Patent Publication Number: US-2017354347-A1

Title: Position detection system and capsule medical apparatus guidance system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of PCT international application Ser. No. PCT/JP2015/079886 filed on Oct. 22, 2015 which designates the United States, incorporated herein by reference, and which claims the benefit of priority from Japanese Patent Applications No. 2015-030002, filed on Feb. 18, 2015, incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     The disclosure relates to a position detection system configured to detect a position or a direction of a capsule medical apparatus by detecting, outside a subject, a magnetic field that is generated from the capsule medical apparatus introduced into the subject and relates to a capsule medical apparatus guidance system configured to guide the capsule medical apparatus. 
     2. Related Art 
     Conventionally, a capsule medical apparatus configured to be introduced into a subject and obtain various types of information on the subject, or configured to administer a drug, or the like, to the subject, has been developed. A known example of this is a capsule endoscope formed into a size that can be introduced into the gastrointestinal tract (lumen) of the subject. The capsule endoscope has an imaging function and wireless communication function inside a capsule-shaped casing. The capsule endoscope is swallowed by the subject and thereafter captures images while moving inside the gastrointestinal tract, and wirelessly transmits image data of the image of an internal portion of an organ of the subject (hereinafter, also referred to as an in-vivo image) in sequence. 
     A system for detecting the position and the direction of such a capsule medical apparatus inside the subject has been developed. For example, JP 2008-132047 A discloses a position detection system configured to provide a coil that generates a magnetic field by receiving power (hereinafter, referred to as a magnetic field generation coil) within the capsule medical apparatus, detect a magnetic field generated from this magnetic field generation coil, using a plurality of magnetic field detection coils (hereinafter, referred to as a detection coil) provided outside the subject, and perform position detection calculation of the capsule medical apparatus on the basis of the intensity of the detected magnetic field. 
     SUMMARY 
     In some embodiments, a position detection system includes: a cylindrical detection coil configured to detect a magnetic field generated by a magnetic field generation unit; and a calculation unit configured to calculate at least one of a position and a direction of the magnetic field generation unit based on the magnetic field detected by the detection coil. A relationship between a diameter Ds and a length Ls in a winding direction, of the detection coil, satisfies Formula (1), and each of coefficients G 1 , G 2 , and G 3  in Formula (1) is respectively given by each of Formulae (2), (3) and (4). 
     
       
         
           
             
               
                 
