Patent Publication Number: US-2023153969-A1

Title: Image processing apparatus, image processing method, and image processing program

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2021-187371, filed Nov. 17, 2021, the disclosure of which is incorporated herein by reference in its entirety. 
     BACKGROUND 
     Technical Field 
     The present disclosure relates to an image processing apparatus, an image processing method, and a non-transitory storage medium storing an image processing program. 
     Related Art 
     A technology of performing contrast imaging of capturing a low-energy image and a high-energy image by irradiating a subject into which a contrast medium has been injected with radiation having different energies and generating a difference image indicating a difference between the high-energy image and the low-energy image to generate a radiation image in which the contrast medium is enhanced is known (see JPWO2020/059306A1, for example). 
     By the way, in a case in which imaging is performed without using a grid, there is a problem that an artifact component remains. 
     SUMMARY 
     The present disclosure has been made in consideration of the above circumstances, and is to provide an image processing apparatus, an image processing method, and a non-transitory storage medium storing an image processing program capable of obtaining a difference image in which a contrast medium is clearly reflected and an artifact component is suppressed. 
     In order to achieve the above object, a first aspect of the present disclosure relates to an image processing apparatus that processes a radiation image captured by a radiography apparatus including a radiation source and a radiation detector, the image processing apparatus including: at least one processor that is configured to: acquire a first low-energy image and a first high-energy image of a subject captured by the radiography apparatus in a state in which a grid for removing scattered rays is inserted between the radiation source and the radiation detector, and the radiation source is disposed at a first position at which an incidence direction of the radiation is a normal direction with respect to the grid; acquire a second low-energy image and a second high-energy image of the subject captured by the radiography apparatus in a state in which the grid retreats from between the radiation source and the radiation detector, and the radiation source is disposed at a second position different from the first position; estimate a thickness of the subject from the first low-energy image and the first high-energy image; and generate a difference image indicating a difference between the second high-energy image and the second low-energy image from the second low-energy image and the second high-energy image based on the estimated thickness, wherein the first and second low-energy images are captured by emitting radiation having first energy from the radiation source to the subject into which a contrast medium has been injected, and the first and second high-energy images are captured by emitting radiation having second energy higher than the first energy from the radiation source to the subject into which the contrast medium has been injected. 
     A second aspect of the present disclosure relates to the image processing apparatus according to the first aspect, in which the processor removes an artifact component from each of the second high-energy image and the second low-energy image, and generates the difference image from the second low-energy image and the second high-energy image from which the artifact components have been removed. 
     A third aspect of the present disclosure relates to the image processing apparatus according to the first aspect, in which the processor generates the difference image from the second low-energy image and the second high-energy image, and removes an artifact component from the difference image. 
     A fourth aspect of the present disclosure relates to the image processing apparatus according to any one of the first to third aspects, in which the processor estimates the thickness from a difference image indicating a difference between the first high-energy image and the first low-energy image. 
     A fifth aspect of the present disclosure relates to the image processing apparatus according to any one of the first to fourth aspects, in which the processor identifies a position of an object-of-interest dyed with the contrast medium in a depth direction from a difference image indicating a difference between the first high-energy image and the first low-energy image, and the difference image indicating the difference between the second high-energy image and the second low-energy image. 
     A sixth aspect of the present disclosure relates to the image processing apparatus according to any one of the first to fifth aspects, in which the artifact component is a component caused by oblique incidence of the radiation. 
     A seventh aspect of the present disclosure relates to the image processing apparatus according to any one of the first to fifth aspects, in which the artifact component is a scattered ray component caused by the scattered ray. 
     In addition, in order to achieve the above object, an eighth aspect of the present disclosure relates to an image processing method in which a computer executes image processing of a radiation image captured by a radiography apparatus including a radiation source and a radiation detector, the image processing method including: acquiring a first low-energy image and a first high-energy image of a subject captured by the radiography apparatus in a state in which a grid for removing scattered rays is inserted between the radiation source and the radiation detector, and the radiation source is disposed at a first position at which an incidence direction of the radiation is a normal direction with respect to the grid; acquiring a second low-energy image and a second high-energy image of the subject captured by the radiography apparatus in a state in which the grid retreats from between the radiation source and the radiation detector, and the radiation source is disposed at a second position different from the first position; estimating a thickness of the subject from the first low-energy image and the first high-energy image; and generating a difference image indicating a difference between the second high-energy image and the second low-energy image from the second low-energy image and the second high-energy image based on the estimated thickness, wherein the first and second low-energy images are captured by emitting radiation having first energy from the radiation source to the subject into which a contrast medium has been injected, and the first and second high-energy images are captured by emitting radiation having second energy higher than the first energy from the radiation source to the subject into which the contrast medium has been injected. 
     In addition, in order to achieve the above object, a ninth aspect of the present disclosure relates to a non-transitory storage medium storing an image processing program causing a computer to execute image processing of a radiation image captured by a radiography apparatus including a radiation source and a radiation detector, the image processing including: acquiring a first low-energy image and a first high-energy image of a subject captured by the radiography apparatus in a state in which a grid for removing scattered rays is inserted between the radiation source and the radiation detector, and the radiation source is disposed at a first position at which an incidence direction of the radiation is a normal direction with respect to the grid; acquiring a second low-energy image and a second high-energy image of the subject captured by the radiography apparatus in a state in which the grid retreats from between the radiation source and the radiation detector, and the radiation source is disposed at a second position different from the first position; estimating a thickness of the subject from the first low-energy image and the first high-energy image; and generating a difference image indicating a difference between the second high-energy image and the second low-energy image from the second low-energy image and the second high-energy image based on the estimated thickness, wherein the first and second low-energy images are captured by emitting radiation having first energy from the radiation source to the subject into which a contrast medium has been injected, and the first and second high-energy images are captured by emitting radiation having second energy higher than the first energy from the radiation source to the subject into which the contrast medium has been injected. 
     According to the present disclosure, it is possible to obtain the difference image in which the contrast medium is clearly reflected and the artifact component is suppressed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a configuration diagram schematically showing an example of an overall configuration of a radiography system according to an embodiment. 
         FIG.  2    is a side view showing an example of an appearance of a mammography apparatus according to the embodiment. 
         FIG.  3    is a diagram for describing movement of a grid according to the embodiment. 
         FIG.  4 A  is a diagram for describing a position of a radiation source in first contrast imaging. 
         FIG.  4 B  is a diagram for describing the position of the radiation source in second contrast imaging. 
         FIG.  5 A  is a diagram for describing a scattered ray component. 
         FIG.  5 B  is a diagram for describing an oblique incidence component. 
         FIG.  6    is a block diagram showing an example of configurations of the mammography apparatus and a console according to the embodiment. 
         FIG.  7    is a functional block diagram showing an example of a function of the console according to the embodiment. 
         FIG.  8    is a flowchart showing an example of a flow of contrast imaging by the radiography system according to the embodiment. 
         FIG.  9    is a flowchart showing an example of a flow of contrast imaging processing executed in the contrast imaging according to the embodiment. 
         FIG.  10    is a flowchart showing an example of a flow of difference image generation processing executed in the contrast imaging according to the embodiment. 
         FIG.  11    is a flowchart showing an example of a flow of the difference image generation processing executed in the contrast imaging according to Modification Example 1. 
         FIG.  12    is a diagram for describing tomosynthesis imaging according to Modification Example 2. 
         FIG.  13    is a flowchart showing an example of a flow of the contrast imaging by the radiography system according to Modification Example 2. 
         FIG.  14    is a flowchart showing an example of a flow of the contrast imaging processing executed in second contrast imaging according to Modification Example 2. 
         FIG.  15    is a flowchart showing an example of a flow of the difference image generation processing executed in the contrast imaging according to Modification Example 2. 
         FIG.  16    is a functional block diagram showing an example of the function of the console according to Modification Example 3. 
         FIG.  17    is a flowchart showing an example of a flow of the difference image generation processing executed in the contrast imaging according to Modification Example 3. 
     
