Patent Publication Number: US-2011069812-A1

Title: Radiation imaging apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a radiation imaging apparatus, and more particularly to a radiation imaging apparatus for obtaining a plurality of radiation images used for generating a tomosynthesis image. 
     2. Description of the Related Art 
     Tomosynthesis imaging in which radiation imaging is performed a plurality of times by shifting a radiation source with respect to an examination region so that radiation is incident on the examination region at different incident angles is known as described, for example, Japanese Unexamined Patent Publication No. 63 (1988)-230152. 
     A method in which a plurality of radiation images obtained by the tomosynthesis imaging is combined, thereby obtaining a radiation tomographic image representing cross-sections of the examination region, i.e., a tomosynthesis image is also known as described, for example, in “The principle and clinical application of Tomosynthesis”, T. Shiomi, Journal of Japan Society Medical Imaging and Information Science, Vol. 24, No. 2, pp. 22-27, 2007. The method obtains a tomosynthesis image by combining a plurality of radiation images such that the contrast of a predetermined cross-section of the examination region is enhanced and blur of images representing cross-sections other than the predetermined cross-section is increased. 
     As a system that performs such tomosynthesis imaging, a general purpose radiation imaging system in which a radiation source supported by an overhead traveling suspension that travels on a rail which is installed on a ceiling is shifted in horizontal and vertical directions is known as described, for example, in U.S. Pat. No. 6,970,531 and Japanese Unexamined Patent Publication No. 2007-244569. 
     The general purpose radiation imaging system allows both tomosynthesis imaging by performing radiation imaging a plurality of times while shifting the radiation source by the overhead traveling suspension and general radiation imaging for obtaining a radiation image by fixing the position of the radiation source by the overhead traveling suspension and emitting radiation to an examination region once. 
     In comparatively small medical facilities, such as private practitioners, clinics, and the like, it is rare to have a dedicated radiation imaging system for tomosynthesis imaging and it is common to perform various types of radiation imaging using a general purpose radiation imaging system. 
     When performing tomosynthesis imaging using such a general purpose radiation imaging system, radiation imaging of an examination region (e.g., a spine) is performed a plurality of times by shifting the radiation source in one direction parallel to a detection surface of a radiation detector and changing the incident angle of the radiation with respect to the examination region. Thereafter, a tomosynthesis image representing a cross-section of the examination region is obtained through image synthesis using image data representing a plurality of images obtained by the tomosynthesis imaging and position data representing the position of the tube focus at each radiation imaging. 
     The position of the tube focus in each radiation imaging obtained in tomosynthesis imaging using such a general purpose radiation imaging system is measured by a position measuring device (e.g., linear scale measuring device) provided adjacent to the ceiling where the overhead traveling suspension is installed. 
     The measurement of the position of the tube focus by such a position measuring device provided adjacent to the ceiling has a large error in the position data of the tube focus because the measurement axis is located near the ceiling while the tube focus, the measurement target, is away from the ceiling (e.g., 1.5 m). 
     That is, when position data obtained by measuring a shift of the overhead traveling suspension by a linear scale measuring device provided along a ceiling are used as the position data of a tube focus away from the ceiling, the influence of shift errors of the overhead traveling suspension (errors of pitching, yawing, rolling, and the like) is increased such that the greater the distance of the tube focus from the ceiling, the greater the influence. Due to the so-called Abbe principle, the measurement error increases with the increase in the distance between the measurement axis (measurement axis of the linear scale measuring device) and measurement target (tube focus). 
     Consequently, the error when combining a plurality of radiation images obtained by the tomosynthesis imaging is also increased, posing a problem that the resolution or contrast of the tomosynthesis image obtained is degraded. 
     When measuring the position of the tube focus, it is preferable that the tube focus is positioned on the measurement axis for the reason described above. There is a problem here that it is difficult to construct a radiation imaging system such that the measurement axis of a position measuring device is located adjacent to the tube focus (measurement target point) of a radiation source held by an overhead traveling suspension that travels on a rail installed on a ceiling. 
     The problem described above is not limited to radiation imaging systems that implement tomosynthesis imaging but is common to radiation imaging systems that perform radiation imaging while shifting the radiation source in a direction parallel to the detection surface of the radiation detector. 
     The present invention is developed in view of the circumstances described above and it is an object of the present invention to provide a radiation imaging apparatus capable of easily improving measurement accuracy of the position of tube focus in radiation imaging. 
     SUMMARY OF THE INVENTION 
     A radiation imaging apparatus of the present invention is an apparatus which includes a radiation source for emitting radiation, a radiation detector for detecting radiation emitted from the radiation source and transmitted through a subject, a shifting means for holding and shifting the radiation source in a direction parallel to a detection surface of the radiation detector, and a position obtaining means for obtaining a position of a tube focus of the radiation source, and which performs tomosynthesis imaging while shifting the radiation source by the shifting means, 
     wherein the position obtaining means comprises an acceleration sensor integrally disposed adjacent to the radiation source. 
     The position of the tube focus is a position of a focus of radiation emitted from the radiation source. That is, it is a position of a focus of a tube provided in the radiation source. 