                   
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     In some embodiments, a capsule medical apparatus guidance system includes the above-described the position detection system. The capsule medical apparatus further incorporates a magnet. The capsule medical apparatus guidance system further includes a guidance magnetic field generation apparatus configured to generate a magnetic field for guiding the capsule medical apparatus by causing the magnet to act. 
     The above and other features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating a position detection system according to a first embodiment of the disclosure; 
         FIG. 2  is a schematic diagram illustrating an exemplary internal structure of a capsule endoscope illustrated in  FIG. 1 ; 
         FIG. 3  is a schematic diagram illustrating a shape of a detection coil illustrated in  FIG. 1 ; 
         FIG. 4  is a plan view illustrating exemplary arrangement of the detection coil on a panel of a magnetic field detection apparatus illustrated in  FIG. 1 ; 
         FIG. 5  is a plan view illustrating another exemplary arrangement of the detection coil on the panel of the magnetic field detection apparatus illustrated in  FIG. 1 ; 
         FIG. 6A  is a schematic diagram illustrating a result of simulation for obtaining a detected magnetic field error (D s =10 mm, L s =30 mm); 
         FIG. 6B  is a schematic diagram illustrating a result of simulation for obtaining a detected magnetic field error (D s =10 mm, L s =25 mm); 
         FIG. 6C  is a schematic diagram illustrating a result of simulation for obtaining a detected magnetic field error (D s =10 mm, L s =20 mm); 
         FIG. 6D  is a schematic diagram illustrating a result of simulation for obtaining a detected magnetic field error (D s =10 mm, L s =15 mm); 
         FIG. 6E  is a schematic diagram illustrating a result of simulation for obtaining a detected magnetic field error (D s =10 mm, L s =10 mm); 
         FIG. 6F  is a schematic diagram illustrating a result of simulation for obtaining a detected magnetic field error (D s =10 mm, L s =5 mm); 
         FIG. 7  is a graph illustrating correlation between a ratio of length/diameter of the detection coil and the detected magnetic field error; 
         FIG. 8  is a schematic diagram illustrating an exemplary configuration of a capsule medical apparatus guidance system according to a second embodiment of the disclosure; 
         FIG. 9  is a schematic diagram illustrating an exemplary internal structure of a capsule endoscope illustrated in  FIG. 8 ; and 
         FIG. 10  is a schematic diagram illustrating an exemplary configuration of a guidance magnetic field generation apparatus illustrated in  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION 
     A position detection system according to embodiments of the disclosure will be described with reference to the drawings. The following description will exemplify a capsule endoscope configured to be introduced into the subject orally and to capture an image of an internal portion (lumen) of the subject as one mode of a capsule medical apparatus as a detection target by the position detection system according to the present embodiment. The disclosure, however, is not limited to this embodiment. In other words, the disclosure is applicable to position detection for various capsule-shaped medical apparatuses such as a capsule endoscope that moves inside the lumen from the esophagus to the anus of the subject, a capsule medical apparatus that delivers a drug, or the like, to internal portions of the subject, and a capsule medical apparatus including a pH sensor for measuring pH within the subject. 
     Note that the drawings in the following description merely schematically illustrate the shapes, sizes, and positional relations to such degrees that the contents of the disclosure are understandable. Accordingly, the disclosure is not limited only to the shapes, sizes, and positional relations exemplified in the individual drawings. In the drawings, same reference signs are attached to the same portions. 
     First Embodiment 
       FIG. 1  is a schematic diagram illustrating a position detection system according to a first embodiment of the disclosure. As illustrated in  FIG. 1 , a position detection system  1  according to the first embodiment includes a capsule endoscope  10 , a magnetic field detection apparatus  30 , and a control apparatus  40 . The capsule endoscope  10 , as an exemplary capsule medical apparatus introduced into a lumen of a subject  2 , transmits image data obtained by capturing the inside of a subject  2 , by superposing the data onto radio signals. The magnetic field detection apparatus  30  is provided below a bed  2   a  on which the subject  2  is placed and detects an alternating magnetic field generated by the capsule endoscope  10 . The control apparatus  40  detects at least any of the position of the capsule endoscope  10  and the direction (posture) of the capsule endoscope  10  on the basis of the alternating magnetic field detected by the magnetic field detection apparatus  30 . 
     Hereinafter, an upper surface of the bed  2   a , that is, a placement surface for the subject  2  is defined as an X-Y plane (horizontal plane), and a direction orthogonal to the X-Y plane is defined as a Z-direction (vertical direction). 