    
    
     DETAILED DESCRIPTION 
     In the following, an embodiment of the present invention will be described in detail with reference to the drawings. It should be noted that the embodiment does not limit the present invention. 
     First, an example of an overall configuration of a radiography system according to the present embodiment will be described.  FIG.  1    shows a configuration diagram showing an example of an overall configuration of a radiography system  1  according to the present embodiment. As shown in  FIG.  1   , the radiography system  1  according to the present embodiment comprises a mammography apparatus  10  and a console  12 . The mammography apparatus  10  according to the present embodiment is an example of a radiography apparatus according to the present disclosure. In addition, the console  12  according to the present embodiment is an example of an image processing apparatus according to the present disclosure. 
     First, the mammography apparatus  10  according to the present embodiment will be described.  FIG.  2    shows a side view showing an example of an appearance of the mammography apparatus  10  according to the present embodiment. It should be noted that  FIG.  2    shows the example of the appearance of the mammography apparatus  10  as viewed from a right side of an examinee. 
     The mammography apparatus  10  according to the present embodiment is an apparatus that uses a breast of the examinee as a subject and captures a radiation image of the breast by irradiating the breast with radiation R (for example, X-rays). It should be noted that the mammography apparatus  10  may be an apparatus that images the breast of the examinee in a state in which the examinee is sitting on a chair (including a wheelchair) or the like (sitting state) in addition to a state in which the examinee is standing (standing state). 
     An operation unit  26  is provided as a plurality of switches on an imaging table  30  of the mammography apparatus  10 , for example. It should be noted that the operation unit  26  may be provided as a touch panel type switch, or may be provided as a foot switch operated by a user, such as a doctor or an engineer with a foot. 
     As shown in  FIG.  2   , the mammography apparatus  10  according to the present embodiment comprises a controller  20 , a storage unit  22 , and an interface (I/F) unit  24  inside the imaging table  30 , which will be described below in detail. 
     In addition, as shown in  FIG.  2   , a radiation detector  28  is disposed inside the imaging table  30 . In the mammography apparatus  10  according to the present embodiment, the user positions the breast of the examinee on an imaging surface  30 A of the imaging table  30  during the imaging. The radiation detector  28  detects the radiation R transmitted through the breast of the examinee and the imaging table  30 , generates a radiation image based on the detected radiation R, and outputs image data representing the generated radiation image. A type of the radiation detector  28  according to the present embodiment is not particularly limited. For example, a radiation detector of an indirect conversion method that converts the radiation R into light and converts the converted light into a charge may be used, and a radiation detector of a direct conversion method that directly converts the radiation R into a charge may be used. 
     In addition, a grid  27  is provided between a radiation source  37 R and the radiation detector  28  to remove scattered rays. As shown in  FIG.  3   , the grid  27  according to the present embodiment can be moved between a position (hereinafter, referred to as “insertion position”) at which the grid  27  is inserted between the radiation source  37 R and the radiation detector  28 , and a position (hereinafter, referred to as “retreat position”) at which the grid  27  retreats from between the radiation source  37 R and the radiation detector  28  by a grid movement unit  31  (see  FIG.  5   ). As an example, in the mammography apparatus  10  according to the present embodiment, as shown in  FIG.  3   , the grid  27  is slid in a direction of a compression unit  36  to be moved from the insertion position to the retreat position. 
     As shown in  FIG.  3   , at the insertion position, specifically, the grid  27  is disposed substantially parallel to the radiation detector  28  between the imaging surface  30 A of the imaging table  30  and the radiation detector  28 , and a normal line with respect to a detection surface  28 A of the radiation detector  28  and a normal line of the grid  27  are the same (see normal line CL in  FIGS.  4 A and  4 B ). At the insertion position, the radiation detector  28  is irradiated with the radiation R emitted from the radiation source  37 R via the grid  27 . On the other hand, at the retreat position, as shown in  FIG.  3   , the radiation detector  28  is irradiated with the radiation R emitted from the radiation source  37 R without passing through the grid  27 . 
     A radiation emitting unit  37  comprises the radiation source  37 R. As shown in  FIG.  2   , the radiation emitting unit  37  is provided in an arm part  32  together with the imaging table  30  and the compression unit  36 . As shown in  FIG.  2   , a face guard  38  is attachably and detachably provided at a position near the examinee on the arm part  32  below the radiation emitting unit  37 . The face guard  38  is a protective member for protecting the examinee from the radiation R emitted from the radiation source  37 R. 
     It should be noted that, as shown in  FIG.  2   , the mammography apparatus  10  according to the present embodiment comprises the arm part  32 , a base  34 , and a shaft part  35 . The arm part  32  is held by the base  34  to be movable in a vertical direction (Z-axis direction). In addition, the arm part  32  can be rotated with respect to the base  34  by the shaft part  35 . The shaft part  35  is fixed to the base  34 , and the shaft part  35  and the arm part  32  are rotated together. 
     A gear is provided for each of the shaft part  35  and the compression unit  36 . By switching between an engaged state and a non-engaged state of the gears, a state in which the compression unit  36  and the shaft part  35  are connected and are integrally rotated and a state in which the shaft part  35  is separated from the compression unit  36  and the imaging table  30  and is idled can be switched. It should be noted that, for switching between transmission and non-transmission of power of the shaft part  35 , various mechanical elements can be used in addition to the gear described above. 
     The arm part  32 , the imaging table  30 , and the compression unit  36  can be separately rotated relative to the base  34  with the shaft part  35  as a rotation axis. In the present embodiment, the base  34 , the arm part  32 , the imaging table  30 , and the compression unit  36  are each provided with an engaging part (not shown), and each of the arm part  32 , the imaging table  30 , and the compression unit  36  is connected to the base  34  by switching a state of the engaging part. One or two of the arm part  32 , the imaging table  30 , or the compression unit  36 , which are connected to the shaft part  35 , are integrally rotated around the shaft part  35 . 
     The compression unit  36  is provided with a compression plate driving unit  41  that moves the compression plate  40  in the vertical direction (Z-axis direction). The compression plate  40  according to the present embodiment has a function of compressing the breast of the examinee. A support part  46  of the compression plate  40  is attachably and detachably attached to the compression plate driving unit  41 , is moved in the vertical direction (Z-axis direction) by the compression plate driving unit  41 , and compresses the breast of the examinee with the imaging table  30 . 
     The mammography apparatus  10  according to the present embodiment has a function of performing general imaging and a function of performing so-called contrast imaging in which the imaging is performed in a state in which a contrast medium has been injected into the breast of the examinee. In the present embodiment, the “contrast imaging” refers to the imaging performed in a state in which the contrast medium is injected into the breast (subject 9) of the examinee, and the “general imaging” refers to imaging that is not the contrast imaging. 
     It should be noted that the mammography apparatus  10  according to the present embodiment has a contrast enhanced digital mammography (CEDM) function of performing the contrast imaging by energy subtraction imaging as the function of performing the contrast imaging. 