     The acceleration sensor integrally disposed adjacent to the radiation source is a sensor disposed adjacent to the radiation source and disposed integrally with the radiation source. 
     Preferably, the acceleration sensor includes a pair of sensors disposed on a straight line passing through the tube focus so as to sandwich the tube focus. 
     The shifting means may be a means that includes an orthogonal shifting section for shifting the radiation source in a direction orthogonal to the detection surface. 
     The radiation source may be a source held by an overhead traveling suspension shifting platform. 
     Another radiation imaging apparatus of the present invention is an apparatus which includes a radiation source for emitting radiation, a radiation detector for detecting radiation emitted from the radiation source and transmitted through a subject, a shifting means for holding and shifting the radiation source in a direction parallel to a detection surface of the radiation detector, and a position obtaining means for obtaining a position of a tube focus of the radiation source, and which performs radiation imaging while shifting the radiation source by the shifting means, 
     wherein the position obtaining means comprises a laser length measuring device integrally disposed adjacent to the radiation source. 
     The radiation imaging described above may be tomosynthesis imaging. In the tomosynthesis imaging, radiation imaging is performed a plurality of times while shifting the radiation source by the shifting means to obtain a plurality of radiation images for use in generating a tomosynthesis image representing a cross-section of a subject and the position of the tube focus at each of the plurality of times of radiation imaging is obtained. 
     The acceleration sensor is a sensor for measuring the position of tube focus in a three-dimensional space. 
     According to the radiation imaging apparatus of the present invention, the position obtaining means includes an acceleration sensor disposed adjacent to and integrally with the radiation source and the position of the tube focus is measured using the acceleration sensor. This may easily improve measurement accuracy of the position of tube focus obtained at each radiation imaging. 
     That is, for example, the acceleration sensor can be easily attached to a radiation source held on the lower side of an overhead traveling suspension and can be shifted in horizontal and vertical directions, because the acceleration sensor is capable of measuring acceleration by itself. Further, the position of acceleration sensor may be obtained by integrating acceleration values measured by the acceleration sensor. 
     This reduces the distance from the measurement axis of the acceleration sensor (position measuring device) to the tube focus (measurement target point) and allows measurements in accordance with the Abbe principle, whereby measurement accuracy of the position of tube focus may be improved. 
     In contrast, a linear scale measuring device, for example, is a device that performs measurement with a scale fixed on a fixed portion and a reader, provided on a moving portion, for reading a scale mark of the scale. Therefore, it is difficult to bring the measurement axis, which is defined by the combination of the fixed and moving portions, near the tube focus. 
     Further, if the acceleration sensor includes a pair of sensors disposed on a straight line passing through the tube focus so as to sandwich the tube focus, errors that occur according to the distance from the measurement axis to the tube focus (errors due to measurements not in accordance with the Abbe principle) may further be reduced. That is, if the distance from the tube focus to each acceleration sensor disposed opposite to each other across the tube focus is known, an error in the measurement performed by the acceleration sensor disposed on one of the sides sandwiching the tube focus can be offset by an error in the measurement performed by the acceleration sensor on the other side, whereby measurement accuracy of the position of tube focus may further be improved. 
     Still further, if the shifting means is a means that includes an orthogonal shifting section for shifting the radiation source in a direction orthogonal to the detection surface, or if the radiation source is a source held by an overhead traveling suspension shifting platform, measuring accuracy of the position of tube focus may be improved significantly. 
     That is, the magnitude of a measurement error that occurs when measuring the position of tube focus by the acceleration sensor disposed on the radiation source is mainly dependent on the distance from the position of tube focus to the acceleration sensor, so that the position of tube focus may be measured without being affected by a shift error that occurs when the radiation is shifted by the shifting means. Consequently, if radiation imaging is performed while shifting the radiation source by a shifting means that causes a relatively large error, a significant effect of improving measurement accuracy of the position of tube focus may be obtained. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a conceptual drawing illustrating a schematic configuration of a radiation imaging apparatus according to an embodiment of the present invention. 
         FIG. 2  illustrates radiation imaging performed in standing position using the radiation imaging apparatus shown in  FIG. 1 . 
         FIG. 3  is an enlarged perspective view of a shift assistance section. 
         FIG. 4  illustrates synthesis of a plurality of images obtained by tomosynthesis imaging. 
         FIG. 5  illustrates a positional relationship between each section when tomosynthesis imaging is performed. 
         FIG. 6  illustrates a tolerance when tomosynthesis imaging is performed with the positional relationship shown in  FIG. 5 . 
         FIG. 7  illustrates deflection of a movable rail extending in an X direction. 
         FIG. 8  shows acceleration values measured by an acceleration sensor. 
         FIG. 9  illustrates speeds obtained by integrating the acceleration values once measured by the acceleration sensor. 
         FIG. 10  illustrates values of speeds obtained by integrating the acceleration values twice measured by the acceleration sensor. 
         FIG. 11  shows position errors in a range in which a horizontal shifting platform is shifted at a constant speed. 
         FIG. 12A  is a flowchart of an acceleration data measurement. 
         FIG. 12B  is a flowchart of tomosynthesis imaging. 
