Patent Publication Number: US-2022233157-A1

Title: Mobile radiography apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of International Application No. PCT/JP2020/039423, filed Oct. 20, 2020, the disclosure of which is incorporated herein by reference in its entirety. Further, this application claims priority from Japanese Patent Application No. 2019-199331, filed on Oct. 31, 2019, the disclosure of which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a mobile radiography apparatus. 
     2. Description of the Related Art 
     A mobile radiography apparatus including an irradiation unit that emits radiation and a carriage portion that can travel is known (see JP2011-87923A). The carriage portion is provided with a plurality of casters, and a support portion that supports an arm provided with the irradiation unit and a main body portion are mounted on the carriage portion. 
     Two types of casters of steering casters (main wheels) and free casters (auxiliary wheels) are provided in the carriage portion of the mobile radiography apparatus (X-ray imaging apparatus) disclosed in JP2011-87923A. The steering caster is a caster to which a steering is connected, and a steering angle is given to the steering caster by the operation of the steering. Therefore, the traveling direction of the carriage portion can be fixed by the operation of the steering. The free caster is not connected to the steering, and a steering angle thereof is changed subordinately according to force applied to the carriage portion. 
     SUMMARY 
     In some cases, this mobile radiography apparatus is used in a hospital room and an operating room as well as in an imaging room. In a case in which a patient who has difficulty in moving to the imaging room is imaged, the mobile radiography apparatus is carried into the patient&#39;s hospital room and used in the hospital room. In addition, in a case in which a moving image of a treatment target part of a patient undergoing surgery is captured, the mobile radiography apparatus is carried into the operating room and used in the operating room. In a case in which the use of the mobile radiography apparatus in a plurality of usage situations is considered, operability required for the carriage portion changes depending on each usage situation. 
     For example, since the hospital room is smaller than the imaging room, in the hospital room, turning in a smaller radius is more required than in the imaging room. In addition, in a case in which the mobile radiography apparatus is used in a narrow hospital room with many obstacles including a bed, there is a demand for easily changing the direction of the carriage portion by pushing and pulling the mobile radiography apparatus in various directions without bothering to operate the steering. On the other hand, in some cases, for example, imaging is performed while sequentially moving an imaging position along the body axis of the patient during surgery. In this situation, in a case in which the carriage portion sways, it is difficult to perform appropriate imaging. Therefore, there is a demand for fixing the position of the steering to fix the traveling direction of the carriage portion. 
     In the mobile radiography apparatus disclosed in JP2011-87923A, since the steering casters are always grounded, the traveling direction of the carriage portion is fixed by the position of the steering. Therefore, since it is difficult to change the direction of the carriage portion without operating the steering, the carriage portion is not turned in a small radius, and the mobile radiography apparatus is not easy to use in a case in which it is used in a narrow hospital room. On the other hand, in a case in which the carriage portion having only the free casters without the steering casters is used, the carriage portion is turned in a small radius. However, in a case in which an operator wants to fix the traveling direction of the carriage portion, the mobile radiography apparatus is not easy to use. As described above, the operability required for the carriage portion changes according to the usage situation. 
     The technology according to the present disclosure provides a mobile radiography apparatus that can change the operability of a carriage portion according to a usage situation. 
     According to a first aspect of the present disclosure, there is provided a mobile radiography apparatus comprising: an irradiation unit that emits radiation; a carriage portion on which the irradiation unit is mounted and which is capable of traveling; steering casters which are provided in the carriage portion and are connected to a steering and whose steering angle is given by an operation of the steering; free casters which are provided in the carriage portion and are not connected to the steering and whose steering angle is changed subordinately according to a direction of force applied to the carriage portion; and a switching mechanism that selectively switches between a first grounded state in which the free casters are separated from a floor and the steering casters are grounded to the floor and a second grounded state in which the free casters are grounded to the floor and the steering casters are separated from the floor. 
     According to the above configuration, the switching mechanism selectively switches the grounded state of the free casters and the steering casters to change the operability of the carriage portion according to the usage situation. 
     That is, for the operability of the carriage portion, the free casters are grounded such that the carriage portion can be turned in a small radius. The steering casters are grounded to fix the traveling direction of the carriage portion. The turning of the carriage portion in a small radius improves usability in a case in which the mobile radiography apparatus is used in a small hospital room. The fixation of the traveling direction improves usability in a case in which continuous imaging is performed while moving an imaging position. 
     According to a second aspect of the present disclosure, in the mobile radiography apparatus according to the first aspect, the switching mechanism may raise and lower the free casters with respect to the steering casters to switch between the first grounded state and the second grounded state. 
     In general, the steering casters are heavier than the free casters since they are connected to the steering. Here, according to the above configuration, the switching mechanism raises and lowers the free casters, which makes it possible to easily switch the grounded state of the casters with a small force as compared to a configuration in which the steering casters are raised and lowered. 
     According to a third aspect of the present disclosure, in the mobile radiography apparatus according to the second aspect, the switching mechanism may have a pedal that raises and lowers the free casters. 
     According to the above configuration, the free casters can be raised and lowered by stepping on the pedal. Therefore, it is possible to switch the grounded state of the casters while manually operating the mobile radiography apparatus. 
     According to a fourth aspect of the present disclosure, in the mobile radiography apparatus according to any one of the first to third aspects, always-grounded casters that are grounded to the floor in both the first grounded state and the second grounded state may be provided, in addition to the free casters and the steering casters. 
     According to the above configuration, the always-grounded casters are provided in addition to the free casters and the steering casters. Therefore, it is possible to stably move the carriage portion even in a case in which one of the free casters and the steering casters is separated from the floor. In addition, in a case in which the grounded state of the free casters and the steering casters is switched, the postural instability of the carriage portion is also suppressed. 
     According to a fifth aspect of the present disclosure, in the mobile radiography apparatus according to the fourth aspect, an arm that supports the irradiation unit and a main body portion including a control unit that controls the irradiation unit may be mounted on the carriage portion, and the always-grounded casters may be casters whose steering angle is changed subordinately similarly to the free casters. In a case in which a side on which the arm is provided is a front side of the carriage portion and a side on which the main body portion is provided is a rear side of the carriage portion, the always-grounded casters, the steering casters, and the free casters may be disposed in this order from the front side to the rear side of the carriage portion. 
     According to the above configuration, the steering casters are disposed between the always-grounded casters and the free casters in the front-rear direction of the main body portion. Therefore, the steering casters can be disposed at a position close to the position of the center of gravity of the main body portion. As a result, it is possible to suppress the postural instability of the main body from in a case in which the steering is operated. 
     According to a sixth aspect of the present disclosure, in the mobile radiography apparatus according to the fifth aspect, the steering casters may be disposed at a position of a center of gravity in a front-rear direction of the carriage portion. 
