R, the new mammary gland amount, and the mammary gland content rate of the breast M. The calculation unit  39  calculates the volume of the breast M from the thickness R and the area of the breast M. The area of the breast M is a range up to the large pectoral muscle of the breast M included in the low voltage image GL or the high voltage image GH. The calculation unit  39  calculates the volume of the mammary gland from the mammary gland amount and the area of the breast M. The calculation unit  39  calculates the mammary gland content rate of the entire breast M by dividing the volume of the mammary gland by the volume of the breast M. By calculating the average value of the mammary gland content rate of each pixel, the mammary gland content rate of the entire breast M may be calculated. 
     The image processing unit  33  performs image processing on the low voltage image GL or the high voltage image GH based on the mammary gland content rate of the entire breast M calculated by the mammary gland amount calculation unit  32 . In the present embodiment, contrast enhancement processing according to the mammary gland content rate is performed on the low voltage image GL. Here, as the mammary gland content rate increases, a lesion structure is hidden in the mammary gland in the radiographic image. For this reason, contrast enhancement processing is performed on the low voltage image GL so that the contrast increases as the mammary gland content rate increases. In addition to the contrast enhancement processing, the image processing unit  33  may perform density correction processing, frequency processing, and the like on the low voltage image GL. 
     The classification unit  34  classifies the state of the breast M based on the mammary gland content rate of the entire breast M calculated by the mammary gland amount calculation unit  32 . Specifically, the state of the breast M is classified into “fatty” in a case where the mammary gland content rate is 0% or more and less than 15%, “mammary gland scattered” in a case where the mammary gland content rate is 15% or more and less than 35%, “uneven high density” in a case where the mammary gland content rate is 35% or more and less than 60%, and “high density” in a case where the mammary gland content rate is 60% or more. 
     The index generation unit  35  generates an index indicating the possibility of disease occurrence according to the mammary gland amount. In the present embodiment, the degree of risk of carcinogenesis according to the mammary gland content rate of the entire breast M calculated by the mammary gland amount calculation unit  32  is generated as an index. Specifically, the index generation unit  35  generates an index of “no risk” in a case where the mammary gland content rate is 0% or more and less than 15%, an index of “low risk” in a case where the mammary gland content rate is 15% or more and less than 35%, “medium risk” in a case where the mammary gland content rate is 35% or more and less than 60%, and “high risk” in a case where the mammary gland content rate is 60% or more. 
     The display control unit  36  displays the low voltage image GL subjected to the image processing, the classification result of the classification unit  34 , and the index generated by the index generation unit  35  on the display unit  3  as the examination result.  FIG. 9  is a diagram showing the examination result displayed on the display unit  3 . As shown in  FIG. 9 , the low voltage image GL subjected to the image processing, a classification result  41 , and an index  42  are included in the display unit  3 . In  FIG. 9 , the classification result is “high density”, and the index is “high risk”. 
     Next, the process performed in the first embodiment will be described.  FIG. 10  is a flowchart showing the process performed in the first embodiment. In a case where the input unit  4  receives an operator&#39;s instruction to start the process, the breast M is imaged so that the image acquisition unit  31  acquires the low voltage image GL and the high voltage image GH (image acquisition: step ST 1 ). Then, the subject information acquisition unit  37  of the mammary gland amount calculation unit  32  acquires the temporary thickness and the temporary mammary gland amount of the breast M based on the low voltage image GL (step ST 2 ). Then, the absorption coefficient acquisition unit  38  acquires a fat absorption coefficient and a mammary gland absorption coefficient with reference to the relationship between the thickness of the breast M and the X-ray absorption coefficient, which is calculated in advance according to the energy distribution and the substance forming the breast M (absorption coefficient acquisition: step ST 3 ). Then, based on the low voltage image GL, the high voltage image GH, the tube voltage at the time of imaging, the fat absorption coefficient, and the mammary gland absorption coefficient, the calculation unit  39  calculates the new thickness R and the new mammary gland amount of the breast M, and calculates the mammary gland content rate (mammary gland amount calculation; step ST 4 ). Then, the image processing unit  33  performs image processing on the low voltage image GL according to the mammary gland amount (step ST 5 ), and the classification unit  34  classifies the state of the breast M based on the mammary gland content rate of the entire breast M calculated by the mammary gland amount calculation unit  32  (step ST 6 ). Then, the index generation unit  35  generates an index indicating the possibility of disease occurrence according to the mammary gland amount (step ST 7 ). Then, the display control unit  36  displays the low voltage image GL subjected to the image processing, the classification result of the classification unit  34 , and the index generated by the index generation unit  35  on the display unit  3  as the examination result (step ST 8 ), and the process is ended. 
     As described above, in the first embodiment, since the amount of mammary gland contained in the breast is calculated based on the low voltage image GL and the high voltage image GH, an error from the actual value of the mammary gland amount can be reduced compared with a case where only one radiographic image is used. As a result, it is possible to accurately calculate the mammary gland amount. 
     In addition, since the registration between the low voltage image GL and the high voltage image GH is performed by the calculation unit  39 , it is possible to calculate the mammary gland amount more accurately. 