         FIG. 13  illustrates radiation imaging apparatus that uses a laser length measuring device for tube position measurement. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.  FIGS. 1 and 2  illustrate schematic configurations of a radiation imaging apparatus according to embodiments of the present invention.  FIG. 1  is a conceptual drawing illustrating radiation imaging performed in spine position.  FIG. 2  is a conceptual drawing illustrating radiation imaging performed in standing position.  FIG. 3  is an enlarged perspective view of a shift assistance section. 
     As shown in  FIG. 1 , radiation imaging apparatus  100  of the present invention installed in an examination room is a general purpose radiation imaging apparatus  100 . It includes radiation source  30  for emitting radiation, radiation detector  20  for detecting radiation emitted from radiation source  30  and transmitted through examination region  1 A of subject  1 , shifting section  10  for holding and shifting radiation source  30  in one direction parallel to detection surface  20 M of radiation detector  20  (X direction, here), and position obtaining section  40  for obtaining the position (in X, Y, and Z directions in  FIG. 1 ) of tube focus  30 F of radiation source  30  shifted by shifting section  10 . It performs tomosynthesis imaging while shifting radiation source  30  by shifting section  10 . 
     Shifting section  10  can be shifted on a rail installed on the ceiling of the examination room, as described later. 
     Radiation detector  20  detects radiation transmitted through examination region  1 A and representing examination region  1 A. 
     Radiation imaging apparatus  100  performs radiation imaging of examination region  1 A a plurality of times by emitting radiation from radiation source  30  while shifting radiation source  30  by shifting section  10 . Through the plurality of times of radiation imaging, image data representing a plurality of radiation images used for generating a tomosynthesis image representing a cross-section of examination region  1 A are obtained via radiation detector  20 . 
     Position obtaining section  40  includes acceleration sensors  40 Sa and  40 Sb (hereinafter, also collectively referred to as acceleration sensors  40 S), which are motion sensors integrally provided adjacent to radiation source  30  for measuring acceleration, and measures the position of tube focus  30 F using acceleration sensors  40 Sa and  40 Sb. 
     Acceleration sensor  40 Sa is a sensor capable of measuring acceleration in three axis directions of X, Y, and Z. Also, acceleration sensor  40 Sb is a sensor capable of measuring acceleration in the three axis directions. 
     Position obtaining section  40  includes calculation section  40 E that obtains amounts of positional displacement of acceleration sensors  40 Sa and  40 Sb by time-integrating acceleration values obtained by respective acceleration sensors  40 Sa and  40 Sb. 
     Acceleration sensors  40 Sa and  40 Sb are disposed on a straight line passing through tube focus  30 F so as to sandwich tube focus  30 F, i.e., each on each side of tube focus  30 F. 
     Here, acceleration sensors  40 Sa and  40 Sb are equidistant from tube focus  30 F, so that calculation section  40 E may obtain the amount of positional displacement of tube focus  30 F by averaging each set of coordinates, obtained by calculation section  40 E, representing the amount of positional displacement of each of acceleration sensors  40 Sa and  40 Sb. 
     Distance La from tube focus  30 F (measurement target point) to acceleration sensor  40 Sa (measurement axis) in Z axis direction and distance Lb from tube focus  30 F (measurement target point) to acceleration sensor  40 Sb (measurement axis) in Z axis direction are equal and they may be, for example, 0.1 m. 
     Note that even in a case where acceleration sensors  40 Sa and  40 Sb are not equidistant from tube focus  30 F, if the ratio of the distance from tube focus  30 F to acceleration sensor  40 Sa and the distance from tube focus  30 F to acceleration sensor  40 Sb is known, the amount of positional displacement of tube focus  30 F may be obtained by a known method as described above. 
     Calculation section  40 E adds the amount of positional displacement obtained in the manner described above to predetermined and inputted coordinates representing the reference position of tube focus  30 F and outputs coordinate data representing the position of tube focus  30 F. 
     More specifically, radiation source  30  is disposed such that tube focus  30 F is placed at a predetermined reference position (e.g., coordinates (0, 0, 0)) in the examination room. Then, the position of tube focus  30 F may be obtained by adding the amount of displacement obtained by calculation section  40 E to the coordinates (0, 0, 0) of the reference position. This allows, when, for example, the position coordinates of examination table  72  with respect to the reference position, the position coordinates of examination region  1 A with respect to the reference position, or the like is obtained and stored in calculation section  40 E in advance, the position of tube focus  30 F with respect to these positions described above may be obtained. In this way, position obtaining section  40  may obtain the position of tube focus  30 F with respect to an object which is stationary relative to the examination room. 
     Here, it is assumed that position obtaining section  40  obtains coordinate data representing the position of tube focus  30 F in three axis directions of one direction parallel to detection surface  20 M (arrow X directions in  FIG. 1 ), a direction orthogonal to the one direction and parallel to detection surface  20 M (arrow Y directions in  FIG. 1 ), and a direction orthogonal to detection surface  20 M (arrow Z directions in  FIG. 1 ). 