       FIG. 2  is a schematic diagram illustrating an exemplary internal structure of the capsule endoscope  10  illustrated in  FIG. 1 . As illustrated in  FIG. 2 , the capsule endoscope  10  includes a casing  100 , an imaging unit  11 , a control unit  12 , a transmitting unit  13 , a magnetic field generation unit  14 , and a power supply unit  15 . The casing  100  is a capsule-shaped casing formed into a size that can easily be introduced into the lumen of the subject  2 . The imaging unit  11  is contained in the casing  100  and captures the inside of the subject  2  to obtain an imaging signal. The control unit  12  controls operation of each of components, including the imaging unit  11 , of the capsule endoscope  10  and performs predetermined signal processing on the imaging signal obtained by the imaging unit  11 . The transmitting unit  13  wirelessly transmits the signal-processed image signal. The magnetic field generation unit  14  generates an alternating magnetic field for position detection of the capsule endoscope  10 . The power supply unit  15  supplies power to each of the components of the capsule endoscope  10 . 
     The casing  100  is an outer casing formed into a size that can be introduced into the inside of the organ of the subject  2 . The casing  100  includes a cylindrical casing  101  having a cylindrical shape, and dome-shaped casings  102  and  103  each having dorm-like shapes, and is formed with both opening ends of the cylindrical casing  101  being closed by the dome-shaped casings  102  and  103 . The cylindrical casing  101  is formed of a colored member substantially opaque for the visible light. At least one of the dome-shaped casings  102  and  103  (the dome-shaped casing  102  on a side of the imaging unit  11  in  FIG. 2 ) is formed of an optical member transparent for the light having a predetermined wavelength band, such as visible light. Note that, while one imaging unit  11  is provided on one side, namely, on the side of the dome-shaped casing  102  in  FIG. 2 , it is allowable to provide two imaging units  11 . In this case, also the dome-shaped casing  103  is formed of a transparent optical member. The casing  100  configured in this manner contains, using fluid-tight sealing, the imaging unit  11 , the control unit  12 , the transmitting unit  13 , the magnetic field generation unit  14 , and the power supply unit  15 . 
     The imaging unit  11  includes an illumination unit  111  such as an LED, an optical system  112  such as a condenser lens, and an imaging element  113 , that is, a CMOS image sensor, a CCD, or the like. The illumination unit  111  projects illumination light such as white light toward an imaging field of the imaging element  113 , thereby illuminating the subject within the imaging field through the dome-shaped casing  102 . The optical system  112  collects reflected light from the imaging field onto an imaging surface of the imaging element  113  and forms an image. The imaging element  113  converts reflected light (optical signal) from the imaging field, received on the imaging surface, into an electrical signal, and outputs it as an image signal. 
     The control unit  12  operates the imaging unit  11  with a predetermined imaging frame rate, and together with this, allows the illumination unit  111  to project light in synchronization with the imaging frame rate. Moreover, the control unit  12  performs A/D conversion or other predetermined signal processing on the imaging signal generated by the imaging unit  11 , thereby generating image data. The control unit  12  further generates an alternating magnetic field from the magnetic field generation unit  14  by allowing the power supply unit  15  to supply power to the magnetic field generation unit  14 . 
     The transmitting unit  13  includes a transmitting antenna, obtains image data signal-processed by the control unit  12  and related information, then performs modulation processing on the data and information, and wirelessly transmits the data and information in sequence to the outside via the transmitting antenna. 
     The magnetic field generation unit  14  constitutes a portion of a resonant circuit and includes a magnetic field generation coil  141  that generates a magnetic field by the current flow, and a capacitor  142  that forms the resonant circuit together with the magnetic field generation coil  141 . The magnetic field generation unit  14  receives power supplied from the power supply unit  15  and generates an alternating magnetic field having a predetermined frequency. The magnetic field generation coil  141  is a cylindrical coil formed by winding metal wire in a fixed direction. 
     The power supply unit  15  is a power storage unit such as a button cell battery and a capacitor, including a switching unit such as a magnetic switch and an optical switch. When configured to include the magnetic switch, the power supply unit  15  switches power supply on/off by the magnetic field applied from the outside, and in a case of on state, appropriately supplies power of the power storage unit to each of the components (the imaging unit  11 , the control unit  12 , and the transmitting unit  13 ) of the capsule endoscope  10 . In the case of off state, the power supply unit  15  stops power supply to each of the components of the capsule endoscope  10 . 
     Referring back to  FIG. 1 , the magnetic field detection apparatus  30  includes a planar panel  31  and a plurality of detection coils  32  arranged on a main surface of the panel  31 , each of the detections coils  32  receiving the alternating magnetic field generated from the capsule endoscope  10  and outputting a detected signal. 
