Patent Publication Number: US-2022218300-A1

Title: Radiation source position estimation system, calibration system and biomagnetic measuring system

Description:
TECHNICAL FIELD 
     The disclosures discussed herein relate to a radiation source position estimation system, a calibration system, and a biomagnetic measuring system. 
     BACKGROUND ART 
     Patent Document 1 proposes a device for measuring a position and a direction of a detection coil. Such a device is typically included in a superconducting quantum interference device sensor. 
     CITATION LIST 
     Patent Literature 
     [PTL 1] Japanese Unexamined Patent Application Publication No. H05-277082 
     SUMMARY OF INVENTION 
     Technical Problem 
     To detect biological magnetism of a subject by using a superconducting quantum interference device sensor, imaging of a subject is performed using radiation such as plain X-rays, and a sensing result of the biological magnetism of the subject is superimposed on an imaging result of the subject. The imaging result is affected by a position of a radiation source. Hence, a position of the radiation source is important. However, the technique disclosed in Patent Document 1 is unable to measure a position of the radiation source. 
     The present disclosure is intended to provide a radiation source position estimation system, a calibration system, and a biomagnetic measuring system, which are capable of accurately estimating a position of the radiation source. 
     Solution to Problem 
     According to one aspect of the present disclosure, a radiation source position estimation system is provided. The radiation source position estimation system includes a first position information specifier configured to specify position information of one or more elements included in a position measuring member, an imager configured to acquire images of the one or more elements formed by radiation emitted by a radiation source, and a second position information specifier configured to specify position information of the radiation source, based on the position information specified by the first position information specifier and the images acquired by the imager. 
     Advantageous Effect of the Invention 
     According to the present disclosure, it is possible to estimate a position of a radiation source with high accuracy. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view (Part  1 ) illustrating a configuration of a biomagnetic measuring system according to a first embodiment; 
         FIG. 2  is a perspective view (Part  2 ) illustrating a configuration of the biomagnetic measuring system according to the first embodiment; 
         FIG. 3  is a side view illustrating a configuration of the biomagnetic measuring system according to the first embodiment; 
         FIG. 4  is a front view illustrating a configuration of the biomagnetic measuring system according to the first embodiment; 
         FIG. 5  is a top view illustrating a configuration of the biomagnetic measuring system according to the first embodiment; 
         FIG. 6  is a cross-sectional view illustrating a configuration of a magnetic field measuring device; 
         FIG. 7  is a schematic view illustrating a configuration of a calibration tool; 
         FIG. 8A  is a schematic view illustrating a configuration of a modified example of the calibration tool; 
         FIG. 8B  is a schematic view illustrating a configuration of a modified example of the calibration tool; 
         FIG. 8C  is a schematic view illustrating a configuration of a modified example of the calibration tool; 
         FIG. 9  is a diagram illustrating a configuration of a control device; 
         FIG. 10  is a diagram illustrating a functional configuration of a control device at time of estimating a position of a radiation source; 
         FIG. 11  is a flowchart illustrating a method of estimating a position of a radiation source; 
         FIG. 12  is a flowchart illustrating a method of calculating a position of a radiation source; 
         FIG. 13  is a schematic view illustrating an example of a display; 
         FIG. 14  is a diagram illustrating a functional configuration of a control device upon biological measurement of a subject; 
         FIG. 15  is a perspective view illustrating a configuration of a biomagnetic measuring system according to a second embodiment; 
         FIG. 16  is a side view illustrating a configuration of the biomagnetic measuring system according to the second embodiment; 
         FIG. 17  is a front view illustrating a configuration of the biomagnetic measuring system according to the second embodiment; 
         FIG. 18  is a top view illustrating a configuration of the biomagnetic measuring system according to the second embodiment; and 
         FIG. 19  is a front view illustrating a configuration of a biomagnetic measuring system according to a modification of the second embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings. In the present specification and the drawings, duplicated illustration will be omitted by assigning the same reference numerals to components having substantially the same functional configurations. 
     First Embodiment 
     &lt;Overview of Biomagnetic Measuring System&gt; 
     In a biomagnetic measuring system according to a first embodiment, a radiation source and an imaging device are disposed along a substantially horizontal direction such that a magnetic field measuring device is interposed between the radiation source and the imaging device. When a subject is located in the biomagnetic measuring system, the subject is also interposed between the radiation source and the imaging device along a substantially horizontal direction.  FIGS. 1 and 2  are perspective views illustrating a configuration of the biomagnetic measuring system according to the first embodiment.  FIG. 1  illustrates a configuration upon estimating a position of a radiation source, and  FIG. 2  illustrates a configuration upon biomagnetic measurement of the subject.  FIG. 3  is a side view illustrating a configuration of the biomagnetic measuring system according to the first embodiment.  FIG. 4  is a front view illustrating a configuration of the biomagnetic measuring system according to the first embodiment.  FIG. 5  is a top view illustrating a configuration of the biomagnetic measuring system according to the first embodiment.  FIGS. 3 to 5  illustrate a configuration upon estimating a position of the radiation source, similar to  FIG. 1 . 
     As illustrated in  FIGS. 1 to 5 , the biomagnetic measuring system  100  according to the first embodiment includes an imaging device  110 , a magnetic field measuring device  120 , a calibration tool  130 , a support  140 , a calibration tool stabilizer  150 , a radiation exposure device  160  including a radiation source  161 , a table  170 , and a control device  180  (see  FIG. 10 , etc.). In the present specification and drawings, an X axis indicates a normal to an imaging surface  111  of the imaging device  110 , and a Z axis indicates a vertically downward direction, and a Y axis indicates a direction orthogonal to the X axis and the Z axis in the right-handed system. 
     Imaging Device  110   
     The imaging device  110  is configured to acquire, as morphological images, digital image data of radiation R, which passes through a measuring area of a subject S or a calibration tool  130 . Signals detected by the imaging device  110  are transmitted to the control device  180 . The imaging device  110  also acquires a captured image of calibration tool  130 . The imaging device  110  is an example of an imager. 