     As shown in  FIG. 1 , shifting section  10  includes fixed rails  11  installed on ceiling  9 , movable rail  12  extending in a direction (an X direction) orthogonal to the extending direction of fixed rails  11  (a Y direction), which is suspended by fixed rails  11  and movable in the extending direction of fixed rails  11  (a Y direction), horizontal shifting platform  13  which is suspended by movable rail  12  and movable in the extending direction of movable rail  12  (X direction), extensible/retractable support  14  which is attached to the underside of horizontal shifting platform  13  and is extensible/retractable in a vertical direction (a Z direction) by a multistage extension mechanism, drive motors (not shown) for shifting horizontal shifting platform  13  in X and Y directions and extending/retracting extendable/retractable support  14  in a Z direction. 
     Radiation source  30  is attached to a second end of extendable/retractable support  14 , a first end of which is fixed to horizontal shifting platform  13 . 
     This allows shifting section  10  to shift radiation source  30  in two mutually orthogonal directions (X and Y directions) which are parallel to detection surface  20 M and an orthogonal direction (a Z direction) orthogonal to detection surface  20 M. 
     Radiation source  30  attached to the second end of extendable/retractable support  14  is rotatably supported about horizontal axis Hj (arrow RA directions in  FIG. 3 ) so as to be changeable in position with respect to extendable/retractable support  14 , thereby allowing radiation imaging in spine position ( FIG. 1 ) and radiation imaging in standing position ( FIGS. 2 and 3 ). 
     In addition to acceleration sensors  40 S, position obtaining section  40  includes linear scale measuring devices  40 L. 
     Linear scale measuring device  40  includes linear scale measuring device  40 Ly for Y axis constituted by a linear scale fixed along fixed rail  11  which is installed on the ceiling and extending in a Y direction and a reading section, fixed to movable rail  12 , for reading a scale mark of the linear scale. Linear scale measuring device  40  also includes linear scale measuring device  40 Lx for position measurement in an X axis direction constituted by a linear scale fixed along movable rail  12  which is installed on the ceiling and a reading section, fixed to horizontal shifting platform  13 , for reading a scale mark of the linear scale. Linear scale measuring device  40  further includes linear scale measuring device  40 Lz for measuring the position of the tip of extendable/retractable support  14  in a Z axis direction. Note that these linear scales are omitted in  FIG. 1 . 
     Linear scale measuring devices  40 Lx and  40 Ly may accurately measure positions of horizontal shifting platform  13  in X and Y directions since horizontal shifting platform  13 , which is the measurement target point, is located adjacent to the measurement axes thereof. When the measurement target point is tube focus  30 F, however, the measurement axes of linear scale measuring devices  40 Lx and  40 Ly are far away from the measurement target point. Distance Ls in a Z axis direction from tube focus  30 F (measurement target point) to the measurement axis of linear scale measuring device  40 Lx is 1.5 m. 
     Consequently, measurement error is larger when the position of tube focus  30 F in X and Y directions is obtained by linear scale measuring devices  40 Lx and  40 Ly than in the case in which the position of horizontal shifting platform  13  in X and Y directions is measured by linear scale measuring devices  40 Lx and  40 Ly due to the so-called Abbe principle. 
     Including such error, the measurement of the shifting of tube focus  30 F in X, Y, and Z directions by linear scale measuring devices  40 Lx,  40 Ly,  40 Lz (also, collectively referred to as linear scale measuring devices  40 L) will be described later. 
     Note that acceleration sensors  40 Sa and  40 Sb are located adjacent to the measurement target point of tube focus  30 F and each of distances La and Lb from tube focus  30 F (measurement target point) to acceleration sensors  40 Sa and  40 Sb in a Z axis direction is 0.1 m, while distance Ls from tube focus  30 F (measurement target point) to the measurement axis of linear scale measuring device  40 Lx is 1.5 m. 
     Therefore, when the position of tube focus  30 F is measured using acceleration sensors  40 Sa and  40 Sb, measurement errors included in measurement values arising from the influence of shifting errors of shifting section  10  (pitching, rolling, yawing) are reduced to 1/15 of measurement errors included in measurement values arising from the influence of shifting errors of shifting section  10  (pitching, rolling, yawing) when tube focus  30 F is measured by linear scale measuring device  40 Lx. 
     Further, calculation section  40 E may perform calculation such that an error (Abbe principle error) occurred in the position measurement of tube focus  30 F by acceleration sensor  40 Sa is offset by an error (Abbe principle error) occurred in the position measurement of tube focus  30 F by acceleration sensor  40 Sb. This allows position obtaining section  40  to accurately measure the position of tube focus  30 F. 
     Image data obtained by radiation detector  20  and position data obtained by position obtaining section  40  of radiation imaging apparatus  100  are temporarily stored in storage section  76  and then inputted to tomosynthesis image generation section  210  of radiation image reproducing apparatus  200 . Then, tomosynthesis image generation section  210  generates a tomosynthesis image using the position data obtained by position obtaining section  40  and image data representing a plurality of radiation images obtained by radiation detector  20 . Note that the position of examination region  1 A of subject  1  on imaging platform  25  measured separately is inputted to tomosynthesis image generation section  210  via console  70 , to be described later, and storage section  76 . 
