Abstract:
The present invention pertains to a radiography system and a radiography method. A radiation output device has a plurality of radiation sources disposed along a predetermined plane. Also, in accordance with whether an imaging state is a still image mode or a moving picture mode, at least one of the radiation sources that outputs radiation among the plurality of radiation sources is selected.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS AND PRIORITY CLAIM 
       [0001]    This application is a Continuation of International Application No. PCT/JP2012/068321 filed on Jul. 19, 2012, which was published under PCT Article 21(2) in Japanese, which is based upon and claims the benefit of priority from Japanese Patent Application No. 2011-214547 filed on Sep. 29, 2011, the contents all of which are incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a radiographic image capturing system and a radiographic image capturing method (radiography system and radiography method) using a radiation output device having a plurality of radiation sources arranged along a predetermined surface. 
       BACKGROUND ART 
       [0003]    Recently, a variety of radiation output devices have been proposed that incorporate plural radiation sources arranged along a predetermined surface. Various radiographic image capturing methods using such radiation output devices have also been proposed. 
         [0004]    Japanese Laid-Open Patent Publication No. 2010-115270 states that an observational area can be changed, e.g., scaled up and down, by controlling the size and position of a collimator in a fluoroscopy (moving image capturing) technique. 
       SUMMARY OF INVENTION 
       [0005]    Radiographic image capturing processes are roughly classified into two image capturing modes depending on whether a temporal element is involved or not. More specifically, one image capturing mode is defined by a still image mode for capturing a radiographic image in a single image capturing event, and another image capturing mode is defined by a moving image mode for capturing a succession of radiographic images in successive image capturing events. Japanese Laid-Open Patent Publication No. 2010-115270 makes no specific proposals in relation to performing a radiographic image capturing process using both a still image mode and a moving image mode. Therefore, an unsolved problem remains as to how a plurality of radiation sources should be used. 
         [0006]    The present invention has been made in view of the aforementioned problem. It is an object of the present invention to provide a radiographic image capturing system and a radiographic image capturing method, which are capable of greatly increasing the efficiency with which an operator is able to work in carrying out a radiographic image capturing process using both a still image mode and a moving image mode. 
         [0007]    According to the present invention, there is provided a radiographic image capturing system comprising a radiation output device having a plurality of radiation sources arranged along a predetermined surface, and an output control portion for controlling emissions of radiation, respectively, from the radiation sources of the radiation output device, wherein the output control portion includes a radiation source selecting portion for selecting at least one of the radiation sources for emitting the radiation depending on whether an image capturing mode is a still image mode or a moving image mode. 
         [0008]    Since the output control portion selects at least one of the radiation sources for emitting radiation depending on whether the image capturing mode is a still image mode or a moving image mode, it is possible to emit radiation from a position suitable for the image capturing mode, without the need for moving the radiation sources and positioning the subject differently. Consequently, the efficiency with which an operator works in capturing images in the still image mode and the moving image mode is greatly increased. 
         [0009]    Preferably, the predetermined surface comprises a planar surface, and the radiation sources are arranged in a matrix. 
         [0010]    Preferably, the output control portion further includes a group determining portion for determining a first radiation source group to be used in the still image mode from among the radiation sources, and the radiation source selecting portion selects at least one of the radiation sources from among the first radiation source group determined by the group determining portion in a case where the image capturing mode is the still image mode. Consequently, it is possible to emit radiation from at least one position, which is suitable for a radiographic image capturing process in the still image mode. 
         [0011]    Preferably, the group determining portion determines a second radiation source group to be used in the moving image mode from among the radiation sources, and the radiation source selecting portion selects at least one of the radiation sources from among the second radiation source group determined by the group determining portion in a case where the image capturing mode is the moving image mode. Consequently, it is possible to emit radiation from at least one position, which is suitable for a radiographic image capturing process in the moving image mode. 
         [0012]    Preferably, the group determining portion determines the first radiation source group and the second radiation source group such that each of the radiation sources belongs to either one of the first radiation source group and the second radiation source group. 
         [0013]    Preferably, the radiation source selecting portion selects at least one of the radiation sources depending on a positional relationship between a region of interest of a subject as a target to be imaged and each of the radiation sources. Consequently, it is possible to perform a radiographic image capturing process that is suitable for the region of interest, regardless of whether the image capturing mode is the still image mode or the moving image mode. 
         [0014]    Preferably, in the case where the image capturing mode is the still image mode, the radiation source selecting portion selects at least one of the radiation sources, which resides at a position located a short distance with respect to the region of interest. 
         [0015]    Preferably, in the case where the image capturing mode is the still image mode, the radiation source selecting portion selects at least one of the radiation sources, which resides at a position having a small irradiation angle with respect to the region of interest. 
         [0016]    Preferably, the radiation source selecting portion selects a cluster of at least two of the radiation sources, as the predetermined surface is viewed in plan. In this manner, the respective emissions of radiation, which are emitted simultaneously from the selected radiation sources, are closely bundled, so as to prevent a resultant radiographic image from suffering from geometric distortions. 
         [0017]    Preferably, the output control portion successively controls the emissions of radiation depending on the still image mode at longer time intervals than a frame interval of the moving image mode. Consequently, images can reliably be captured in the still image mode without a loss in timing, even though states of the target to be imaged (characteristics of the radiographic images) are changed from time to time. 
         [0018]    Preferably, the radiographic image capturing system further comprises a moving mechanism for moving the radiation sources in unison with each other. 
         [0019]    Preferably, the radiographic image capturing system further comprises a radiographic image capturing apparatus for converting the radiation emitted respectively from the radiation sources into a radiographic image. 
         [0020]    According to the present invention, there also is provided a radiographic image capturing method using a radiation output device having a plurality of radiation sources arranged along a predetermined surface, comprising the step of selecting at least one of the radiation sources for emitting radiation from among the plurality of radiation sources of the radiation output device, depending on whether an image capturing mode is a still image mode or a moving image mode. 