     In addition, the mammography apparatus  10  according to the present embodiment has a function of performing, as the contrast imaging, so-called stereo imaging in which two types of the contrast imaging of first contrast imaging and second contrast imaging in which a position of the radiation source  37 R varies are performed. Two types of the contrast imaging will be described with reference to  FIGS.  4 A and  4 B . 
     As shown in  FIG.  4 A , the first contrast imaging is the contrast imaging performed with the radiation source  37 R at a first position at which an incidence direction of the radiation R is a normal direction with respect to the grid  27  and the radiation detector  28 . That is, in the first contrast imaging, the normal line CL with respect to the grid  27  and the detection surface  28 A of the radiation detector  28  matches a radiation axis RC. In the first contrast imaging, a first low-energy image is captured by irradiating a breast W into which the contrast medium has been injected with the radiation R having first energy. In addition, in the first contrast imaging, a first high-energy image is captured by emitting the radiation R having second energy higher than the first energy. It should be noted that, in the present embodiment, the radiation image captured by emitting the radiation R having the first energy is referred to as a “low-energy image”, and the radiation image captured by emitting the radiation R having the second energy is referred to as a “high-energy image”. In addition, in a case in which the low-energy image, the high-energy image, and a difference image described below generically refer to a mammography image obtained by the mammography apparatus  10  without distinguishing the types thereof, the low-energy image, the high-energy image, and the difference image are simply referred to as a “radiation image” 
     For example, an iodine contrast medium with a k-absorption edge of 32 keV is generally used as the contrast medium for the contrast imaging. In the contrast imaging in this case, the low-energy image is captured by emitting the radiation R having the first energy lower than the k-absorption edge of the iodine contrast medium. In addition, the high-energy image is captured by emitting the radiation R having the second energy higher than the k-absorption edge of the iodine contrast medium. 
     The contrast medium and a body tissue, such as a mammary gland, have different absorption characteristics of the radiation. Therefore, in the high-energy image captured as described above, the body tissue, such as the mammary gland or fat, is reflected, and the contrast medium is clearly reflected. In addition, in the low-energy image, almost no contrast medium is reflected, and the body tissue, such as the mammary gland, is clearly reflected. Therefore, the difference image indicating a difference between the low-energy image and the high-energy image can be an image in which a mammary gland structure has been removed and the contrast medium is clearly reflected. 
     As shown in  FIG.  4 A , in a case of performing the first contrast imaging, the grid  27  is at the insertion position, and the grid  27  is inserted between the radiation source  37 R and the radiation detector  28 . In a case in which the grid  27  is at the insertion position, the detection surface  28 A of the radiation detector  28  is irradiated with the radiation R emitted from the radiation source  37 R and transmitted through the breast W via the grid  27 . Therefore, the radiation image obtained by the radiation detector  28  is an image in which a scattered ray component caused by the scattered ray and the component caused by the oblique incidence of the radiation are suppressed by the grid  27 . Therefore, the first low-energy image and the first high-energy image are images in which an artifact component including the scattered ray component and the component caused by the oblique incidence of the radiation are suppressed. 
     On the other hand, as shown in  FIG.  4 B , the second contrast imaging is the contrast imaging with the radiation source  37 R at a position different from the first position. In other words, the second contrast imaging is the contrast imaging performed by causing the radiation R emitted from the radiation source  37 R to be obliquely incident on the detection surface  28 A of the radiation detector  28 . That is, in the second contrast imaging, the normal line CL of the detection surface  28 A of the radiation detector  28  and the radiation axis RC do not match. In the second contrast imaging, a second low-energy image is captured by irradiating the breast W into which the contrast medium has been injected with the radiation R having the first energy. In addition, in the second contrast imaging, a second high-energy image is captured by emitting the radiation R having the second energy. 
     As shown in  FIG.  4 B , in a case of performing the second contrast imaging, the grid  27  is at the retreat position, and the grid  27  retreats from between the radiation source  37 R and the radiation detector  28 . In a case in which the grid  27  is at the retreat position, the detection surface  28 A of the radiation detector  28  is irradiated with the radiation R emitted from the radiation source  37 R and transmitted through the breast W without passing through the grid  27 . Therefore, the radiation image obtained by the radiation detector  28  is an image in which the scattered ray component caused by the scattered rays and the oblique incidence component caused by the oblique incidence of the radiation are included. Therefore, the second low-energy image and the second high-energy image are images including the scattered ray component and the oblique incidence component, which are the artifact component. Such an artifact component appears as an image that appears white in the radiation image. 
     Such an artifact component will be described. As described above, the artifact component mainly includes the scattered ray component and the oblique incidence component caused by the oblique incidence of the radiation. 
     First, with reference to  FIG.  5 A , the scattered ray component caused by the scattered rays of the radiation R will be described.  FIG.  5 A  shows a schematic diagram for describing an example of an artifact due to the scattered ray component of the radiation R. An image of an inside region of the breast W has a large dose because the scattered rays generated by the breast W overlap each other. Therefore, the image of the inside region of the breast W has a large pixel value and the image is black. On the other hand, in the image of an end part region of the breast W, a part of the scattered rays escapes to an outside of the breast W, so that the dose is reduced. Therefore, a cool image on an end part side of the breast W, in other words, on a skin line side, has a small pixel value and a small image. In other words, in the vicinity of the skin line of the breast W, the pixel value is smaller than in the inside region. In a case of the breast W, since the scattered rays are less likely to escape to the outside on a chest wall side, the dose is increased and the pixel value is increased, so the image tends to be black. On the other hand, since the scattered rays are more likely to escape to the outside on a nipple side, the dose is decreased and the pixel value is decreased, so that the image tends to be white. Therefore, the scattered ray component of the radiation R is the artifact component. It should be noted that an amount of the generated scattered rays are larger as a thickness h of the breast W is larger. 
     Next, with reference to  FIG.  5 B , the oblique incidence component caused by the oblique incidence of the radiation R on the breast W will be described.  FIG.  5 B  shows a schematic diagram for describing an example of the artifact due to the oblique incidence component of the radiation R. In a case in which radiation R1 having an incidence angle of 0 degrees is incident on the breast W having the thickness h from the radiation source  37 R, a length of a transmission path through which the radiation R1 is transmitted through the breast W is h. On the other hand, in a case in which radiation R2 having an incidence angle θ is incident from the radiation source  37 R, the length of the transmission path is h/cosθ. Therefore, the transmission path in a case in which the radiation R2 is incident is longer than the transmission path in a case in which the radiation R1 is incident by a length x represented by Expression ( 1 ). 
     
