Patent Publication Number: US-2018031715-A1

Title: Radiography system, radiography method, and radiography program

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 U.S.C §119 to Japanese Patent Application No. 2016-150591, filed on Jul. 29, 2016, which is hereby expressly incorporated by reference, in its entirety, into the present application. 
     BACKGROUND 
     Technical Field 
     The present disclosure relates to a radiography system, a radiography method, and a radiography program. 
     Related Art 
     For example, as disclosed in WO2013/047193A, a radiography apparatus has been known that comprises two radiation detectors each of which includes a plurality of pixels that accumulate charge corresponding to the amount of radiation emitted and which are provided so as to be stacked. 
     In addition, a technique has been known which detects a predetermined time related to the emission of radiation, such as the time when the emission of radiation starts and the time when the emission of radiation ends, on the basis of an electric signal of which the level generally increases as the amount of charge output from each pixel of a radiation detector of a radiography apparatus increases. 
     However, in a case in which radiographic images are captured by two radiation detectors disclosed in, for example, WO2013/047193A, radiation that has been transmitted through the radiation detector provided on the incident side of the radiation reaches the radiation detector provided on the emission side of the radiation. Therefore, the amount of radiation that reaches the radiation detector provided on the emission side of the radiation is less than the amount of radiation that reaches the radiation detector provided on the incident side and the amount of radiation used to generate a radiographic image is reduced. 
     Therefore, in the radiation detector provided on the incident side of the radiation and the radiation detector provided on the emission side of the radiation, the detection results of the predetermined time related to the emission of radiation are different from each other. As a result, in some cases, it is difficult to appropriately detect the emission of radiation in the entire radiography apparatus. 
     SUMMARY 
     The present disclosure has been made in view of the above-mentioned problems and an object of the present disclosure is to provide a technique that can appropriately detect the emission of radiation even when the amount of radiation emitted to a second radiation detector is less than the amount of radiation emitted to a first radiation detector. 
     In order to achieve the object, according to an aspect of the invention, there is provided a radiography system comprising: a radiography apparatus comprising a first radiation detector in which a plurality of pixels, each of which includes a conversion element that generates a larger amount of charge as it is irradiated with a larger amount of radiation, are two-dimensionally arranged and a second radiation detector which is provided so as to be stacked on a side of the first radiation detector from which the radiation is transmitted and emitted and in which a plurality of pixels, each of which includes a conversion element that generates a larger amount of charge as it is irradiated with a larger amount of radiation, are two-dimensionally arranged; and a specification unit that specifies a predetermined time related to the emission of the radiation, on the basis of a first detection result that is a detection result of a predetermined time related to the emission of the radiation using a first electric signal which is obtained by converting charge generated in the pixels of the first radiation detector and of which the level increases as the amount of charge increases and a second detection result that is a detection result of a predetermined time related to the emission of the radiation using a second electric signal which is obtained by converting charge generated in the pixels of the second radiation detector and of which the level increases as the amount of charge increases. 
     In the radiography system according to the present disclosure, in a case in which the first detection result and the second detection result are different from each other, the specification unit may specify the predetermined time related to the emission of the radiation on the basis of a predetermined detection result of the first and second detection results. 
     In the radiography system according to the present disclosure, the predetermined detection result may be the first detection result. 
     The radiography system according to the present disclosure may further comprise a detection result setting unit that sets the predetermined detection result. 
     In the radiography system according to the present disclosure, the specification unit may further specify whether to continue to perform an operation of accumulating charge in the plurality of pixels of the first radiation detector and an operation of accumulating charge in the plurality of pixels of the second radiation detector, using a first noise detection result which is a detection result of noise included in the first electric signal and a second noise detection result which is a detection result of noise included in the second electric signal after the operation of accumulating charge in the plurality of pixels of the first radiation detector and the operation of accumulating charge in the plurality of pixels of the second radiation detector start. 
     In the radiography system according to the present disclosure, the first noise detection result and the second noise detection result that the specification unit uses to specify whether to continue to perform the operation of accumulating charge in the plurality of pixels of the first radiation detector and the operation of accumulating charge in the plurality of pixels of the second radiation detector may be a detection result of noise included in the first electric signal using the first electric signal and a detection result of noise included in the second electric signal using the second electric signal, respectively. 
     The radiography system according to the present disclosure may further comprise: a first detection unit that detects at least one of an impact or an electromagnetic wave which is applied from the outside to the first radiation detector; and a second detection unit that detects at least one of an impact or an electromagnetic wave which is applied from the outside to the second radiation detector. The first noise detection result and the second noise detection result that the specification unit uses to specify whether to continue to perform the operation of accumulating charge in the plurality of pixels of the first radiation detector and the operation of accumulating charge in the plurality of pixels of the second radiation detector may be a detection result of noise included in the first electric signal using a detection result of the first detection unit and a detection result of noise included in the second electric signal using a detection result of the second detection unit, respectively. 
     In the radiography system according to the present disclosure, in a case in which the first noise detection result and the second noise detection result are different from each other, the specification unit may specify whether to continue to perform the operation of accumulating charge in the plurality of pixels of the first radiation detector and the operation of accumulating charge in the plurality of pixels of the second radiation detector, using a predetermined noise detection result of the first and second noise detection results. 
     In the radiography system according to the present disclosure, the predetermined noise detection result may be the first noise detection result. 
     The radiography system according to the present disclosure may further comprise a noise detection result setting unit that sets the predetermined noise detection result. 
     In the radiography system according to the present disclosure, in a case in which at least one of the first noise detection result or the second noise detection result indicates that noise has been detected, the specification unit may specify to stop the operation of accumulating charge in the plurality of pixels of the first radiation detector and the operation of accumulating charge in the plurality of pixels of the second radiation detector. 
     In the radiography system according to the present disclosure, the specification unit may specify a time when the emission of the radiation starts as the predetermined time related to the emission of the radiation. 
     In the radiography system according to the present disclosure, the radiography apparatus may further comprise the specification unit. 
     In the radiography system according to the present disclosure, each of the first radiation detector and the second radiation detector may comprise a light emitting layer that is irradiated with radiation and emits light. The plurality of pixels of each of the first radiation detector and the second radiation detector may receive the light, generate the charge, and accumulate the charge. The light emitting layer of the first radiation detector and the light emitting layer of the second radiation detector may have different compositions. 
     In the radiography system according to the present disclosure, the light emitting layer of the first radiation detector may include CsI and the light emitting layer of the second radiation detector may include GOS. 
     The radiography system according to the present disclosure may further comprise a derivation unit that derives at least one of bone mineral content or bone density, using a first radiographic image captured by the first radiation detector and a second radiographic image captured by the second radiation detector. 
     In order to achieve the object, according to another aspect of the present disclosure, there is provided a radiography method that is performed by a radiography apparatus comprising a first radiation detector in which a plurality of pixels, each of which includes a conversion element that generates a larger amount of charge as it is irradiated with a larger amount of radiation, are two-dimensionally arranged and a second radiation detector which is provided so as to be stacked on a side of the first radiation detector from which the radiation is transmitted and emitted and in which a plurality of pixels, each of which includes a conversion element that generates a larger amount of charge as it is irradiated with a larger amount of radiation, are two-dimensionally arranged. The radiography method comprises specifying a predetermined time related to the emission of the radiation, on the basis of a first detection result that is a detection result of a predetermined time related to the emission of the radiation using a first electric signal which is obtained by converting charge generated in the pixels of the first radiation detector and of which the level increases as the amount of charge increases and a second detection result that is a detection result of a predetermined time related to the emission of the radiation using a second electric signal which is obtained by converting charge generated in the pixels of the second radiation detector and of which the level increases as the amount of charge increases. 
     In order to achieve the object, according to still another aspect of the present disclosure, there is provided a radiography program that causes a computer controlling a radiography apparatus comprising a first radiation detector in which a plurality of pixels, each of which includes a conversion element that generates a larger amount of charge as it is irradiated with a larger amount of radiation, are two-dimensionally arranged and a second radiation detector which is provided so as to be stacked on a side of the first radiation detector from which the radiation is transmitted and emitted and in which a plurality of pixels, each of which includes a conversion element that generates a larger amount of charge as it is irradiated with a larger amount of radiation, are two-dimensionally arranged to perform specifying a predetermined time related to the emission of the radiation, on the basis of a first detection result that is a detection result of a predetermined time related to the emission of the radiation using a first electric signal which is obtained by converting charge generated in the pixels of the first radiation detector and of which the level increases as the amount of charge increases and a second detection result that is a detection result of a predetermined time related to the emission of the radiation using a second electric signal which is obtained by converting charge generated in the pixels of the second radiation detector and of which the level increases as the amount of charge increases. 