         [0021]    With the radiographic image capturing system and the radiographic image capturing method according to the present invention, inasmuch as at least one radiation source for emitting radiation is selected from among the plural radiation sources depending on whether the image capturing mode is the still image mode or the moving image mode, it is possible to emit radiation from a position that is suitable for the image capturing mode, without the need for moving the radiation sources and positioning the subject differently. Consequently, the efficiency with which an operator works in capturing images in the still image mode and the moving image mode is significantly increased. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0022]      FIG. 1  is a diagram of a radiographic image capturing system according to an embodiment of the present invention; 
           [0023]      FIG. 2  is a view of a radiation output device and an electronic cassette shown in  FIG. 1 ; 
           [0024]      FIGS. 3A and 3B  are schematic plan views showing by way of example a layout of radiation sources of the radiation output device shown in  FIG. 2 ; 
           [0025]      FIG. 4  is an electric block diagram of the radiographic image capturing system shown in  FIG. 1 ; 
           [0026]      FIG. 5  is a circuit diagram of a circuit arrangement of the electronic cassette shown in  FIG. 4 ; 
           [0027]      FIG. 6  is a flowchart of an operation sequence of the radiographic image capturing system shown in  FIG. 1 ; 
           [0028]      FIG. 7A  is a diagram showing by way of example first and second radiation source groups determined for capturing an image of a region of interest located in a first position; 
           [0029]      FIG. 7B  is a diagram showing a manner in which radiation is emitted toward the region of interest located in the first position, in the case that the radiographic image capturing system is in a still image mode; 
           [0030]      FIG. 7C  is a diagram showing a manner in which radiation is emitted toward the region of interest located in the first position, in the case that the radiographic image capturing system is in a moving image mode; 
           [0031]      FIG. 8A  is a diagram showing by way of example first and second radiation source groups determined for capturing an image of a region of interest located in a second position; 
           [0032]      FIG. 8B  is a diagram showing a manner in which radiation is emitted toward the region of interest located in the second position, in the case that the radiographic image capturing system is in a still image mode; 
           [0033]      FIG. 8C  is a diagram showing a manner in which radiation is emitted toward the region of interest located in the second position, in the case that the radiographic image capturing system is in a moving image mode; 
           [0034]      FIG. 9A  is a diagram showing by way of example first and second radiation source groups determined for capturing an image of a region of interest located in a third position; 
           [0035]      FIG. 9B  is a diagram showing a manner in which radiation is emitted toward the region of interest located in the third position, in the case that the radiographic image capturing system is in a still image mode; 
           [0036]      FIG. 9C  is a diagram showing a manner in which radiation is emitted toward the region of interest located in the third position, in the case that the radiographic image capturing system is in a moving image mode; 
           [0037]      FIG. 10A  is a diagram showing a manner in which radiation is emitted toward plural regions of interest, in the case that the radiographic image capturing system is in a still image mode; 
           [0038]      FIG. 10B  is a diagram showing by way of example first and second radiation source groups determined for capturing an image of plural regions of interest; 
           [0039]      FIG. 11  is a schematic plan view showing by way of example a layout of radiation sources of a radiation output device according to a first modification; and 
           [0040]      FIG. 12  is an electric block diagram of a radiographic image capturing system according to a second modification. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0041]    A radiographic image capturing method according to a preferred embodiment of the present invention, which is carried out by way of a radiographic image capturing system, will be described below with reference to the accompanying drawings. 
       [Arrangement of Radiographic Image Capturing System  10 ] 
       [0042]    As shown in  FIG. 1 , a radiographic image capturing system  10  includes a radiation output device  18  for applying radiation  16  to a patient as a subject  14  lying on an image capturing table  12  such as a bed or the like, an electronic cassette  20  for detecting radiation  16  that has passed through the subject  14  and converting the detected radiation  16  into a radiographic image, a console  22  for controlling the radiation output device  18  and the electronic cassette  20 , and a display device  24  for displaying the radiographic image. 
         [0043]    The console  22 , the radiation output device  18 , the electronic cassette  20 , and the display device  24  send signals to and receive signals from each other via UWB (Ultra Wide Band) communications, over a wireless LAN (Local Area Network) according to standards such as IEEE 802.11.a/b/g/n, or by way of millimeter-wave communications. Alternatively, such signals may be sent and received mutually via wired communications using cables. 
         [0044]    The console  22  is connected to a radiology information system (RIS)  26 , which generally manages radiographic images and other information that are handled in a hospital radiological department. The RIS  26  is connected to a hospital information system (HIS)  28 , which generally manages medical information in the hospital. 
         [0045]    As shown in  FIGS. 2 and 3A , the radiation output device  18  is a multiple-radiation-source radiation output device equipped with a radiation output portion  29  comprising a plurality of radiation sources  30   a  through  30   p . The respective radiation sources are denoted by  30   a  to  30   p  in alphabetical order. Reference numerals with suffixes appended thereto in alphabetical order also are applied similarly to other elements, to be described below.  FIG. 2  shows by way of example four radiation sources  30   a  through  30   d  as viewed on a side of the radiation output device  18 , which lies perpendicular to the direction along which respective emissions of radiation  16   a  through  16   p  are output from the radiation output device  18 . In a case where the radiation sources or the radiation emissions are referred to collectively rather than individually, the radiation sources or the radiation emissions may be denoted by the numerals “ 30 ” or “ 16 ” without alphabetical suffixes (“a” or the like) appended thereto. 
         [0046]    Each of the radiation sources  30  comprises afield-electron-emission radiation source or a thermionic-emission radiation source. More specifically, each of the radiation sources  30  has a non-illustrated electron beam generating portion that applies an electron beam to a target. The target emits radiation  16  toward the subject  14  from an area (focused area) that is bombarded by the electron beam. 
         [0047]      FIG. 3A  is a schematic plan view showing by way of example a layout of radiation sources  30  of the radiation output device  18  shown in  FIG. 2 . In  FIG. 3A , the radiation sources  30   a  through  30   p  are arranged in a matrix of four rows and four columns along a predetermined output surface  31  (planar surface in  FIG. 3A ). For illustrative purposes, as shown in  FIG. 3B , positions of the radiation sources  30  are shown schematically as square cells in some of the figures, such as  FIG. 7A . In  FIG. 3B , the square cells are denoted by respective alphabetical letters corresponding to the suffixes added to the radiation sources  30  or the emissions of radiation  16 . 