     The imaging device  110  may, for example, be a flat panel detector (hereinafter referred to as “FPD”). The FPD includes so-called “direct conversion system” and “indirect conversion system”. In the direct conversion system, electric charges are generated by sensing elements according to a dose of applied radiation, and the generated electric charges are converted to electric signals. In the indirect conversion system, radiation applied is converted to electromagnetic waves with a different wavelength such as visible light by a scintillator or the like, electric charges are then generated by photoelectric conversion elements such as photodiodes according to energy of the converted electromagnetic waves, and the generated electric charges are converted to electric signals. 
     In addition, a so-called imaging plate (hereinafter, referred to as an IP) may be preferably used as the imaging device  110 . The imaging plate is a film that is coated with photostimulable phosphor powder and housed in a cassette. Radiation R passing through the measurement area of the subject S is applied to the imaging plate, and radiation energy is stored in the photostimulable phosphor. The morphological images can then be acquired as digital image data by irradiating the imaging plate with laser light with a particular wavelength and scanning the irradiated laser with a scanner. 
     Magnetic Field Measuring Device  120   
       FIG. 6  is a cross-sectional view illustrating a configuration of the magnetic field measuring device  120 . As illustrated in  FIG. 6 , the magnetic field measuring device  120  includes a magnetic sensor array having a plurality of magnetic sensors  121  configured to detect biological magnetism. The plurality of magnetic sensors  121  are held in a thermal insulated container  122  having a temperature control mechanism. The magnetic field measuring device  120  is an example of a detector. 
     (Magnetic Sensors  121 ) 
     The magnetic sensors  121  are each configured to detect biological magnetism generated from a subject S. Specifically, the magnetic sensors  121  each include a superconducting quantum interference device (SQUID) or an optical pumped atomic magnetometer (OPAM). These SQUID sensors and OPAM sensors have detection sensitivity sufficient to detect extremely weak biological magnetism on the order of 10 −18  T. The magnetic sensors  121  are also configured to detect magnetic fields generated by the magnetic field generators  131  (see  FIG. 7 , etc.) included in the calibration tool  130 . 
     The magnetic sensors  121  are typically arranged in a thermal insulated container  122  having a temperature control mechanism, as illustrated in  FIG. 6 . Signals from the respective magnetic sensors  121  are transmitted to a control device  180  for conversion to biomagnetic information. The plurality of magnetic sensors  121  not only provide a large amount of biomagnetic information, but also provide more detailed bioinformation by two-dimensional mapping of measured magnetic information or the like. Also, when the magnetic sensors  121  operate at room temperature, the temperature control mechanism and the thermal insulated container  122  are not required. The number and arrangement method of the magnetic sensors  121  are not particularly limited, and may be appropriately set according to the measurement region of the subject S. 
     (Temperature Control Mechanism) 
     The temperature control mechanism is a mechanism configured to adjust temperatures of the magnetic sensors  121  to predetermined temperatures suitable for operations of the magnetic sensors  121 . The temperature control mechanism may be a known cooling or heating device. For example, if the magnetic sensors  121  are SQUID sensors, the magnetic sensors  121  operate at temperatures close to absolute zero in order to achieve a superconducting state. In this embodiment, the thermal insulated container  122  partially functions as the temperature control mechanism. 
     (Thermal Insulated Container  122 ) 
     As illustrated in  FIG. 6 , the thermal insulated container  122  is provided with an inner container  221  and an outer container  222 , for example. The inner container  221  includes the plurality of magnetic sensors  121 , and vacuum space is provided between the inner container  221  and the outer container  222 . A coolant, such as liquid helium, is supplied to the inner container  221 . Accordingly, the magnetic field measuring device  120  is controlled to a temperature suitable for operating the magnetic sensors  121 . 
     The shape of the thermal insulated container  122  is not particularly specified, but the thermal insulated container  122  may preferably have a surface (hereinafter referred to as a facing surface  122   a ) shaping along a body surface of the measurement area of the subject S, where the facing surface  122   a  faces the subject S. The facing surface  122   a  is preferably planar or curved. For example, when the neck of the subject S is placed on the magnetic field measuring device  120  in order to perform the biomagnetic measurement, the facing surface  122   a  of the thermal insulated container  122  may preferably be curved along the arc of the cervical spinal cord. 
     The thermal insulated container  122  is not limited to the vacuum thermal insulated container as illustrated in  FIG. 6 . The thermal insulated container  122  may be made of foam or the like. The thermal insulated container  122  is preferably made of a nonmagnetic material with low magnetic permeability. The use of the thermal insulated container  122  made of a non-magnetic material can prevent adverse effects, which are caused by fluctuations in the environmental magnetism, on the magnetic sensors  121  while the thermal insulated container  122  vibrates. Examples of non-magnetic materials include plastic materials such as acrylic resins; inorganic materials such as silica and alumina; non-ferrous metals such as copper, brass, aluminum and titanium; and mixtures of these materials. 
     Calibration Tool  130   
     The calibration tool  130  is disposed above the magnetic field measuring device  120  when estimating a position of the radiation source included in the radiation exposure device  160 .  FIG. 7  is a schematic view illustrating a configuration of the calibration tool  130 . As illustrated in  FIG. 7 , the calibration tool  130  includes a plurality of magnetic field generators  131  configured to generate magnetic fields, a plurality of absorbers  132  configured to absorb radiation emitted by the radiation source  161 , and a support  133  configured to support the magnetic field generators  131  and the absorbers  132 . The magnetic field generators  131  are each, for example, a coil configured to receive electric current supplied. The absorbers  132  are each, for example, a sphere made of iron or made of metal having density higher than iron such as tungsten. The absorbers  132  may have a shape such as a cylinder other than a sphere. The support  133  is configured to let through radiation more than the absorbers  132  let through, upon receiving the radiation emitted by the radiation source  161 . The support  133  is, for example, made of plastic. 
     As illustrated in  FIG. 7 , the outer shape of the support  133  is substantially rectangular. For example, one face  133 A is provided with a plurality of magnetic field generators  131 , and a plurality of absorbers  132  is disposed at positions distanced from the face  133 A. For example, as illustrated in  FIG. 8A , the absorbers  132  may be disposed in an irregular manner, or as illustrated in  FIG. 8B , the magnetic field generators  131  may be disposed on separate faces  133 B and  133 C at positions close to the face  133 A, where the faces  133 B and  133 C face each other. Further, as illustrated in  FIG. 8C , the outer shape of the support  133  may be substantially cylindrical. The calibration tool  130  is an example of a position measuring member. The absorbers  132  are each an example of an element. 