     According to the present disclosure, it is possible to appropriately detect the emission of radiation even when the amount of radiation emitted to the second radiation detector is less than the amount of radiation emitted to the first radiation detector. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating an example of the structure of a radiography system according to an embodiment. 
         FIG. 2  is a side cross-sectional view illustrating an example of the structure of a radiography apparatus according to this embodiment. 
         FIG. 3  is a block diagram illustrating an example of the structure of a main portion of an electric system of the radiography apparatus according to this embodiment. 
         FIG. 4  is a circuit diagram illustrating an example of the structure of a signal processing unit according to this embodiment. 
         FIG. 5  is a block diagram illustrating an example of the structure of a main portion of an electric system of a console according to this embodiment. 
         FIG. 6  is a graph illustrating the amount of radiation that reaches each of a first radiation detector and a second radiation detector according to this embodiment. 
         FIG. 7  is a flowchart illustrating an example of the flow of an overall imaging process according to this embodiment. 
         FIG. 8  is a flowchart illustrating an example of the flow of an image generation process in the overall imaging process according to this embodiment. 
         FIG. 9  is a front view schematically illustrating a bone tissue region and a soft tissue region according to this embodiment. 
         FIG. 10  is a flowchart illustrating an example of the flow of an imaging control process according to this embodiment. 
         FIG. 11  is a diagram schematically illustrating an example of a selection screen for selecting a detection result having priority. 
         FIG. 12  is a flowchart illustrating an example of the flow of a first imaging process and a second imaging process according to this embodiment. 
         FIG. 13  is a diagram schematically illustrating a variation in the amount of radiation emitted from a radiation source over an irradiation time. 
         FIG. 14  is a diagram schematically illustrating an example of a selection screen for selecting a noise detection result having priority. 
         FIG. 15  is a block diagram illustrating another example of the structure of the main portion of the electric system of the radiography apparatus according to this embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, an embodiment of the invention will be described in detail with reference to the drawings. 
     First, the structure of a radiography system  10  according to this embodiment will be described with reference to  FIG. 1 . As illustrated in  FIG. 1 , the radiography system  10  comprises a radiation emitting apparatus  12 , a radiography apparatus  16 , and a console  18 . The console  18  according to this embodiment is an example of an image processing apparatus according to the invention. 
     The radiation emitting apparatus  12  according to this embodiment comprises a radiation source  14  that irradiates a subject W, which is an example of an imaging target, with radiation R such as X-rays. An example of the radiation emitting apparatus  12  is a treatment cart. A method for instructing the radiation emitting apparatus  12  to emit the radiation R is not particularly limited. For example, in a case in which the radiation emitting apparatus  12  comprises an irradiation button, a user, such as a doctor or a radiology technician, may press the irradiation button to instruct the emission of the radiation R such that the radiation R is emitted from the radiation emitting apparatus  12 . In addition, for example, the user may operate the console  18  to instruct the emission of the radiation R such that the radiation R is emitted from the radiation emitting apparatus  12 . 
     When receiving a command to start the emission of the radiation R, the radiation emitting apparatus  12  emits the radiation R from the radiation source  14  according to emission conditions, such as a tube voltage, a tube current, and an irradiation period. 
     The radiography apparatus  16  according to this embodiment comprises a first radiation detector  20 A and a second radiation detector  20 B that detect the radiation R which has been emitted from the radiation emitting apparatus  12  and then transmitted through the subject W. The radiography apparatus  16  captures radiographic images of the subject W using the first radiation detector  20 A and the second radiation detector  20 B. Hereinafter, in a case in which the first radiation detector  20 A and the second radiation detector  20 B do not need to be distinguished from each other, they are generically referred to as “radiation detectors  20 ”. 
     Next, the structure of the radiography apparatus  16  according to this embodiment will be described with reference to  FIG. 2 . As illustrated in  FIG. 2 , the radiography apparatus  16  comprises a plate-shaped housing  21  that transmits the radiation R and has a waterproof, antibacterial, and airtight structure. The housing  21  includes the first radiation detector  20 A, the second radiation detector  20 B, a radiation limitation member  24 , a control board  25 , a control board  26 A, a control board  26 B, and a case  28 . 
     The first radiation detector  20 A is provided on the incident side of the radiation R and the second radiation detector  20 B is provided so as to be stacked on the side of the first radiation detector  20 A from which the radiation R is transmitted and emitted in the radiography apparatus  16 . The first radiation detector  20 A comprises a thin film transistor (TFT) substrate  30 A and a scintillator  22 A which is an example of a light emitting layer that is irradiated with the radiation R and emits light corresponding to the amount of radiation R emitted. The TFT substrate  30 A and the scintillator  22 A are stacked in the order of the TFT substrate  30 A and the scintillator  22 A from the incident side of the radiation R. The term “stacked” means a state in which the first radiation detector  20 A and the second radiation detector  20 B overlap each other in a case in which the first radiation detector  20 A and the second radiation detector  20 B are seen from the incident side or the emission side of the radiation R in the radiography apparatus  16  and it does not matter how they overlap each other. For example, the first radiation detector  20 A and the second radiation detector  20 B, or the first radiation detector  20 A, the radiation limitation member  24 , and the second radiation detector  20 B may overlap while coming into contact with each other or may overlap with a gap therebetween in the stacking direction. 
     The second radiation detector  20 B comprises a TFT substrate  30 B and a scintillator  22 B which is an example of the light emitting layer. The TFT substrate  30 B and the scintillator  22 B are stacked in the order of the TFT substrate  30 B and the scintillator  22 B from the incident side of the radiation R. 
     That is, the first radiation detector  20 A and the second radiation detector  20 B are so-called irradiation side sampling (ISS) radiation detectors that are irradiated with the radiation R from the side of the TFT substrates  30 A and  30 B. 
     In the radiography apparatus  16  according to this embodiment, the scintillator  22 A of the first radiation detector  20 A and the scintillator  22 B of the second radiation detector  20 B have different compositions. Specifically, for example, the scintillator  22 A includes CsI (Tl) (cesium iodide having thallium added thereto) as a main component and the scintillator  22 B includes gadolinium oxysulfide (GOS) as a main component. GOS has a higher sensitivity to the high-energy radiation R than CsI. In addition, a combination of the composition of the scintillator  22 A and the composition of the scintillator  22 B is not limited to the above-mentioned example and may be a combination of other compositions or a combination of the same compositions. 
     The radiation limitation member  24  that limits the transmission of the radiation R is provided between the first radiation detector  20 A and the second radiation detector  20 B. An example of the radiation limitation member  24  is a metal plate made of, for example, copper or tin. It is preferable that a variation in the thickness of the radiation limitation member  24  in the incident direction of the radiation R is equal to or less than 1% in order to uniformize limitations (transmissivity) on the radiation. 
     An electronic circuit, such as an integrated control unit  71  (see  FIG. 3 ) which will be described below, is formed on the control board  25 . The control board  26 A is provided so as to correspond to the first radiation detector  20 A and electronic circuits, such as an image memory  56 A and a control unit  58 A which will be described below, are formed on the control board  26 A. The control board  26 B is provided so as to correspond to the second radiation detector  20 B and electronic circuits, such as an image memory  56 B and a control unit  58 B which will be described below, are formed on the control board  26 B. The control board  25 , the control board  26 A, and the control board  26 B are provided on the side of the second radiation detector  20 B which is opposite to the incident side of the radiation R. 
     As illustrated in  FIG. 2 , the case  28  is provided at a position (that is, outside the range of an imaging region) that does not overlap the radiation detector  20  at one end of the housing  21 . For example, a power supply unit  70  which will be described below is accommodated in the case  28 . The installation position of the case  28  is not particularly limited. For example, the case  28  may be provided at a position that overlaps the radiation detector  20  on the side of the second radiation detector  20 B which is opposite to the incident side of the radiation R. 
     Next, the structure of a main portion of an electric system of the radiography apparatus  16  according to this embodiment will be described with reference to  FIG. 3 . 
     As illustrated in  FIG. 3 , a plurality of pixels  32  are two-dimensionally provided in one direction (a row direction in  FIG. 3 ) and an intersection direction (a column direction in  FIG. 3 ) that intersects the one direction on the TFT substrate  30 A. 