       
         
           
             x =  
             
               
                 1 
                 / 
                 ​ 
                 cos 
                 θ 
                 − 
                 1 
               
             
             × 
             h 
           
         
       
     
     As the transmission path is longer, the dose of the radiation R reaching the radiation detector  28  is smaller. Therefore, the radiation R2 that is transmitted through the breast W and reaches the radiation detector  28  has a smaller dose than the radiation R1. Due to the smaller dose, the image generated by the radiation detector  28  in response to the radiation R2 will be whiter than the image generated by the radiation detector  28  in response to the radiation R1. As shown in Expression ( 1 ), the pixel value of the radiation image is decreased due to the oblique incidence of the radiation R. In addition, as the thickness h of the breast W is thicker, the transmission path is longer and the pixel value of the radiation image is smaller. Therefore, the oblique incidence component of the radiation R is the artifact component. 
     Further, configurations of the mammography apparatus  10  and the console  12  will be described with reference to  FIG.  6   .  FIG.  6    shows a block diagram showing an example of the configurations of the mammography apparatus  10  and the console  12  according to the present embodiment. As shown in  FIG.  6   , in the mammography apparatus  10  according to the present embodiment, the controller  20 , the storage unit  22 , the I/F unit  24 , the operation unit  26 , the grid movement unit  31 , a radiation source movement unit  39 , and the compression plate driving unit  41  are connected to each other via a bus  29 , such as a system bus or a control bus, such that various types of information can be exchanged. 
     The controller  20  controls an overall operation of the mammography apparatus  10  under the control of the console  12 . The controller  20  comprises a central processing unit (CPU)  20 A, a read only memory (ROM)  20 B, and a random access memory (RAM)  20 C. The ROM  20 B stores, in advance, various programs, including an imaging processing program for controlling radiation image capturing, which is executed by the CPU  20 A. The RAM  20 C transitorily stores various data. 
     The storage unit  22  stores the image data of the radiation image captured by the radiation detector  28  or various types of other information. Specific examples of the storage unit  22  include a hard disk drive (HDD) and a solid state drive (SSD). The I/F unit  24  performs communication of various types of information with the console  12  by wireless communication or wired communication. The image data of the radiation image captured by the radiation detector  28  in the mammography apparatus  10  is transmitted to the console  12  via the I/F unit  24  by wireless communication or wired communication. 
     The grid movement unit  31  has a function of moving the grid  27  between the insertion position and the retreat position in accordance with an instruction from the controller  20 , as described above. The radiation source movement unit  39  has a function of rotating the arm part  32  by rotating the shaft part  35  to move the position of the radiation source  37 R from the first position to a second position or from the second position to the first position. The compression plate driving unit  41  has a function of moving the compression plate  40  in the vertical direction (from a direction of compressing the breast to a direction away from the breast), as described above. 
     On the other hand, the console  12  according to the present embodiment has a function of controlling the mammography apparatus  10  by using an imaging order and various types of information acquired from a radiology information system (RIS) 2 via a wireless communication local area network (LAN) and the like, and an instruction performed by the user by an operation unit  56  and the like. 
     The console  12  according to the present embodiment is, for example, a server computer. As shown in  FIG.  5   , the console  12  comprises a controller  50 , a storage unit  52 , an I/F unit  54 , the operation unit  56 , and a display unit  58 . The controller  50 , the storage unit  52 , the I/F unit  54 , the operation unit  56 , and the display unit  58  are connected to each other via a bus  59 , such as a system bus or a control bus, such that various types of information can be exchanged. 
     The controller  50  according to the present embodiment controls an overall operation of the console  12 . The controller  50  comprises a CPU  50 A, a ROM  50 B, and a RAM  50 C. The ROM  50 B stores, in advance, various programs including a contrast imaging processing program  51 A and a difference image generation program  51 B, which are executed by the CPU  50 A and will be described below. The RAM  50 C transitorily stores various data. The CPU  50 A according to the present embodiment is an example of a processor according to the present disclosure. The difference image generation program  51 B according to the present embodiment is an example of an image processing program according to the present disclosure. 
     The storage unit  52  stores the image data of the radiation image captured by the mammography apparatus  10  and various types of other information. Specific examples of the storage unit  52  include an HDD and an SSD. 
     The operation unit  56  is used by the user to input the instruction, various types of information, and the like related to the radiation image capturing and the like, including an irradiation instruction of the radiation R. The operation unit  56  is not particularly limited, and examples thereof include various switches, a touch panel, a touch pen, and a mouse. The display unit  58  displays various types of information. It should be noted that the operation unit  56  and the display unit  58  may be integrated to form a touch panel display. 
     The I/F unit  54  performs communication of various types of information between the mammography apparatus  10  and the RIS 2 by wireless communication or wired communication. In the radiography system  1  according to the present embodiment, the console  12  receives the image data of the radiation image captured by the mammography apparatus  10  from the mammography apparatus  10  via the I/F unit  54  by wireless communication or wired communication. 
     Further,  FIG.  7    shows a functional block diagram of an example of the configuration of the console  12  according to the present embodiment. As shown in  FIG.  7   , the console  12  comprises a first acquisition unit  60 , a second acquisition unit  62 , a first difference image generation unit  64 , a thickness estimation unit  66 , a second difference image generation unit  68 , and a display controller  70 . As an example, in the console  12  according to the present embodiment, the CPU  50 A of the controller  50  executes the difference image generation program  51 B stored in the ROM  50 B, so that the CPU  50 A functions as the first acquisition unit  60 , the second acquisition unit  62 , the first difference image generation unit  64 , the thickness estimation unit  66 , the second difference image generation unit  68 , and the display controller  70 . 
     The first acquisition unit  60  has a function of acquiring the first low-energy image and the first high-energy image obtained by the first contrast imaging. Specifically, the first acquisition unit  60  acquires image data representing the first low-energy image and image data representing the first high-energy image captured in the first contrast imaging by the radiation detector  28  of the mammography apparatus  10  via the I/F unit  24  and the I/F unit  54 . The first acquisition unit  60  outputs the acquired first low-energy image and first high-energy image to the first difference image generation unit  64 . 
     The first difference image generation unit  64  has a function of generating a first difference image indicating a difference between the first high-energy image and the first low-energy image. As an example, the first difference image generation unit  64  according to the present embodiment generates image data of the first difference image by subtracting image data obtained by multiplying the first low-energy image by a predetermined coefficient from image data obtained by multiplying the first high-energy image by a predetermined coefficient for each corresponding pixel. As an example of a weighting coefficient, a weighting coefficient based on an absorption coefficient µg L  of the mammary gland for the radiation R having the first energy, an absorption coefficient µa L  of the fat for the radiation R having the first energy, an absorption coefficient µg H  of the mammary gland for the radiation R having the second energy, and an absorption coefficient µa H  of the fat for the radiation R having the second energy is used. Specifically, a weighting coefficient γ obtained by Expression (2) is used. 
     
       
         
           
             γ 
             = 
             
               
                 μ 
                 ​ 
                 
                   g 
                   L 
                 
                 − 
                 μ 
                 ​ 
                 
                   a 
                   L 
                 
               
             
             / 
             
               
                 μ 
                 ​ 
                 
                   g 
                   H 
                 
                 − 
                 μ 
                 ​ 
                 
                   a 
                   H 
                 
               
             
           
         
       
     