     According to the above configuration, since the steering casters are disposed at the position of the center of gravity in the entire mobile radiography apparatus, it is possible to further suppress the postural instability of the main body portion in a case in which the steering is operated. 
     According to a seventh aspect of the present disclosure, in the mobile radiography apparatus according to the fifth or sixth aspect, the arm may have two end portions. The irradiation unit may be provided at one end of the arm, and an image receiving unit that receives the radiation, which has been emitted from the irradiation unit and transmitted through the subject, may be provided at the other end of the arm. 
     According to the above configuration, the irradiation unit and the image receiving unit can be integrally held by the arm. Therefore, for example, it is possible to perform imaging while moving the carriage portion in the body axis direction of the subject during surgery. 
     According to an eighth aspect of the present disclosure, the mobile radiography apparatus according to any one of the first to seventh aspects may further comprise a steering lock mechanism that locks an operation by the steering in the second grounded state in which the steering casters are separated from the floor. 
     According to the above configuration, the steering lock mechanism that locks the operation by the steering in a case in which the steering casters are separated from the floor is provided. Therefore, it is possible to the postural instability of the mobile radiography apparatus caused by the movement of the steering casters in a case in which the main body portion is moved by the free casters. 
     According to a ninth aspect of the present disclosure, in the mobile radiography apparatus according to any one of the first to eighth aspects, a handle for pushing and pulling the carriage portion may be provided separately from the steering. 
     According to the above configuration, since the handle for pushing and pulling the carriage portion separately from the steering is provided, it is easy to operate the mobile radiography apparatus in a case in which the carriage portion is moved by the free casters. 
     According to the technology of the present disclosure, it is possible to change the operability of the carriage portion according to the usage situation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments according to the technique of the present disclosure will be described in detail based on the following figures, wherein: 
         FIG. 1  is an overall perspective view illustrating a mobile radiography apparatus according to an example of an embodiment, 
         FIG. 2A  is a side view illustrating the mobile radiography apparatus according to an example of the embodiment, 
         FIG. 2B  is a side view illustrating a state in which an arm of the mobile radiography apparatus illustrated in  FIG. 2A  is rotated in a direction of an arrow M 1 , 
         FIG. 2C  is a side view illustrating a state in which the arm of the mobile radiography apparatus illustrated in  FIG. 2A  is rotated in a direction of an arrow M 2 , 
         FIG. 3A  is a front view illustrating the mobile radiography apparatus according to an example of the embodiment, 
         FIG. 3B  is a front view illustrating a state in which the arm of the mobile radiography apparatus illustrated in  FIG. 3A  is rotated in a direction of an arrow N 1 , 
         FIG. 3C  is a front view illustrating a state in which the arm of the mobile radiography apparatus illustrated in  FIG. 3A  is rotated 180° in a direction of an arrow N 2 , 
         FIG. 4  is an overall side view illustrating a first grounded state of the mobile radiography apparatus according to an example of the embodiment, 
         FIG. 5  is an overall side view illustrating a second grounded state of the mobile radiography apparatus according to an example of the embodiment, 
         FIG. 6  is a plan view illustrating the mobile radiography apparatus according to an example of the embodiment, 
         FIG. 7A  is a plan view illustrating a state in which a steering of the mobile radiography apparatus illustrated in  FIG. 6  is operated in one direction, 
         FIG. 7B  is a plan view illustrating a state in which the steering of the mobile radiography apparatus illustrated in  FIG. 6  is operated in the other direction, 
         FIG. 8  is a perspective view illustrating a switching mechanism of the mobile radiography apparatus according to an example of the embodiment, 
         FIG. 9  is an exploded perspective view illustrating the switching mechanism illustrated in  FIG. 8 , 
         FIG. 10  is a side view illustrating the switching mechanism in the first grounded state, 
         FIG. 11  is a side view illustrating the switching mechanism in the second grounded state, 
         FIG. 12A  is a perspective view illustrating an unlocked state of the steering by the steering lock mechanism, and 
         FIG. 12B  is a perspective view illustrating a locked state of the steering by the steering lock mechanism. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, a mobile radiography apparatus according to an example of an embodiment of the present disclosure will be described with reference to the drawings. In addition, in the drawings, an arrow X indicates a front-rear direction of the mobile radiography apparatus, an arrow Y indicates a width direction of the mobile radiography apparatus, and an arrow Z indicates a vertical direction. 
     (Overall Configuration of Mobile Radiography Apparatus) 
     A mobile radiography apparatus  10  according to this embodiment illustrated in  FIG. 1  is an apparatus that captures a radiographic image of a subject H. The mobile radiography apparatus  10  can capture, for example, moving images and still images of the subject H. The capture of the moving image is performed, for example, in a case in which a treatment target part of the subject H is displayed as a moving image during surgery (also referred to as fluoroscopy or the like). In the capture of the moving image, for example, the moving image of the subject H is displayed on a monitor (not illustrated) that is provided separately from the mobile radiography apparatus  10 . Of course, data of the captured moving image may be stored in a memory of the mobile radiography apparatus  10 . In addition, in the case of the capture of the still image, the captured still image may be displayed on the monitor or may be stored in the memory of the mobile radiography apparatus  10 . 
     As illustrated in  FIG. 1 , the mobile radiography apparatus  10  includes an arm  12  (referred to as a C-arm or the like) having a C-shape (an arc shape) in a side view and a main body portion  16  to which a support portion  14  is attached. The arm  12 , the support portion  14 , and the main body portion  16  are mounted on a carriage portion  17 . In addition, hereinafter, it is assumed that the side of the mobile radiography apparatus  10  on which the arm  12  is provided is the front side of the mobile radiography apparatus  10  and the side on which the main body portion  16  is provided is the rear side of the mobile radiography apparatus  10 . 
     (Configuration of Arm) 
     The arm  12  has two end portions. An irradiation unit  18  is provided at one end of the arm  12 , and an image receiving unit  20  is provided at the other end. The arm  12  can hold the irradiation unit  18  and the image receiving unit  20  in a posture in which they face each other. A space, into which the subject H and a bed S on which the subject H lies supine can be inserted, is ensured between the irradiation unit  18  and the image receiving unit  20 . Further, in the following description, in some cases, in a side view of the arm  12  (as viewed from the Y direction in  FIG. 1 ), a direction in which the irradiation unit  18  and the image receiving unit  20  are provided is referred to as the front side of the arm  12 , and a side close to the support portion  14  is referred to as the rear side of the arm  12 . The irradiation unit  18  is an example of an irradiator according to the present disclosure. The image receiving unit  20  is an example of an image receiver according to the present disclosure. 