     In this embodiment, among the plurality of pixels  32 , pixels  32 A for capturing a radiographic image and pixels  32 B for detecting radiation are predetermined. The pixel  32 A for capturing a radiographic image is a pixel  32  that detects the radiation R and is used to generate an image indicated by the radiation R. The pixel  32 B for detecting radiation is a pixel  32  that is used to detect, for example, the start of the emission of the radiation R and outputs charge even for a charge accumulation period (which will be described in detail below). 
     The pixel  32  includes a sensor unit  33 A, a capacitor  33 B, and a field effect thin film transistor (TFT; hereinafter, simply referred to as a “thin film transistor”)  33 C. The sensor unit  33 A according to this embodiment is an example of a conversion element according to the invention. In the pixel  32 A for capturing a radiographic image and the pixel  32 B for detecting radiation, the thin film transistors  33 C have different structures. 
     The sensor unit  33 A includes, for example, an upper electrode, a lower electrode, and a photoelectric conversion film which are not illustrated, absorbs the light emitted from the scintillator  22 A, and generates charge. The capacitor  33 B accumulates the charge generated by the sensor unit  33 A. The thin film transistor  33 C of the pixel  32 A for capturing a radiographic image reads the charge accumulated in the capacitor  33 B and outputs the charge in response to a control signal. In contrast, the thin film transistor  33 C of the pixel  32 B for detecting radiation has a source and drain which are short-circuited. Therefore, in the pixel  32 B for detecting radiation, the charge generated by the sensor unit  33 A flows to a data line  36 , regardless of the switching state of the thin film transistor  33 C. 
     According to this structure, as the pixel  32  according to this embodiment is irradiated with a larger amount of radiation, a larger amount of charge is accumulated in the pixel  32 . 
     A plurality of gate lines  34  which extend in the one direction and are used to turn on and off each thin film transistor  33 C are provided on the TFT substrate  30 A. In addition, a plurality of data lines  36  which extend in the intersection direction and to which the charge read by the thin film transistors  33 C in an on state is output are provided on the TFT substrate  30 A. 
     A gate line driver  52 A is provided on one side of two adjacent sides of the TFT substrate  30 A and a signal processing unit  54 A is provided on the other side. Each gate line  34  of the TFT substrate  30 A is connected to the gate line driver  52 A and each data line  36  of the TFT substrate  30 A is connected to the signal processing unit  54 A. 
     The thin film transistors  33 C corresponding to each gate line  34  on the TFT substrate  30 A are sequentially turned on (in units of row illustrated in  FIG. 3  in this embodiment) by control signals which are supplied from the gate line driver  52 A through the gate lines  34 . The charge which is read by the thin film transistor  33 C of the pixel  32 A for capturing a radiographic image in an on state is transmitted as an electric signal through the data line  36  and is input to the signal processing unit  54 A. In this way, charge is sequentially read from each gate line  34  (in units of row illustrated in  FIG. 3  in this embodiment) and image data indicating a two-dimensional radiographic image is generated by the signal processing unit  54 A. In addition, the charge which is read by the thin film transistor  33 C of the pixel  32 B for detecting radiation is transmitted as an electric signal through the data line  36  and is input to the signal processing unit  54 A. However, image data indicating a radiographic image is not generated and the charge is output to the control unit  58 A. 
     As illustrated in  FIG. 4 , the signal processing unit  54 A comprises a variable gain pre-amplifier (charge amplifier)  82  and a sample-and-hold circuit  84  which correspond to each data line  36 . 
     The variable gain pre-amplifier  82  includes an operational amplifier  82 A that has a positive input side grounded and a capacitor  82 B and a reset switch  82 C that are connected in parallel to each other between a negative input side and an output side of the operational amplifier  82 A. The reset switch  82 C is turned on and off by the control unit  58 A. 
     In addition, the signal processing unit  54 A according to this embodiment comprises a multiplexer  86  and an analog/digital (A/D) converter  88 . The sampling time of the sample-and-hold circuit  84  and the turn-on and turn-off of a switch  86 A provided in the multiplexer  86  are controlled by the control unit  58 A. 
     When a radiographic image is detected, first, the control unit  58 A maintains the reset switch  82 C of the variable gain pre-amplifier  82  in an on state for a predetermined period to release the charge accumulated in the capacitor  82 B. 
     In contrast, the charge generated in the pixel  32 B for detecting radiation due to irradiation with the radiation R is read to the data line  36  by the thin film transistor  33 C, regardless of the switching state of the thin film transistor  33 C. In addition, the charge generated in the pixel  32 A for capturing a radiographic image is accumulated in the capacitor  33 B and is read to the data line  36  by the thin film transistor  33 C in an on state. The charge read to the data line  36  is transmitted as an electric signal and is then amplified by the corresponding variable gain pre-amplifier  82  at a predetermined gain. 
     After the above-mentioned discharging is performed, the control unit  58 A drives the sample-and-hold circuit  84  for a predetermined period such that the level of the electric signal amplified by the variable gain pre-amplifier  82  is held and sampled by the sample-and-hold circuit  84 . 
     Then, the signal levels sampled by each sample-and-hold circuit  84  are sequentially selected by the multiplexer  86  and are then converted into digital signal levels by the A/D converter  88  under the control of the control unit  58 A. In this way, image data indicating the captured radiographic image is acquired. Hereinafter, the digital signal obtained by converting the electric signal (first electric signal) using the A/D converter  88  in the signal processing unit  54 A is referred to as a “first digital signal” and the digital signal obtained by converting the electric signal (second electric signal) using the A/D converter  88  in the signal processing unit  54 B is referred to as a “second digital signal”. In addition, in a case in which the first digital signal and the second digital signal do not need to be distinguished from each other, they are generically referred to as “digital signals”. 
     The control unit  58 A which will be described below is connected to the signal processing unit  54 A. The image data output from the A/D converter of the signal processing unit  54 A is sequentially output the control unit  58 A. The image memory  56 A is connected to the control unit  58 A. The image data sequentially output from the signal processing unit  54 A is sequentially stored in the image memory  56 A under the control of the control unit  58 A. The image memory  56 A has memory capacity that can store a predetermined amount of image data. Whenever a radiographic image is captured, captured image data is sequentially stored in the image memory  56 A. 
     The control unit  58 A comprises a central processing unit (CPU)  60 , a memory  62  including, for example, a read only memory (ROM) and a random access memory (RAM), and a non-volatile storage unit  64  such as a flash memory. An example of the control unit  58 A is a microcomputer. 
     The control unit  58 A according to this embodiment has a function that outputs a first detection result, which is the detection result of the time when the emission of the radiation R has started, to the integrated control unit  71  according to whether the value of the first digital signal is equal to or greater than a predetermined start threshold value, which will be described in detail below. In some cases, the control unit  58 A according to this embodiment erroneously detects the time when the emission of the radiation R has started, on the basis of charge that is generated as noise due to disturbance, such as impact and electromagnetic waves, particularly, vibration. Therefore, the control unit  58 A according to this embodiment has a function that outputs a first noise detection result, which is the detection result of the generation of noise using the first digital signal, to the integrated control unit  71 , which will be described in detail below. 
     The integrated control unit  71  comprises a CPU  72 , a memory  74  including, for example, a ROM and a RAM, and a non-volatile storage unit  76  such as a flash memory. An example of the integrated control unit  71  is a microcomputer. The control unit  58 A and the integrated control unit  71  are connected such that they can communicate with each other. 
     The integrated control unit  71  according to this embodiment has a function that specifies the time when the emission of the radiation R has started, using a predetermined result with priority of the first detection result and a second detection result output from the control unit  58 A, which will be described in detail below. In addition, the integrated control unit  71  according to this embodiment has a function that controls the control unit  58 A and the control unit  58 B such that the accumulation of charge in each pixel  32  is stopped in a case in which at least one of the first noise detection result or a second noise detection result indicates that the generation of noise has been detected, which will be described in detail below. 
     A communication unit  66  is connected to the control unit  58 A and the integrated control unit  71  and transmits and receives various kinds of information to and from external apparatuses, such as the radiation emitting apparatus  12  and the console  18 , using at least one of wireless communication or wired communication. The power supply unit  70  supplies power to each of the above-mentioned various circuits or elements (for example, the gate line driver  52 A, the signal processing unit  54 A, the image memory  56 A, the control unit  58 A, the communication unit  66 , and the integrated control unit  71 ). In  FIG. 3 , lines for connecting the power supply unit  70  to various circuits or elements are not illustrated in order to avoid complication. 
     Components of the TFT substrate  30 B, the gate line driver  52 B, the signal processing unit  54 B, the image memory  56 B, and the control unit  58 B of the second radiation detector  20 B have the same structures as the corresponding components of the first radiation detector  20 A and thus the description thereof will not be repeated here. 