         [0048]    As shown in  FIG. 2 , the electronic cassette  20  serves as a low-profile, portable radiographic image capturing apparatus, which is placed between the image capturing table  12  and the subject  14 . The electronic cassette  20  includes a thin-walled housing  32  made of a resin or metal material permeable to radiation  16 , a radiation conversion panel  34  disposed in the housing  32  for converting radiation  16  that has passed through the housing  32  into a radiographic image, and a cassette control portion  38  disposed in the housing  32  for controlling the radiation conversion panel  34  through a flexible printed circuit board (FPC)  36   a , and for reading electric signals depending on the radiographic image from the radiation conversion panel  34  through another FPC  36   b.    
         [0049]    The radiation conversion panel  34  is an indirect-conversion radiation detector comprising a scintillator  42  for converting radiation  16  into electromagnetic waves having another wavelength, such as visible light, and a photoelectric transducer layer  40  for converting the electromagnetic waves into electric signals with a plurality of solid-state detecting elements (hereinafter referred to as “pixels”) made of a material such as amorphous silicon (a-Si) or the like. 
         [0050]    In  FIG. 2 , the radiation conversion panel  34  is of a face side reading type, i.e., an ISS (Irradiation Side Sampling) type, in which the photoelectric transducer layer  40  and the scintillator  42 , which includes columnar crystals  44  of cesium iodide (CsI), are successively disposed along the irradiating direction (output direction, incident direction) in which the radiation  16  is applied. The columnar crystals  44  are formed along the irradiating direction. With such an ISS type of radiation conversion panel  34 , since radiation  16  passes through the photoelectric transducer layer  40  to the scintillator  42 , absorption of radiation  16  by the photoelectric transducer layer  40  should be minimized. 
         [0051]    The photoelectric transducer layer  40  is constructed from a non-illustrated insulative substrate, plural TFTs (Thin-Film Transistors), and a photoelectric transducer, which are stacked successively along the irradiating direction. The photoelectric transducer, which is positioned near the scintillator  42 , absorbs electromagnetic waves, e.g., visible light, which is emitted from the scintillator  42 , and generates electric charges depending on the absorbed visible light. More specifically, the photoelectric transducer preferably includes a photoelectric transducer film made of a-Si or an organic photoconductor (OPC) material, or the like, for example, which absorbs visible light and generates electric charges responsive to the visible light. The TFTs, which read the electric charges generated by the photoelectric transducer, preferably include an active layer of a-Si, an amorphous oxide, an organic semiconductor material, carbon nanotubes, or the like. The insulative substrate, which is disposed proximate the subject  14 , preferably is constituted by a flexible substrate of plastic, a substrate of aramid, or a substrate of bionanofibers. The photoelectric transducer layer  40 , which includes such materials, can be fabricated according to a low-temperature process, is flexible, and minimizes absorption of radiation  16 . 
         [0052]    The scintillator  42  is fabricated by forming the columnar crystals  44  of CsI along the irradiation direction on a non-illustrated evaporated substrate, which is disposed on the surface of the photoelectric transducer layer  40  and faces toward the bottom surface of the housing  32 . In a case where the scintillator  42  is made of columnar crystals  44  of thallium-added cesium iodide (CsI:Tl), and the photoelectric transducer film is made of a quinacridone based optical photoconductor (OPC), then a difference between the peak wavelength of light emitted by the scintillator  42  and the peak wavelength of light absorbed by the photoelectric transducer film can be reduced to 5 nm or smaller, thereby maximizing the amount of electric charge generated by the photoelectric transducer film. The evaporated substrate may comprise a thin aluminum (Al) substrate, which is both inexpensive and highly resistant to heat. 
         [0053]    The material of the scintillator  42  is not limited to CsI or CsI:Tl, but may be CsI:Na (sodium-activated cesium iodide), GOS (gadolinium oxide sulfur, Gd 2 O 2 S:Tb), or the like. According to the present embodiment, the radiation conversion panel  34  may be of a reverse side reading type, i.e., a PSS (Penetration Side Sampling) type, in which the scintillator  42  and the photoelectric transducer layer  40  are disposed successively along the irradiating direction of the radiation  16 . Alternatively, the radiation conversion panel  34  may be of the direct conversion type, which directly converts radiation  16  into electric signals with a plurality of pixels made of amorphous selenium (a-Se) or the like. 
         [0054]      FIG. 4  is a block diagram of the radiation output device  18 , the electronic cassette  20 , and the console  22 , which collectively make up the radiographic image capturing system  10 . 
         [0055]    In addition to the radiation output portion  29 , the radiation output device  18  includes a radiation source control portion  50  (output control portion) for controlling output of the respective emissions of radiation  16  from the radiation sources  30 , a communication portion  52  for sending signals to and receiving signals from the console  22 , a battery  54  for supplying electric power to the radiation output portion  29 , the radiation source control portion  50 , the communication portion  52 , and a moving mechanism  55  for moving the radiation sources  30  in unison along the directions indicated by the arrows A. 
         [0056]    The radiation source control portion  50  controls, i.e., turns on and off, each of the radiation sources  30  in addition to controlling the radiation dose of the respective emissions of radiation  16 . In addition, the radiation source control portion  50  functions as a group determining portion  56  for determining from among the radiation sources  30  either one or both of a first radiation source group  112  (see  FIG. 7A ) which is used during a still image mode and a second radiation source group  114  (see also  FIG. 7A ) which is used during a moving image mode, a position information acquiring portion  58  for acquiring position information concerning a region of interest  110  (see  FIG. 7B ) of the subject  14  and the radiation output portion  29 , and a radiation source selecting portion  60  for selecting at least one of the radiation sources  30  used for capturing an image. The radiation source control portion  50  supplies the moving mechanism  55  with a signal representing the distance that the radiation output portion  29  moves along the directions indicated by the arrows A. 