       FIG. 3  is a schematic diagram illustrating the shape of each of the detection coils  32 . Each of the detection coils  32  is formed by winding meal wire in a fixed direction and has a cylindrical shape in general, as illustrated in  FIG. 3 . Hereinafter, the diameter of the cylindrical detection coil  32  (cylinder diameter) is defined as D s , the length in the winding direction (cylinder height) is defined as L s , and the ratio of the length L s  to the diameter D s , namely, L s /D s , is defined as a parameter indicating a shape of the detection coil  32 . 
       FIGS. 4 and 5  are plan views illustrating exemplary arrangement of the detection coil  32  on the panel  31 . The detection coil  32  may be arranged in a matrix in which adjacent detection coils  32  have a uniform interval between each other, as illustrated in  FIG. 4 . Alternatively, the detection coil  32  may be arranged such that the adjacent detection coils  32  have greater intervals between each other in accordance with a distance from the center of the panel  31 , as illustrated in  FIG. 5 . Moreover, the detection coil  32  may be arranged in the direction in which a rotation center axis A (refer to  FIG. 3 ) is located in parallel with the Z-axis in each of all the detection coils  32  as illustrated in  FIG. 4 . Alternatively, the direction of the detection coil  32  may be changed so as to allow the rotation center axis A to be in parallel with any of the X-axis, Y-axis, and Z-axis in accordance with the position of the detection coil  32 . The detection coil  32  is capable of detecting, with high accuracy, the change in the magnetic field in the direction parallel with the rotation center axis A. Accordingly, by arranging three detection coils  32  in which each of the rotation center axes A is arranged in parallel with each of the X-axis, Y-axis, and Z-axis, as one unit (coil set  33 ), it is possible to three-dimensionally detect the change in the magnetic field at the corresponding position.  FIG. 5  illustrates an exemplary case where a plurality of detection coils  32  is arranged on the inner peripheral side of the panel  31  such that the rotation center axis A is in parallel with the Z-axis, and each of coil sets  33  is arranged at each of ends of the panel  31 . 
     The above-configured magnetic field detection apparatus  30  is arranged in the vicinity of the subject  2  under examination. In the first embodiment, the magnetic field detection apparatus  30  is arranged below the bed  2   a  such that a main surface of the panel  31  is arranged horizontally. 
     A region in which the position or the direction of the capsule endoscope  10  can be detected by the magnetic field detection apparatus  30  is defined as a detection target region R. The detection target region R is three-dimensional closed region including a range in which the capsule endoscope  10  is movable within the subject  2  (that is, a range of observation target organ). The detection target region R is preset in accordance with conditions such as the arrangement of the plurality of detections coils  32  on the magnetic field detection apparatus  30  and with magnetic field intensity that can be generated by the magnetic field generation unit  14  within the capsule endoscope  10 . 
     Referring back to  FIG. 1 , the control apparatus  40  includes a receiving unit  41 , an output unit  42 , a storage unit  43 , a signal processing unit  44 , and a calculation unit  45 . The receiving unit  41  receives a radio signal transmitted from the capsule endoscope  10  via a receiving antenna  41   a . The output unit  42  outputs and displays various types of information, or the like, processed by the control apparatus  40 , onto a display device, or the like. The signal processing unit  44  performs various types of signal processing onto detected signals output from each of the detection coils  32  and generates magnetic field information. The calculation unit  45  performs image generation on the basis of image data received by the receiving unit  41  or performs various types of calculation processing including position or direction detection of the capsule endoscope  10  on the basis of the magnetic field information generated by the signal processing unit  44 . 
     When examination is performed with the capsule endoscope  10 , a plurality of the receiving antennas  41   a  is attached on the body surface of the subject  2 . Each of the receiving antennas  41   a  receives radio signals transmitted from the capsule endoscope  10 . The receiving unit  41  selects the receiving antenna  41   a  having the highest reception intensity toward radio signals, among these receiving antennas  41   a , and performs demodulation processing, or the like, onto the radio signals received via the selected receiving antenna  41   a , thereby obtaining image data of in-vivo images and related information. 
     The output unit  42  includes various displays such as liquid crystal display and an organic EL display, and displays in-vivo image of the subject  2  and information on the position and direction of the capsule endoscope  10  when the in-vivo image is captured. 
     The storage unit  43  is configured with a storage medium and a read/write apparatus for rewritably storing information, such as a flash memory and a hard disk. The storage unit  43  stores various programs, parameters used for controlling components of the control apparatus  40  by the calculation unit  45 , image data of the in-vivo image captured by the capsule endoscope  10 , and information on the position and direction of the capsule endoscope  10  within the subject  2 , or the like. 
     The signal processing unit  44  includes a filter unit  441 , an amplifier  442 , and an A/D converter  443 . The filter unit  441  shapes the waveform of the detected signal output from the magnetic field detection apparatus  30 . The A/D converter  443  performs A/D conversion processing on the detected signal. 