     The first difference image is an image in which a mammary gland tissue has been removed and the contrast medium is enhanced. The first difference image generation unit  64  outputs the generated first difference image to the thickness estimation unit  66  and the display controller  70 . 
     The thickness estimation unit  66  has a function of estimating the thickness of the breast from the first low-energy image and the first high-energy image. In other words, the thickness estimation unit  66  has a function of estimating the thickness of the breast in a state of being compressed by the compression plate  40  based on the first difference image. As an example, the thickness estimation unit  66  according to the present embodiment derives the thickness of the breast W by calculating a coefficient that eliminates the contrast between the mammary gland and the fat. As a specific method, for example, a method disclosed in JP6667462B can be applied. It should be noted that the method of estimating the thickness of the breast by the thickness estimation unit  66  is not limited to the present form, and an existing method can be used. For example, as disclosed in JP2007-300964A, the thickness of the breast may be estimated by simulating radiation scattering based on thickness information and density distribution of the subject. The thickness estimation unit  66  outputs the estimated thickness of the breast to the second difference image generation unit  68 . 
     In addition, the second acquisition unit  62  has a function of acquiring the second low-energy image and the second high-energy image obtained by the second contrast imaging. Specifically, the second acquisition unit  62  acquires image data representing the second low-energy image and image data representing the second high-energy image captured in the second contrast imaging by the radiation detector  28  of the mammography apparatus  10  via the I/F unit  24  and the I/F unit  54 . The second acquisition unit  62  outputs the acquired second low-energy image and second high-energy image to the second difference image generation unit  68 . 
     A second difference image generation unit  68  has a function of generating a second difference image indicating a difference between the second high-energy image and the second low-energy image in which the artifact components are suppressed, based on the thickness of the breast estimated by the thickness estimation unit  66 . As an example, the second difference image generation unit  68  according to the present embodiment generates the second difference image from the second low-energy image and the second high-energy image by the same method as the method of generating the first difference image by the first difference image generation unit  64  as described above. In addition, the second difference image generation unit  68  removes the artifact component described above from the generated second difference image based on the thickness of the breast estimated by the thickness estimation unit  66 . Although the method of removing the artifact component from the second difference image by the second difference image generation unit  68  is not particularly limited, as an example, the second difference image generation unit  68  according to the present embodiment removes the scattered ray component and the oblique incidence component by applying a low-frequency removal filter corresponding to the scattered ray component and the oblique incidence component corresponding to the thickness of the breast to the second difference image. 
     The scattered ray component and the oblique incidence component are generated as low-frequency unevenness. Therefore, by applying the low-frequency removal filter that removes a low-frequency component corresponding to the scattered ray component and the oblique incidence component to the second difference image, the scattered ray component and the oblique incidence component have been removed from the second difference image. It should be noted that a degree of the scattered ray component and the oblique incidence component varies in accordance with the thickness of the breast. Specifically, as the breast is thicker, the scattered ray component and the oblique incidence component are larger. Therefore, it is preferable to apply the low-frequency removal filter that removes a lower-frequency component as the breast is thicker. The second difference image generation unit  68  according to the present embodiment prepares a plurality of low-frequency removal filters corresponding to the thickness of the breast. Then, the second difference image generation unit  68  applies the low-frequency removal filter corresponding to the thickness of the breast estimated by the thickness estimation unit  66  to remove the scattered ray component and the oblique incidence component from the second difference image. It should be noted that a threshold value of the low-frequency removal filter may be changeable in accordance with the scattered ray component, the oblique incidence component, the instruction of the user, or the like. As described above, the second difference image generation unit  68  generates the second difference image in which the artifact component is suppressed. The second difference image generation unit  68  outputs the generated second difference image to the display controller  70 . 
     The display controller  70  has a function of displaying the first difference image and the second difference image on the display unit  58 . 
     Next, an action of the console  12  in the contrast imaging by the radiography system  1  according to the present embodiment will be described with reference to the drawings. 
       FIG.  8    shows a flowchart showing an example of a flow of the contrast imaging by the radiography system  1  according to the present embodiment. In a case in which the contrast imaging is performed, first, the user injects the contrast medium into the breast, which is the subject, as shown in step S 10  of  FIG.  8   . Next, as shown in step S 12 , the user positions the breast of the examinee on the imaging table  30  of the mammography apparatus  10  and compresses the breast with the compression plate  40 . 
     Next, in step S 14 , the console  12  performs first contrast imaging processing shown in  FIG.  9    for performing the first contrast imaging described above by the mammography apparatus  10 . It should be noted that, since the first contrast imaging processing and second contrast imaging processing, which will be described below in detail, are the same processing, both the first contrast imaging processing and the second contrast imaging processing will be described with reference to  FIG.  9   . 
     As described above, in the first contrast imaging (see  FIGS.  3  and  4 A ), the grid  27  is at the insertion position, and the grid  27  is inserted between the radiation source  37 R and the detection surface  28 A of the radiation detector  28 . 
     In the console  12  according to the present embodiment, as an example, the CPU  50 A of the controller  50  executes the contrast imaging processing program  51 A stored in the ROM  50 B, thereby executing the first contrast imaging processing whose example is shown in  FIG.  9   .  FIG.  9    shows a flowchart showing an example of a flow of the first contrast imaging processing executed in the console  12  according to the present embodiment. 
     In step S 100  of  FIG.  9   , the controller  50  determines whether or not the irradiation instruction of the radiation R is received. A negative determination is made in the determination in step S 100  until the irradiation instruction is received. On the other hand, in a case in which the irradiation instruction is received, a positive determination is made in the determination in step S 100 , and the processing proceeds to step S 102 . 
     In step S 102 , the controller  50  outputs the instruction to emit the radiation R having the first energy to the mammography apparatus  10 . In the mammography apparatus  10 , the controller  20  emits the radiation R having the first energy from the radiation source  37 R toward the breast based on the instruction input from the console  12 , and the first low-energy image is captured by the radiation detector  28 . 
     In next step S 104 , the controller  50  outputs the instruction to emit the radiation R having the second energy to the mammography apparatus  10 . In the mammography apparatus  10 , the controller  20  emits the radiation R having the second energy from the radiation source  37 R toward the breast based on the instruction input from the console  12 , and the first high-energy image is captured by the radiation detector  28 . In a case in which the processing of step S 104  ends, the first contrast imaging processing ends. 
     It should be noted that the order of capturing the first low-energy image and the first high-energy image is not limited to the present embodiment, and the first high-energy image may be captured before the first low-energy image. That is, the order of the processing of step S 102  and the processing of step S 104  may be switched. 
     In a case in which the first contrast imaging processing shown in  FIG.  9    ends as described above, the first contrast imaging processing in step S 14  shown in  FIG.  8    ends. It should be noted that the controller  50  may notify the user that the first contrast imaging ends. 
     In next step S 16 , the mammography apparatus  10  moves the grid  27  from the insertion position to the retreat position and causes the grid  27  to retreat from between the radiation source  37 R and the radiation detector  28  by using the grid movement unit  31  (see  FIGS.  3  and  4 A ). 
     In next step S 18 , the mammography apparatus  10  moves the radiation source  37 R to the second position by using the radiation source movement unit  39  (see  FIGS.  3  and  4 A ). 
     It should be noted that the order of the processing of step S 16  and the processing of step S 18  may be switched, or the processing of step S 16  and the processing of step S 18  may be performed in parallel. 
     In next step S 20 , the console  12  performs the second contrast imaging processing for performing the second contrast imaging described above by the mammography apparatus  10 . As described above, a flow of the second contrast imaging processing is the same as that of the first contrast imaging processing, and the second contrast imaging processing shown in  FIG.  9    is performed by the console  12 . 
     Specifically, in the second contrast imaging processing, in step S 100  of  FIG.  9   , the controller  50  determines whether or not the irradiation instruction of the radiation R is received. A negative determination is made in the determination in step S 100  until the irradiation instruction is received. On the other hand, in a case in which the irradiation instruction is received, a positive determination is made in the determination in step S 100 , and the processing proceeds to step S 102 . 
     In step S 102 , the controller  50  outputs the instruction to emit the radiation R having the first energy to the mammography apparatus  10 . In the mammography apparatus  10 , the second low-energy image is captured by the radiation detector  28 . 
     In next step S  104 , the controller  50  outputs the instruction to emit the radiation R having the second energy to the mammography apparatus  10 . In the mammography apparatus  10 , the second high-energy image is captured by the radiation detector  28 . In a case in which the processing of step S 104  ends, the second contrast imaging processing ends. 
     In a case in which the second contrast imaging processing shown in  FIG.  9    ends as described above, the second contrast imaging processing in step S 20  shown in  FIG.  8    ends. It should be noted that the controller  50  may notify the user that the second contrast imaging ends. 
     In next step S 22 , the console  12  performs the difference image generation processing shown in  FIG.  10   . In the console  12  according to the present embodiment, as an example, the CPU  50 A of the controller  50  executes the difference image generation program  51 B stored in the ROM  50 B, thereby executing the difference image generation processing whose example is shown in  FIG.  10   .  FIG.  10    shows a flowchart showing an example of a flow of the difference image generation processing executed in the console  12  according to the present embodiment. 
     In step S 200 , the first acquisition unit  60  acquires the first low-energy image and the first high-energy image obtained by the first contrast imaging, as described above. 