         [0057]    The electronic cassette  20  includes, in addition to the radiation conversion panel  34  and the cassette control portion  38 , a communication portion  62  for sending signals to and receiving signals from the console  22 , and a battery  64  for supplying electric power to the photoelectric transducer layer  40 , the cassette control portion  38 , and the communication portion  62 . The cassette control portion  38  has an address signal processing portion  66  for supplying address signals for enabling reading out of a radiographic image to the photoelectric transducer layer  40  through the FPC  36   a  (see  FIG. 2 ), an image memory  68  for storing the radiographic image read from the photoelectric transducer layer  40  through the FPC  36   b , and a cassette ID memory  70  for storing cassette ID information, which specifies the electronic cassette  20 . 
         [0058]    The console  22  has a communication portion  72  for sending signals to and receiving signals from the communication portions  52 ,  62 , a control processing portion  74  for performing a predetermined control processing sequence, an order information memory portion  75  for storing order information concerning a request for capturing radiographic images of the subject  14 , an image capturing condition memory portion  76  for storing image capturing conditions under which the subject  14  with is irradiated with radiation  16 , an operating portion  77  such as a keyboard, a mouse, etc., an exposure switch  78  operated by a doctor or a radiological technician (hereinafter referred to as an “operator”) to indicate a start timing to start emission of radiation  16  from the radiation sources  30 , and an image memory  79  for storing a radiographic image, which is received from the communication portion  62  by the communication portion  72 , and a radiographic image that is processed by the control processing portion  74 . 
         [0059]    The order information is generated by a doctor in charge of the RIS  26  or the HIS  28  (see  FIG. 1 ). The order information includes subject information for identifying the subject  14 , such as the name, age, gender, etc., of the subject  14 , information concerning the radiation output device  18  and the electronic cassette  20  used for capturing radiographic images, and information concerning an area to be imaged of the subject  14 . The image capturing conditions refer to various conditions required to apply radiation  16  to the area to be imaged of the subject  14 , such as tube voltages and tube currents of the respective radiation sources  30 , irradiation times of the respective radiation sources  30 , etc., for example. 
         [0060]      FIG. 5  is a circuit diagram of a circuit arrangement of the electronic cassette  20 . 
         [0061]    The photoelectric transducer layer  40  referred to above comprises an array of TFTs  86  arranged in rows and columns, and a plurality of pixels  80  made of a material such as a-Si or the like for converting electromagnetic waves (visible light), which have been converted from the radiation  16  by the scintillator  42 , into electric signals. The pixels  80  are disposed on the array of TFTs  86 . The pixels  80 , which are supplied with a bias voltage Vb from the battery  64  (see  FIG. 4 ), store electric charges that are generated in a case where the pixels  80  convert electromagnetic waves into electric signals (analog signals). The TFTs  86  are turned on successively along each row at a time, whereupon the stored electric charges are read from the pixels  80  as an image signal. 
         [0062]    The TFTs  86  are connected to the respective pixels  80 . Gate lines  82 , which extend parallel to the rows, and signal lines  84 , which extend parallel to the columns, are connected to the TFTs  86 . The gate lines  82  are connected to a line scanning driver  90 , and the signal lines  84  are connected to a multiplexer  92 . The gate lines  82  are supplied with control signals Von, Voff from the line scanning driver  90  for turning on and off the TFTs  86  along the rows. The line scanning driver  90  includes a plurality of switches SW 1  for switching between the gate lines  82 , and an address decoder  94  for supplying a selection signal for selecting one of the switches SW 1  at a time. The address decoder  94  is supplied with an address signal from the address signal processing portion  66  (see  FIG. 4 ) of the cassette control portion  38 . 
         [0063]    The signal lines  84  are supplied with electric charges through the TFTs  86 , which are arranged in columns, and the electric charges are stored in the pixels  80 . The electric charges supplied to the signal lines  84  are amplified by amplifiers  96 . The amplifiers  96  are connected through respective sample and hold circuits  98  to the multiplexer  92 . The multiplexer  92  includes a plurality of switches SW 2  for successively switching between the signal lines  84 , and an address decoder  100  for outputting selection signals for selecting one of the switches SW 2  at a time. The address decoder  100  is supplied with address signals from the address signal processing portion  66 . The multiplexer  92  has an output terminal connected to an A/D converter  102 . The A/D converter  102  converts radiographic image information into digital image signals, which are supplied to the cassette control portion  38 . 
         [0064]    The TFTs  86 , which function as switching devices, may be combined with another image capturing device such as a CMOS (Complementary Metal-Oxide Semiconductor) image sensor or the like. Alternatively, the TFTs  86  may be replaced with a CCD (Charge-Coupled Device) image sensor for shifting and transferring electric charges with shift pulses that correspond to gate signals in the TFTs  86 . 
         [0065]    In the present description, an image capturing mode for acquiring a radiographic image in a single image capturing event is referred to as a “still image mode”, whereas an image capturing mode for capturing a succession of radiographic images (including tomosynthesis images) in successive image capturing events is referred to as a “moving image mode (fluoroscopic image mode)”. In the moving image mode, radiation  16  is emitted, whereby radiographic images are acquired repeatedly in succession at relatively short time intervals (at a predetermined frame rate). In the moving mode, various known moving image capturing techniques, such as a progressive scanning technique, an interlaced scanning technique, a binning technique, etc., may be applied. 
       [Operations of Radiographic Image Capturing System  10 ] 
       [0066]    Operations of the radiographic image capturing system  10  according to the present embodiment will be described below primarily with reference to the flowchart shown in  FIG. 6  and the block diagram shown in  FIG. 4 . 
         [0067]    In step S 1  of  FIG. 6 , the control processing portion  74  of the console  22  sets image capturing conditions (tube voltage, tube current, irradiation time) for applying radiation  16  from the respective radiation sources  30  to an area to be imaged of the subject  14 , based on the order information. Such order information refers to information acquired from the RIS  26  (or the HIS  28 ), which is temporarily stored in the order information memory portion  75 . 