     Support  140   
     The support  140  is, for example, a cylinder. The magnetic field measuring device  120  is fixed to the support  140 . The imaging device  110  is removably disposed on the support  140 . 
     Calibration Tool Stabilizer  150   
     The calibration tool stabilizer  150  is configured to stabilize a position of the calibration tool  130  on the magnetic field measuring device  120 . That is, the calibration tool stabilizer  150  prevents misalignment and wobbles of the calibration tool  130 . An example of the calibration tool stabilizer  150  may include a non-skid pad. As the calibration tool stabilizer  150 , a structure, which has a flat face facing the calibration tool  130  and a face along the facing surface  122   a  of the magnetic field measuring device  120 , may be used. The calibration tool stabilizer  150  is not necessarily required when misalignment and wobbles of the calibration tool  130  are unlikely to occur. Illustration of the calibration tool stabilizer  150  is omitted from  FIGS. 3 to 5 . 
     Radiation Exposure Device  160   
     The radiation exposure device  160  includes a radiation source  161 . The radiation source  161  may be any known radiation source configured to apply radiation to a living body. In the present invention, “radiation” not only indicates commonly used plain X-rays, but also indicates a broader concept of radiation. Examples of such radiation may include beams made of particles (including photons) such as α-rays, β-rays, and γ-rays, which are released due to radioactive decay; and beams having the same or higher energy level than plain X-rays such as particle rays and cosmic rays. In view of the high versatility, the plain X-rays may preferably be used as radiation. 
     The radiation exposure device  160  is, for example, disposed on a movable carriage  162  provided with casters  163  in order to facilitate the movement of the radiation exposure device  160 . The movable carriage  162  may preferably have a lifting mechanism configured to adjust the height of a surface on which the radiation exposure device  160  is placed. The movable carriage  162  may have the ability to switch between locking and unlocking of the casters  163 . Without considering the movement facilitation of the radiation exposure device  160 , the movable carriage  162  may not necessarily be provided with casters  163 . As materials of the movable carriage  162 , metal that can withstand the weight of the radiation exposure device  160  may be used. 
     (Table  170 ) 
     The table  170  is not particularly specified in shape insofar as the table  170  on which the subject S is placed can support the subject S; however, as illustrated in  FIG. 2 , the table  170  may be formed by a plurality of section-separated tables, such as a head table  171  for supporting the head of the subject S and a body table  172  for supporting the body of the subject S. The magnetic field measuring device  120  may be disposed between the head table  171  and the body table  172  so as to face a measurement area of the subject S. 
     It is preferable that those members forming the table  170  should be made of a nonmagnetic material having low magnetic permeability. The table  170  made of a nonmagnetic material may be able to prevent adverse effects caused by fluctuations of environmental magnetism on the magnetic sensors  121  even when the subject S vibrates. As with the thermal insulated container  122 , non-magnetic materials to be used for the members forming the table  170  include plastic materials such as acrylic resin; inorganic materials such as silica and alumina; non-ferrous metals such as copper, brass, aluminum and titanium; and mixtures of these materials. The table  170  is required to have load resistance, impact resistance, and the like in order to support part of or all of the subject S. Thus, it is preferable that the table  170  should be made of metal parts with high mechanical strength or engineering plastic. 
     Control Device  180   
     The control device  180  includes a CPU (Central Processing Unit)  181 , a ROM (Read Only Memory)  182 , a RAM (Random Access Memory)  183 , and an auxiliary storage unit  184 , as illustrated in  FIG. 9 . The CPU  181 , the ROM  182 , the RAM  183 , and the auxiliary storage unit  184  constitute a so-called computer. These components of the control device  180  are interconnected via a bus  185 . 
     The CPU  181  is configured to execute various programs (e.g., a program for estimating a position of a radiation source) stored in the auxiliary storage unit  184 . 
     The ROM  182  is a non-volatile primary storage device. The ROM  182  stores various programs, data and the like, which are necessary for causing the CPU  181  to execute various programs stored in the auxiliary storage unit  184 . Specifically, the ROM  182  stores boot programs such as BIOS (Basic Input/Output System) and EFI (Extensible Firmware Interface). 
     The RAM  183  is a volatile primary storage device such as a DRAM (Dynamic Random Access Memory) or an SRAM (Static Random Access Memory). The RAM  183  functions as a work area for loading programs upon various programs stored in the auxiliary storage unit  184  being executed by the CPU  181 . 
     The auxiliary storage unit  184  is an auxiliary storage device. The auxiliary storage unit  184  stores various programs executed by the CPU  181  and various data generated upon various programs being executed by the CPU  181 . 
     &lt;Functional Configuration of Control Device  180  at Time of Estimating a Position of the Radiation Source  161 &gt; 
     As illustrated in  FIG. 10 , at time of estimating a position of the radiation source  161 , the control device  180  functionally includes a radiation source controller  281 , a relative position acquisition unit  282 , a magnetic field generator position acquisition unit  283 , an absorber position calculator  284 , an absorber image detector  285 , a radiation source position calculator  286 , and a display controller  287 . 
     (Radiation Source Controller  281 ) 
     The radiation source controller  281  is configured to control a timing of radiation emission performed by the radiation source  161 . 
     (Relative Position Acquisition Unit  282 ) 
     The relative position acquisition unit  282  is configured to acquire relative position relationships between the magnetic field generators  131  and the absorbers  132  within the calibration tool  130 . The relative position acquisition unit  282  acquires relative position relationships from, for example, design values of the calibration tool  130 . The relative position acquisition unit  282  may, for example, acquire relative position relationships between the magnetic field generators  131  and the absorbers  132 , based on measurements of the internal structure of the completed calibration tool  130 . 
     (Magnetic Field Generator Position Acquisition Unit  283 ) 
     The magnetic field generator position acquisition unit  283  is configured to receive signals output from the magnetic sensors  121  of the magnetic field measuring device  120  and acquire positions of the magnetic field generators  131  included in the calibration tool  130 . 
     (Absorber Position Calculator  284 ) 
     The absorber position calculator  284  is configured to compare respective positions of the magnetic field generators  131  acquired by the magnetic field generator position acquisition unit  283  and relative positions acquired by the relative position acquisition unit  282 , and to calculate positions of the absorbers  132 . The absorber position calculator  284  may use the ICP (iterative close point) algorithm to calculate the positions of the absorbers  132 . The absorber position calculator  284  is an example of a first position information specifier. 