     The control unit  58 B according to this embodiment has a function that outputs the second detection result, which is the detection result of the time when the emission of the radiation R has started, to the integrated control unit  71  according to whether the value of the second digital signal is equal to or greater than a predetermined start threshold value, which will be described in detail below. In addition, the control unit  58 B according to this embodiment has a function that outputs the second noise detection result, which is the detection result of the generation of noise using the second digital signal, to the integrated control unit  71 , which will be described in detail below. 
     The control unit  58 A and the control unit  58 B are connected such that they can communicate with each other. 
     According to the above-mentioned structure, the radiography apparatus  16  according to this embodiment captures radiographic images using the first radiation detector  20 A and the second radiation detector  20 B. 
     Next, the structure of the console  18  according to this embodiment will be described with reference to  FIG. 5 . As illustrated in  FIG. 5 , the console  18  comprises a control unit  90 . The control unit  90  comprises a CPU  90 A that controls the overall operation of the console  18 , a ROM  90 B in which, for example, various programs or various parameters are stored in advance, and a RAM  90 C that is used as, for example, a work area when the CPU  90 A executes various programs. 
     In addition, the console  18  comprises a non-volatile storage unit  92  such as a hard disk drive (HDD). The storage unit  92  stores and holds image data indicating a radiographic image captured by the first radiation detector  20 A, image data indicating a radiographic image captured by the second radiation detector  20 B, and various other data. Hereinafter, the radiographic image captured by the first radiation detector  20 A is referred to as a “first radiographic image” and image data indicating the first radiographic image is referred to as “first radiographic image data”. In addition, hereinafter, the radiographic image captured by the second radiation detector  20 B is referred to as a “second radiographic image” and image data indicating the second radiographic image is referred to as “second radiographic image data”. In a case in which the “first radiographic image” and the “second radiographic image” are generically named, they are simply referred to as “radiographic images”. 
     The console  18  further comprises a display unit  94 , an operation unit  96 , and a communication unit  98 . The display unit  94  displays, for example, information related to imaging and a captured radiographic image. The user uses the operation unit  96  to input, for example, a command to capture a radiographic image and a command related to image processing for a captured radiographic image. For example, the operation unit  96  may have the form of a keyboard or may have the form of a touch panel that is integrated with the display unit  94 . The communication unit  98  transmits and receives various kinds of information to and from the radiation emitting apparatus  12  and the radiography apparatus  16 , using at least one of wireless communication or wired communication. In addition, the communication unit  98  transmits and receives various kinds of information to and from external systems, such as a picture archiving and communication system (PACS) and a radiology information system (RIS), using at least one of wireless communication or wired communication. 
     The control unit  90 , the storage unit  92 , the display unit  94 , the operation unit  96 , and the communication unit  98  are connected to each other through a bus  99 . 
     As described above, in the radiography apparatus  16  according to this embodiment, the amount of radiation that reaches the second radiation detector  20 B is less than the amount of radiation that reaches the first radiation detector  20 A. In addition, the radiation limitation member  24  generally has the characteristic that it absorbs a larger number of low-energy components than high-energy components in energy forming the radiation R, which depends on the material forming the radiation limitation member  24 . Therefore, the energy distribution of the radiation R that reaches the second radiation detector  20 B has a larger number of high-energy components than the energy distribution of the radiation R that reaches the first radiation detector  20 A. 
     In this embodiment, for example, about 50% of the radiation R that has reached the first radiation detector  20 A is absorbed by the first radiation detector  20 A and is used to capture a radiographic image. In addition, about 60% of the radiation R that has passed through the first radiation detector  20 A and reached the radiation limitation member  24  is absorbed by the radiation limitation member  24 . About 50% of the radiation R that has passed through the first radiation detector  20 A and the radiation limitation member  24  and reached the second radiation detector  20 B is absorbed by the second radiation detector  20 B and is used to capture a radiographic image. 
     That is, the amount of radiation (the amount of charge generated by the second radiation detector  20 B) used to capture a radiographic image by the second radiation detector  20 B is about 20% of the amount of radiation used to capture a radiographic image by the first radiation detector  20 A. In addition, the ratio of the amount of radiation used to capture a radiographic image by the second radiation detector  20 B to the amount of radiation used to capture a radiographic image by the first radiation detector  20 A is not limited to the above-mentioned ratio. However, it is preferable that the amount of radiation used to capture a radiographic image by the second radiation detector  20 B is equal to or greater than 10% of the amount of radiation used to capture a radiographic image by the first radiation detector  20 A in terms of diagnosis. 
     The radiation R is absorbed from a low-energy component. Therefore, for example, as illustrated in  FIG. 6 , the energy components of the radiation R that reaches the second radiation detector  20 B do not include the low-energy components of the energy components of the radiation R that reaches the first radiation detector  20 A. In  FIG. 6 , the vertical axis indicates the amount of radiation R absorbed per unit area and the horizontal axis indicates the energy of the radiation R in a case in which the tube voltage of the radiation source  14  is 80 kV. In addition, in  FIG. 6 , a solid line L 1  indicates the relationship between the energy of the radiation R absorbed by the first radiation detector  20 A and the amount of radiation R absorbed per unit area. In  FIG. 6 , a solid line L 2  indicates the relationship between the energy of the radiation R absorbed by the second radiation detector  20 B and the amount of radiation R absorbed per unit area. 
     Next, the operation of the radiography system  10  according to this embodiment will be described. 
     First, the operation of the console  18  will be described.  FIG. 7  is a flowchart illustrating an example of the flow of an overall imaging process performed by the control unit  90  of the console  18 . Specifically, the CPU  90 A of the control unit  90  executes an overall imaging processing program to perform the overall imaging process illustrated in  FIG. 7 . The control unit  90  executes the overall imaging processing program to function as an example of a derivation unit according to the invention. 
     In this embodiment, the overall imaging process illustrated in  FIG. 7  is performed in a case in which the control unit  90  of the console  18  acquires an imaging menu including, for example, the name of the subject W, an imaging part, and the emission conditions of the radiation R from the user through the operation unit  96 . The control unit  90  may acquire the imaging menu from an external system, such as an RIS, or may acquire the imaging menu input by the user through the operation unit  96 . 
     In Step S 100  of  FIG. 7 , the control unit  90  of the console  18  transmits information included in the imaging menu as an imaging start command to the radiography apparatus  16  through the communication unit  98  and transmits the emission conditions of the radiation R to the radiation emitting apparatus  12  through the communication unit  98 . 
     Then, in Step S 102 , the control unit  90  transmits a command to start the emission of the radiation R to the radiography apparatus  16  and the radiation emitting apparatus  12  through the communication unit  98 . When receiving the emission conditions and the emission start command transmitted from the console  18 , the radiation emitting apparatus  12  starts the emission of the radiation R according to the received emission conditions. The radiation emitting apparatus  12  may comprise an irradiation button. In this case, the radiation emitting apparatus  12  receives the emission conditions and the emission start command transmitted from the console  18  and starts the emission of the radiation R according to the received emission conditions in a case in which the irradiation button is pressed. 
     In the radiography apparatus  16 , the first radiation detector  20 A captures the first radiographic image and the second radiation detector  20 B captures the second radiographic image, on the basis of the information in the imaging menu transmitted from the console  18 , in response to the imaging start command, which will be described in detail below. In the radiography apparatus  16 , the control units  58 A and  58 B perform various correction processes, such as offset correction and gain correction, for first radiographic image data indicating the captured first radiographic image and second radiographic image data indicating the captured second radiographic image, respectively, and store the corrected radiographic image data in the storage unit  64 . 
     Then, in Step S 104 , the control unit  90  determines whether the capture of the radiographic images has ended in the radiography apparatus  16 . A method for determining whether the capture of the radiographic images has ended is not particularly limited. For example, each of the control units  58 A and  58 B of the radiography apparatus  16  transmits end information indicating that imaging has ended to the console  18  through the communication unit  66 . In a case in which the end information is received, the control unit  90  of the console  18  determines that the capture of the radiographic images has ended in the radiography apparatus  16 . 
     For example, each of the control units  58 A and  58 B transmits the first radiographic image data and the second radiographic image data to the console  18  through the communication unit  66  after imaging ends. In a case in which the first radiographic image data and the second radiographic image data are received, the control unit  90  determines that the capture of the radiographic images by the radiography apparatus  16  has ended. In addition, in a case in which the first radiographic image data and the second radiographic image data are received, the console  18  stores the received first radiographic image data and the received second radiographic image data in the storage unit  92 . 