     Here, as the mammary gland amount increases, a lesion is hidden in the mammary gland in the radiographic image. For this reason, by performing image processing according to the mammary gland amount, it is possible to increase the contrast or the like as the mammary gland amount increases. Therefore, a lesion can be easily found in the radiographic image regardless of the mammary gland amount. 
     The low voltage image GL has a higher contrast than the high voltage image GH. Therefore, based on the low voltage image GL, it is possible to accurately acquire the thickness and the mammary gland amount of the breast M. 
     In the first embodiment described above, in order to acquire the more accurate thickness R and mammary gland amount of the breast M, which are suitable for the subject, repetitive calculation may be performed. Hereinafter, this will be described as a second embodiment.  FIG. 11  is a flowchart of repetitive calculation in the second embodiment. 
     In the second embodiment, as in the first embodiment described above, the subject information acquisition unit  37  acquires the temporary thickness and the temporary mammary gland amount of the breast M (step ST 11 ), and the absorption coefficient acquisition unit  38  acquires a fat absorption coefficient and a mammary gland absorption coefficient with these as initial values (absorption coefficient acquisition using initial values: step ST 12 ). Then, the calculation unit  39  calculates the thickness R and the mammary gland amount r g  of the breast M as in the first embodiment (step ST 13 ). In the second embodiment, using the thickness R and the mammary gland amount r g  calculated by the calculation unit  39 , the fat absorption coefficient and the mammary gland absorption coefficient are acquired with reference to the relationship shown in  FIGS. 6 and 7  (absorption coefficient acquisition: step ST 14 ). Then, the calculation unit  39  determines whether or not a difference value between each of the fat absorption coefficient and the mammary gland absorption coefficient acquired in step ST 14  and each of the fat absorption coefficient and the mammary gland absorption coefficient acquired in the previous processing is less than a threshold value Th 1  (step ST 15 ). In the first processing, the difference value is a difference between each of the fat absorption coefficient and the mammary gland absorption coefficient acquired in step ST 14  and each of the fat absorption coefficient and the mammary gland absorption coefficient acquired using the thickness and the mammary gland amount of the breast M acquired by the subject information acquisition unit  37 . 
     Here, the fat absorption coefficient μ a   L  for low energy X-rays, the mammary gland absorption coefficient μ g   L  for low energy X-rays, the fat absorption coefficient μ a   H  for high energy X-rays, and the mammary gland absorption coefficient μ g   H  for high energy X-rays are acquired as fat absorption coefficients and mammary gland absorption coefficients. Therefore, in the processing of step ST 15 , it is determined whether or not difference values for all the four absorption coefficients are less than the threshold value Th 1 . The threshold value Th 1  may be set for each absorption coefficient. Here, it is assumed that step ST 15  is affirmed in a case where all the four difference values are less than the threshold value Th 1 . 
     In a case where step ST 15  is denied, the calculation unit  39  returns to step ST 13  to calculate the new thickness R and the new mammary gland amount r g  of the breast M and acquire a new fat absorption coefficient and a new mammary gland absorption coefficient using the thickness R and the mammary gland amount r g  of the breast M calculated in step ST 13  and the fat absorption coefficient and the mammary gland absorption coefficient acquired in step ST 14 . Then, the processing of steps ST 13  and ST 14  is repeated until step ST 15  is affirmed. In a case where step ST 15  is affirmed, the mammary gland amount r g  calculated at that point in time is determined as a final mammary gland amount (step ST 16 ). After step ST 16 , the processing from step ST 5  in the first embodiment is performed. 
     As described above, in the second embodiment, since the thickness R and the mammary gland amount r g  of the breast M are calculated by repetitive calculation and the fat absorption coefficient and the mammary gland absorption coefficient are acquired, it is possible to reduce errors between the calculated values and the actual values. As a result, it is possible to calculate the mammary gland amount more accurately. 
     In the second embodiment described above, every time the processing of steps ST 13  and ST 14  is repeated, it is determined whether or not the difference value between the absorption coefficients is less than the threshold value Th 1  in step ST 15 . However, the processing of step ST 15  may be performed every time the processing of steps ST 13  and ST 14  is repeated a predetermined first number of times. Alternatively, the processing of steps ST 13  and ST 14  may be repeated without performing the processing of step ST 15  until a predetermined second number of times from the start of the process, and the processing of step ST 15  may be performed every time the processing of steps ST 13  and ST 14  may be repeated after repeating the second number of times. Also in this case, after repeating the processing of steps ST 13  and ST 14  a second number of times, the processing of step ST 15  may be performed every time the processing of steps ST 13  and ST 14  is repeated a predetermined first number of times. 
     In each of the embodiments described above, the low voltage image GL and the high voltage image GH are acquired. However, in order to perform the positioning of the subject or the like, pre-shot imaging may be performed in which imaging is performed with a low dose of radiation. In this case, the pre-shot imaging may be performed by using X-rays having energy higher than in the subsequent imaging (assumed to be main imaging), and a pre-shot image acquired by the pre-shot imaging may be used as the high voltage image GH. In this case, the radiographic image acquired by the main imaging may be used as the low voltage image GL. 