     (Absorber Image Detector  285 ) 
     The absorber image detector  285  is configured to detect images of the absorbers  132  from a captured image, which is output from the imaging device  110 . When the absorbers  132  each have a spherical shape, images of the absorbers  132  in the captured image are circles. In this case, the absorber image detector  285  performs circle detection to detect all the circle images of the absorbers  132  in the captured image, and to acquire positions of the absorbers  132  from the centers of respective circle images. The Hough transform may be used for circle detection. The absorber image detector  285  is an example of an image detector. 
     (Radiation Source Position Calculator  286 ) 
     The radiation source position calculator  286  is configured to calculate a position of the radiation source  161  using the positions of the absorbers  132  calculated by the absorber position calculator  284  and the positions of the absorbers  132  detected by the absorber image detector  285 . A method of calculating a position of the radiation source  161  will be described in detail below. The radiation source position calculator  286  outputs a calculated position of the radiation source  161  to a server  192  and delivers the calculated position of the radiation source  161  to the display controller  287 . The server  192  stores the position of the radiation source  161 . The radiation source position calculator  286  is an example of a second position information specifier. 
     (Display Controller  287 ) 
     The display controller  287  is configured to display on the display device  193  a position of the radiation source  161  calculated by the radiation source position calculator  286 . 
     &lt;Method of Estimating a Position of Radiation Source  161 &gt; 
     Next, a method of estimating a position of the radiation source  161  will be described. To estimate a position of the radiation source  161 , a calibration tool  130  is disposed between the radiation exposure device  160  and the imaging device  110 , and a magnetic field measuring device  120  is disposed beneath the calibration tool  130 . The calibration tool  130  is disposed such that magnetic fields generated by the magnetic field generators  131  are measured by the magnetic field measuring device  120 , and the absorbers  132  are displayed on the imaging surface  111 . For example, in a case where a distance from the radiation source  161  to the imaging surface  111  is 1500 mm, a diameter of each absorber  132  is 1.2 mm, a Z-axis dimension of the imaging surface  111  is 290.4 mm, a Y-axis dimension of the imaging surface  111  is 176.4 mm, and intervals between the absorbers  132  are each 100 mm, the calibration tool  130  is preferably disposed at a position 450 mm to 1450 mm distanced from the radiation source  161 . This arrangement is preferable because the absorbers  132  are prevented from being superimposed on each other on the imaging surface  111 .  FIG. 11  is a flowchart illustrating a method of estimating a position of the radiation source  161 . 
     First, in step S 11 , the relative position acquisition unit  282  acquires relative positions between the magnetic field generators  131  and the absorbers  132  included in the calibration tool  130 . 
     Also, the magnetic field generators  131  are caused to generate magnetic fields, and the magnetic field measuring device  120  measures the magnetic fields generated by the magnetic field generators  131  (step S 12 ). Subsequently, in step S 13 , the magnetic field generator position acquisition unit  283  receives signals output from the magnetic sensors  121  of the magnetic field measuring device  120 , and acquires positions of the magnetic field generators  131  included in the calibration tool  130 . Thereafter, in step S 14 , the absorber position calculator  284  compares positions of the magnetic field generators  131  acquired by the magnetic field generator position acquisition unit  283  and the relative positions acquired by the relative position acquisition unit  282  to calculate positions of the absorbers  132 . 
     Also, the radiation source  161  applies radiation to the calibration tool  130 , based on the control of the radiation source  161  performed by the radiation source controller  281  (step S 15 ). The imaging device  110  then acquires image data from the radiation, which has passed through the calibration tool  130  (step S 16 ). A portion of the radiation applied to the calibration tool  130  passes through the support  133 , and another portion of radiation applied to the calibration tool  130  is absorbed by the absorbers  132 . Accordingly, shadows of the absorbers  132  are displayed as respective images on the imaging surface  111  of the imaging device  110 . The image data thus includes the images of the absorbers  132 . Then, in step S 17 , the absorber image detector  285  detects the images of the absorbers  132  from the image data. 
     Subsequently, the radiation source position calculator  286  calculates a position of the radiation source  161  using the positions of the absorbers  132  calculated by the absorber position calculator  284  and the positions of the absorbers  132  detected by the absorber image detector  285  (step S 18 ). 
     Herein, a method of calculating a position of the radiation source  161  will be described.  FIG. 12  is a flowchart illustrating a method of calculating a position of the radiation source  161 . Hereinafter, coordinates of a position of each absorber  132  calculated by the absorber position calculator  284  are referred to as “subject coordinates”, and coordinates of a position of each absorber  132  detected by the absorber image detector  285  are referred to as “projection coordinates”. 
     First, for each of a plurality of subject coordinates, the radiation source position calculator  286  calculates the center of gravity of subject coordinates. The radiation source position calculator  286  sets an initial position of the radiation source at a given position on a positive side of the X axis on a straight line that passes through the center of gravity of the subject coordinates. The radiation source position calculator  286  sets a reference point of projection coordinates at the center of gravity of the projection coordinates. The radiation source position calculator  286  sets an initial position of the projection coordinates at a given position on a negative side of the X axis on the straight line that passes through the reference point of the projection coordinates and the center of gravity of the subject coordinates (step S 21 ). 
     Subsequently, the radiation source position calculator  286  sets a cost function of the least squares method (step S 22 ). That is, the radiation source position calculator  286  sets a cost function so as to minimize a distance between an intersection position of each of the subject coordinates intersecting the imaging surface  111  and a position of a counterpart one of the projection coordinates. Note that the intersection position of each of the subject coordinates intersecting the imaging surface  111  is a position at which a straight line extending from the radiation source  161  passes through the corresponding subject coordinates and intersects the imaging surface  111 . 
     The following illustrates a cost function. For example, assuming that position coordinates of the radiation source  161  are (x0, y0, z0), reference coordinates of the projection coordinates are (xb1, yb1, zb1), the i-th projection coordinates with respect to the reference coordinates are (ni, li, mi), and the slope of the imaging surface is (θ, φ, ψ). When the slope of the imaging surface is (θ, φ, ψ), the rotation matrix is expressed by Math. 1. 
     