     The calculation unit  45  is formed with a central processing unit (CPU), for example, integrally controls operation of the control apparatus  40 , specifically, by reading a program from the storage unit  43 , transferring instructions and data to each of components constituting the control apparatus  40 , or performing other operation. The calculation unit  45  further includes an image processing unit  451  and a position detection calculation unit  452 . 
     The image processing unit  451  performs predetermined image processing such as white balance processing, demosaicing, gamma conversion, smoothing (noise removal, etc.) toward the image data input from the receiving unit  41 , and thereby generating image data for display. 
     The position detection calculation unit  452  obtains information representing the position and direction of the capsule endoscope  10  (hereinafter, collectively referred to as positional information) on the basis of the detected signal output from the signal processing unit  44 . More specifically, the position detection calculation unit  452  includes an FFT processing unit  452   a  and a position calculation unit  452   b . The FFT processing unit  452   a  extracts magnification field information such as amplitude and phase of the alternating magnetic field by performing fast Fourier transform (hereinafter, referred to as FFT processing) on the detection data output from the signal processing unit  44 . The position calculation unit  452   b  calculates at least any of the position and the direction of the capsule endoscope  10  on the basis of the magnetic field information extracted by the FFT processing unit  452   a.    
     Nest, the shape of the detection coil  32  disposed on the magnetic field detection apparatus  30  will be described. In many cases, position detection errors for the capsule endoscope  10  is attributable to an error arising between distribution of a theoretical magnetic field (hereinafter, referred to as an ideal magnetic field) when the position of the magnetic field generation coil  141  is assumed to be a magnetic field generation source, and distribution of a magnetic field based on the actual magnetic field (hereinafter, referred to as a detected magnetic field) actually detected by the plurality of detection coils  32 . This error arises from the fact that the position calculation unit  452   b  calculates the position or the direction using magnetic field distribution in an ideal condition in which the magnetic field generation coil  141  and the detection coil  32  are assumed as points, without taking the size and shape of the magnetic field generation coil  141  and the detection coil  32  into consideration. 
     Accordingly, the present inventor has simulated to determine the error (detected magnetic field error) between the intensity of the detected magnetic field and the intensity of the ideal magnetic field in the following procedure. Specifically, it is assumed that the detection coil  32  is arranged at one position within an arrangement surface of the detection coil  32 , and a center point (geometrical center point) in each of the radial direction and the length direction of the detection coil  32  is defined as an origin (X, Y, Z)=(0, 0, 0). Subsequently, a magnetic field intensity detected by the detection coil  32  at the time when the magnetic field generation coil  141  is arranged at a predetermined measurement point within the detection target region, is calculated. The magnetic field intensity in a case where the magnetic field generation coil  141  and the detection coil  32  are assumed as points (microscopic points) is defined as the ideal magnetic field intensity, and magnetic field intensity in a case where the magnetic field generation coil  141  and the detection coil  32  have their actual sizes and shapes is defined as the detected magnetic field intensity. Subsequently, the detected magnetic field error was calculated from a difference between the above-described detected magnetic field intensity and the ideal magnetic field intensity (detected magnetic field intensity−ideal magnetic field intensity). 
     The ideal magnetic field intensity was obtained by calculating magnetic field distribution on the assumption that a microscopic magnetic power generation source exists at each of the measurement points. 
     Meanwhile, the following model was set for the detected magnetic field intensity. The position is set such that the centers in the radial direction and in the length direction coincide with the origin, and the magnetic field intensity on the origin is obtained on the assumption that the detection coil  32  is a set of circular detection coils having a diameter D s  aggregated across a length L s , without taking the helical shape of the wound metal wire into consideration. The direction of the detection coil  32  is determined such that the rotation center axis of the detection coil  32  is vertical (parallel to the Z-axis, namely, opening end surface is horizontal). The shape of the detection coil  32  was determined such that the diameter D s  was set to four types of 10 mm, 20 mm, 30 mm, and 40 mm, and the length L s  was varied in a range 5 mm to 30 mm with respect to each of the diameters. 