     In a case in which the capture of the radiographic images by the radiography apparatus  16  has not ended, the determination result in Step S 104  is “No” and the control unit  90  waits until the capture of the radiographic images by the radiography apparatus  16  ends. On the other hand, in a case in which the capture of the radiographic images by the radiography apparatus  16  has ended, the determination result in Step S 104  is “Yes” and the control unit  90  proceeds to Step S 106 . 
     In Step S 106 , the control unit  90  performs an image generation process illustrated in  FIG. 8  and ends the overall imaging process. 
     Next, the image generation process performed in Step S 106  of the overall imaging process (see  FIG. 7 ) will be described with reference to  FIG. 8 . 
     In Step S 150  of  FIG. 8 , the control unit  90  of the console  18  acquires the first radiographic image data and the second radiographic image data. In a case in which the first radiographic image data and the second radiographic image data have been stored in the storage unit  92 , the control unit  90  reads and acquires the first radiographic image data and the second radiographic image data from the storage unit  92 . In a case in which the first radiographic image data and the second radiographic image data have not been stored in the storage unit  92 , the control unit  90  acquires the first radiographic image data from the first radiation detector  20 A and acquires the second radiographic image data from the second radiation detector  20 B. 
     Then, in Step S 152 , the control unit  90  generates image data indicating an energy subtraction image, using the first radiographic image data and the second radiographic image data. Hereinafter, the energy subtraction image is referred to as an “ES image” and the image data indicating the energy subtraction image is referred to as “ES image data”. 
     In this embodiment, the control unit  90  subtracts image data obtained by multiplying the first radiographic image data by a predetermined coefficient from image data obtained by multiplying the second radiographic image data by a predetermined coefficient for each corresponding pixel. The control unit  90  generates ES image data indicating an ES image in which soft tissues have been removed and bone tissues have been highlighted, using the subtraction. A method for determining the corresponding pixels of the first radiographic image data and the second radiographic image data is not particularly limited. For example, the amount of positional deviation between the first radiographic image data and the second radiographic image data, which are captured by the radiography apparatus  16  in a state in which a marker is put in advance, may be calculated from the difference between the positions of the marker in the first radiographic image data and the second radiographic image data. Then, the corresponding pixels of the first radiographic image data and the second radiographic image data may be determined on the basis of the calculated amount of positional deviation. 
     In this case, for example, the amount of positional deviation between the first radiographic image data and the second radiographic image data, which are obtained by capturing the image of both the subject W and the marker when the image of the subject W is captured, may be calculated from the difference between the positions of the marker in the first radiographic image data and the second radiographic image data. In addition, for example, the amount of positional deviation between the first radiographic image data and the second radiographic image data may be calculated on the basis of the structure of the subject W in the first radiographic image data and the second radiographic image data obtained by capturing the image of the subject W. 
     Then, in Step S 154 , the control unit  90  determines a bone tissue region (hereinafter, referred to as a “bone region”) in the ES image that is indicated by the ES image data generated in Step S 152 . In this embodiment, for example, the control unit  90  estimates the approximate range of the bone region on the basis of the imaging part included in the imaging menu. Then, the control unit  90  detects pixels that are disposed in the vicinity of the pixels, of which the differential values are equal to or greater than a predetermined value, as the pixels forming the edge (end) of the bone region in the estimated range to determine the bone region. 
     For example, as illustrated in  FIG. 9 , in Step S 154 , the control unit  90  detects the edge E of a bone region B and determines a region in the edge E as the bone region B. For example,  FIG. 9  illustrates an ES image in a case in which the image of a backbone part of the upper half of the body of the subject W is captured. 
     A method for determining the bone region B is not limited to the above-mentioned example. For example, the control unit  90  displays the ES image that is indicated by the ES image data generated in Step S 152  on the display unit  94 . The user designates the edge E of the bone region B in the ES image displayed on the display unit  94  through the operation unit  96 . Then, the control unit  90  may determine a region in the edge E designated by the user as the bone region B. 
     The control unit  90  may display an image in which the ES image and the edge E detected in Step S 154  overlap each other on the display unit  94 . In this case, in a case in which it is necessary to correct the edge E displayed on the display unit  94 , the user corrects the position of the edge E through the operation unit  96 . Then, the control unit  90  may determine a region in the edge E corrected by the user as the bone region B. 
     Then, in Step S 156 , the control unit  90  determines a soft tissue region (hereinafter, referred to as a “soft region”) in the ES image that is indicated by the ES image data generated in Step S 152 . In this embodiment, for example, the control unit  90  determines a region, which is other than the bone region B and has a predetermined area including pixels that are separated from the edge E by a distance corresponding to a predetermined number of pixels in a predetermined direction, as the soft region. For example, as illustrated in  FIG. 9 , in Step S 156 , the control unit  90  determines a plurality of (in the example illustrated in  FIG. 9 , six) soft regions S. 
     The predetermined direction and the predetermined number of pixels may be predetermined by, for example, experiments using the actual radiography apparatus  16  according to the imaging part. The predetermined area may be predetermined or may be designated by the user. In addition, for example, the control unit  90  may determine, as the soft region S, the pixels with pixel values in a predetermined range having the minimum pixel value (a pixel value corresponding to a position where the body thickness of the subject W is the maximum except the bone region B) as the lower limit in the ES image data. In addition, it goes without saying that the number of soft regions S determined in Step S 156  is not limited to that illustrated in  FIG. 9 . 
     Then, in Step S 158 , the control unit  90  corrects the ES image data generated in Step S 152  such that a variation in the ES image in each imaging operation is within an allowable range. In this embodiment, for example, the control unit  90  performs a correction process of removing image blur in the entire frequency band of the ES image data. The image data corrected in Step S 158  is used to calculate bone density in a process from Step S 160  to Step S 164  which will be described below. Therefore, hereinafter, the corrected image data is referred to as “dual-energy X-ray absorptiometry (DXA) image data”. 
     Then, in Step S 160 , the control unit  90  calculates an average value A 1  of the pixel values of the bone region B in the D×A image data. Then, in Step S 162 , the control unit  90  calculates an average value A 2  of the pixel values of all of the soft regions S in the D×A image data. Here, in this embodiment, for example, the control unit  90  performs weighting such that the soft region S which is further away from the edge E has a smaller pixel value and calculates the average value A 2 . Before the average values A 1  and A 2  are calculated in Step S 160  and Step S 162 , respectively, abnormal values of the pixel values of the bone region B and the pixel values of the soft region S may be removed by, for example, a median filter. 
     Then, in Step S 164 , the control unit  90  calculates the bone density of the imaging part of the subject W. In this embodiment, for example, the control unit  90  calculates the difference between the average value A 1  calculated in Step S 160  and the average value A 2  calculated in Step S 162 . In addition, the control unit  90  multiplies the calculated difference by a conversion coefficient for converting the pixel value into bone mass [g] to calculate the bone mass. Then, the control unit  90  divides the calculated bone mass by the area [cm 2 ] of the bone region B to calculate bone density [g/cm 2 ]. The conversion coefficient may be predetermined by, for example, experiments using the actual radiography apparatus  16  according to the imaging part. 
     Then, in Step S 166 , the control unit  90  stores the ES image data generated in Step S 152  and the bone density calculated in Step S 164  in the storage unit  92  so as to be associated with information for identifying the subject W. In addition, for example, the control unit  90  may store the ES image data generated in Step S 152 , the bone density calculated in Step S 164 , the first radiographic image data, and the second radiographic image data in the storage unit  92  so as to be associated with the information for identifying the subject W. 
     Then, in Step S 168 , the control unit  90  displays the ES image indicated by the ES image data generated in Step S 152  and the bone density calculated in Step S 164  on the display unit  94  and then ends the image generation process. 
     Next, the operation of the radiography apparatus  16  according to this embodiment will be described. 
     As described above, when the radiography apparatus  16  according to this embodiment receives an imaging start command from the console  18 , the first radiation detector  20 A captures the first radiographic image and the second radiation detector  20 B captures the second radiographic image under the control of the integrated control unit  71 . 
       FIG. 10  is a flowchart illustrating an example of the flow of an imaging control process performed by the integrated control unit  71 . Specifically, when the imaging start command is received from the console  18 , the CPU  72  of the integrated control unit  71  executes an imaging control processing program that is stored in the ROM of the memory  74  in advance to perform the imaging control process illustrated in  FIG. 10 . The imaging control processing program is an example of a program including a radiography program according to the invention. In addition, the integrated control unit  71  executes an imaging processing program to function as an example of a specification unit according to the invention and to make the radiography apparatus  16  function as the radiography system  10  according to the invention. 