     The operation, synchronization of operational timing, and the like of each section of radiation imaging apparatus  100  that performs tomosynthesis imaging for obtaining image data and position data used for generating a tomosynthesis image are controlled by controller  72 . Controller  72  controls each section according to an instruction inputted from console  70 . 
     An operation of radiation imaging apparatus  100  when performing tomosynthesis imaging will now be described. 
     First, a case in which the position of tube focus  30 F is measured using only acceleration sensors  40 Sa and  40 Sb will be described. Then, a case in which the position of tube focus  30 F is measured using only linear scale measuring devices  40 L will be described. Finally, a case in which the position of tube focus  30 F is measured in combination with acceleration sensors  40 S and linear scale measuring devices  40 L will be described. 
     &lt;Case in which Position of Tube Focus is Measured Using Only Acceleration Sensors&gt; 
     When performing tomosynthesis imaging in spine position, radiation detector  20  is accommodated in spine position imaging platform  25 , and subject  1  is placed in spine position on the upper side of imaging platform  25 , i.e., on the side of detection surface  20 M of radiation detector  20  accommodated in spine position imaging platform  25  ( FIG. 1 ). 
     Then, operator  2  enters position data obtained by measuring the position of examination region  1 A of subject  1  in spine position, imaging conditions of tomosynthesis imaging, and the like in console  70 . 
     Next, operator  2  operates console  70  to shift radiation source  30  such that tube focus  30 F is placed at the reference position of coordinates (0, 0, 0). The reference position of coordinates (0, 0, 0) may be, for example, a position of tube focus  30 F when radiation source  30  is abutted to a predetermined abutment surface. 
     Then, operator  2  operates console  70  to start tomosynthesis imaging. This causes an instruction to perform predetermined tomosynthesis imaging to be inputted to controller  72  from console  70 . Controller  72  controls each section so that tomosynthesis imaging for obtaining a tomographic image (tomosynthesis image) of examination region  1 A of subject  1  is performed. 
     First, radiation source  30  is shifted through control of controller  72  such that tube focus  30 F located at the reference position of coordinates (0, 0, 0) is placed at the initial position of coordinates (X 0 , Y 0 , Z 0 ). That is, radiation source  30  is shifted such that the position of tube focus  30 F obtained by position obtaining section through measurement using only acceleration sensors  40 S corresponds to coordinates (X 0 , Y 0 , Z 0 ) ( FIG. 1 ). Thereafter, tomosynthesis imaging by radiation imaging apparatus  100  is started. 
     As shown in  FIG. 1 , horizontal shifting platform  13  is shifted by shifting section  10  along movable rail  12  extending in an X direction. This causes radiation source  30 , attached to the tip of extensible/retractable support  14 , to be shifted also in the X direction. 
     Here, the shifting section  10  shifts horizontal shifting platform  13  only on movable rail  12  extending in an X direction without extending/retracting extensible/retractable support  14  extending in a Z direction and without shifting horizontal shifting platform  13  on fixed rails  11  extending in a Y direction. 
     While being shifted in the X direction which is one direction parallel to detection surface  20 M of radiation detector  20 , radiation source  30  emits radiation onto examination region  1 A at first imaging position P 1 . First imaging position P 1  is a position where X coordinate of the position of tube focus  30 F obtained by position obtaining section  40  is X 1 . The position of tube focus  30 F obtained by position obtaining section  40  at first imaging position P 1  corresponds to coordinates (X 1 , Y 1 , Z 1 ). 
     Here, if shifting section  10  could shift radiation source  30  in one direction (X direction) without any error, the position of tube focus  30 F obtained by position obtaining section  40  would be coordinates (X 1 , Y 0 , Z 0 ). But, tube focus  30 F is shifted with slightly being displaced in Y and Z directions. Consequently, when tube focus  30 F arrives at first imaging position P 1  (position where the coordinate in X direction obtained by position obtaining section  40  is X 1 ), the position of tube focus  30 F is displaced in a Y direction (amount of displacement (Y 1 −Y 0 )) and a Z direction (amount of displacement (Z 1 −Z 0 )), so that the position of tube focus  30 F is obtained as coordinates (X 1 , Y 1 , Z 1 ). 
     That is, due to deformation of movable rail  12  extending in an X direction along the ceiling, deformation of ceiling itself, or the like, horizontal shifting platform  13  shifted on movable rail  12  in the X direction is displaced in Y and Z directions or slightly rotated about X axis, Y axis, and Z axis. That is, along with the shift of horizontal shifting platform  13 , pitching, rolling, and yawing occur in horizontal shifting platform  13 . 
     Then, while being shifted in the X direction, radiation source  30  emits radiation toward examination region  1 A at second imaging position P 2 . Second imaging position P 2  is a position where X coordinate of the position of tube focus  30 F obtained by position obtaining section  40  is predetermined coordinate X 2 . The position of tube focus  30 F obtained by position obtaining section  40  at second imaging position P 2  corresponds to coordinates (X 2 , Y 2 , Z 2 ). 