         [0068]    Thereafter, the control processing portion  74  stores the set image capturing conditions in the image capturing condition memory portion  76 , and sends the set image capturing conditions through the communication portion  72  and the communication portion  52  to the radiation output device  18 . The control processing portion  74  also sends the set image capturing conditions or the order information through the communication portion  72  and the communication portion  62  to the electronic cassette  20  via a wireless link. Thereafter, the cassette control portion  38  stores the image capturing conditions, etc., which are received through the communication portion  62 , in at least one of the image memory  68  and the cassette ID memory  70 . The image capturing conditions or the order information may be sent either immediately after the image capturing conditions have been set, or in response to a request to send the image capturing conditions and the order information from the radiation output device  18  or the electronic cassette  20 . 
         [0069]    In step S 2 , the position information acquiring portion  58  of the radiation output device  18  acquires position information concerning the radiation output portion  29  and the region of interest  110  (see  FIG. 7B ). The radiation output portion  29  may be in a predetermined reference position, e.g., a central position on the output surface  31 , or an absolute position in reference to each of the radiation sources  30   a  through  30   p . The region of interest  110  may be in an absolute position, which is measured by some means, or a position that is estimated from the order information, which represents the position and posture of the subject  14  and the area to be imaged of the subject  14 . 
         [0070]    In step S 3 , the group determining portion  56  of the radiation output device  18  determines, from among the radiation sources  30 , at least one of a first radiation source group  112  which is used in the still image mode and a second radiation source group  114  which is used in the moving image mode. 
         [0071]    As shown in  FIG. 7A , the radiation sources  30  that belong to the first radiation source group  112  are cells shown in hatching, i.e., the four radiation sources  30   f ,  30   g ,  30   j , and  30   k , which are centrally located on the radiation output portion  29 . The radiation sources  30  that belong to the second radiation source group  114  are cells shown as stippled, i.e., the remaining twelve radiation sources  30   a  through  30   d ,  30   e ,  30   h ,  30   i ,  30   l , and  30   m  through  30   p . Details of a process for determining the first radiation source group  112  and the second radiation source group  114  will be described later. 
         [0072]    In step S 4 , in response to a command to start capturing a radiographic image of the subject  14 , the radiation source control portion  50  of the radiation output device  18  judges whether the image capturing mode to be carried out is a still image mode or a moving image mode. 
         [0073]    The operator inserts the electronic cassette  20  (see  FIG. 1 ) between the subject  14  and the image capturing table  12 , positions the subject  14 , and then turns on the exposure switch  78 . The control processing portion  74  synchronizes the start of emission of radiation  16  from the radiation output portion  29  with the detection and conversion of radiation  16  by the radiation conversion panel  34  into a radiographic image. The control processing portion  74  generates a synchronizing control signal for capturing a radiographic image of the area to be imaged of the subject  14 . The control processing portion  74  sends the generated synchronizing control signal from the communication portion  72  to the communication portions  52 ,  62  via wireless links. In a case where the radiation source control portion  50  receives the synchronizing control signal through the communication portion  52 , a radiographic image of the area to be imaged of the subject  14  starts to be captured under the image capturing conditions set in step S 1 . Thereafter, by referring to the order information received from the console  22 , the radiation source control portion  50  determines the image capturing mode. 
         [0074]    In a case where the radiation source control portion  50  judges that the image capturing mode is a still image mode, then control proceeds to step S 5 , whereupon the radiation source selecting portion  60  selects at least one radiation source  30  from among the first radiation source group  112 , which is used in the still image mode. It is assumed that the radiation source selecting portion  60  selects all of the radiation sources  30  belonging to the first radiation source group  112 , i.e., the four radiation sources  30   f ,  30   g ,  30   j , and  30   k.    
         [0075]    In a case where the radiation source control portion  50  judges that the image capturing mode is a moving image mode, then control proceeds to step S 6 , whereupon the radiation source selecting portion  60  selects at least one radiation source  30  from among the second radiation source group  114 , which is used in the moving image mode. It is assumed that the radiation source selecting portion  60  selects all of the radiation sources  30  belonging to the second radiation source group  114 , i.e., the twelve radiation sources  30   a  through  30   d ,  30   e ,  30   h ,  30   i , and  30   l  through  30   p.    
         [0076]    In step S 7 , the radiographic image capturing system  10  captures a radiographic image of the subject  14  by emitting radiation  16  from the radiation sources  30  that were selected in step S 5  or step S 6 . More specifically, the radiation source control portion  50  controls the radiation sources  30  to apply radiation  16  at a predetermined dose from the radiation sources  30  to the area to be imaged of the subject  14  for a predetermined irradiation time according to the image capturing conditions acquired from the console  22 . 
         [0077]    Preferred image capturing processes carried out during the image capturing modes will be described in detail below with reference to  FIGS. 7A through 10B . For the sake of brevity, only certain ones of the radiation sources  30 , i.e., the radiation sources  30  belonging to the second row of the matrix shown in  FIG. 3B , will be described by way of illustrative example. 
         [0078]    According to a first example, it is assumed that, in a case where the region of interest  110  is projected onto the output surface  31  along a direction normal to the electronic cassette  20 , a projected image  110   p  (indicated by the two-dot-and-dash line in  FIG. 7A ) of the region of interest  110  lies on four cells (f, g, j, k), as shown in  FIG. 3B . An actual position of the region of interest  110  shown in  FIGS. 7B and 7C  is referred to as a first position. Stated otherwise, the radiation sources  30   f ,  30   g ,  30   j ,  30   k  are present directly above the first position along a direction normal to the principal surface of the electronic cassette  20 . 
         [0079]    As shown in  FIG. 7B , during an image capturing process in the still image mode, two radiation sources  30   f ,  30   g  of the four radiation sources  30   e  through  30   h  that belong to the second row emit respective emissions of radiation  16   f ,  16   g . The emissions of radiation  16   f ,  16   g  pass through the region of interest  110  substantially from the front side thereof, and are applied substantially perpendicularly to the electronic cassette  20 . Consequently, it is possible to obtain a high-quality radiographic image, which is almost free of geometrical distortions. In order to achieve higher image quality and reduce the dose of radiation to which the subject  14  is exposed, the image capturing process may be carried out with a smaller irradiation field, which is made up of the radiation sources  30   f ,  30   g , than in the moving image mode. 