     In Step S 200  of  FIG. 10 , the integrated control unit  71  determines which of the first detection result indicating the detection result of the start of the emission of the radiation R by the control unit  58 A and the second detection result indicating the detection result of the start of the emission of the radiation R by the control unit  58 B priority is given to. The determination in Step S 200  may be performed only in a case in which the first detection result and the second detection result are different from each other. 
     In this embodiment, a method for determining the detection result with higher priority is not particularly limited. For example, in a case in which information indicating the priority of the detection results is set in the storage unit  76  of the integrated control unit  71  in advance, the set detection results may be read. In this case, as described above, since the amount of radiation R that reaches the second radiation detector  20 B is less than the amount of radiation R that reaches the first radiation detector  20 A, it is preferable that settings for giving priority to the first detection result obtained by the first radiation detector  20 A are performed. 
     For example, as in the example illustrated in  FIG. 11 , the integrated control unit  71  may display a selection screen  100  that allows the user to select the detection result with priority on the display unit  94  of the console  18  through the communication unit  66  and perform the determination on the basis of the selection result of the user through the operation unit  96 . According to the selection screen  100  illustrated in  FIG. 11 , in a case in which the user selects the first detection result obtained by the first radiation detector  20 A, the user selects a selection box  100 A through the operation unit  96 . In a case in which the user selects the second detection result obtained by the second radiation detector  20 B, the user selects a selection box  100 B through the operation unit  96  and operates a decision button  100 C through the operation unit  96 . Then, the operation result is output from the console  18  to the radiography apparatus  16  through the communication unit  98 . In this case, the operation unit  96  is an example of a detection result setting unit according to the invention. 
     In a case in which priority is given to the first detection result, the determination result in Step S 200  is “Yes” and the process proceeds to Step S 202 . In Step S 202 , the integrated control unit  71  determines whether the first detection result has been received from the control unit  58 A. In a case in which the first detection result has not been received, the determination result in Step S 202  is “No” and the integrated control unit  71  waits until the first detection result is received. On the other hand, in a case in which the first detection result has been received, the determination result in Step S 202  is “Yes” and the proceeds to Step S 206 . 
     In contrast, in a case in which priority is given to the second detection result, the determination result in Step S 200  is “No” and the process proceeds to Step S 204 . In Step S 204 , the integrated control unit  71  determines whether the second detection result has been received from the control unit  58 B. In a case in which the second detection result has not been received, the determination result in Step S 204  is “No” and the integrated control unit  71  waits until the second detection result is received. On the other hand, in a case in which the second detection result has been received, the determination result in Step S 204  is “Yes” and the proceeds to Step S 206 . 
     In Step S 206 , the integrated control unit  71  outputs an accumulation start command to the control unit  58 A and the control unit  58 B. 
     Then, in Step S 208 , the integrated control unit  71  determines whether a first noise detection result indicating the detection result of the generation of noise by the control unit  58 A has been received from the control unit  58 A or a second noise detection result indicating the detection result of the generation of noise by the control unit  58 B has been received from the control unit  58 B. 
     In a case in which at least one of the first noise detection result or the second noise detection result has been received, the determination result in Step S 208  is “Yes” and the process proceeds to Step S 210 . In Step S 210 , the integrated control unit  71  outputs an accumulation stop command to the control unit  58 A and the control unit  58 B, returns to Step S 200 , and repeatedly performs the process in Steps S 200  to S 208 . 
     On the other hand, in a case in which neither the first noise detection result nor the second noise detection result has been received after a predetermined period of time has elapsed, the determination result in Step S 208  is “No” and the integrated control unit  71  ends the imaging control process. The predetermined period of time in this step is not particularly limited. An example of the predetermined period of time is a charge accumulation period in the radiation detector  20 , which will be described in detail below. 
       FIG. 12  is a flowchart illustrating an example of the flow of a first imaging process performed by the control unit  58 A and an example of the flow of a second imaging process performed by the control unit  58 B in the radiography apparatus  16 . Specifically, when an imaging start command is received from the console  18 , the CPU  60  of the control unit  58 A executes a first imaging processing program that is stored in the ROM of the memory  62  in advance to perform the first imaging process illustrated in  FIG. 12 . In addition, when the imaging start command is received from the console  18 , the CPU  60  of the control unit  58 B executes a second imaging processing program that is stored in the ROM of the memory  62  in advance to perform the second imaging process illustrated in  FIG. 12 . 
     In Step S 250  of  FIG. 12 , the control unit  58 A determines whether the value of the first digital signal is equal to or greater than a predetermined start threshold value for detecting the start of the emission of the radiation R. Until image data is read in Step S 270  which will be described below or until a reset operation is performed in Step S 266 , all of the thin film transistors  33 C of the pixels  32  in the first radiation detector  20 A are in an off state. However, as described above, a first electric signal corresponding to the charge which is read from the pixel  32 B for detecting radiation regardless of a switching state is transmitted through the data line  36 , is converted into the first digital signal by the signal processing unit  54 A, and is output to the control unit  58 A. 
     In a case in which the value of the first digital signal is equal to or greater than the predetermined start threshold value for detecting the start of the emission of the radiation R, the determination result in Step S 250  is “Yes” and the process proceeds to Step S 252 . In Step S 252 , the control unit  58 A outputs the first detection result indicating that the start of the emission of the radiation R has been detected to the integrated control unit  71  and proceeds to Step S 254 . 
     On the other hand, in a case in which the value of the first digital signal is less than the start threshold value in Step S 250 , the determination result is “No” and the process proceeds to Step S 254 . As such, the control unit  58 A according to this embodiment uses a method that detects the time when the value of the first digital signal is equal to or greater than the start threshold value as the time when the emission of the radiation R has started. However, a method for detecting the time when the emission of the radiation R has started is not limited thereto. For example, the time when the value of the first digital signal is greater than the start threshold value may be detected as the time when the emission of the radiation R has started or the time when a variation in the first digital signal per unit time is equal to or greater than a predetermined start threshold value may be detected as the time when the emission of the radiation R has started. 
     In this embodiment, the time when the emission of the radiation R has started is an example of a predetermined time related to the emission of radiation according to the invention. As in the example illustrated in  FIG. 13 , the amount of radiation R emitted from the radiation source  14  of the radiation emitting apparatus  12  varies depending on the irradiation time. In the radiography apparatus  16  according to this embodiment, a period from a time T 1  to a time T 2  illustrated in  FIG. 13  is used as an accumulation period for which the above-mentioned accumulation operation is performed, according to the amount of radiation R that is emitted from the radiation source  14  to the radiography apparatus  16 . Therefore, the time T 1  is detected the time when the emission of the radiation R has started. Thus, the time when the radiation source  14  actually starts to emit the radiation R is different from the time when the radiography apparatus  16  starts to be irradiated with the radiation R. In addition, for example, the time T 1  is determined in terms of an error in the detection of time. 
     In Step S 254 , the control unit  58 A determines whether an accumulation start command has been received from the integrated control unit  71 . In a case in which the accumulation start command has not been received, the determination result in Step S 254  is “No” and the control unit  58 A returns to Step S 250 . In a case in which the process proceeds to Step S 254  after Step S 252  and the accumulation start command has not been received, the control unit  58 A may not return to Step S 250  and wait until the accumulation start command is received. On the other hand, in a case in which the accumulation start command has been received, the determination result in Step S 254  is “Yes” and the control unit  58 A proceeds to Step S 256 . 
     In Step S 256 , the control unit  58 A starts an accumulation operation. When the accumulation operation starts, the first radiation detector  20 A proceeds to the accumulation period for which charge generated by the emitted radiation R is accumulated in the pixel  32 . Specifically, the control unit  58 A controls the gate line driver  52 A such that an off signal is output from the gate line driver  52 A to each gate line  34  of the first radiation detector  20 A. Then, each thin film transistor  33 C connected to each gate line  34  is turned off. As described above, after the accumulation operation starts, an electric signal corresponding to the charge that is read from the pixel  32 B for detecting radiation is transmitted through the data line  36 , is converted into the first digital signal by the signal processing unit  54 A, and is output to the control unit  58 A. 
     Then, in Step S 258 , the control unit  58 A determines whether the inclusion of noise in the first digital signal has been detected. A method for detecting the inclusion of noise in the first digital signal in the control unit  58 A is not particularly limited. Noise generated in the radiation detector  20  is disclosed in, for example, JP2014-023957A and a noise detection method disclosed in JP2014-023957A may be applied to this embodiment. 