       
         
           
             
               
                 
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     Accordingly, the i-th projection coordinates with respect to the reference coordinates are rotated by (θ, φ, ψ) about the X axis, the Y axis, and the Z axis, so that i-th projection coordinates are moved to coordinates (xt, yt, zt) represented by Math. 2, where the i-th projection coordinates are moved with respect to the reference coordinates of the projection coordinates, which are used as a reference position. 
     
       
         
           
             
               
                 
                   
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                   [ 
                   
                     Math 
                     . 
                     
                         
                     
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                     2 
                   
                   ] 
                 
               
             
           
         
       
     
     Further, a plane is represented by Math. 3 using a point (xd, yd, zd) on the plane and the normal vector (a, b, c) to the plane. 
     
       
         
           
             
               
                 
                   
                     
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                       ⁡ 
                       
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                     Math 
                     . 
                     
                         
                     
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                     3 
                   
                   ] 
                 
               
             
           
         
       
     
     Accordingly, assuming that the imaging surface  111  is oriented along the X axis at an initial state, a normal vector to the imaging surface  111  is (1, 0, 0), and the imaging surface  111  is represented by Math. 4 using the reference point of the projection coordinates and the normal vector. 
     
       
         
           
             
               
                 
                   
                     
                       
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                     . 
                     
                         
                     
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     Further, assuming that the i-th subject coordinates are (xai, yai, zai), a straight line connecting the radiation source  161  and the i-th subject coordinates is represented by Math. 5. 
     
       
         
           
             
               
                 
                   
                     
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                     . 
                     
                         
                     
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     Accordingly, coordinates (xp, yp, zp) at an intersection of the plane represented by Math. 4 and the straight line represented by Math. 5 can be obtained. Then, a value D of the square of the three-dimensional Euclidean distance between the projection coordinates (xt, yt, zt) and the coordinates (xp, yp, zp) at the intersection of the imaging surface  111  and the straight line is represented by Math. 6. 
     