     As illustrated in  FIG. 2A , the arm  12  is orbitally rotatable about an axis line M (an axis line parallel to the Y axis) with respect to a track portion  22 B that is provided in the support portion  14 . Further, the arm  12  is axially rotatable about an axis line N (an axis line parallel to the X axis) with respect to a bearing portion  23  that is provided in the main body portion  16 . 
     Specifically, the track portion  22 B has an arc shape that has the same radius as the arc of the arm  12 . Moreover, a fitting portion  22 A that is fitted to the track portion  22 B is provided in an outer peripheral portion of the arm  12 . The fitting portion  22 A has an arc shape following the shape of the arm  12 . The track portion  22 B has, for example, a groove shape, and the fitting portion  22 A having a protruding shape is fitted to the track portion  22 B. 
     As illustrated in  FIG. 2A , the fitting portion  22 A formed in the arm  12  slides along the track portion  22 B formed on the support portion  14 . Therefore, the arm  12  can be orbitally rotated about the axis line M at the center of the arc of the arm  12  as a rotation center with respect to the support portion  14  and the main body portion  16 . 
     That is, as illustrated in  FIGS. 2B and 2C , it is possible to orbitally rotate the arm  12  about the axis line M in the direction of an arrow M 1  (counterclockwise in  FIG. 2B ) and the direction of an arrow M 2  (clockwise in  FIG. 2C ). Therefore, the irradiation unit  18  and the image receiving unit  20  provided at both ends of the arm  12  can be rotated about the body axis (an axis parallel to the Y axis) of the subject H (see  FIG. 1 ) in a posture in which they face each other. 
     Further, as illustrated in  FIG. 2A , one end of a support shaft  24  that extends in the front-rear direction (X direction) of the mobile radiography apparatus  10  is fixed to the arm  12 . The other end of the support shaft  24  is supported by the main body portion  16  through the bearing portion  23 . The support shaft  24  is rotated about the axis line N with respect to the bearing portion  23  such that the arm  12  and the support portion  14  are rotatable about the axis line N of the support shaft  24  as a rotation center with respect to the main body portion  16  as illustrated in  FIGS. 3A to 3C . 
     That is, as illustrated in  FIGS. 3B and 3C , it is possible to rotate the arm  12  about the axis line N in the direction of an arrow N 1  (counterclockwise in  FIG. 3B ) and the direction of an arrow N 2  (clockwise in  FIG. 3C ). Therefore, it is possible to reverse the positions of the irradiation unit  18  and the image receiving unit  20  provided at both ends of the arm  12  with respect to the subject H (see  FIG. 1 ) in the vertical direction (Z-axis direction). 
     Here, in the posture of the arm  12  in which the irradiation unit  18  is disposed above the image receiving unit  20  as illustrated in  FIG. 3A , a radiation tube  32  (see  FIG. 1 ) included in the irradiation unit  18  is located above the subject H. Therefore, this posture is called, for example, an overtube posture. In addition, in the posture of the arm  12  in which the irradiation unit  18  is disposed below the image receiving unit  20  illustrated in  FIG. 3C , the radiation tube  32  is located below the subject H. Therefore, this posture is called, for example, an undertube posture. 
     In the overtube posture, it is possible to increase a distance between the irradiation unit  18  and the subject H (see  FIG. 1 ), as compared to the undertube posture. This makes it possible to image a relatively wide region in the overtube posture. Therefore, the overtube posture is mainly used to capture the still image of the subject H. On the other hand, in the undertube posture, the radiation emitted from the irradiation unit  18  is partially blocked by, for example, the bed S. Therefore, in the undertube posture, it is possible to reduce the amount of radiation exposure to, for example, a radiology technician or an operator (not illustrated) around the subject H (see  FIG. 1 ). Therefore, the undertube posture is used for the capture of the moving image of the subject H in which radiation is continuously emitted. 
     (Configuration of Main Body Portion) 
     As illustrated in  FIG. 1 , the main body portion  16  of the mobile radiography apparatus  10  includes a control unit  28  that controls each unit, such as the irradiation unit  18 , in the mobile radiography apparatus  10  and an operation panel  30  of, for example, a touch panel type. In addition, the main body portion  16  comprises various switches (not illustrated) including, for example, a power switch of the mobile radiography apparatus  10 , a power circuit that supplies power to each unit of the mobile radiography apparatus  10 , a battery, and the like. 
     The operation panel  30  functions as an operation unit that inputs an operation instruction to each unit of the mobile radiography apparatus  10  to operate each unit and a display unit that displays various kinds of information, such as a warning message and a radiographic image output from the image receiving unit  20 . The control unit  28  is an example of a controller according to the present disclosure. 
     (Configuration of Control Unit) 
     The control unit  28  transmits a control signal to the radiation tube  32  of the irradiation unit  18 , which will be described below, to control, for example, the tube voltage, tube current, and irradiation time of radiation of the radiation tube  32 . The control unit  28  controls the tube voltage to control the energy of the radiation. In addition, the control unit  28  controls the tube current and the irradiation time to control a radiation dose. In practice, since a high voltage is applied to the radiation tube  32 , the control unit  28  controls the radiation tube  32  through a high-voltage generation device (not illustrated). In imaging, imaging conditions including, for example, the tube voltage, the tube current, and the irradiation time are set through the operation panel  30 . The control unit  28  operates the irradiation unit  18  on the basis of the set imaging conditions. 
     The control unit  28  directs the irradiation unit  18  to perform moving image capture irradiation in which the irradiation unit  18  continuously emits radiation such that a moving image of the subject H can be captured. In a case in which a moving image is captured, the control unit  28  operates a detector of the image receiving unit  20  which will be described below in synchronization with the moving image capture irradiation by the irradiation unit  18 . In a case in which a moving image is captured, basically, the irradiation time is not set as the imaging condition, and instructions to start and end the capture of the moving image are input through the operation panel  30 . In a case in which the instruction to start the capture of a moving image is input, the control unit  28  directs the irradiation unit  18  to start the emission of radiation under preset imaging conditions. 
     In the capture of a moving image, the detector repeats an image detection operation at a preset frame rate while the moving image capture irradiation is performed. The image output by the detector is transmitted to the control unit  28 . The control unit  28  sequentially outputs the received images to a monitor (not illustrated). Therefore, the moving image of the subject H is displayed on the monitor. 
     In addition, the control unit  28  directs the irradiation unit  18  to perform still image capture irradiation in which the irradiation unit  18  emits radiation for a shorter time than in the moving image capture irradiation such that a still image of the subject H can be captured. 
     In the capture of a still image, the control unit  28  operates the detector of the image receiving unit  20  in synchronization with the irradiation timing in the still image capture irradiation by the irradiation unit  18 . For example, an instruction to capture a still image is input through an irradiation switch (not illustrated) that is connected to the control unit  28 . In the capture of a still image, the irradiation time is, for example, in the order of several tens of milliseconds to several hundreds of milliseconds. In a case in which the instruction to capture a still image is input, the control unit  28  operates the irradiation unit  18  on the basis of preset imaging conditions. In the capture of a still image, in a case in which the set irradiation time elapses, the irradiation operation of the irradiation unit  18  ends since the irradiation time is set in the imaging conditions. 