     In each of the embodiments described above, in the case of performing tomosynthesis imaging of the breast M, X-rays having energy higher than in the case of acquiring the low voltage image GL may be used, and one projection image acquired by tomosynthesis imaging may be used as the high voltage image GH. In this case, it is preferable to use a projection image acquired at a radiation source position, which is closest to that in the case of acquiring the low voltage image GL, as the high voltage image GH. A composite image that is generated by generating a plurality of tomographic images by reconstructing the projection image acquired by the tomosynthesis imaging, adding the plurality of tomographic images at corresponding pixel positions, and performing enlargement and reduction so as to have the same size as the low voltage image GL may be used as the high voltage image GH. In this case, it is preferable to generate a composite image by geometrically converting the plurality of tomographic images so that the high voltage image GH is acquired at the same radiation source position as in the case of acquiring the low voltage image GL. 
     At the time of imaging the breast M, as shown in  FIG. 5 , the vicinity of the skin line is not in close contact with the imaging table  13 . For this reason, in a region between the broken line and the skin line in  FIG. 4 , the thickness R of the breast M is reduced. Therefore, in the low voltage image GL displayed on the display unit  3 , the density increases in the vicinity of the skin line of the breast M. For this reason, the density of the low voltage image GL may be corrected by the image processing unit  33  using the thickness R of the breast M calculated according to the above Equation (5). In this case, the image processing unit  33  compares the thickness R of the breast M calculated by the calculation unit  39  for each pixel of the low voltage image GL. In the second embodiment, the thickness R of the breast M is the final thickness R. Then, in a pixel having a smaller thickness than other pixels, correction is made so that the density of the pixel value is close to the densities of the other pixels. Therefore, in the corrected low voltage image GL, the density in the vicinity of the skin line can be made to be the same as the densities in the other regions. As a result, it is possible to generate the low voltage image GL with higher image quality. 
     In each of the embodiments described above, an energy subtraction image may be generated by performing energy subtraction processing on the low voltage image GL and the high voltage image GH using the calculated absorption coefficients. In this case, the weighting coefficients for the low voltage image GL and the high voltage image GH are calculated based on the absorption coefficients acquired by the absorption coefficient acquisition unit  38  or the absorption coefficients calculated by the calculation unit  39 . In the case of calculating a subtraction image Gs by subtracting the low voltage image GL from the high voltage image GH weighted by the weighting coefficient α according to the following Equation (7), the weighting coefficient α is calculated by the following Equation (8).
 
 Gs=α·GH−GL   (7)
 
α=(μ g   H −μ a   H )/(μ g   L −μ a   L )  (8)
 
The image processing unit  33  may perform image processing according to the mammary gland amount on the subtraction image Gs generated as described above, in the same manner as in the case of performing image processing on the low voltage image GL.
 
     Hereinafter, the effect of the present embodiment will be described. 
     By acquiring a new fat absorption coefficient and a new mammary gland absorption coefficient based on the calculated thickness and mammary gland amount of the breast and calculating the thickness and the mammary gland amount of the breast by repeating the calculation of a new thickness and a new mammary gland amount of the breast based on the new fat absorption coefficient and the new mammary gland absorption coefficient and the acquisition of a newer fat absorption coefficient and a newer mammary gland absorption coefficient based on the new thickness and the new mammary gland amount of the breast, it is possible to reduce the error between the value of the calculated mammary gland amount and the value of the actual mammary gland amount. As a result, it is possible to calculate the mammary gland amount more accurately. 
     By calculating the mammary gland amount by performing registration between the two radiographic images, it is possible to calculate the mammary gland amount more accurately. 
     Here, as the mammary gland amount increases, a lesion is hidden in the mammary gland in the radiographic image. For this reason, by performing image processing according to the mammary gland amount, it is possible to increase the contrast or the like as the mammary gland amount increases. Therefore, a lesion can be easily found in the radiographic image regardless of the mammary gland amount. 
     EXPLANATION OF REFERENCES 
       1 : radiographic image capturing apparatus 
       2 : computer 
       3 : display unit 
       4 : input unit 
       10 : imaging unit 
       11 : rotary shaft 
       12 : arm unit 
       13 : imaging table 
       14 : irradiation unit 
       15 : radiation detector 
       16 : X-ray source 
       17 : compression plate 
       18 : support unit 
       19 : moving mechanism 
       21 : CPU 
       22 : memory 
       23 : storage 
       31 : image acquisition unit 
       32 : mammary gland amount calculation unit 
       33 : image processing unit 
       34 : classification unit 
       35 : index generation unit 
       36 : display control unit 
       37 : subject information acquisition unit 
       38 : absorption coefficient acquisition unit 
       39 : calculation unit 
     A 1 : breast region 
     A 2 : blank region 
     M: breast 
     Ma: fat 
     Mg: mammary gland 
     R: height of compression plate 
     GL: low voltage image 
     GH: high voltage image