       
         
           
             
               
                 
                   D 
                   = 
                   
                     
                       
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                       2 
                     
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     The value D of Math. 6 is calculated for each of the absorbers  132 , and the sum of these values D results in a cost function. 
     After setting the cost function in step S 22 , the radiation source position calculator  286  optimizes the cost function (step S 23 ). In cost function optimization, the radiation source position calculator  286  differentiates the cost function set in step S 22  with respect to unknowns. When the above-described cost function is used, derivatives of the cost function are obtained with respect to nine parameters x0, y0, z0, xb1, yb1, zb1, θ, φ, and ψ, which are unknowns. 
     Subsequently, the radiation source position calculator  286  updates the position of the radiation source  161  and the reference position of the imaging surface  111  to optimized positions obtained by the optimization (step S 24 ). Thereafter, the radiation source position calculator  286  calculates updated values of the cost function (step S 25 ). The radiation source position calculator  286  repeatedly performs the processing of step S 22  to S 25 , and determines, upon predetermined conditions being satisfied, values of the above-described nine parameters (x0, y0, z0, xb1, yb1, zb1, θ, φ, and ψ) at the time of the predetermined conditions being satisfied (step S 26 ). The radiation source position calculator  286  then acquires the position of the radiation source  161 , the reference position of the imaging surface  111 , and the slope of the imaging surface  111  (step S 27 ). Note that thresholds may be set in advance for the updated values of the cost function, and the repeated processing may be terminated when the values of the cost function are less than the preset thresholds. Optimization may be performed using Adam (Adaptive Moment Estimation), which is a gradient descent method. 
     The position of the radiation source  161  is calculated in this manner (step S 18 ). 
     Thereafter, the radiation source position calculator  286  outputs, to the server  192 , the values of the above-described nine parameters (x0, y0, z0, xb1, yb1, zb1, θ, φ, and ψ) including parameters indicating the position of the radiation source  161 , and delivers these values to the display controller  287 . The display controller  287  displays the position of the radiation source  161  calculated by the radiation source position calculator  286  on the display device  193 . 
     The position of the radiation source  161  can be estimated in this manner. That is, the biomagnetic measuring system  100  partially functions as a radiation source position estimation system, which is configured to estimate a position of the radiation source  161 . 
     The display controller  287  may display, on the display device  193 , position coordinates of the magnetic field generators  131  calculated by the magnetic field generator position acquisition unit  283  and image data acquired by the imaging device  110 .  FIG. 13  is a schematic view illustrating a display example, which indicates the position coordinates of the magnetic field generators  131  calculated by the magnetic field generator position acquisition unit  283  and the image data acquired by the imaging device  110 . 
     According to the display example illustrated in  FIG. 13 , a left region  300 L of a display  300  displays calculation results indicating how images of the subject coordinates projected on the imaging surface  111  are presented when viewed from the initial position of the radiation source  161 . A right region  300 R of the display  300  displays a captured image acquired by the imaging device  110 . For example, the left region  300 L displays numbers (not illustrated) assigned to respective point groups. The right region  300 R is provided with a user interface (UI) configured to acquire a position of each of the spherical images from the captured image by clicking with the input device  191  around a corresponding one of the spherical images, which are associated in the order of the numbers assigned to the respective point groups. The projection coordinates may be rearranged by using the UI in the same order as the numbers assigned to the subject coordinates. Further, the detection range of an absorber  132  may be designated by clicking or dragging around a spherical image with the input device  191  upon detecting the absorber  132 . In the display example illustrated in  FIG. 13 , the display of the left region  300 L may be replaced with the display of the right region  300 R, or the display of the left region  300 L may be vertically aligned with the display of the right region  300 R. Alternatively, only one of the left region  300 L and the right region  300 R may be displayed. 
     In the present embodiment, the processing of the absorber position calculator  284  (the first position information acquisition unit) and the processing of the radiation source position calculator  286  (the second position information acquisition unit) are performed by the same control device  180 . However, the above-described processing may be performed by separate devices. 
     &lt;Functional Configuration of the Control Device  180  at Time of Measuring Biomagnetic Information of Subject S&gt; 
     At time of measuring biomagnetic information of a subject S, the control device  180  functionally includes a radiation source controller  381 , a biomagnetic information acquisition unit  382 , a calibrator  383 , a superimposition unit  384 , and a display controller  385 , as illustrated in  FIG. 14 . 
     (Radiation Source Controller  381 ) 
     The radiation source controller  381  is configured to control the timing of radiation emission performed by the radiation source  161 . 
     (Biomagnetic Information Acquisition Unit  382 ) 
     The biomagnetic information acquisition unit  382  is configured to receive signals output from the magnetic sensors  121  of the magnetic field measuring device  120 , and acquire biomagnetic detection results of the subject S as biomagnetic information. 
     (Calibrator  383 ) 
     The calibrator  383  is configured to calibrate morphological images output from the imaging device  110  using values of the nine parameters (x0, y0, z0, xb1, yb1, zb1, θ, φ, and ψ) stored in the server  192 . 
     (Superimposition Unit  384 ) 
     The superimposition unit  384  is configured to superimpose the biomagnetic information acquired by the biomagnetic information acquisition unit  382  on the morphological images calibrated by the calibrator  383 , and deliver the superimposed images to the display controller  385 . 
     (Display Controller  385 ) 
     The display controller  385  is configured to display the superimposed images obtained from the superimposition unit  384  on the display device  193 . The display controller  385  may display not only the superimposed images obtained from the superimposition unit  384  but may also display, on the display device  193 , the morphological images calibrated by the calibrator  383  or the biomagnetic information acquired by the biomagnetic information acquisition unit  382 , or both the morphological images and the biomagnetic information. 