     For example, in some cases, charge which will be noise is generated in the sensor unit  33 A due to disturbance, such as impact and electromagnetic waves, particularly, vibration. An electric signal caused by noise (charge) that is generated due to disturbance has the characteristic that it is different from an electric signal caused by charge that is generated by irradiation with the radiation R in a general radiographic image. Particularly, the electric signals are different in a variation over time. For example, in a case in which noise is included, charge flows in the opposite direction. As a result, the polarity of the electric signal is likely to be opposite to the general polarity. In addition, in a case in which noise is included, a waveform indicating a variation in the electric signal over time has an amplitude. 
     In this embodiment, after the accumulation operation starts in Step S 256 , the control unit  58 A detects whether noise has been generated, on the basis of whether a variation in a digital signal over time within a predetermined detection period has the above-mentioned characteristic of noise. As a detailed detection method, for example, the following methods are used: a method that detects whether noise has been generated, on the basis of whether a digital signal has a polarity opposite to the general polarity; a method that detects whether noise has been generated, on the basis of whether a gradient is reduced when differentiation (for example, first-order differentiation or second-order differentiation) is performed for a digital signal output within a predetermined period, for example, a method that differentiates the digital signal and detects that no noise has been generated in a case in which the gradient is substantially constant or is expected to gradually increase; and a method that detects whether noise has been generated, using a noise determination threshold value. In addition, it is preferable that a combination of a plurality of kinds of detection methods is used in order to increase the accuracy of detecting noise. 
     In a case in which the inclusion of noise in the first digital signal has been detected, the determination result in Step S 258  is “Yes” and the control unit  58 A proceeds to Step S 260 . In Step S 260 , the control unit  58 A outputs the first noise detection result indicating that the inclusion of noise has been detected to the integrated control unit  71  and proceeds to Step S 262 . 
     On the other hand, in a case in which the inclusion of noise in the first digital signal has not been detected in Step S 258 , the determination result is “No” and the control unit  58 A proceeds to Step S 262 . 
     Then, in Step S 262 , the control unit  58 A determines whether an accumulation stop command has been received from the integrated control unit  71 . In a case in which the accumulation stop command has been received, the determination result in Step S 262  is “Yes” and the control unit  58 A proceeds to Step S 264 . 
     In Step S 264 , the control unit  58 A stops the operation of accumulating charge in the pixel  32 . Then, in Step S 266 , the control unit  58 A performs a reset operation of resetting the charge accumulated in the pixel  32  and returns to Step S 250 . Specifically, the control unit  58 A controls the gate line driver  52 A such that an on signal is output from the gate line driver  52 A to each gate line  34  of the first radiation detector  20 A. Then, each thin film transistor  33 C connected to each gate line  34  is turned on and the charge accumulated in the capacitor  33 B is output to the data line  36 . 
     The period for which the reset operation is performed is a dead period (non-detection period) for which the start of the emission of the radiation R is not detected. Therefore, it is preferable to perform the reset operation for a plurality of gate lines  34  at the same time in order to shorten the dead period. In addition, during the reset operation, a command to stop the emission of the radiation R may be output to the radiation emitting apparatus  12  through the communication unit  66 . 
     In contrast, in a case in which the accumulation stop command has not been received in Step S 262 , the determination result is “No” and the process proceeds to Step S 268 . In a case in which the process proceeds to Step S 262  after Step S 260  and the accumulation start command has not been received, the control unit  58 A may wait until the accumulation start command is received, without proceeding to Step S 268 . 
     In Step S 268 , the control unit  58 A determines whether to end the accumulation of charge. A method for determining whether to end the accumulation of charge is not particularly limited. For example, in a case in which a predetermined accumulation period has elapsed since the accumulation start command has been received, the control unit  58 A may determine to end the accumulation of charge. In this case, in a case in which the predetermined accumulation period has not elapsed, the determination result in Step S 268  is “No” and the process returns to Step S 258 . On the other hand, in a case in which the predetermined accumulation period has elapsed, the determination result in Step S 268  is “Yes” and the process proceeds to Step S 270 . 
     Then, in Step S 270 , the control unit  58 A ends the accumulation operation, proceeds to a reading period for which the charge accumulated in the pixel  32  is read, starts a reading operation, and controls the gate line driver  52 A such that an on signal is sequentially output from the gate line driver  52 A to each gate line  34  of the first radiation detector  20 A. Then, the lines of the thin film transistors  33 C connected to each gate line  34  are sequentially turned on and charge accumulated in each line of the capacitors  33 B sequentially flows as an electric signal to each data line  36 . Specifically, charge accumulated in the capacitors  33 B of the pixels  32 A for capturing a radiographic image flows as an electric signal to the data line  36 . Then, the electric signal that has flowed to each data line  36  is converted into digital image data by the signal processing unit  54 A, is output from the control unit  58 A to the image memory  56 A, and is then stored in the image memory  56 A. 
     Then, in Step S 272 , the control unit  58 A performs image processing including various correction processes, such as offset correction and gain correction, for the image data stored in the image memory  56 A in Step S 270 . Then, in Step S 274 , the control unit  58 A transmits the image data (first radiographic image data) processed in Step S 272  to the integrated control unit  71  and ends the first imaging process. 
     As illustrated in  FIG. 12 , the first imaging process and the second imaging process are the same process. In the second imaging process, the control unit  58 B may replace the control unit  58 A, the second digital signal may replace the first digital signal, the second detection result may replace the first detection result, and the second noise detection result may replace the first noise detection result. In addition, the gate line driver  52 B may replace the gate line driver  52 A, the signal processing unit  54 B may replace the signal processing unit  54 A, and the image memory  56 B may replace the image memory  56 A. Therefore, the description of the components will not be repeated. 
     As described above, since the amount of radiation R that reaches the second radiation detector  20 B is less than the amount of radiation R that reaches the first radiation detector  20 A, the start threshold value used by the first radiation detector  20 A may be different from the start threshold value used by the second radiation detector  20 B. 
     As described above, the radiography system  10  according to this embodiment comprises: the radiography apparatus  16  comprising the first radiation detector  20 A in which a plurality of pixels  32 , each of which includes the sensor unit  33 A that generates a larger amount of charge as it is irradiated with a larger amount of radiation R, are two-dimensionally arranged and the second radiation detector  20 B which is provided so as to be stacked on the side of the first radiation detector  20 A from which the radiation R is transmitted and emitted and in which a plurality of pixels  32 , each of which includes the sensor unit  33 A that generates a larger amount of charge as it is irradiated with a larger amount of radiation R, are two-dimensionally arranged; and the integrated control unit  71  that specifies a predetermined time related to the emission of the radiation R, on the basis of a first detection result that is a detection result of a predetermined time related to the emission of the radiation R using a first electric signal (first digital signal) which is obtained by converting charge generated in the pixels  32  of the first radiation detector  20 A and of which the level increases as the amount of charge generated increases and a second detection result that is a detection result of a predetermined time related to the emission of the radiation R using a second electric signal (second digital signal) which is obtained by converting charge generated in the pixels  32  of the second radiation detector  20 B and of which the level increases as the amount of charge generated increases. 
     In the radiography apparatus  16  according to this embodiment, the amount of radiation that reaches the second radiation detector  20 B is less than the amount of radiation that reaches the first radiation detector  20 A. Therefore, in some cases, the first detection result which is the detection result of the predetermined time related to the emission of the radiation R using the first digital signal output from the first radiation detector  20 A is different from the second detection result which is the detection result of the predetermined time related to the emission of the radiation R using the second digital signal output from the second radiation detector  20 B. 
     In the radiography apparatus  16  according to this embodiment, the integrated control unit  71  specifies the time when the emission of the radiation R starts, using the first detection result and the second detection result, specifically, one of the first detection result and the second detection result which has higher priority. 
     Therefore, according to the radiography system  10  of each of the above-described embodiments, it is possible to appropriately detect the emission of the radiation R even when the amount of radiation R emitted to the second radiation detector is less than the amount of radiation R emitted to the first radiation detector. 
     In this embodiment, the case in which the control unit  58 A and the control unit  58 B detects the time when the emission of the radiation R starts as the predetermined time related to the emission of the radiation R has been described. However, the invention is not limited thereto. For example, the control unit  58 A and the control unit  58 B may detect the time when the emission of the radiation R is stopped like the time T 2  illustrated in  FIG. 13 . In this case, for example, the control unit  58 A and the control unit  58 B may compare the value of the above-mentioned digital signal with a predetermined stop threshold value for detecting the stop of the emission of the radiation R and may determine that it is time to stop the emission of the radiation R in a case in which the value of the digital signal is less than the stop threshold value. In addition, as such, in a case in which the time when the emission of the radiation R is stopped is detected, the control unit  58 A and the control unit  58 B may end the accumulation of charge in the pixel  32  and may proceed to the reading period. 