     In a case in which the irradiation ends, the detector starts to output the detected image. The image output by the detector is transmitted to the control unit  28 . The control unit  28  stores data of the still image in a memory (not illustrated). Then, the stored still image is displayed on the monitor (not illustrated). Therefore, the still image of the subject H is displayed on the monitor. Further, the still image may be displayed on the operation panel  30  in order to check the captured still image immediately after imaging. 
     (Configuration of Irradiation Unit) 
     The irradiation unit  18  comprises a radiation source  31  and an irradiation field limiter  34 . The radiation source  31  comprises the radiation tube  32  that generates radiation. The radiation is, for example, X-rays. The radiation tube  32  generates radiation by colliding electrons generated from a cathode with a target (anode). The position where the electrons collide with the target is a focus where radiation is emitted. 
     In addition, the irradiation field limiter  34  is provided below the radiation source  31 . The irradiation field limiter  34  (also referred to as a collimator or the like) has a rectangular irradiation opening  34 A. The radiation generated by the radiation tube  32  is emitted to the subject H through the irradiation opening  34 A. The irradiation field limiter  34  can adjust the opening area of the irradiation opening  34 A. The irradiation field limiter  34  has, for example, four shielding plates (not illustrated) that shield radiation. In each of the four shielding plates, each side corresponds to each side of the irradiation opening  34 A and defines the irradiation opening  34 A. The position of the shielding plates is changed to adjust the opening area of the irradiation opening  34 A, and the irradiation field of the radiation emitted from the irradiation unit  18  is changed. 
     Further, the irradiation unit  18  can be rotated about an axis line of a rotation shaft  36  that extends in the width direction (the Y direction in  FIG. 1 ) of the mobile radiography apparatus  10  as a rotation center with respect to the arm  12 . Specifically, a pair of attachment plates  38  (only one attachment plate is illustrated in  FIG. 1 ) are fixed to one end of the arm  12 . 
     The pair of attachment plates  38  are disposed such that both sides of the irradiation unit  18  in the width direction are interposed therebetween and are connected to both side surfaces of the irradiation unit  18  in the width direction. The rotation shafts  36  are provided on each of the side surfaces of the irradiation unit  18  facing the attachment plates  38  so as to protrude. The rotation shafts  36  are supported by the pair of attachment plates  38  through bearings (not illustrated). Therefore, the irradiation unit  18  can be rotated about the axis line of the rotation shaft  36  as the rotation center with respect to the attachment plates  38 , and the orientation of the irradiation opening  34 A of the irradiation unit  18  can be changed in the front-rear direction of the arm  12 . The orientation of the irradiation opening  34 A can be changed to change the irradiation direction of radiation. 
     Further, the irradiation unit  18  is connected to, for example, the control unit  28  and a power circuit (not illustrated) of the main body portion  16  by a cable (not illustrated) including a signal line for transmitting a control signal and a power line for supplying power. 
     (Configuration of Image Receiving Unit) 
     As illustrated in  FIG. 1 , the image receiving unit  20  is provided at the other end of the arm  12  which is a position facing the irradiation unit  18 . Furthermore, in this embodiment, the image receiving unit  20  is fixed to the other end of the arm  12  so as not to be detachable. However, the image receiving unit  20  may be attached to the other end of the arm  12  so as to be detachable. 
     The image receiving unit  20  comprises the detector provided in a housing. The image receiving unit  20  has an image receiving surface  20 A that receives the radiation which has been emitted from the irradiation unit  18  and then transmitted through the subject H. The radiation carrying the information of the subject H is incident on the image receiving surface  20 A. 
     The detector is, for example, a flat panel detector (FPD) of a digital radiography (DR) type. The FPD has a detection surface in which a plurality of pixels are two-dimensionally arranged and a thin film transistor (TFT) panel (not illustrated) for driving the pixels. The radiation is incident on the detection surface of the detector through the image receiving surface  20 A. The detector converts the incident radiation into an electric signal and outputs a radiographic image indicating the subject H on the basis of the converted electric signal. For example, the detector is an indirect conversion type that converts radiation into visible light using a scintillator and converts the converted visible light into an electric signal. In addition, the detector may be a direct conversion type that directly converts radiation into an electric signal. Further, the image receiving unit  20  may have any configuration other than the configuration using the FPD. For example, the image receiving unit  20  may have a configuration in which an image intensifier (II) and a camera are combined. 
     Further, the image receiving unit  20  is connected to, for example, the control unit  28  and a power circuit (not illustrated) of the main body portion  16  by a cable (not illustrated) including a signal line for transmitting a control signal and a power line for supplying power. 
     (Configuration of Carriage Portion) 
     As illustrated in  FIGS. 4 to 6 , the carriage portion  17  has a rectangular shape in a plan view and is attached to a lower surface of the main body portion  16 . All of the main body portion  16 , the arm  12 , and the irradiation unit  18  and the image receiving unit  20  supported by the arm  12  of the mobile radiography apparatus  10  are mounted on the carriage portion  17 . 
     The carriage portion  17  has a plurality of casters  26  provided in a lower part and can travel. Therefore, an operator pushes the mobile radiography apparatus  10  by hand such that the mobile radiography apparatus  10  can travel in, for example, an operating room or a ward. 
     As illustrated in  FIG. 6 , the casters  26  include a pair of steering casters  40 , a pair of free casters  42 , and a pair of always-grounded casters  44 . That is, the carriage portion  17  has a total of six casters  26 . The casters  26  are disposed in the carriage portion  17  in the order of the always-grounded casters  44 , the steering casters  40 , and the free casters  42  from the front to the rear of the carriage portion  17 , that is, from the front to the rear of the mobile radiography apparatus  10 . 
     Specifically, the pair of always-grounded casters  44  are provided in both end portions of the carriage portion  17  in the width direction (the Y direction in  FIG. 6 ) in a front part of the carriage portion  17 . Further, the pair of steering casters  40  are provided in both end portions of the carriage portion  17  in the width direction (the Y direction in  FIG. 6 ) substantially at the center of the carriage portion  17  in the front-rear direction, that is, at the position of the center of gravity of the entire mobile radiography apparatus  10  in the front-rear direction. Furthermore, the pair of free casters  42  are provided in both end portions of the carriage portion  17  in the width direction (the Y direction in  FIG. 6 ) in a rear part of the carriage portion  17 . 