     In measurements of a subject S using the biomagnetic measuring system  100 , the calibration tool  130  is removed from the biomagnetic measuring system  100 , and a measurement area of the subject S is located on the magnetic field measuring device  120  as illustrated in  FIG. 2 . In this state, biomagnetic measurements using the magnetic field measuring device  120  and capturing of a radiation image such as a plain X-ray image are performed using the radiation exposure device  160  and the imaging device  110 . Either biomagnetic measurements or capturing of the radiation image may be performed first. 
     The biomagnetic detection results and the morphological images are input to the control device  180 , where the biomagnetic detection results are obtained from the magnetic field measuring device  120 , and the morphological images are digital image data of the radiation image obtained from the imaging device  110 . 
     In the control device  180 , the radiation source controller  381  causes the radiation source  161  to deliver radiation during capturing the radiation image. Then, the calibrator  383  calibrates the morphological images output from the imaging device  110  using the values of the nine parameters (x0, y0, z0, xb1, yb1, zb1, θ, φ, and ψ) stored in the server  192 . The biomagnetic information acquisition unit  382  acquires biomagnetic detection results of the subject S from the magnetic field measuring device  120  as the biomagnetic information. The superimposition unit  384  superimposes the biomagnetic information acquired by the biomagnetic information acquisition unit  382  on the morphological images that have been calibrated by the calibrator  383 , and delivers the superimposed images to the display controller  385 . Thereafter, the display controller  385  displays the superimposed images obtained from the superimposition unit  384  on the display device  193 . 
     In this manner, the biomagnetic measuring system  100  can perform biomagnetic measurements. In addition, during biomagnetic measurements, the biomagnetic measuring system  100  partially functions as a configuration system to calibrate a position of the radiation source  161 . 
     The position and angle of one of or both of the radiation exposure device  160  including the radiation source  161  and the imaging surface  111  may be adjusted based on the position of the radiation source  161  calculated by the radiation source position calculator  286 , the reference position of the imaging surface  111 , and the slope of the imaging surface  111 . 
     Second Embodiment 
     &lt;Overview of the Biomagnetic Measuring System&gt; 
     In the biomagnetic measuring system according to a second embodiment, a radiation source is disposed vertically above the subject.  FIG. 15  is a perspective view illustrating a configuration of a biomagnetic measuring system according to the second embodiment.  FIG. 16  is a side view illustrating a configuration of the biomagnetic measuring system according to a second embodiment.  FIG. 17  is a front view illustrating a configuration of the biomagnetic measuring system according to the second embodiment.  FIG. 18  is a top view illustrating a configuration of the biomagnetic measuring system according to the second embodiment.  FIGS. 15 to 18  illustrate configurations upon the position of the radiation source being estimated. 
     As illustrated in  FIGS. 15 to 18 , the biomagnetic measuring system  400  according to a second embodiment includes an imaging device  410 , a magnetic field measuring device  120 , a calibration tool  130 , a support  140 , a calibration tool stabilizer  150 , a radiation exposure device  460  having a radiation source  461 , a table  170  (see  FIG. 2 ), and a control device  180  (see  FIG. 10 , etc.). In the present specification and the drawings, a Z axis represents a vertically downward direction, an X axis represents a direction viewing from the support  140  toward the magnetic field measuring device  120 , and a Y axis represents a direction orthogonal to the X axis and the Z axis in the right-handed system. 
     Imaging Device  410   
     The imaging device  410  is configured to acquire morphological images as digital image data of radiation R, which passes through a measuring area of a subject S (see  FIG. 2 ) or the calibration tool  130 . Signals detected by the imaging device  410  are sent to the control device  180 . The imaging device  410  is also configured to acquire a captured image of the calibration tool  130 . The imaging device  410  is an example of an imager. 
     The imaging device  410  may use an FPD in a manner similar to the imaging device  110 . The imaging device  410  may also preferably use a film coated with a photostimulable phosphor powder. When a film coated with a photostimulable phosphor powder is used, the film is preferably fixed by an imaging plane fixture  434  so as not to bend the film. A preferable material used for the imaging plane fixture  434  may be a non-magnetic material, such as acrylic resin so as not to interfere with the measuring magnetic field. 
     Radiation Exposure Device  460   
     The radiation exposure device  460  is, for example, attached to a rail  462  for facilitating the movement of the radiation exposure device  460 , and the rail  462  is secured to a ceiling or the like. The rail  462  is configured to hang a radiation source  461  so as to allow the radiation exposure device  460  to move along the X axis. The rail  462  may preferably be provided with a mechanism configured to switch between locking and unlocking of the radiation exposure device  460  with respect to the rail  462 . The rail  462  may also be provided with screws or the like so as to secure the radiation exposure device  460  with respect to the rail  462 . As a material used for the rail  462 , a material such as a metal having strength capable of suspending the radiation exposure device  460  without deformation can be used. A mechanism, such as an arm, may be used in place of the rail  462  when the radiation exposure device  460  can be movably supported above the subject S. 
     The control device  180  is configured to control the imaging device  410  and the radiation exposure device  460  including the radiation source  461 , instead of the imaging device  110  and the radiation exposure device  160  including the radiation source  161  in the biomagnetic measuring system  100 . 
     Other configurations are similar to those of the first embodiment. 
     &lt;Method of Estimating a Position of Radiation Source  461 &gt; 
     Next, a method of estimating a position of the radiation source  461  will be described. In estimating a position of the radiation source  461 , a calibration tool  130  is disposed between the radiation exposure device  460  and the imaging device  410 , and a magnetic field measuring device  120  is disposed beneath the calibration tool  130 . The imaging device  410  is sandwiched by the imaging plane fixture  434 . Then, as in the first embodiment, a position of the radiation source  461  is estimated along the flowchart illustrated in  FIG. 11 . 
     Regarding a cost function, in the second embodiment, assuming that the imaging surface  411  is oriented toward the positive z axis at an initial state, and a normal vector to the imaging surface  411  is (0, 0, −1), the imaging surface  411  is represented by Math. 7 using a reference point of the projection coordinates and the normal vector. 
     