         [0080]    As shown in  FIG. 7C , during an image capturing process in the moving image mode, two radiation sources  30   e ,  30   h  of the four radiation sources  30   e  through  30   h  that belong to the second row emit respective emissions of radiation  16   e ,  16   h . The emissions of radiation  16   e ,  16   h  pass through the region of interest  110  in a slightly oblique manner, and are applied substantially perpendicularly to the electronic cassette  20 . A moving image captured in this manner primarily is used in order to monitor a change in the state of the subject  14 , and hence, the moving image is sufficiently useful for this purpose. In order to reduce the dose of radiation to which the subject  14  is exposed, the image capturing process may be carried out with a smaller dose per unit time of the radiation  16   f ,  16   g  than in the still image mode. 
         [0081]    The group determining portion  56  determines the first radiation source group  112  and the second radiation source group  114 , such that each of the radiation sources  30  belongs to either one of the groups. Therefore, both a still image and a moving image can simultaneously be captured of the same subject  14  and the same region of interest  110 . In this case, the radiographic image capturing system  10  requires an arrangement, such as an arrangement having two radiation detectors, for separately acquiring the still image and the moving image. 
         [0082]    According to a typical example, which involves frequent switching between the still image mode and the moving image mode, the operator may initially select the moving image mode and carry out an image capturing process in the moving image mode. Then, the operator may position the subject  14  while visually confirming the moving image of the subject  14 , and thereafter, switch from the moving image mode to the still image mode in order to carry out a main image capturing mode (and repeat the whole process). 
         [0083]    Even in a case where the position of the region of interest  110  differs from that of the first position (see  FIGS. 7B and 7C ) described above, suitable radiation sources  30  can be selected based on the above principles. 
         [0084]    According to a second example, it is assumed that, in a case where the region of interest  110  is projected onto the output surface  31  along a direction normal to the electronic cassette  20 , a projected image  110   p  (indicated by the two-dot-and-dash line in  FIG. 8A ) of the region of interest  110  lies on four cells (e, f, i, j), as shown in  FIG. 3B . The actual position of the region of interest  110  shown in  FIGS. 8B and 8C  is referred to as a second position. 
         [0085]    The group determining portion  56  classifies radiation sources  30 , the actual distances of which from the region of interest  110  (to the projected image  110   p  on the output surface  31 ) are relatively small, into a first radiation source group (see step S 3 ). As a result, as shown in  FIG. 8A , the radiation sources  30 , which belong to a first radiation source group  116 , are the four radiation sources  30   e ,  30   f ,  30   i , and  30   j  that reside in a central left area of the radiation output portion  29 . The radiation sources  30 , which belong to a second radiation source group  118 , are the remaining twelve radiation sources  30   a  through  30   d ,  30   g ,  30   h ,  30   k ,  30   l , and  30   m  through  30   p.    
         [0086]    For an image capturing process in the still image mode, the radiation source selecting portion  60  selects at least one of the radiation sources  30  to be used in the image capturing process from among the first radiation source group  116  (see step S 5 ). For example, it is assumed that the radiation source selecting portion  60  selects all four of the radiation sources  30   e ,  30   f ,  30   i ,  30   j . Then, as shown in  FIG. 8B , two radiation sources  30   e ,  30   f  from among the four radiation sources  30   e  through  30   h  that belong to the second row emit respective emissions of radiation  16   e ,  16   f.    
         [0087]    For an image capturing process in the moving image mode, the radiation source selecting portion  60  selects at least one of the radiation sources  30  to be used in the image capturing process from among the second radiation source group  118  (see step S 6 ). For example, it is assumed that the radiation source selecting portion  60  selects eight radiation sources  30   a  through  30   c ,  30   g ,  30   k , and  30   m  through  30   o  having relatively small irradiation angles with respect to the region of interest  110 , from among the remaining radiation sources  30   a  through  30   d ,  30   g ,  30   h ,  30   k ,  30   l , and  30   m  through  30   p . Then, as shown in  FIG. 8C , one radiation source  30   g  from among the four radiation sources  30   e  through  30   h  that belong to the second row emits radiation  16   g.    
         [0088]    Consequently, depending on the positional relationship between the region of interest  110  (second position) and each of the radiation sources  30 , at least one radiation source  30  is selected as a radiation source to be used for the image capturing process, thereby making it possible to capture a radiographic image of the region of interest  110  in a manner that is suitable for the position of the region of interest  110 , regardless of whether the image capturing mode is the still image mode or the moving image mode. 
         [0089]    According to a third example, it is assumed that, in a case where the region of interest  110  is projected onto the output surface  31  along a direction normal to the electronic cassette  20 , a projected image  110   p  (indicated by the two-dot-and-dash line in  FIG. 9A ) of the region of interest  110  lies on two cells (e, i), as shown in  FIG. 3B . The actual position of the region of interest  110  shown in  FIGS. 9B and 9C  is referred to as a third position. 
         [0090]    The radiation source control portion  50  controls the moving mechanism  55  in order to move the radiation output portion  29  a predetermined distance to the left (commensurate with one and a half cells, for example) along the direction indicated by the arrow A. At this time, the radiation output portion  29  and the region of interest  110  have the same positional relationship as shown in  FIGS. 8B and 8C . Thereafter, radiation sources  30  can be selected in the same manner as though the region of interest  110  were present in the second position. 
         [0091]    According to a fourth example, as shown in  FIG. 10A , the subject  14  has a plurality of regions of interest, or more specifically, has another region of interest  120  in addition to the region of interest  110  in the first position. It is assumed that, in a case where the region of interest  120  is projected onto the output surface  31  along a direction normal to the electronic cassette  20 , a projected image  120   p  (indicated by the two-dot-and-dash line in  FIG. 10B ) of the region of interest  120  lies on four cells (a, b, e, f), as shown in  FIG. 3B . The actual position of the region of interest  120  shown in  FIG. 10B  is referred to as a fourth position. 