     Here, if shifting section  10  could shift radiation source  30  in one direction (X direction) without any error, the position of tube focus  30 F obtained by position obtaining section  40  at second imaging position P 2  would be coordinates (X 2 , Y 0 , Z 0 ). But, as in the above, when tube focus  30 F arrives at second imaging position P 2  (position where the coordinate in X direction obtained by position obtaining section  40  is X 2 ), the position of tube focus  30 F is displaced in a Y direction (amount of displacement (Y 2 −Y 0 )) and a Z direction (amount of displacement (Z 2 −Z 0 )), so that the position of tube focus  30 F is obtained as coordinates (X 2 , Y 2 , Z 2 ). 
     Then, while being shifted in the X direction, radiation source  30  emits radiation toward examination region  1 A at third imaging position P 3 . Third imaging position P 3  is a position where X coordinate of the position of tube focus  30 F obtained by position obtaining section  40  is predetermined coordinate X 3 . The position of tube focus  30 F obtained by position obtaining section  40  at third imaging position P 3  corresponds to coordinates (X 3 , Y 3 , Z 3 ). 
     Here, if shifting section  10  could shift radiation source  30  in one direction (X direction) without any error, the position of tube focus  30 F obtained by position obtaining section  40  at third imaging position P 3  would be coordinates (X 3 , Y 0 , Z 0 ). But, as in the above, when tube focus  30 F arrives at third imaging position P 3  (position where the coordinate in X direction obtained by position obtaining section  40  is X 3 ), the position of tube focus  30 F is displaced in a Y direction (amount of displacement (Y 3 −Y 0 )) and a Z direction (amount of displacement (Z 3 −Z 0 )), so that the position of tube focus  30 F is obtained as coordinates (X 3 , Y 3 , Z 3 ). 
     Through the description above, image data representing a radiation image taken at each of first to third imaging positions P 1  to P 3  are obtained via radiation detector  20  and position data representing the position of tube focus  30 F at each of imaging positions P 1  to P 3  are obtained by position obtaining section  40 , whereby the tomosynthesis imaging is completed. 
     The position data obtained by position obtaining section  40  and the image data representing a plurality of radiation images obtained via radiation detector  20  in the tomosynthesis imaging described above are temporarily stored in storage section  76  and then inputted to tomosynthesis image generation section  210 . Thereafter, tomosynthesis image generation section  210  generates a tomosynthesis image representing a cross-section of examination region  1 A by reconstructing the image data using the inputted position data. 
     The tomosynthesis image generated in the manner described above is displayed on display device  220  of radiation image reproducing apparatus  200 . 
     A method of obtaining a tomosynthesis image by combining a plurality of radiation images obtained by the tomosynthesis imaging is described in detail, for example, in “The principle and clinical application of Tomosynthesis”, T. Shiomi, Journal of Japan Society Medical Imaging and Information Science, Vol. 24, No. 2, pp. 22-27, 2007. 
     &lt;Case in which Position of Tube Focus is Measured Using Only Linear Scale Measuring Devices&gt; 
     Next, a case in which the position of tube focus is measured using only the linear scale measuring devices will be described. 
     Steps to the start of tomosynthesis imaging by radiation imaging apparatus  100  are identical to those of the case in which the imaging is performed using only the acceleration sensors and, therefore, they will not be elaborated upon further here. Further, in the steps that follow, only the measuring method of the position of tube focus  30  is different, so that the measurement using only the linear scale measuring devices will be described with reference to  FIG. 1 . 
     As illustrated in  FIG. 1 , when tomosynthesis imaging by radiation imaging apparatus  100  is started, horizontal shifting platform  13  is shifted by shifting section  10  along movable rail  12  extending in an X direction. This causes radiation source  30 , attached to the tip of extensible/retractable support  14 , to be shifted also in the X direction, whereby the tube focus is also shifted in the X direction. 
     Here, the shifting section  10  shifts horizontal shifting platform  13  only on movable rail  12  extending in an X direction without extending/retracting extensible/retractable support  14  extending in a Z direction and without shifting horizontal shifting platform  13  on fixed rails  11  extending in a Y direction. 
     While being shifted in the X direction which is one direction parallel to detection surface  20 M of radiation detector  20 , radiation source  30  emits radiation onto examination region  1 A at first imaging position P 1 ′. First imaging position P 1 ′ is a position where X coordinate of the position of tube focus  30 F obtained by position obtaining section  40  using the linear scale measuring devices is X 1 . The position of tube focus  30 F obtained by position obtaining section  40  at first imaging position P 1 ′ corresponds to coordinates (X 1 , Y 0 , Z 0 ). 
     Here, shifting section  10  does not shift radiation source  30  in the X direction without any error, but horizontal shifting platform  13  shifted on movable rail  12  in the X direction is displaced in Y and Z directions or slightly rotated about X axis, Y axis, and Z axis (pitching, rolling, yawing). The position of tube focus  30 F varies with such shifting errors. 
     Consequently, the position of tube focus  30 F is displaced, in actuality, from the position represented by the coordinates obtained by position obtaining section  40 . Such shifting errors, however, are not detected when the measurement is performed using only the linear scale measuring devices attached in the manner described above. 