       
         
           
             
               
                 
                   
                     
                       
                         - 
                         
                           cos 
                           ⁡ 
                           
                             ( 
                             φ 
                             ) 
                           
                         
                       
                       ⁢ 
                       
                         cos 
                         ⁡ 
                         
                           ( 
                           ψ 
                           ) 
                         
                       
                       ⁢ 
                       
                         ( 
                         
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                             ⁢ 
                             
                                 
                             
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                         ) 
                       
                     
                     + 
                     
                       
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                                 ( 
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                                 ) 
                               
                             
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                                 ( 
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                       ⁢ 
                       
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                               ⁡ 
                               
                                 ( 
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                                 ) 
                               
                             
                           
                         
                         ) 
                       
                       ⁢ 
                       
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                             ⁢ 
                             
                                 
                             
                             ⁢ 
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                         ) 
                       
                     
                   
                   = 
                   0 
                 
               
               
                 
                   [ 
                   
                     Math 
                     . 
                     
                         
                     
                     ⁢ 
                     7 
                   
                   ] 
                 
               
             
           
         
       
     
     Then, assuming that the i-th subject coordinates are (xai, yai, zai), a straight line connecting the radiation source  461  and the i-th subject coordinates is represented by Math. 5, as in the first embodiment. Accordingly, a position of the radiation source  461  can be estimated in the same manner as in the first embodiment. 
     The biomagnetic measuring system  400  can perform biomagnetic measurements, in a manner similar to the biomagnetic measuring system  100 , using the radiation exposure device  460  including the imaging device  410  and the radiation source  461 , in place of the radiation exposure device  160  including the imaging device  110  and the radiation source  161 . Also, at the time of biomagnetic measurements, the biomagnetic measuring system  400  can partially function as a calibration system to calibrate a position of the radiation source  461 . 
     The position and angle of the radiation exposure device  460  including the radiation source  461  or the imaging surface  411 , or of both the radiation exposure device  460  and the imaging surface  411  may be adjusted based on the position of the radiation source  461  calculated by the radiation source position calculator  286 , the reference position of the imaging surface  411 , and the slope of the imaging surface  411 . 
     (Modification of Second Embodiment) 
     Next, a modification of the second embodiment will be described. The modification of the second embodiment differs from the second embodiment primarily in the arrangement of the magnetic field measuring device  120 , the imaging device  410 , and the radiation exposure device  460 .  FIG. 19  is a front view illustrating a configuration of a biomagnetic measuring system according to the modification of the second embodiment. 
     In the modification of the second embodiment, an upper surface of the magnetic field measuring device  120  is tilted from a horizontal plane (X-Y plane) as illustrated in  FIG. 19 . As a result, the imaging surface  411  of the imaging device  410  is also tilted from the horizontal plane. The radiation source  461  included in the radiation exposure device  460  is then located on the normal to the imaging surface  411 . For example, a line connecting the center of the imaging surface  411  and the center of the radiation source  461  intersects the imaging surface  411  at right angles. The magnitude of the tilt is, for example, 10 degrees. 
     In the modification of the second embodiment, it is possible to provide the same effects as in the second embodiment. 
     Note that in the first embodiment, the upper surface of the magnetic field measuring device  120  may be tilted from the horizontal plane, the imaging surface  111  of the imaging device  110  may be tilted from the horizontal plane (the X-Y plane), and the radiation source  161  included in the radiation exposure device  160  may be located on the normal to the imaging surface  111 , in a manner similar to the modification of the second embodiment. 
     Although the preferred embodiments have been described in detail above, various modifications and substitutions may be made to the above-described embodiments without departing from the scope of the claims. 
     REFERENCE SIGNS LIST 
     
         
         
           
               100 ,  400  biomagnetic measuring system 
               110 ,  410  imaging device 
               111 ,  411  imaging surface 
               120  magnetic field measuring device 
               121  magnetic sensor 
               130  calibration device 
               131  magnetic field generator 
               132  absorber 
               133  support 
               160 ,  460  radiation exposure device 
               161 ,  461  radiation source 
               180  control device 
               191  input device 
               192  server 
               193  display device 
               281  radiation source controller 
               282  relative position acquisition unit 
               283  magnetic field generator position acquisition unit 
               284  absorber position calculator 
               285  absorber image detector 
               286  radiation source position calculator 
               287  display controller 
               381  radiation source controller 
               382  biomagnetic information acquisition unit 
               383  calibrator 
               384  superimposition unit 
               385  display controller 
           
         
       
    
     The present application is based on Japanese Priority Application No. 2019-120422 filed on Jun. 27, 2019, and Japanese Priority Application No. 2019-166562 filed on Sep. 12, 2019, the entire contents of which are hereby incorporated herein by reference.