         [0092]    The group determining portion  56  classifies radiation sources  30 , the actual distances of which from the region of interest  110  (first position) and the region of interest  120  (fourth position) (to the projected images  110   p ,  120   p  on the output surface  31 ) are relatively small, into a first radiation source group (see step S 3 ). As a result, the radiation sources  30  that belong to a first radiation source group  122  are the seven radiation sources  30   a ,  30   b ,  30   e  through  30   g ,  30   j , and  30   k , which reside in an upper left area of the radiation output portion  29 . The radiation sources  30  that belong to a second radiation source group  124  are the remaining nine radiation sources  30   c ,  30   d ,  30   h ,  30   i , and  30   l  through  30   p.    
         [0093]    Then, as shown in  FIG. 10A , three radiation sources  30   e  through  30   g  from among the four radiation sources  30   e  through  30   h  that belong to the second row emit respective emissions of radiation  16   e  through  16   g.    
         [0094]    In common with the first through fourth examples, in the event that the radiation source control portion  50  selects two or more radiation sources  30 , the radiation source control portion  50  may select a cluster of radiation sources  30 , as the output surface  31  is viewed in plan. In such a cluster of radiation sources  30 , each of the radiation sources  30  is positioned adjacent to at least one other radiation source  30 . In this manner, the respective emissions of radiation  16 , which are emitted simultaneously from the selected radiation sources  30  to the subject  14 , are closely bundled, so as to prevent the resultant radiographic image from suffering from geometric distortions. 
         [0095]    For an image capturing process in the moving image mode, the radiation source control portion  50  may switch between the radiation sources  30 , such that selected radiation sources  30  are used at predetermined time intervals. For example, in the second radiation source group  114  shown in  FIG. 7A , the radiation source control portion  50  successively selects the radiation sources  30   a ,  30   c ,  30   e ,  30   h ,  30   i , . . . , one at a time, within respective frames. The radiation source control portion  50  thus selects the radiation source  30   c , which is in a non-adjacent position to the radiation source  30   a  used immediately before and uses the radiation sources  30  substantially at the same frequency as each other, for thereby dissipating heat efficiently from the target of the radiation sources  30 , and effectively preventing the radiation output device  18  from becoming overheated. 
         [0096]    As descried above, the radiographic image capturing system  10  emits radiation  16  from radiation sources  30  selected depending on the image capturing mode, thereby capturing a radiographic image of the subject  14  (step S 7 ). The radiation output device  18  may automatically change the radiation sources  30  that are used for capturing the radiographic image depending on instructions to change image capturing conditions such as the image capturing mode, the region of interest, etc. Alternatively, the radiation output device  18  may automatically change the radiation sources  30  that are used for capturing a radiographic image depending on a manual action made by the operator. 
         [0097]    In step S 8 , the radiographic image capturing system  10  acquires a radiographic image of the subject  14 . Operations of various components of the radiographic image capturing system  10  will be described in detail below. 
         [0098]    Radiation  16  that has passed through the subject  14  (imaged region) reaches the radiation conversion panel  34  of the electronic cassette  20 . The scintillator  42  having the columnar crystals  44  of CsI or CsI:Tl emits visible light at an intensity depending on the intensity of the radiation  16 , and the pixels  80  of the photoelectric transducer layer  40  convert the visible light into electric signals and store the signals as electric charges. 
         [0099]    In step S 4 , the control processing portion  74  sends the synchronizing control signal through the communication portion  72  to the communication portion  62 . After the cassette control portion  38  has received the synchronizing control signal from the communication portion  62 , the address signal processing portion  66  supplies address signals to the line scanning driver  90  and the multiplexer  92 , so as to initiate reading of the electric charge information representing a radiographic image of the subject  14  from the pixels  80 . 
         [0100]    More specifically, the address decoder  94  of the line scanning driver  90  supplies a selection signal for selecting one of the switches SW 1  according to the address signals sent from the address signal processing portion  66 , and supplies control signals Von to the gates of the TFTs  86  connected to the corresponding gate lines  82 . The address decoder  100  of the multiplexer  92  supplies selection signals according to the address signals sent from the address signal processing portion  66 , thereby successively selecting the switches SW 2 . Through the signal lines  84 , the address decoder  100  successively reads a radiographic image represented by the electric charge information held by the pixels  80  connected to the gate lines  82 , which are selected by the line scanning driver  90 . 
         [0101]    The radiographic image read from the pixels  80  connected to the selected gate lines  82  is amplified by the amplifiers  96 , sampled by the sample and hold circuits  98 , and supplied through the multiplexer  92  to the A/D converter  102 , which converts the radiographic image into digital signals. The digital signals, which represent the radiographic image, are temporarily stored in the image memory  68  of the cassette control portion  38 . 
         [0102]    Similarly, the address decoder  94  of the line scanning driver  90  successively selects switches SW 1  according to the address signals sent from the address signal processing portion  66 . Through the signal lines  84 , the address decoder  94  reads a radiographic image represented by the electric charge information held by the pixels  80  connected to the gate lines  82 , and stores the radiographic image in the image memory  68  of the cassette control portion  38  through the multiplexer  92  and the A/D converter  102 . 
         [0103]    In a case where the image capturing mode is the moving image mode, the cassette control portion  38  acquires a radiographic image from the electric charges stored by the pixels  80  at a preset frame rate, e.g., a frame rate of 15 to 60 frames/second. In a case where the electric charges are read by an interlaced scanning technique, for example, the frame rate can be increased, and the burden on the signal processing system can be reduced. 
         [0104]    In step S 9 , the display device  24  receives the radiographic image, which is acquired from the electronic cassette  20  via the console  22 , and displays the received radiographic image as an image to be interpreted by the operator. 
         [0105]    More specifically, in response to a transmission request from the console  22 , which is received through the communication portion  62 , the cassette control portion  38  sends the radiographic image stored in the image memory  68  and the cassette ID information stored in the cassette ID memory  70  through the communication portion  62  to the console  22  via a wireless link. Thereafter, the control processing portion  74  performs a predetermined image processing technique, e.g., a known type of image reconstructing process, on the received radiographic image in order to generate an image that can be interpreted by the operator. The control processing portion  74  sends the generated image through the communication portion  72  to the display device  24  via a wireless link. 