     In this embodiment, the case in which an indirect-conversion-type radiation detector that converts radiation into light and converts the converted light into charge is applied to both the first radiation detector  20 A and the second radiation detector  20 B has been described. However, the invention is not limited thereto. For example, a direct-conversion-type radiation detector that directly converts radiation into charge may be applied to at least one of the first radiation detector  20 A or the second radiation detector  20 B. 
     In the radiography apparatus  16  according to this embodiment, the aspect in which the pixels  32  comprise the pixel  32 B for detecting radiation in which the thin film transistor  33 C is short-circuited and the predetermined time related to the emission of the radiation R is detected using the electric signal generated by charge output from the pixel  32 B for detecting radiation has been described. However, the invention is not limited thereto. For example, a technique disclosed in JP2014-023957A can be applied to detect the predetermined time related to the emission of the radiation R. Specifically, for example, all of the pixels  32  connected to a specific gate line  34  may be used as the pixels  32 B for detecting radiation. In this case, the pixel  32 B for detecting radiation comprises a thin film transistor  33 C that is not short-circuited. In a case in which the predetermined time related to the emission of the radiation R is detected, the control unit  58 A and the control unit  58 B control the gate line driver  52 A and the gate line driver  52 B such that the on signals are output from the gate line driver  52 A and the gate line driver  52 B to the gate lines  34  connected to the pixels  32 B for detecting radiation in the first radiation detector  20 A and the second radiation detector  20 B, respectively. In addition, for example, a first electric signal output from a sensor that is provided so as to correspond to the first radiation detector  20 A and outputs the first electric signal of which the level increases as the amount of radiation R detected increases and a second electric signal output from a sensor that is provided so as to correspond to the second radiation detector  20 B and outputs the second electric signal of which the level increases as the amount of radiation R detected increases may be used. 
     In this embodiment, in a case in which at least one of the first noise detection result or the second noise detection result indicates that the generation of noise has been detected, the integrated control unit  71  directs the control unit  58 A and the control unit  58 B to stop the accumulation of charge in each pixel  32 . However, the invention is not limited thereto. For example, similarly to the first detection result and the second detection result, in a case in which the first noise detection result and the second noise detection result are different from each other, priority may be given to one of the noise detection results. In this case, for example, in a case in which information indicating which of the noise detection results has priority is set in the storage unit  76  of the integrated control unit  71  in advance, the set noise detection result may be read. In this case, as described above, since the amount of radiation R that reaches the second radiation detector  20 B is less than the amount of radiation R that reaches the first radiation detector  20 A, it is preferable that settings for giving priority to the first noise detection result obtained by the first radiation detector  20 A are performed. 
     For example, as in the example illustrated in  FIG. 14 , the integrated control unit  71  may display a selection screen  102  that allows the user to select the noise detection result having priority on the display unit  94  of the console  18  through the communication unit  66  and may perform determination on the basis of the selection result selected by the user through the operation unit  96 . According to the selection screen  102  illustrated in  FIG. 14 , in a case in which the user selects the first noise detection result obtained by the first radiation detector  20 A, the user selects a selection box  102 A through the operation unit  96 . In a case in which the user selects the second noise detection result obtained by the second radiation detector  20 B, the user selects a selection box  102 B through the operation unit  96  and operates a decision button  102 C through the operation unit  96 . Then, the operation result is output from the console  18  to the radiography apparatus  16  through the communication unit  98 . In this case, the operation unit  96  is an example of a noise detection result setting unit according to the invention. 
     In this embodiment, as described above, the case in which the control unit  58 A detects noise included in the first digital signal, using the first digital signal, and the control unit  58 B detects noise included in the second digital signal, using the second digital signal has been described. However, a structure for detecting noise is not limited thereto. 
     For example, as illustrated in  FIG. 15 , the radiography apparatus  16  may further comprise a detection unit  59 A and a detection unit  59 B. The control unit  58 A may detect that noise is included in the first digital signal, using the detection result of the detection unit  59 A, and the control unit  58 B may detect that noise is included in the second digital signal, using the detection result of the detection unit  59 B. 
     The detection unit  59 A is not particularly limited as long as it can detect at least one of an impact or electromagnetic waves applied from the outside to the first radiation detector  20 A. In addition, the detection unit  59 B is not particularly limited as long as it can detect at least one of impact or electromagnetic waves applied from the outside to the second radiation detector  20 B. In this case, the term “outside” may be the outside of each of the first radiation detector  20 A and the second radiation detector  20 B or may be one of the inside and the outside of the radiography apparatus  16 . In this case, the detection unit  59 A is an example of a first detection unit according to the invention and the detection unit  59 B is an example of a second detection unit according to the invention. 
     For example, an impact sensor that directly detects an impact or an electromagnetic wave sensor that detects electromagnetic waves may be used as the detection unit  59 A and the detection unit  59 B. In a case in which the detection unit  59 A and the detection unit  59 B are impact sensors, an example of the detection unit  59 A and the detection unit  59 B is an acceleration sensor. In a case in which the impact sensor is used, it is preferable that the impact sensor is electro-magnetically shielded. 
     For example, in a case in which the detection unit  59 A is the impact sensor, when detecting that the generation of an impact on the first radiation detector  20 A is detected, the detection unit  59 A outputs a signal indicating the generation of the impact as the detection result to the control unit  58 A. In a case in which the detection unit  59 B is the impact sensor, similarly, when detecting that the generation of an impact on the second radiation detector  20 B is detected, the detection unit  59 B outputs a signal indicating the generation of the impact as the detection result to the control unit  58 B. 
     Then, the control unit  58 A detects whether noise is included in the first digital signal, using the detection result of the detection unit  59 A, specifically, on the basis of whether the signal indicating the generation of the impact is input from the detection unit  59 A, in Step S 258  (see  FIG. 12 ) of the first imaging process. Similarly, the control unit  58 B detects whether noise is included in the second digital signal, using the detection result of the detection unit  59 B, specifically, on the basis of whether the signal indicating the generation of the impact is input from the detection unit  59 B, in Step S 258  (see  FIG. 12 ) of the second imaging process. 
     In this embodiment, the case in which the irradiation side sampling radiation detectors in which the radiation R is incident from the side of the TFT substrates  30 A and  30 B are applied to the first radiation detector  20 A and the second radiation detector  20 B, respectively, has been described. However, the invention is not limited thereto. For example, a so-called penetration side sampling (PSS) radiation detector in which the radiation R is incident from the side of the scintillator  22 A or  22 B may be applied to at least one of the first radiation detector  20 A or the second radiation detector  20 B. 
     In this embodiment, the case in which the radiography apparatus  16  is controlled by three control units (control units  58 A,  58 B, and  71 ) has been described. However, the invention is not limited thereto. For example, one of the control unit  58 A and the control unit  58 B may have the functions of the integrated control unit  71  or the integrated control unit  71  may have the functions of the control unit  58 A and the control unit  58 B. In addition, the radiography apparatus  16  may be controlled by one control unit. 
     In this embodiment, for example, the case in which the integrated control unit  71  of the radiography apparatus  16  has the functions of the specification unit according to the invention has been described. However, the invention is not limited thereto. For example, the control unit  90  of the console  18  may execute the imaging control processing program (see  FIG. 10 ) to function as an example of the specification unit according to the invention. 
     In this embodiment, the case in which bone density is derived using the first radiographic image and the second radiographic image has been described. However, the invention is not limited thereto. For example, bone mineral content or both bone density and bone mineral content may be derived using the first radiographic image and the second radiographic image. 
     In this embodiment, the aspect in which the overall imaging processing program is stored (installed) in the ROM  90 B in advance, the imaging control processing program is stored in the memory  74  in advance, the first imaging processing program is stored in the memory  62  in advance, and the second imaging processing program is stored in the memory  62  in advance has been described. However, the invention is not limited thereto. Each of the overall imaging processing program, the imaging control processing program, the first imaging processing program, and the second imaging processing program may be recorded in a recording medium, such as a compact disk read only memory (CD-ROM), a digital versatile disk read only memory (DVD-ROM), or a universal serial bus (USB) memory, and then provided. In addition, each of the overall imaging processing program, the imaging control processing program, the first imaging processing program, and the second imaging processing program may be downloaded from an external apparatus through a network.