     That is, shifting errors occurred when horizontal shifting platform  13  arrives at first imaging position P 1 ′ (position where the coordinate in the X direction obtained by linear scale measuring device  40 Lx is X 1 ) and first imaging is performed, so that the position of first imaging position P 1 ′ obtained by position obtaining section  40  corresponds to coordinates (X 1 , Y 0 , Z 0 ). 
     Note that, the coordinate value in the X direction of first imaging position P 1 ′ obtained by position obtaining section  40  is X 1 , but the value X 1  also includes a measurement error due to a measurement not in accordance with the Abbe principle. 
     Then, when radiation source  30  is shifted in the X direction and arrives at second imaging position P 2 ′ (position where the coordinate in the X direction obtained by linear scale measuring device  40 Lx of position obtaining section  40  is X 2 ), second imaging is performed. Since shifting errors described above are not detected, the position of second imaging position P 2 ′ obtained by position obtaining section  40  corresponds to coordinates (X 2 , Y 0 , Z 0 ). 
     Then, when radiation source  30  is shifted in the X direction and arrives at third imaging position P 3 ′ (position where the coordinate in the X direction obtained by linear scale measuring device  40 Lx of position obtaining section  40  is X 3 ), third imaging is performed. Here also, shifting errors described above are not detected, so that the position of third imaging position P 2 ′ obtained by position obtaining section  40  corresponds to coordinates (X 3 , Y 0 , Z 0 ). 
     As described above, the positions of tube focus  30 F at first imaging position P 1 ′, second imaging position P 2 ′, and third imaging position P 3 ′ are obtained by position obtaining section as coordinates (X 1 , Y 0 , Z 0 ), coordinates (X 2 , Y 0 , Z 0 ), and coordinates (X 3 , Y 0 , Z 0 ) respectively. These coordinates, however, include errors due to a measurement not in accordance with the Abbe principle. 
     Through the description above, image data representing a radiation image taken at each of first to third imaging positions P 1 ′ to P 3 ′ are obtained via radiation detector  20  and position data (position data measured using only the linear scale measuring device) representing the position of tube focus  30 F at each of imaging positions P 1 ′ to P 3 ′ are obtained by position obtaining section  40 , whereby the tomosynthesis imaging is completed. 
     The image data and position data obtained by the tomosynthesis imaging described above are temporarily stored in storage section  76  and then inputted to tomosynthesis image generation section  210 . Thereafter, tomosynthesis image generation section  210  generates a tomosynthesis image representing a cross-section of examination region  1 A by reconstructing the image data using the inputted position data. The tomosynthesis image generated in the manner described above is displayed on display device  220 . 
     &lt;Case in which Position of Tube Focus is Measured in Combination With Acceleration sensors  40 S and Linear Scale Measuring Devices  40 L&gt; 
     When measuring the position of tube focus in combination with acceleration sensors  40 S and linear scale measuring devices  40 L, it is necessary to combine an acceleration sensor and linear scale measuring device to be actually used in consideration of the performance thereof. For example, when measuring the position in an X direction, an arrangement may be adopted in which measurement result of linear scale measuring device  40 Lx is corrected by measurement result of acceleration sensors  40 S. 
     &lt;Generation of Tomosynthesis Image&gt; 
       FIG. 4  illustrates combining of a plurality of images obtained by tomosynthesis imaging. 
     For example, if image G 1  obtained by the imaging at first imaging position P 1  and image G 2  obtained by the imaging at second imaging position P 2  are simply superimposed on top of each other to combine them without correcting the positional relationship, images  1 Ag representing examination region  1 A are misaligned and superimposed, as superimposed image G 12 . 
     In such a case, a correction is performed on images G 1  and G 2  using coordinates (X 1 , Y 1 , Z 1 ) and (X 2 , Y 2 , Z 2 ) respectively representing first imaging position P 1  and second imaging position P 2  obtained by the measurement of acceleration sensors  40 S. That is, the correction is performed such that image G 1  and image G 2  are obtained at predetermined positions of coordinates (X 1 , Y 0 , Z 0 ) and coordinates (X 2 , Y 0 , Z 0 ) respectively. That is, in tomosynthesis image generation section  210 , image reconstruction is performed on the assumption that radiation imaging is performed at predetermined positions of coordinates (X 1 , Y 0 , Z 0 ) and coordinates (X 2 , Y 0 , Z 0 ) Therefore, the image actually obtained is corrected so as to become close to the radiation image to be obtained when radiation imaging is performed at the predetermined position. 
     In superimposed image G 12 ′ of image G 1 ′ and image G 2 ′ corrected in the manner described above, images  1 Ag representing examination region  1 A substantially overlap with each other. In this way, correction of each image obtained by tomosynthesis imaging prior to reconstruction may result in a high quality tomosynthesis image. 
     Note that a positional displacement in an X direction or in a Y direction can be corrected by shifting the image, and a positional displacement in a Z direction can be corrected by enlarging or reducing the image. 
     Hereinafter, a drawing illustrating tomosynthesis imaging, a drawing illustrating an error that occurs in tomosynthesis imaging, a drawing illustrating measurement results of acceleration sensors, and flowcharts of an acceleration measurement and tomosynthesis imaging will be described. 
       FIG. 5  illustrates a positional relationship between each section when tomosynthesis imaging is performed. 