     The pair of free casters  42  are rotated independently about an axle D 1  that extends in the horizontal direction (Y direction) and are turned independently about turning shafts H 1  that extend in the vertical direction (Z direction). Further, the pair of free casters  42  are not connected to a steering  48  which is provided in the main body portion  16  and will be described below, and the steering angle of the free caster  42  is subordinately changed according to the direction of the force applied to the carriage portion  17 . 
     The pair of always-grounded casters  44  are casters that are always grounded to a floor  46  illustrated in  FIG. 4 . Here, the term “always” means that the caster is grounded in both a first grounded state and a second grounded state which will be described below. Therefore, for example, in a case in which the carriage portion  17  is inclined, the always-grounded casters  44  are also separated from the floor  46 . In this embodiment, the always-grounded casters  44  have the same configuration as the free casters  42 . That is, the pair of always-grounded casters  44  are rotated independently about an axle D 2  that extends in the horizontal direction (Y direction) and are turned independently about turning shafts H 2  that extend in the vertical direction (Z direction). Then, similarly to the free caster  42 , the steering angle of the always-grounded caster  44  is subordinately changed according to the direction of the force applied to the carriage portion  17 . 
     The pair of steering casters  40  are rotated independently about an axle D 3  that extends in the horizontal direction (Y direction). In addition, the pair of steering casters  40  are connected to the steering  48  which is provided in the main body portion  16  and will be described below, and the steering angle of the steering caster  40  is given by the operation of the steering  48 . The pair of steering casters  40  are turned in the same direction in operative association with each other by the operation of the steering  48 . 
     (Configuration of Steering) 
     As illustrated in  FIGS. 4 to 6 , the main body portion  16  is provided with the steering  48  that gives a steering angle to the steering casters  40 . The steering  48  comprises a lever  50  and a link mechanism  52  that transmits the rotation of the lever  50  to the pair of steering casters  40 . 
     As illustrated in  FIGS. 4 and 5 , the lever  50  is attached to the main body portion  16  through, for example, a base  54  fixed to an upper surface of the main body portion  16 . The lever  50  is vertically provided on the base  54  and comprises a rotation shaft  50 A that is attached to the base  54  so as to be rotatable about an axis line and a grip portion  50 B that extends from the rotation shaft  50 A outward (horizontally) in a radial direction of the rotation shaft  50 A. The operator holds the grip portion  50 B by hand and rotates the grip portion  50 B about the axis line of the rotation shaft  50 A to operate the lever  50 . 
     The lever  50  can be rotated about the axis line of the rotation shaft  50 A in a range of about 90° to the left and right from a reference position where the grip portion  50 B of the lever  50  extends in the front-rear direction (X direction) of the mobile radiography apparatus  10 . The rotation angle of the lever  50  can be adjusted to any angle within a rotatable range. 
     The link mechanism  52  comprises a shaft  56  having an upper end fixed to the rotation shaft  50 A of the lever  50 , a pinion  58  that is fixed to a lower end of the shaft  56 , a rack  60  that is engaged with the pinion  58 , and a tie rod  62  that connects the rack  60  and the steering casters  40 . 
     In a case in which the operator rotates the grip portion  50 B of the lever  50  to the left and the right (in the direction of an arrow R in  FIG. 6 ) about the axis line of the rotation shaft  50 A, the rotation shaft  50 A of the lever  50  is rotated about the axis line, and the shaft  56  of the link mechanism  52  fixed to the rotation shaft  50 A and the pinion  58  fixed to the shaft  56  are rotated with the rotation of the rotation shaft  50 A. In a case in which the pinion  58  is rotated, the rack  60  engaged with the pinion  58  is moved in the width direction (the Y direction in  FIG. 6 ) of the carriage portion  17  in operative association with the rotation of the pinion  58 . The movement of the rack  60  is transmitted to the steering casters  40  through the tie rod  62 , and the steering casters  40  are turned. 
     Specifically, as illustrated in  FIG. 7A , in a case in which the lever  50  is rotated in the left direction (the direction of an arrow R 1 ), the rack  60  of the link mechanism  52  is moved to the left (arrow Y 1 ), and each of the pair of steering casters  40  is rotated in the right direction (the direction of an arrow  51 ). This makes it possible to move the carriage portion  17  in the right direction. 
     On the other hand, as illustrated in  FIG. 7B , in a case in which the lever  50  is rotated in the right direction (the direction of an arrow R 2 ), the rack  60  of the link mechanism  52  is moved to the right (the direction of an arrow Y 2 ), and each of the pair of steering casters  40  is moved in the left direction (arrow S 2 ). This makes it possible to move the carriage portion  17  in the left direction. 
     (Configuration of Switching Mechanism) 
     Further, as illustrated in  FIGS. 4 to 6 , the carriage portion  17  is provided with a switching mechanism  64 . The switching mechanism  64  raises and lowers the free casters  42  with respect to the steering casters  40  to selectively switch between the first grounded state and the second grounded state. 
     Here, the first grounded state is a grounded state in which the free casters  42  are separated from the floor  46  and the steering casters  40  are grounded to the floor  46  as illustrated in  FIG. 4 . In addition, the second grounded state is a grounded state in which the free casters  42  are grounded to the floor  46  and the steering casters  40  are separated from the floor  46  as illustrated in  FIG. 5 . 
     Specifically, as illustrated in  FIGS. 8 and 9 , the switching mechanism  64  comprises a movable frame  66 , and a first pedal  68  and a second pedal  70  as pedals for raising and lowering the free caster  42 . 
     A front end portion (an end portion on the front side) of the movable frame  66  is fixed to a shaft  72  through a bearing portion (not illustrated) so as to be rotatable about an axis line of the shaft  72 . The shaft  72  extends in the width direction (Y direction) of the carriage portion  17  illustrated in  FIG. 4 , and both ends of the shaft  72  in the axial direction are fixed to the carriage portion  17 . On the other hand, the free caster  42  is attached to a rear end portion (an end portion on the rear side) of the movable frame  66 . That is, the free caster  42  is attached to the carriage portion  17  through the movable frame  66 . 
     The movable frame  66  is rotated about the shaft  72  provided in the front end portion. The rear end portion of the movable frame  66  is moved up and down in the vertical direction (Z direction) by the rotation about the shaft  72  while drawing an arc. In addition, the rear end portion of the movable frame  66  is biased upward in the vertical direction (Z direction) by a biasing member (not illustrated). 
     Further, a pair of slopes  74  are formed on an upper surface of the rear end portion of the movable frame  66 . The height of the pair of slopes  74  gradually increases from the rear end portion to the front end portion of the movable frame  66 . Further, a groove  76  is formed between the pair of slopes  74 . 