     In next step S 202 , the first difference image generation unit  64  generates the first difference image from the first low-energy image and the first high-energy image. As described above, the first difference image generation unit  64  according to the present embodiment generates the image data of the first difference image by subtracting the image data obtained by multiplying the first low-energy image by the predetermined coefficient from the image data obtained by multiplying the first high-energy image by the predetermined coefficient for each corresponding pixel. 
     In next step S 204 , the thickness estimation unit  66  estimates the thickness of the breast based on the first difference image generated in step S 204 . As described above, the thickness estimation unit  66  according to the present embodiment derives the thickness of the breast by calculating a coefficient that eliminates the contrast between the mammary gland and the fat in the first difference image. 
     As described above, the thickness of the breast is estimated from the first low-energy image and the first high-energy image obtained by the first contrast imaging using the grid by pieces of processing of steps S 200  to S 204 . 
     In next step S 206 , the second acquisition unit  62  acquires the second low-energy image and the second high-energy image obtained by the second contrast imaging, as described above. 
     In next step S 208 , the second difference image generation unit  68  generates the second difference image including the artifact component from the second low-energy image and the second high-energy image. As described above, the second difference image generation unit  68   according to the present embodiment generates the image data of the second difference image by subtracting the image data obtained by multiplying the second low-energy image by the predetermined coefficient from the image data obtained by multiplying the second high-energy image by the predetermined coefficient for each corresponding pixel. 
     In next step S 210 , the second difference image generation unit  68  removes the artifact component from the second difference image generated in step S 208  based on the thickness of the breast estimated in step S 204 . As described above, the second difference image generation unit  68  according to the present embodiment selects the low-frequency removal filter corresponding to the thickness of the breast estimated by the thickness estimation unit  66  from among the plurality of low-frequency removal filters prepared in accordance with the thickness of the breast, and applies the selected low-frequency removal filter to the second difference image to remove the scattered ray component and the oblique incidence component. 
     In next step S 212 , the display controller  70  performs control of displaying the first difference image generated in step S 202  and the second difference image from which the artifact component has been removed in step S 210  on the display unit  58 . 
     It should be noted that a display form in which the first difference image and the second difference image are displayed on the display unit  58  is not particularly limited. For example, a form may be adopted in which the second difference image before removing the artifact component, that is, the second difference image including the artifact component generated by the processing of step S 208  is displayed on the display unit  58 . In addition, the first difference image and the second difference image may be displayed side by side for medical use without being superimposed, or may be displayed side by side in a partially superimposed state. Alternatively, a form may be adopted in which any one of the first difference image or the second difference image is displayed on the display unit  58  to be switchable in accordance with the instruction of the user. It should be noted that a form may be adopted in which the first low-energy image, the first high-energy image, the first difference image, the second low-energy image, the second high-energy image, the second difference image including the artifact component, the second difference image from which the artifact component has been removed, and the like are stored in the storage unit  52  of the console  12 , picture archiving and communication systems (PACS), or the like. 
     In a case in which the processing of step S 212  ends as described above, the difference image generation processing shown in  FIG.  10    ends, and the difference image generation processing of step S 22  shown in  FIG.  8    ends. 
     In a case in which the contrast imaging being performed is the stereo imaging for biopsy, the user interprets the first difference image and the second difference image displayed on the display unit  58 , and collects a lesion while checking the position of the lesion. 
     In next step S 24 , the compression of the breast is released. Specifically, the controller  50  outputs an instruction to the mammography apparatus  10  to move the compression plate  40  in a direction away from the imaging table  30 . In the mammography apparatus  10 , the controller  50  moves the compression plate  40  in the direction away from the imaging table  30  based on the input instruction. As a result, the compression of the breast is released. It should be noted that the release of the breast compression may be performed in accordance with the instruction of the user, or may be performed automatically in accordance with the end of the contrast imaging. 
     In next step S 26 , the mammography apparatus  10  moves the grid  27  to the insertion position and inserts the grid  27  between the radiation source  37 R and the radiation detector  28  by using the grid movement unit  31 . In the present embodiment, the insertion position of the grid  27  is an initial position. Therefore, the mammography apparatus  10  moves the grid  27 , which is moved to the retreat position after the second contrast imaging, to the insertion position that is the initial position by using the grid movement unit  31 . 
     In next step S 28 , the mammography apparatus  10  moves the radiation source  37 R to the first position by using the radiation source movement unit  39 . In the present embodiment, for the radiation source  37 R, the first position is the initial position. Therefore, the mammography apparatus  10  moves the radiation source  37 R, which is moved to the second position after the second contrast imaging, to the first position that is the initial position by using the radiation source movement unit  39 . 
     In a case in which the processing of step S 28  ends, a series of processing related to the contrast imaging according to the embodiment shown in  FIG.  8    ends. 
     As described above, according to the present embodiment, the second difference image from which the artifact component has been removed is obtained based on the thickness of the breast estimated from the first difference image indicating the difference between the first high-energy image and the first low-energy image in which the artifact components are suppressed by the grid  27 . Therefore, according to the present embodiment, it is possible to obtain the difference image in which the contrast medium is clearly reflected and the artifact component is suppressed. 
     It should be noted that the embodiment described above is an example, and various modification examples are possible. For example, the following modification examples may be used. 
     Modification Example 1: Modification Example of Difference Image Generation Processing 
     In the difference image generation processing (see  FIG.  10   ) of the form described above, the form has been described in which the artifact component has been removed from the second difference image, but a form will be described in the present modification example in which the artifact components are removed from the second low-energy image and the second high-energy image. 
     The second difference image generation unit  68  according to the present embodiment removes the artifact component from each of the second low-energy image and the second high-energy image based on the thickness of the breast estimated by the thickness estimation unit  66 . 
     It should be noted that the method of removing the artifact component from the second low-energy image and the method of removing the artifact component from the second high-energy image by the second difference image generation unit  68  are not particularly limited. For example, as in the method of removing the artifact component from the second difference image, the second difference image generation unit  68  removes the artifact component by applying the low-frequency removal filter corresponding to the scattered ray component and the oblique incidence component corresponding to the thickness of the breast to each of the second low-energy image and the second high-energy image. It should be noted that the low-frequency removal filters applied to the second low-energy image and the low-frequency removal filter applied to the second high-energy image may be the same or different. 
       FIG.  11    shows a flowchart showing an example of a flow of the difference image generation processing according to the present modification example. The difference image generation processing shown in  FIG.  11    is different from the difference image generation processing (see  FIG.  10   ) of the embodiment described above in that pieces of processing of steps S 207  and S 209  are provided instead of steps S 208  and S 210 . 
     In step S 207  of  FIG.  11   , the second difference image generation unit  68  removes the artifact component from each of the second low-energy image and the second high-energy image generated in step S 206  based on the thickness of the breast estimated in step S 204 . As described above, the second difference image generation unit  68  selects the low-frequency removal filter corresponding to the thickness of the breast estimated by the thickness estimation unit  66  from among the plurality of low-frequency removal filters prepared in accordance with the thickness of the breast, and applies the selected low-frequency removal filter to the second low-energy image to remove the scattered ray component and the oblique incidence component. In addition, the second difference image generation unit  68  selects the low-frequency removal filter corresponding to the thickness of the breast estimated by the thickness estimation unit  66  from among the plurality of low-frequency removal filters prepared in accordance with the thickness of the breast, and applies the selected low-frequency removal filter to the second high-energy image to remove the scattered ray component and the oblique incidence component. 
     In next step S 209 , the second difference image generation unit  68  generates the second difference image from which artifact component has been removed, from the second low-energy image and the second high-energy image from which the artifact components have been removed in step S 207 . The second difference image generation unit  68  generates the image data of the second difference image from which the artifact component has been removed, by subtracting the image data obtained by multiplying the second low-energy image from which the artifact component has been removed by the predetermined coefficient from the image data obtained by multiplying the second high-energy image from which the artifact component has been removed by the predetermined coefficient for each corresponding pixel. 
     Thus, according to the present modification example, since the second difference image is generated from the second low-energy image and the second high-energy image from which the artifact components have been removed, the generated second difference image is the second difference image from which the artifact component has been removed. 
     Modification Example 2: Tomosynthesis Imaging 
     Although the form has been described in which the technology of the present disclosure is applied to the stereo imaging in the form described above, a form will be described in which the technology of the present disclosure is applied to tomosynthesis imaging in the present modification example. 