     The position is set such that the center points in the radial direction and in the length direction coincide with coordinates of each of the measurement points, and the magnetic field distribution is calculated on the assumption that the magnetic field generation coil  141  is a set of circular current having a diameter D m  aggregated across a length L m , without taking the helical shape of the wound metal wire into consideration. As the direction of the magnetic field generation coil  141  at each of the measurement points, two patterns were applied, namely, the direction in which the rotation center axis of the magnetic field generation coil  141  is vertical (that is, the same as the direction of the detection coil  32 ) and the direction in which the rotation center axis is parallel to the X-axis (that is, radial direction of the detection coil). The diameter D m  of the magnetic field generation coil  141  is determined to be smaller than any of the above-described diameters D s  of the detection coil  32 . 
     The measurement points to be set within the detection target region were determined to be arranged with a pitch of 50 mm in the range of 0 mm to 450 mm in the +X direction, and with a pitch of 50 mm in the range of 50 mm to 500 mm in the +Z direction. Note that the arrangement in the −X direction and the ±Y directions is symmetrical to the +X direction in relation with arrangement of the detection coil  32 , and thus, the simulation therefor was omitted. 
       FIGS. 6A to 6F  are schematic diagrams illustrating results of the above-described simulation. In this exemplary case, the diameter D s  of the detection coil  32  was 10 mm. The length L s  of the detection coil  32  is 30 mm (L s /D s =3.0) in the case of  FIG. 6A , 25 mm (L s /D s =2.5) in the case of  FIG. 6B , 20 mm (L s /D s =2.0) in the case of  FIG. 6C , 15 mm (L s /D s =1.5) in the case of  FIG. 6D , 10 mm (L s /D s =1.0) in the case of  FIG. 6E , and 5 mm (L s /D s =0.5) in the case of  FIG. 6F . 
     In the graphs illustrated in  FIGS. 6A to 6F , the horizontal axis indicates the coordinates in the radial direction (X-direction) of the detection coil  32 , and the vertical axis indicates the coordinates in the axial direction (Z-direction) of the detection coil  32 . The density of each of the coordinate points within the graph indicates an absolute value of the detected magnetic field error. Specifically, the density indicates such that the higher the density on the coordinate, the greater the detected magnetic field error (absolute value), and the lower the density, the smaller the detected magnetic field error (absolute value). 
     The present inventor has found the following as a result of the simulation. When comparing  FIGS. 6A to 6F  with each other, it is understood that the detected magnetic field error is properly suppressed (there are few high-density regions) in the case of L s /D s =1.0 illustrated in  FIG. 6E . In other words, it would be appropriate to determine that the closer the diameter D s  to the length L s  of the detection coil  32 , the smaller the detected magnetic field error. 
     Moreover, in a case where the same measurement points are used (that is, the ideal magnetic field intensity is the same), it was found that the greater the diameter D s  of the detection coil  32 , the smaller the detected magnetic field intensity. Therefore in this case, the difference from the ideal magnetic field intensity shifts in the negative direction. Meanwhile, in a case where the same measurement points are used (same as above), it was found that the longer the length L s  of the detection coil, the greater the detected magnetic field intensity. Therefore in this case, the difference from the ideal magnetic field intensity shifts in the positive direction. The present inventor thought, from these result, that it would be possible to reduce the detected magnetic field error by adjusting the balance between the diameter D s  and the length L s  of the detection coil  32 , and performed further examination in order to obtain the optimum shape for the detection coil  32 . 
       FIG. 7  is a summary of the simulation related to the above-described four types of detection coils  32  (D s =10 mm, 20 mm, 30 mm, and 40 mm), representing the correlation between the ratio of the length L s  to the diameter D s , namely, L s /D s  (horizontal axis) and the detected magnetic field error (vertical axis). 
     As illustrated in  FIG. 7 , it is observed that, in order to control the detected magnetic field error to ±20% or below, it would be appropriate to set the ratio L s /D s , that is, the ratio of the length to the diameter, of the detection coil  32 , to a value greater than 0 and not greater than 1.3. Furthermore, in order to control the detected magnetic field error to ±10% or below, it would be appropriate to set the ratio L s /D s , that is, the ratio of the length to the diameter, to a value being 0.65 or above and 1.15 or below. Furthermore, in order to control the detected magnetic field error to ±5% or below, it would be appropriate to set the ratio L s /D s , that is, the ratio of the length to the diameter, to a value being 0.8 or above and 1.05 or below. Herein, the detected magnetic field error being ±20% represents a range whereby the error (position detection error) between the position of the capsule endoscope  10  based on the magnetic field detected by the detection coil  32  and the actual position of the capsule endoscope  10  is 2 mm or below. Moreover, the detected magnetic field error being ±10% represents a range whereby the position detection error is 1 mm or below. 
     The present inventor further found, from the above-described simulation, that one can calculate approximately the detected magnetic field error by using the ratio L s /D s , that is, the ratio of the length L s  to the diameter D s , of the detection coil  32 . The following Formula (1) is approximation representing a detected magnetic field error B. 
     