     The first pedal  68  comprises a base portion  68 A that is provided in a front end portion (an end portion on the front side) of the first pedal  68  and a tread  68 B that is provided in a rear end portion (an end portion on the rear side) of the first pedal  68 . The base portion  68 A is fixed to a shaft  78  through a bearing portion (not illustrated) so as to be rotatable about an axis line of the shaft  78 . The shaft  78  extends in the width direction (Y direction) of the carriage portion  17  illustrated in  FIG. 4 , and both end portions of the shaft  78  in the axial direction are fixed to the carriage portion  17 . 
     Further, a bearing shaft  80  that extends in parallel to the shaft  78  is fixed to a lower portion of the shaft  78  in the base portion  68 A of the first pedal  68 . A pair of bearings  82  are fixed to the bearing shaft  80  so as to be rotatable about an axis line of the bearing shaft  80 . 
     The pair of bearings  82  can be rotated coaxially and are provided at an interval in the axial direction of the bearing shaft  80 . Further, the pair of bearings  82  are mounted on the pair of slopes  74  formed in the movable frame  66  so as to be movable along the slopes  74 . 
     The tread  68 B of the first pedal  68  is integrally fixed to the base  68 A and can be rotated about the axis line of the shaft  78  together with the base  68 A. In addition, as illustrated in  FIGS. 4 to 6 , the tread  68 B of the first pedal  68  protrudes to the rear side of the carriage portion  17 . This enables the operator to step on the tread  68 B of the first pedal  68  from the outside of the mobile radiography apparatus  10 . 
     As illustrated in  FIGS. 8 and 9 , the second pedal  70  comprises a base portion  70 A that is provided in a front end portion (an end portion on the front side) of the second pedal  70  and a tread  70 B that is provided in a rear end portion (an end portion on the rear side) of the second pedal  70 . The base portion  70 A is disposed in the groove  76  formed between the pair of slopes  74  of the movable frame  66  and is fixed to a shaft  84  through a bearing portion (not illustrated) so as to be rotatable about an axis line of the shaft  84 . The shaft  84  extends in the width direction (Y direction) of the carriage portion  17  illustrated in  FIG. 4 , and both end portions of the shaft  84  in the axial direction are fixed to the movable frame  66 . 
     Further, a contact portion  86  is provided above the shaft  84  in the base portion  70 A of the second pedal  70 . The contact portion  86  protrudes upward from the groove  76  of the movable frame  66  in the vertical direction (Z direction) and is located between the pair of slopes  74 . Therefore, the contact portion  86  can come into contact with the pair of bearings  82  moved on the pair of slopes  74 . 
     The tread  70 B of the second pedal  70  is integrally fixed to the base portion  70 A and can be rotated about the axis line of the shaft  84  together with the base portion  70 A. Further, as illustrated in  FIG. 6 , the tread  70 B of the second pedal  70  protrudes to the rear side of the carriage portion  17 , similarly to the tread  68 B of the first pedal  68 . This enables the operator to step on the tread  70 B of the second pedal  70  from the outside of the mobile radiography apparatus  10 . 
     In a case in which the grounded state is switched from the first grounded state to the second grounded state, the operator steps on the tread  68 B of the first pedal  68  to lower the free caster  42  with respect to the steering caster  40 . Specifically, as illustrated in  FIG. 10 , in a case in which the operator steps on the tread  68 B of the first pedal  68 , the first pedal  68  is rotated about the axis line of the shaft  78  in the direction of an arrow J 1  (counterclockwise in  FIG. 10 ). In this case, the bearings  82  provided in the base portion  68 A of the first pedal  68  are moved forward (in the direction of an arrow K 1  in  FIG. 10 ) along the slopes  74  of the movable frame  66 . 
     The height of the slopes  74  gradually increases from the rear end portion to the front end portion of the movable frame  66 . Therefore, in a case in which the bearings  82  are moved forward, the slopes  74  are pushed down in the vertical direction (Z direction) by the bearings  82 . Then, the movable frame  66  is rotated about the axis line of the shaft  72  in the direction of an arrow L 1  (counterclockwise in  FIG. 10 ), and the rear end portion of the movable frame  66  is lowered. 
     Further, in a case in which the bearings  82  of the first pedal  68  are moved forward along the slopes  74  of the movable frame  66 , the bearings  82  come into contact with the contact portion  86  of the second pedal  70  which is provided between the pair of slopes  74  (see  FIG. 11 ). In a case in which the contact portion  86  is pushed forward by the bearings  82 , the second pedal  70  is rotated about the axis line of the shaft  84  in the direction of an arrow T 1  (clockwise in  FIG. 10 ). Then, the tread  70 B of the second pedal  70  is raised and is located above the tread  68 B of the first pedal  68  in the vertical direction (Z direction) (see  FIG. 11 ). 
     The free caster  42  is attached to the lower surface of the rear end portion of the movable frame  66 . Therefore, in a case in which the rear end portion of the movable frame  66  is lowered, the free caster  42  is lowered with respect to the carriage portion  17 , that is, the steering caster  40  illustrated in  FIG. 5 . Then, as illustrated in  FIGS. 5 and 11 , the free caster  42  is grounded to the floor  46 . In addition, the free caster  42  is located below the steering caster  40 . Therefore, the steering caster  40  is separated from the floor  46 . 
     On the other hand, in a case in which the grounded state is switched from the second grounded state to the first grounded state, the operator steps on the tread  70 B of the second pedal  70  to raise the free caster  42  with respect to the steering caster  40 . Specifically, as illustrated in  FIG. 11 , in a case in which the operator steps on the tread  70 B of the second pedal  70 , the second pedal  70  is rotated about the axis line of the shaft  84  in the direction of an arrow T 2  (counterclockwise in  FIG. 11 ). In this case, the contact portion  86  provided in the base portion  70 A of the second pedal  70  is also rotated about the axis line of the shaft  84  in the direction of the arrow T 2 . 
     In a case in which the contact portion  86  is moved in the direction of the arrow T 2 , the bearing  82  of the first pedal  68  is pushed by the contact portion  86 , and the first pedal  68  is rotated about the axis line of the shaft  78  in the direction of an arrow J 2  (clockwise in  FIG. 11 ). Then, the bearing  82  is moved backward (in the direction of an arrow K 2  in  FIG. 11 ) along the slope  74  of the movable frame  66 , and the tread  68 B of the first pedal  68  is raised. 
     In a case in which the bearing  82  is moved backward, the slope  74  pushed down in the vertical direction (Z direction) by the bearing  82  is raised upward in the vertical direction (Z direction) by the biasing force of a biasing member (not illustrated). Then, the movable frame  66  is rotated about the axis line of the shaft  72  in the direction of an arrow L 2  (clockwise in  FIG. 11 ), and the rear end portion of the movable frame  66  is raised. 