     The mammography apparatus  10  according to the present embodiment has a function of performing the tomosynthesis imaging. In a case in which the tomosynthesis imaging is performed in the mammography apparatus  10 , the radiation source  37 R is sequentially moved to each of a plurality of irradiation positions with different irradiation angles due to the rotation of the arm part  32 . The radiation source  37 R has a radiation tube (not shown) that generates the radiation R, and the radiation tube is moved to each of the plurality of irradiation positions in accordance with the movement of the radiation source  37 R.  FIG.  12    shows a diagram for describing an example of the tomosynthesis imaging. It should be noted that, in  FIG.  12   , the compression plate  40  is not shown. In the present embodiment, as shown in  FIG.  12   , the radiation source  37 R is moved to the irradiation position 19 t  (t = 1, 2, ..., the maximum value in  FIG.  12    is 7) at which the irradiation angles differ by each predetermined angle β, in other words, a position at which the irradiation angle of the radiation R with respect to the detection surface  28 A of the radiation detector  28  differs. At each irradiation position 19 t , the radiation R is emitted from the radiation source  37 R toward the breast W in accordance with the instruction of the console  12 , and the radiation image is captured by the radiation detector  28 . In the radiography system  1 , in a case in which the tomosynthesis imaging is performed by moving the radiation source  37 R to each irradiation position 19 t  to capture the radiation image at each irradiation position 19 t , seven radiation images are obtained in the example of  FIG.  12   . It should be noted that, in the following description, the radiation image captured at each irradiation position 19 t  in the tomosynthesis imaging is also referred to as a “projection image” in a case of being distinguished from other radiation images. 
     It should be noted that, as shown in  FIG.  12   , the irradiation angle of the radiation R refers to an angle α formed by the normal line CL of the detection surface  28 A of the radiation detector  28 , and the radiation axis RC. The radiation axis RC is an axis that connects a focal point of the radiation source  37 R at each irradiation position 19 t  and a preset position, such as the center of the detection surface  28 A. In addition, here, the detection surface  28 A of the radiation detector  28  is a substantially parallel surface to an imaging surface  30 A. 
     In a case of the contrast imaging and the tomosynthesis imaging, the mammography apparatus  10  according to the present embodiment captures the projection image, which is the first low-energy image, and the projection image, which is the first high-energy image, in a state in which the radiation source  37 R is disposed at the irradiation position 19 4 , as the first contrast imaging. In addition, the mammography apparatus  10  captures the projection image, which is the second low-energy image, and the projection image, which is the second high-energy image, at each irradiation position 19t (t ≠ 4) in a state in which the radiation source  37 R is disposed at each of the irradiation positions  19   1  to 19 3  and 19 5  to  19   7 , as the second contrast imaging. That is, in the example shown in  FIG.  12   , in a case of the contrast imaging and the tomosynthesis imaging, a set of the first low-energy image and the first high-energy image, and six sets of the second low-energy image and the second high-energy image are obtained. 
       FIG.  13    shows a flowchart showing an example of a flow of the contrast imaging by the radiography system  1  according to the present modification example. The flow of the contrast imaging in the present modification example is different from the flow of the contrast imaging according to the embodiment described above (see  FIG.  8   ) in that processing of step S 21  is provided instead of step S 20 , and processing of step S 23  is provided instead of step S 22 . That is, in the present modification example, the second contrast imaging processing and the difference image generation processing are different from the second contrast imaging processing and the difference image generation processing (see  FIG.  9   ) according to the embodiment described above. 
       FIG.  14    shows a flowchart showing an example of a flow of the second contrast imaging processing executed in the console  12  according to the present modification example. It should be noted that, in the present modification example, in a case in which the second contrast imaging processing is started, that is, in step S 18 , the radiation source  37 R is moved to a position at which the movement of the radiation source  37 R is started in the tomosynthesis imaging, for example, to the irradiation position  19   1  in the example shown in  FIG.  12   . 
     In step S 100  of  FIG.  14   , the controller  50  determines whether or not the irradiation instruction of the radiation R is received, as described above. A negative determination is made in the determination in step S  100  until the irradiation instruction is received. On the other hand, in a case in which the irradiation instruction is received, a positive determination is made in the determination in step S  100 , and the processing proceeds to step S 105 . 
     In step S 105 , the controller  50  starts the movement of the radiation source  37 R. For example, in the example shown in  FIG.  12   , the movement of the radiation source  37 R is started from the irradiation position  19   1  toward the irradiation position  19   7  which is an end position. 
     In step S 107, the controller  50  outputs the instruction to emit the radiation R having the first energy to the mammography apparatus  10 , as described above. In the mammography apparatus  10 , the controller  20  emits the radiation R having the first energy from the radiation source  37 R toward the breast, and the second low-energy image is captured by the radiation detector  28 . 
     In next step S 109 , the controller  50  outputs the instruction to emit the radiation R having the second energy to the mammography apparatus  10 , as described above. In the mammography apparatus  10 , the controller  20  emits the radiation R having the second energy from the radiation source  37 R toward the breast, and the second high-energy image is captured by the radiation detector  28 . 
     In next step S 111 , the controller  50  determines whether or not the position of the radiation source  37 R is the end position. In the example shown in  FIG.  12   , it is determined whether or not the position of the radiation source  37 R is the irradiation position  19   7 . In other words, it is determined whether or not pieces of processing of steps S 107  and S 109  are performed with the position of the radiation source  37 R at the irradiation position  19   7 . In a case in which the position of the radiation source  37 R is not the irradiation position  19   7 , a negative determination is made in the determination in step S 111 , and the processing proceeds to step S 113 . 
     In step S 113 , the controller  50  determines whether or not the position of the moving radiation source  37 R reaches the irradiation position 19 t  (t ≠ 4). A negative determination is made in the determination in step S 113  until the radiation source  37 R reaches the irradiation position 19 t  (t ≠ 4). On the other hand, in a case in which the radiation source  37 R reaches the irradiation position 19 t  (t ≠ 4), a positive determination is made in the determination in step S 113 , and the processing returns to step S 107 , pieces of processing of steps S 107  and S 109  are repeated at the reached irradiation position 19 t  (t ≠ 4). 
     On the other hand, in a case in which the position of the radiation source  37 R is the end position in step S 111 , in the example shown in  FIG.  12   , in a case in which the position of the radiation source  37 R is the irradiation position  19   7 , a positive determination is made, and the processing proceeds to step S 115 . 
     In step S 115 , the controller  50  stops the movement of the radiation source  37 R. In a case in which the processing of step S 115  ends, the second contrast imaging processing shown in  FIG.  14    ends. 
     On the other hand,  FIG.  15    shows a flowchart showing an example of a flow of the difference image generation processing executed in the console  12  according to the present modification example. 
     The difference image generation processing of the present modification example shown in  FIG.  15    is different from the difference image generation processing of the embodiment described above (see  FIG.  10   ) in that pieces of processing of steps S 213  and S 215  are provided instead of the processing of step S 212 . 
     In step S 213  of  FIG.  15   , the second difference image generation unit  68  further generates a tomographic image from the first difference image generated in step S 202  and the plurality of second difference images generated in step S 208 . It should be noted that the method of generating the tomographic image from the first difference image and the plurality of second difference images by the second difference image generation unit  68  is not particularly limited. For example, the second difference image generation unit  68  can generate a plurality of tomographic images by reconstructing the first difference image and the plurality of second difference images by a back projection method, such as a filter back projection (FBP) method or a successive approximation reconstruction method. 
     In next step S 215 , the display controller  70  performs control of displaying the tomographic image generated in step S 213  on the display unit  58 . In a case in which the processing of step S 215  ends, the difference image generation processing shown in  FIG.  15    ends. 
     Thus, the first difference image of the present modification example is the projection image captured by using the grid  27 , and is the projection image in which the artifact components including the scattered ray component and the component caused by the oblique incidence of the radiation are suppressed. In addition, each of the plurality of second difference images is the projection image from which the artifact component has been removed. Therefore, according to the present modification example, the tomographic image in which the artifact component is suppressed can be obtained. 
     It should be noted that in the present modification example, as in Modification Example  1 , a form may be adopted in which the second difference image is generated after removing the artifact component from each of the second low-energy image and the second high-energy image. 
     Modification Example 3: Tomosynthesis Imaging 
     In addition to the form described above, a position of an object-of-interest dyed with the contrast medium may further be specified.  FIG.  16    shows a functional block diagram of an example of a configuration of the console  12  of the present modification example. As shown in  FIG.  16   , the console  12  of the present modification example is different from the console  12  of the form described above in that a position specifying unit  69  is further provided. 
     The position specifying unit  69  has a function of specifying the position of the object-of-interest dyed with the contrast medium, more specifically, the position in a depth direction from the first difference image and the second difference image. Since both the first difference image and the second difference image are images in which the mammary gland tissue has been removed and the contrast medium is enhanced as described above, the object-of-interest, such as a tumor dyed with the contrast medium, is easily detected. The position specifying unit  69  detects the object-of-interest from each of the first difference image and the second difference image. It should be noted that the method of detecting the object-of-interest from each of the first difference image and the second difference image by the position specifying unit  69  is not particularly limited. For example, the position specifying unit  69  may detect the object-of-interest by applying a known computer-aided diagnosis (CAD) algorithm to each of the first difference image and the second difference image. 
     In addition, the position specifying unit  69  specifies the position of the object-of-interest in the depth direction, that is, a height from the detection surface  28 A of the radiation detector  28  from the position of the object-of-interest in the first difference image and the position of the object-of-interest in the second difference image. It should be noted that the method of specifying the position of the object-of-interest in the depth direction by the position specifying unit  69  is also not particularly limited. For example, the position specifying unit  69  may specify the position of the object-of-interest in the depth direction from the position of the object-of-interest in the first difference image and the position of the object-of-interest in the second difference image by a method that applies triangular survey by stereo matching. The position specifying unit  69  outputs the specified position of the object-of-interest in the depth direction to the display controller  70 . 