         [0106]    In step S 10 , the operator observes the image displayed on the display device  24  in order to judge whether or not a desired radiographic image has been obtained. In a case where the operator determines that a desired radiographic image has been obtained (YES), then the image capturing process on the subject  14  is brought to an end. In a case where the operator judges that a desired radiographic image has not been obtained (NO), then control returns to step S 4 , and an image capturing process is carried out again on the subject  14 . 
         [0107]    As described above, inasmuch as at least one radiation source  30  for emitting radiation  16  is selected from among the plural radiation sources  30  depending on whether the image capturing mode is the still image mode or the moving image mode, it is possible to emit radiation  16  from positions suitable for the image capturing mode, without the need for moving the radiation sources  30  or positioning the subject  14  differently. Consequently, the efficiency with which the operator works in capturing images in the still image mode and the moving image mode is significantly increased. 
       [Applications of the Radiographic Image Capturing Process] 
       [0108]    Applications of the radiographic image capturing process in the still image mode and the moving image mode will be described below. An example is presented in which a contrast agent for highlighting a portion of a radiographic image is used for capturing an image of a particular region (e.g., the heart, a blood vessel, or the like) having a minute structure. 
         [0109]    First, the operator injects a contrast agent into the subject  14  through a blood vessel (artery or vein) of the subject  14 . Immediately after injecting the contrast agent, the operator captures a radiographic image of the subject  14  in the moving image mode, and checks the timing for the contrast agent to reach the particular region while viewing the moving image of the particular region. During the moving image mode, radiographic image capturing processes are performed sequentially at a wide angle (see  FIGS. 8C and 7C ) and with low resolution. Upon flowing through the blood vessels, the injected contrast agent circulates through the body of the subject  14 , and after elapse of a predetermined time, the contrast agent reaches the particular region such as the heart or the like. 
         [0110]    In a case where it is judged that the contrast agent has reached the particular region, then the radiation source control portion  50  switches from the moving image mode to a continuous still image capturing mode, in response to an action made on the operating portion  77  by the operator. The continuous still image capturing mode refers to an image capturing mode for capturing successive images in a still image mode at longer time intervals than the frame interval of the moving image mode. In the continuous still image capturing mode, capturing of radiographic images is performed at a narrow angle (see  FIGS. 7B and 8B ) and with high resolution. In other words, images of the particular region are captured in the still image mode during a time zone in which a large amount of the contrast agent has reached the particular region, thereby providing diagnostic images that are high in contrast and resolution. Since the still images are successively captured in the continuous still image capturing mode, no loss in timing occurs, even though the images are captured using the contrast agent. Furthermore, the obtained still images are successively connected to allow the operator to easily grasp the particular region during movement thereof. 
         [0111]    Since the radiation source control portion  50  successively controls the emissions of radiation  16  depending on the still image mode and at a longer time interval than the frame interval of the moving image mode, images can reliably be captured in the still image mode without any loss in timing, even though the states of the target to be imaged (characteristics of the radiographic images) change from time to time. 
       Modifications of the Present Embodiment 
       [0112]    Modifications (first and second modifications) of the present embodiment will be described below with reference to  FIGS. 11 and 12 . Parts of the modifications, which are identical to those of the present embodiment, are denoted by identical reference characters, and such features will not be described in detail below. 
       (First Modification) 
       [0113]    A radiation output portion  130  according to a first modification differs from the present embodiment (see  FIG. 3A ) in relation to the layout (number and positions) of the radiation sources  30 . 
         [0114]    As shown in  FIG. 11 , the radiation output portion  130  has a circular output surface  132  for emitting radiation  16 . The radiation output portion  130  includes a radiation source  30   a  disposed at a central position of the output surface  132 , and eight radiation sources  30   b  through  30   i  disposed respectively at equal intervals along the circumferential edge of the output surface  132 . The size of the radiation source  30   a  is greater than the size of the radiation sources  30   b  through  30   i . The group determining portion  56  classifies the radiation source  30   a  into the first radiation source group, which may be used solely in the still image mode, whereas the other radiation sources  30   b  through  30   i  may be selected depending on at least one of the image capturing mode (still image mode, moving image mode) and the positional relationship with respect to the region of interest  110 . 
         [0115]    According to the present embodiment, the output surfaces  31 ,  132  are planar in shape. However, the output surfaces  31 ,  132  may be of a curved shape or another shape. In addition, the number or positions of the radiation sources  30  may be changed appropriately as required. 
       (Second Modification) 
       [0116]    A radiographic image capturing system  140  according to a second modification differs from the present embodiment (see  FIG. 4 ) as to the arrangement of the output control portion for controlling how the radiation  16  is output. 
         [0117]    As shown in  FIG. 12 , the radiation output device  18  includes a radiation source control portion  142  having functions to turn on and off the radiation sources  30 , so as to control the dose of radiation  16 . The console  22  includes a radiation source determining portion  144  having the functions of a group determining portion  56 , a position information acquiring portion  58 , and a radiation source selecting portion  60 , in addition to a communication portion  72 , a control processing portion  74 , an order information memory portion  75 , an image capturing condition memory portion  76 , an operating portion  77 , an exposure switch  78 , and an image memory  79 . 
         [0118]    Similar to the present embodiment, the radiation source determining portion  144  selects at least one radiation source  30 , which is used in a main image capturing process, from among the radiation sources  30   a  through  30   p . The control processing portion  74  sends information (hereinafter referred to as “radiation source selection information”) concerning the selection result made by the radiation source determining portion  144  to the radiation source control portion  142  through the communication portion  72  and the communication portion  52 . Based on the received radiation source selection information, the radiation source control portion  142  controls the radiation sources  30  to emit radiation  16 . 
         [0119]    More specifically, the output control portion according to the second modification comprises the radiation source control portion  142  of the radiation output device  18  and the radiation source determining portion  144  of the console  22 . The output control portion, which is constructed in the foregoing manner from a plurality of devices (the radiation output device  18  and the console  22 ), offers the same advantages as those of the present embodiment. 
         [0120]    The present invention is not limited to the embodiment described above. It is a matter of course that various changes can freely be made to the embodiment without departing from the scope of the present invention.