     The free caster  42  is attached to the lower surface of the rear end portion of the movable frame  66 . Therefore, as the rear end portion of the movable frame  66  is raised, the free caster  42  is raised with respect to the carriage portion  17 , that is, the steering caster  40  illustrated in  FIG. 4 . Then, as illustrated in  FIGS. 4 and 10 , the free casters  42  are separated from the floor  46 . In addition, the free casters  42  are located above the steering casters  40 . Therefore, the steering casters  40  are grounded to the floor  46 . 
     (Configuration of Handle) 
     Further, as illustrated in  FIGS. 4 and 5 , a pair of handles  88  for pushing and pulling the main body portion  16  (carriage portion  17 ) are fixed to the upper surface of the main body portion  16 . The handle  88  is, for example, a bar handle that is composed of a cylindrical bar having a substantially circular shape in a cross-sectional view. 
     The handle  88  extends in the front-rear direction (X direction) of the main body portion  16 , and both end portions of the handle  88  in an extending direction are fixed to the upper surface of the main body portion  16 . Further, as illustrated in  FIG. 6 , the pair of handles  88  are provided in both end portions in the width direction (the Y direction in  FIG. 6 ) in the rear part of the main body portion  16 . 
     The pair of handles  88  are held and pushed and pulled by the operator in a case in which the mobile radiography apparatus  10  is moved. In addition, for example, the shape and fixation position of the handle  88  are not limited to the embodiment, and a shape that is easy to grip and a position that is easy to grip are appropriately selected. 
     (Configuration of Steering Lock Mechanism) 
     Further, as illustrated in  FIGS. 12A and 12B , the steering  48  is provided with a steering lock mechanism  90  that locks the operation by the steering  48  in the second grounded state in which the steering casters  40  illustrated in  FIG. 4  are separated from the floor  46 . 
     In this embodiment, for example, the steering lock mechanism  90  locks the rotation of the lever  50  constituting the steering  48  with respect to the base  54 . Specifically, the steering lock mechanism  90  comprises a pin hole  92 A that is formed in the upper surface of the base  54 , a pin hole  92 B that is formed in a flange portion  94  which is a contact portion with the base  54  of the lever  50 , and a locking pin  96  that is inserted into the pin hole  92 B of the lever  50 . The pin hole  92 B in the flange portion  94  of the lever  50  has substantially the same diameter as the pin hole  92 A of the base  54  and passes through the flange portion  94  in the vertical direction (Z direction). 
     Here, for example, the pin hole  92 B of the lever  50  is formed at a position that communicates with the pin hole  92 A of the base  54  in a case in which the grip portion  50 B of the lever  50  is located at the reference position (the position where the grip portion  50 B of the lever  50  extends in the front-rear direction (X direction) of the mobile radiography apparatus  10 ). 
     As illustrated in  FIG. 12A , in a state in which the pin hole  92 A of the base  54  and the pin hole  92 B of the lever  50  do not communicate with each other, that is, in a state in which the grip portion  50 B of the lever  50  is not located at the reference position, the locking pin  96  inserted into the pin hole  92 B of the lever  50  is not capable of being inserted into the pin hole  92 A of the base  54 . 
     Therefore, the lever  50  can be rotated about the axis line of the rotation shaft  50 A with respect to the base  54 . In this case, the steering casters  40  (see  FIG. 4 ) can be operated by the lever  50  constituting the steering  48 . 
     On the other hand, as illustrated in  FIG. 12B , in a state in which the pin hole  92 A of the base  54  and the pin hole  92 B of the lever  50  communicate with each other, that is, in a state in which the grip portion  50 B of the lever  50  is located at the reference position, the locking pin  96  inserted into the pin hole  92 B of the lever  50  can be inserted into the pin hole  92 A of the base  54 . 
     Therefore, the lever  50  is not rotatable with respect to the base  54  by pushing the locking pin  96  from the upper side such that the tip of the locking pin  96  is located inside the pin hole  92 A of the base  54 . In this case, the operation of the steering casters  40  (see  FIG. 4 ) by the steering  48  is locked. In addition, the locking pin  96  may be manually inserted into the pin hole  92 A or may be electrically inserted into the pin hole  92 A by, for example, a solenoid (not illustrated). 
     (Operation and Effect) 
     According to the mobile radiography apparatus  10  of this embodiment, the carriage portion  17  is provided with the steering casters  40  whose steering angle is given by the operation of the steering  48  and the free casters  42  whose steering angle is changed subordinately according to the direction of the force applied to the carriage portion  17 . Further, the switching mechanism  64  can selectively switch the casters  26  between the first grounded state and the second grounded state. 
     Therefore, the switching mechanism  64  selectively switches the grounded state of the free casters  42  and the steering casters  40  to change the operability of the carriage portion  17  according to the usage situation. 
     That is, the traveling direction of the carriage portion  17  can be fixed by changing the grounded state to the first grounded state in which the free casters  42  are separated from the floor  46  and the steering casters  40  are grounded to the floor  46 . In a case in which the traveling direction of the carriage portion  17  can be fixed, it is convenient to perform continuous imaging while moving the imaging position. 
     For example, in a case in which imaging is performed while an imaging part is changed along the body axis direction of the subject H, it is convenient that the carriage portion  17  can be stably moved along the body axis direction of the subject H. The steering angle of the steering casters  40  can be fixed by the operation of the steering  48 . Therefore, in a case in which the carriage portion  17  is moved, the carriage portion  17  does not wobble, and the traveling direction is stable. 
     On the other hand, the carriage portion  17  is turned in a small radius by changing the grounded state to the second grounded state in which the free casters  42  are grounded to the floor  46  and the steering casters  40  are separated from the floor  46 . In a case in which the carriage portion  17  is turned in a small radius, it is possible to change the direction of the carriage portion  17  in a narrow space. Therefore, it is convenient that the mobile radiography apparatus  10  is used in a narrow hospital room. 
     Further, since the switching mechanism  64  switches the grounded state of the free casters  42  and the steering casters  40 , the configuration is simple. For example, as the switching mechanism, a method is considered which switches between a valid state and an invalid state of the input of an operating force from the steering  48  to the steering casters  40  in a state in which the steering casters  40  are grounded. The invalid state is, in short, a state in which the steering casters  40  function as the free casters. It is easier to simplify the configuration of the mechanism for switching the grounded state than the configuration of the mechanism for switching between the valid state and the invalid state of the input of the operating force from the steering  48  as described above. 
     Furthermore, according to this embodiment, the switching mechanism  64  raises and lowers the free casters  42  with respect to the steering casters  40  to switch between the first grounded state and the second grounded state. In general, since the steering casters  40  are connected to the steering  48 , the weight of the steering casters  40  is larger than that of the free casters  42 . According to this embodiment, since the switching mechanism  64  raises and lowers the free caster  42 , it is possible to easily switch the grounded state of the casters  26  with a small force, as compared to a configuration in which the steering casters  40  are raised and lowered. 