       
         
           
             
               
                 
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     Coefficients G 1 , G 2 , and G 3  illustrated in Formula (1) change in accordance with the diameter D s  of the detection coil  32 . Each of the following Formulae (2) to (4) is approximation that respectively gives each of the coefficients G 1 , G 2 , and G 3 . 
         G   1 =−1.73×10 −5   ×D   s   2 +7.36×10 −3   ×D   s −4.71×10 −2   (2)
 
         G   2 =3.74×10 −5   ×D   s   2 −1.54×10 −3   ×D   s +1.16×10 −2   (3)
 
         G   3 =−8.96×10 −5   ×D   s   2 −1.74×10 −3   ×D   s +1.30×10 −2   (4)
 
     Accordingly, in order to achieve the detected magnetic field error B that is a desired value or below, it would be sufficient to obtain the ratio L s /D s  that satisfies the following Formula (5). 
     
       
         
           
             
               
                 
                   
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                           2 
                         
                       
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                           G 
                           2 
                         
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                           ( 
                           
                             
                               L 
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                               D 
                               S 
                             
                           
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                   B 
                 
               
               
                 
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     In a case where it is desired to achieve the detected magnetic field error of 10% or below, for example, and when the diameter D s  of the detection coil  32  is fixed, it would be possible to obtain the length L s  of the detection coil  32 , capable of achieving the detected magnetic field error of 10% or below, by solving Formula (5) while applying B=0.1 onto the right side of Formula (5). In contrast, when the length L s  of the detection coil  32  is fixed, it would be possible to obtain the diameter D s  of the detection coil  32 , capable of achieving the detected magnetic field error of 10% or below, by solving Formula (5) while entering B=0.1 on the right side of Formula (5). 
     As described above, according to the first embodiment of the disclosure, by setting the ratio L s /D s , that is, the ratio of the length L s  to the diameter D s , of the detection coil  32 , to a value greater than 0 and not greater than 1.3, preferably, 0.65 or above and 1.15 or less, further preferably, 0.8 or above and 1.05 or less, it is possible to sufficiently and stably reduce the detected magnetic field error at the position of the detection coil  32 . Accordingly, by using above-designed the detection coil  32 , it is possible to execute position detection of the capsule endoscope  10  with high accuracy. 
     Second Embodiment 
     Next, a second embodiment of the disclosure will be described.  FIG. 8  is a schematic diagram illustrating an exemplary configuration of a capsule medical apparatus guidance system according to the second embodiment of the disclosure. As illustrated in  FIG. 8 , a capsule medical apparatus guidance system  3  according to the second embodiment includes a capsule endoscope  10 A, the magnetic field detection apparatus  30 , a guidance magnetic field generation apparatus  50 , and a control apparatus  60 . The guidance magnetic field generation apparatus  50  generates a magnetic field for guiding the capsule endoscope  10 A. The control apparatus  60  detects the position or direction of the capsule endoscope  10 A and controls operation of the guidance magnetic field generation apparatus  50 . Among these, the configuration of the magnetic field detection apparatus  30  is similar to the configuration in the first embodiment. 
       FIG. 9  is a schematic diagram illustrating an exemplary internal structure of the capsule endoscope  10 A. As illustrated in  FIG. 9 , the capsule endoscope  10 A further includes a permanent magnet  16 , in addition to the capsule endoscope  10  illustrated in  FIG. 2 . Configurations and operation of each of the components of the capsule endoscope  10 A other than the permanent magnet  16  are similar to the case in the first embodiment. 
     The permanent magnet  16  is provided to enable magnetic guidance of the capsule endoscope  10 A by the magnetic field generated by the guidance magnetic field generation apparatus  50 . The permanent magnet  16  is fixed inside the casing  100  such that the magnetization direction has inclination toward a long axis La of the casing  100 . Note that in  FIG. 9 , the magnetization direction of the permanent magnet  16  is indicated with an arrow. In the second embodiment, the permanent magnet  16  is arranged such that the magnetization direction is orthogonal to the long axis La. The permanent magnet  16  operates to follow the magnetic field applied from the outside, making it possible to achieve magnetic guidance of the capsule endoscope  10 A by the guidance magnetic field generation apparatus  50 . 