     As an example, in the console  12  according to the present embodiment, the CPU  50 A of the controller  50  also functions as the position specifying unit  69  by the CPU  50 A executing the difference image generation program  51 B stored in the ROM  50 B. 
     Further, the difference image generation processing of specifying the position of the object-of-interest, more specifically, the position in the depth direction will be described.  FIG.  17    shows a flowchart showing an example of a flow of the difference image generation processing according to the present modification example. The difference image generation processing shown in  FIG.  17    is different from the difference image generation processing (see  FIG.  10   ) of the embodiment described above in that pieces of processing of steps S 214  and S 216  are provided instead of step S 212 . 
     In step S 214  of  FIG.  17   , as described above, the position specifying unit  69  specifies the position of the object-of-interest dyed with the contrast medium in the depth direction from the first difference image generated in step S 202  and the second difference image generated in step S 208 . 
     In next step S 216 , the display controller  70  performs control of displaying the first difference image generated in step S 202 , the second difference image from which the artifact component has been removed in step S 210 , and the position of the object-of-interest in the depth direction specified in step S 214  on the display unit  58 . 
     It should be noted that the display form in which the position of the object-of-interest in the depth direction is displayed on the display unit  58  is not particularly limited. For example, information indicating the position of the object-of-interest in the depth direction may be displayed by being superimposed on the first difference image and the second difference image. It should be noted that a form may be adopted in which the position of the object-of-interest in the depth direction is associated with at least one of the first low-energy image, the first high-energy image, the first difference image, the second low-energy image, the second high-energy image, the second difference image including the artifact component, the second difference image from which the artifact component has been removed, or the like, and is stored in the storage unit  52  of the console  12 , picture archiving and communication systems (PACS), or the like. 
     In a case in which the processing of step S 216  ends as described above, the difference image generation processing shown in  FIG.  17    ends. 
     As described above, according to the present modification example, the position of the object-of-interest in the depth direction is derived by using the first difference image in which the artifact component is suppressed by using the grid  27  and the second difference image in which the artifact component has been removed in accordance with the thickness of the breast, so that the derivation accuracy can be improved. 
     It should be noted that, in the present modification example, in a case in which the tomosynthesis imaging is performed as in Modification Example 3 described above, in which a case of performing the stereo imaging is described, the position specifying unit  69  need only derive the position of the object-of-interest in the depth direction from the generated tomographic image. For example, the position specifying unit  69  may need only detect the object-of-interest from each of the plurality of tomographic images to specify the position of the object-of-interest in the depth direction based on the height of the tomographic image in which the object-of-interest is detected. 
     As described above, the mammography apparatus  10  according to each form described above performs the imaging in which the low-energy image is acquired by the radiation detector  28  by emitting the radiation having the first energy from the radiation source  37 R to the subject into which the contrast medium has been injected, and the imaging in which the high-energy image is acquired by the radiation detector  28  by emitting the radiation having the second energy higher than the first energy from the radiation source  37 R to the subject into which the contrast medium has been injected. 
     The console  12  processes the radiation image captured by the mammography apparatus  10 . The first acquisition unit  60  acquires the first low-energy image and the first high-energy image captured by the mammography apparatus  10  in a state in which the grid  27  for removing the scattered rays is inserted between the radiation source  37 R and the radiation detector  28 , and the radiation source  37 R is disposed at the first position at which the incidence direction of the radiation is the normal direction with respect to the grid  27 . The second acquisition unit  62  acquires the second low-energy image and the second high-energy image captured by the mammography apparatus  10  in a state in which the grid  27  retreats from between the radiation source  37 R and the radiation detector  28 , and the radiation source  37 R is disposed at the second position different from the first position. The thickness estimation unit  66  estimates the thickness of the subject from the first low-energy image and the first high-energy image. The second difference image generation unit  68  generates the second difference image indicating the difference between the second high-energy image and the second low-energy image from the second low-energy image and the second high-energy image based on the estimated thickness. 
     Thus, the console  12  according to each form described above estimates the thickness of the breast from the first low-energy image and the first high-energy image captured by using the grid  27 . In addition, the console  12  generates the second difference image from which the artifact component has been removed, from the second low-energy image and the second high-energy image captured without using the grid  27  based on the estimated thickness of the breast. 
     In a case in which the oblique incidence of the radiation is performed on the radiation detector  28 , in other words, in a case in which the radiation source  37 R is the position different from the first position at which the incidence direction of the radiation is the normal direction, the imaging using the grid  27  cannot be performed. However, with the console  12  according to each form described above, it is possible to obtain the second difference image in which the artifact component is suppressed, from the second low-energy image and the second high-energy image captured without using the grid  27 . Therefore, with the console  12  according to each form described above, it is possible to obtain the difference image in which the contrast medium is clearly reflected and obtain artifact component is suppressed. 
     It should be noted that, in each form described above, the form has been described in which the second difference image from which the artifact component has been removed is generated based on the thickness of the breast, but a ratio (compositional ratio) between the fat and the mammary gland for each pixel may be further derived to correct the influence of the scattered rays. 
     In addition, in each form described above, the form has been described in which the difference image generation processing is performed as step S 20  after the processing of step S 20  in  FIG.  8    ends, a timing of performing the difference image generation processing, that is, a timing of generating the first difference image and the second difference image is not limited to each form described above. For example, as soon as the first contrast imaging processing in step S 14  ends, the difference image generation processing may be started in parallel with the processing of step S 16  and subsequent steps. In addition, the difference image generation processing may be performed at a timing in accordance with a desire of the user after the contrast imaging. 
     In addition, in each form described above, the form has been described in which the breast is applied as an example of the subject according to the present disclosure, and the mammography apparatus  10  is applied as an example of the radiography apparatus according to the present disclosure, but the subject is not limited to the breast, and the radiography apparatus is not limited to the mammography apparatus. For example, the subject may be a chest, an abdomen, or the like, and a form may be adopted in which a radiography apparatus other than the mammography apparatus is applied as the radiography apparatus. 
     In addition, in each form described above, the form has been described in which the console  12  is an example of the image processing apparatus according to the present disclosure, but an apparatus other than the console  12  may have the function of the image processing apparatus according to the present disclosure. In other words, some or all of the functions of the first acquisition unit  60 , the second acquisition unit  62 , the first difference image generation unit  64 , the thickness estimation unit  66 , the second difference image generation unit  68 , and the display controller  70  may be provided in an apparatus other than the console  12 , for example, the mammography apparatus  10  or an external apparatus. 
     In addition, in each form described above, various processors shown below can be used as the hardware structure of processing units that execute various pieces of processing, such as the first acquisition unit  60 , the second acquisition unit  62 , the first difference image generation unit  64 , the thickness estimation unit  66 , the second difference image generation unit  68 , and the display controller  70 . As described above, the various processors include, in addition to the CPU which is a general-purpose processor which executes software (program) and functions as various processing units, a programmable logic device (PLD) which is a processor of which a circuit configuration can be changed after manufacture, such as a field programmable gate array (FPGA), and a dedicated electric circuit which is a processor having a circuit configuration which is designed for exclusive use in order to execute specific processing, such as an application specific integrated circuit (ASIC). 
     One processing unit may be composed of one of the various processors or may be composed of a combination of two or more processors of the same type or different types (for example, a combination of a plurality of FPGAs or a combination of a CPU and an FPGA). In addition, a plurality of processing units may be composed of one processor. 
     A first example of the configuration in which the plurality of processing units are composed of one processor is a form in which one processor is composed of a combination of one or more CPUs and software and the processor functions as the plurality of processing units, as represented by the computer, such as a client and a server. Second, as represented by a system on chip (SoC) or the like, there is a form of using a processor that realizes the function of the entire system including the plurality of processing units by one integrated circuit (IC) chip. As described above, various processing units are composed of one or more of the various processors as the hardware structure. 
     Further, more specifically, an electric circuit (circuitry) obtained by combining circuit elements, such as semiconductor elements, can be used as the hardware structure of the various processors. 
     In addition, in each embodiment described above, the aspect has been described in which the contrast imaging processing program  51 A and the difference image generation program  51 B are stored (installed) in advance in the ROM  50 B, but the present disclosure is not limited to this. An aspect may be provided in which each of the contrast imaging processing program  51 A and the difference image generation program  51 B is recorded in the recording medium, such as a compact disc read only memory (CD-ROM), a digital versatile disc read only memory (DVD-ROM), and a universal serial bus (USB) memory. In addition, a form may be adopted in which each of the contrast imaging processing program  51 A and the difference image generation program  51 B is downloaded from an external apparatus via a network.