     Moreover, the switching mechanism  64  has the first pedal  68  and the second pedal  70  as the pedal for raising and lowering the free casters  42 . Therefore, since the operator steps on the first pedal  68  or the second pedal  70  to raise and lower the free casters  42 , it is possible to switch the grounded state of the casters  26  while manually operating the mobile radiography apparatus  10 . 
     In particular, according to this embodiment, the pedal includes two pedals of the first pedal  68  and the second pedal  70 . In a case in which one of the pedals is lowered, the other pedal is raised. In general, in a case in which the free casters  42  are raised and lowered by one pedal, the pedal is lowered to move the free casters  42  down, and the pedal is raised to move the free casters  42  up. However, the load on the operation of pulling up the pedal larger than that on the operation of pushing the pedal down. 
     Here, according to this embodiment, the two pedals are combined such that both the operation of raising the free casters  42  and the operation of lowering the free casters  42  can be achieved by stepping on (depressing) the pedal. Therefore, it is easy to switch the grounded state of the casters  26 . 
     Further, in this embodiment, the carriage portion  17  is provided with the always-grounded casters  44  that are grounded to the floor  46  in both the first grounded state and the second grounded state, in addition to the free casters  42  and the steering casters  40 . 
     Therefore, even in a case in which one of the free casters  42  and the steering casters  40  is separated from the floor  46 , the carriage portion  17  can travel stably. In addition, in a case in which the grounded state of the free casters  42  and the steering casters  40  is switched, it is possible to suppress the postural instability of the carriage portion  17 . 
     Further, according to this embodiment, the always-grounded casters  44 , the steering casters  40 , and the free casters  42  are disposed in this order from the front to the rear of the carriage portion  17 . As described above, since the steering casters  40  are disposed between the always-grounded casters  44  and the free casters  42 , the steering casters  40  can be disposed at a position close to the position of the center of gravity of the main body portion  16 . Therefore, it is possible to suppress the postural instability of the main body portion  16  in a case in which the steering  48  is operated. 
     In particular, according to this embodiment, the steering casters  40  are disposed at the position of the center of gravity of the entire mobile radiography apparatus  10  in the front-rear direction of the carriage portion  17 . Therefore, it is possible to further suppress the postural instability of the main body portion  16  in a case in which the steering  48  is operated. 
     Further, the arm  12  of the mobile radiography apparatus  10  according to this embodiment has two end portions. The irradiation unit  18  is provided at one end, and the image receiving unit  20  is provided at the other end. Therefore, the irradiation unit  18  and the image receiving unit  20  can be integrally held by the arm  12 . This configuration makes it possible to perform imaging while moving the carriage portion  17  in the body axis direction of the subject H, for example, during surgery. 
     Furthermore, the mobile radiography apparatus  10  according to this embodiment includes the steering lock mechanism  90  that locks the operation by the steering  48 . Therefore, for example, in the second grounded state in which the steering casters  40  are separated from the floor  46 , the steering lock mechanism  90  prevents the lever  50  from being rotated with respect to the base  54  (main body portion  16 ), which makes it possible to prevent the rotation of the steering casters  40 . As a result, it is possible to suppress the postural instability of the mobile radiography apparatus  10  caused by the movement of the steering casters  40  in a case in which the main body portion  16  is moved by the free casters  42 . 
     Further, according to this embodiment, the handle  88  for pushing and pulling the carriage portion  17  is provided separately from the lever  50  constituting the steering  48 . As described above, since the handle  88  is provided separately from the lever  50 , it is easy to operate in a case in which the carriage portion  17  is moved by the free casters  42 . 
     OTHER EMBODIMENTS 
     An example of the embodiment of the present disclosure has been described above. However, the present disclosure is not limited to the above-described embodiment, and various modifications and changes can be made without departing from the gist of the present disclosure. 
     For example, in the above-described embodiment, the always-grounded caster  44  is a caster whose steering angle is changed subordinately, similarly to the free caster  42 . However, the always-grounded caster  44  may be a fixed caster that is not capable of being turned. 
     In addition, the carriage portion  17  is not necessarily provided with the always-grounded casters  44 . In a case in which the always-grounded casters  44  are not provided, for example, three or more steering casters  40  and three or more free casters  42  are provided in the carriage portion  17  to stably move the carriage portion  17 . Further, the disposition of each of the casters  26  is not limited to the above-described embodiment. For example, the steering casters  40  may be disposed behind the always-grounded casters  44  and the free casters  42 . 
     Furthermore, in the above-described embodiment, the free casters  42  are raised and lowered by the switching mechanism  64 . However, the steering casters  40  may be raised and lowered by the switching mechanism  64 . 
     Moreover, in the above-described embodiment, the handle  88  is provided on the upper surface of the main body portion  16 . However, the handle  88  may not be provided, and the lever  50  of the steering  48  may be pushed and pulled or the main body portion  16  may be directly pushed and pulled to move the mobile radiography apparatus  10 . 
     In addition, in the above-described embodiment, the link mechanism  52  of the steering  48  is configured to include the shaft  56 , the pinion  58 , the rack  60 , and the tie rod  62 . However, the configuration of the link mechanism  52  is not limited to the above-described embodiment, and it is possible to use other known link mechanisms. 
     Further, in the above-described embodiment, the steering lock mechanism  90  is configured to lock the rotation of the lever  50  constituting the steering  48 . However, the steering lock mechanism  90  may be configured to lock the rotation of any of the mechanisms constituting the steering  48  and the steering casters  40  and may be, for example, a stopper that locks the turning of the steering casters  40 . 
     Furthermore, in a case in which the steering casters  40  can be allowed to be moved in the movement of the main body portion  16  by the free casters  42 , the steering lock mechanism  90  is not necessarily provided. 
     Moreover, in the above-described embodiment, the C-arm having a C-shape in a side view has been described as an example of the arm  12 . However, a U-arm having a U-shape in a side view may be used. In addition, the arm  12  may be an I-arm having an I-shape in a side view in which only the irradiation unit  18  is provided at one end. 
     Further, in the above-described embodiment, the irradiation unit  18  and the image receiving unit  20  are provided at both ends of the arm  12 . However, the image receiving unit  20  may not necessarily be provided in the arm  12 , and at least only the irradiation unit  18  may be provided in the arm  12 . 
     In addition, in the above-described embodiment, X-rays have been described as an example of the radiation. However, the radiation is not limited to the X-rays and may be, for example, γ-rays. 
     The disclosure of JP2019-199331 filed on Oct. 31, 2019 is incorporated herein by reference in its entirety. All of the documents, patent applications, and technical standards described in the specification are incorporated herein by references to the same extent as the incorporation of the individual documents, patent applications, and technical standards by references are described specifically and individually.