       FIG. 10  is a schematic diagram illustrating an exemplary configuration of the guidance magnetic field generation apparatus  50 . As illustrated in  FIG. 10 , the guidance magnetic field generation apparatus  50  generates a magnetic field for changing, relative to the subject  2 , the position, an inclination angle of the long axis La with respect to the vertical direction, and azimuth, of the capsule endoscope  10 A introduced into the subject  2 . More specifically, the guidance magnetic field generation apparatus  50  includes an external permanent magnet  51  and a magnet drive unit  52 . The external permanent magnet  51  functions as a guidance magnetic field generation unit. The magnet drive unit  52  changes the position and posture of the external permanent magnet  51 . Among these, the magnet drive unit  52  includes a horizontal position changing unit  521 , a vertical position changing unit  522 , an elevation changing unit  523 , and a pivot angle changing unit  524 . 
     The external permanent magnet  51  is preferably formed with a bar magnet having a rectangular parallelepiped shape, and contains the capsule endoscope  10 A within a region formed by one of four planes parallel to the magnetization direction of the magnet, being projected to the horizontal plane. Note that it is allowable to provide an electromagnet that generates a magnetic field by current flow, instead of the external permanent magnet  51 . 
     The magnet drive unit  52  operates in accordance with a control signal output from a guidance magnetic field control unit  62  described below. Specifically, the horizontal position changing unit  521  translates the external permanent magnet  51  within the X-Y plane. That is, the external permanent magnet  51  is moved within the horizontal plane while the relative positions of two magnetic poles magnetized on the external permanent magnet  51  being maintained. The vertical position changing unit  522  translates the external permanent magnet  51  along the Z-direction. That is, the external permanent magnet  51  is moved along the vertical direction while the relative positions of two magnetic poles magnetized on the external permanent magnet  51  being maintained. The elevation changing unit  523  changes the magnetization direction angle with respect to the horizontal plane by rotating the external permanent magnet  51  within the vertical plane including the magnetization direction, on the external permanent magnet  51 . The pivot angle changing unit  524  pivots the external permanent magnet  51  around a vertical direction axis passing through the center of the external permanent magnet  51 . 
     Referring back to  FIG. 8 , the control apparatus  60  further includes an operation input unit  61  and a guidance magnetic field control unit  62 , in addition to the control apparatus  40  illustrated in  FIG. 1 . Configurations and operation of each of the components of the control apparatus  60  other than the operation input unit  61  and the guidance magnetic field control unit  62  are similar to the case in the first embodiment. 
     The operation input unit  61  is configured with input devices such as various buttons, a switch, and a keyboard, pointing devices such as a mouse and a touch panel, and a joystick, or the like. The operation input unit  61  is used to input various types of information and commands into the control apparatus  60 . The operation input unit  61  inputs various types of information into the calculation unit  45  in response to input operation by the user. The information input by the operation input unit  61  includes, for example, information for guiding the capsule endoscope  10 A to the position and posture desired by the user (hereinafter, referred to as guidance operation information). 
     The guidance magnetic field control unit  62  performs control for guiding the capsule endoscope  10 A. Specifically, in a case where guidance operation information is input from the operation input unit  61 , the guidance magnetic field control unit  62  controls operation of each of components of the magnet drive unit  52  such that the capsule endoscope  10 A is arranged in the position and direction desired by the user, on the basis of the guidance operation information and the position and direction of the capsule endoscope  10 A calculated by the position detection calculation unit  452 . In other words, the magnetic field in space, including the position of the capsule endoscope  10 A is changed by changing the position, elevation, and the pivot angle of the external permanent magnet  51 , thereby guiding the capsule endoscope  10 A. 
     The first and second embodiments of the disclosure have been described hereinabove merely as examples for implementation of the disclosure, and thus, the disclosure is not intended to be limited to these embodiments. Furthermore, the disclosure can be modified in various manners in accordance with the specification, or the like, and it is apparent from the description given above that various other modes can be implemented within the scope of the disclosure. 
     According to some embodiments, the magnetic field generated by the magnetic field generation unit is detected by a plurality of detection coils each having the ratio of the length to the diameter being more than zero and not more than 1.3. Accordingly, it is possible to sufficiently and stably reduce a detected magnetic field error on each of the detection coils. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.