     As illustrated in  FIG. 5 , the distance from tube focus  30 F of radiation source  30  to examination region  1 A is 1100 mm and the distance from examination region  1 A to detection surface  20 M of radiation detector  20  is 200 mm. The angle formed between the radiation emitted toward examination region  1 A from first imaging position P 1  and the radiation emitted toward examination region  1 A from third imaging position P 3  is 40°. 
       FIG. 6  illustrates a tolerance when tomosynthesis imaging is performed with the positional relationship shown in  FIG. 5 . 
     As illustrated in  FIG. 6 , the pixel pitch of radiation detector  20  is 0.15 mm, so that the acceptable positional displacement of a radiation image formed on detection surface  20 M is not greater than 0.15 nun. In order to limit the position error of a radiation image formed on the detection surface  20 M to not greater than 0.15 mm when tomosynthesis imaging is performed with the positional relationship shown in  FIG. 5 , it is necessary to limit the positional displacement of tube focus  30 F in an X direction to not greater than 0.9 mm. 
       FIG. 7  illustrates deflection of the movable rail extending in an X direction. 
     As illustrated in  FIG. 7 , when shifting section  10 , holding radiation source  30 , is shifted along movable rail  12  extending in an X direction by 3.3 m, deflection δz of movable rail  12  reaches up to 2 mm. Further, along with the deflection, horizontal shifting platform  13  suspended by movable rail  12  is inclined in a Z-X plane, causing a positional displacement in an X direction. Still further, horizontal shifting platform  13  suspended by movable rail  12  is inclined in a Z-Y plane due to deformation of movable rail  12 , causing a positional displacement in a Y direction. 
       FIG. 8  shows acceleration data measured by the acceleration sensor  40 Sa ( 40 Sb) when radiation source  30  is shifted in an X direction in a coordinate system with the vertical axis representing acceleration and the horizontal axis representing elapsed time. The reason why acceleration values in a Z direction remain constant at −1 is that the gravity is detected. 
       FIG. 9  illustrates speed data obtained by integrating once the acceleration values measured by the acceleration sensor  40 Sa ( 40 Sb) when radiation source  30  is shifted in an X direction ( FIG. 8  above) in a coordinate system with the vertical axis representing speed and the horizontal axis representing elapsed time. 
       FIG. 10  illustrates positions obtained by integrating the acceleration values twice measured by the acceleration sensor  40 Sa ( 40 Sb) when radiation source  30  is shifted in an X direction ( FIG. 8  above) in a coordinate system with the vertical axis representing position and the horizontal axis representing elapsed time. 
     It is known from  FIGS. 8 to 10  that the amount of displacement from the reference position, i.e., the position of the tube focus may be measured only by the acceleration sensors attached to the radiation source. 
       FIG. 11  shows position errors (displacements) of tube focus in a range in which horizontal shifting platform  13  is shifted at a constant speed (indicated by a reference symbol W in  FIG. 10 ). 
     As shown in  FIG. 11 , the position error of tube focus in a Z direction (a vertical direction) and the position error of tube focus in a Y direction are not so large, but the position error (displacement from a predetermined position) in an X direction (a shifting direction) is large. 
       FIG. 12A  is a flowchart of an acceleration data measurement. 
       FIG. 12B  is a flowchart of tomosynthesis imaging. 
     The acceleration data measurement and tomosynthesis imaging are performed in the manner described in  FIGS. 12A and 12B  respectively. 
     Hereinafter, a radiation imaging apparatus that uses a laser length measuring device for tube position measurement will be described.  FIG. 13  illustrates a front view and a side view of a radiation imaging apparatus that uses a laser length measuring device for tube position measurement. In  FIG. 13 , the front view is shown on the left and the side view is shown on the right. 
     As illustrated in  FIG. 13 , radiation imaging apparatus  300  that uses a laser length measuring device for tube position measurement performs the measurement by reflecting laser light on each of wall surface Hx serving as the measurement reference in an X direction, wall surface Hy serving as the measurement reference in a Y direction, and floor surface Hz serving as the measurement reference in a Z direction. 
     When tomosynthesis imaging by radiation imaging apparatus  300  is started, horizontal shifting platform  13  is shifted by shifting section  10  on movable rail  12  extending in an X direction. This causes radiation source  30 , attached to the tip of extensible/retractable support  14 , to be shifted also in the X direction, whereby the tube focus is also shifted in the X direction. 
     Laser light emitted from laser oscillator  40 U attached to radiation source  30  passes through optical system  40 J, serving as the base of laser length measurement, reflected on each of reference surfaces (wall surface Hx, wall surface Hy, floor surface Hz), and enters in optical system  40 J again, whereby the position of tube focus  30 F of radiation source  30  is measured. 
     Tomosynthesis imaging performed by radiation imaging apparatus  300  is substantially identical to the tomosynthesis imaging performed by radiation imaging apparatus described above. 
     Instead of using linear scale measuring devices  40 L or laser oscillator  40 U, a rotary encoder (not shown) may be attached to the rotary shaft of each of three drive motors, provided in shifting section  10 , for shifting radiation source in three axis directions in order to measure the position of tube focus in three axis directions.