Abstract:
A magnetic resonance imaging apparatus and the magnet apparatus thereof comprise plural pairs of magnetic field generation sources, arranged face to face with each other, for forming a magnetic field space  3;  and magnetic field homogeneity adjusting apparatuses  17  and  18  for providing homogeneity adjustment of the magnetic field formed in the magnetic field space  3.  The magnetic field homogeneity adjusting apparatuses  17  and  18  contain a plurality of tapped holes  24  for installing a magnetic field homogeneity adjusting member  26  of smaller volume in a plate-formed tray made of a non-magnetic substance; and a plurality of mounts  25  for installing the magnetic field homogeneity adjusting member  27  of greater volume, these mounts being formed over the area containing a plurality of through-holes for permitting installation of a magnetic field homogeneity adjusting member  26  of smaller volume, whereby major magnetic field adjusting work can be performed independently of minor magnetic field adjusting work.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a magnetic resonance imaging apparatus equipped with a magnet apparatus, preferably used to form a homogeneity magnetic field in a space, wherein magnetic field generation sources are arranged face to face with each other; and a magnet apparatus used as a magnetic field generation source therefor.  
         [0002]     A magnetic resonance imaging apparatus provides an image representing the physical and chemical properties of a test object (sample), using the nuclear magnetic resonance that occurs when a high frequency pulse is applied to the test object placed in a homogeneity static magnetic field space. It is mainly used for medical services. The magnetic resonance imaging apparatus (also called the MRI) commonly comprises a magnetic field generation source for applying a homogeneity static magnetic field in the imaging area where the test object is brought, an RF coil for applying a high frequency pulse to the imaging area, a receiving coil for receiving a response from the imaging area, and a gradient coil for applying a gradient magnetic field in the imaging area.  
         [0003]     One of the requirements to improve the image quality of a magnetic resonance imaging apparatus is to improve the homogeneity of static magnetic field. To make homogeneity the static magnetic field applied to the imaging area from the magnetic field generation source, the magnetic apparatus used in the magnetic resonance imaging apparatus has the magnetic field adjusted in each phase of designing, assembling and installation.  
         [0004]     In the adjustment of the magnetic field in the phase of designing, the magnetic field is made homogeneity, for example, by changing the magnetic properties of the magnetic pole material, depending on the place where the magnetic pole of the magnetic field generation source is machined. The Japanese Application Patent Laid-open Publication No. 2002-253530 discloses the method of generating a homogeneity magnetic field by a magnetic pole provided with a concentric annular cavity. The Japanese Application Patent Laid-open Publication No. Hei 05-243037 discloses the method of changing the thickness of the magnetic pole that changes the saturated magnetization characteristic of a magnetic substance, according to the center and periphery of the magnetic pole surface, whereby a homogeneity magnetic field is created. Japanese Application Patent Laid-open Publication No. 2003-24299 discloses the method of forming a groove and projection on the surface of the magnetic pole, thereby generating a homogeneity magnetic field.  
         [0005]     The methods described in Japanese Application Patent Laid-open Publication No. 2002-253530, Japanese Application Patent Laid-open Publication No. Hei 05-243037 and Japanese Application Patent Laid-open Publication No. 2003-24299, however, are the methods for magnetic field adjustment in the so-called phase of designing. These methods fail to create the homogeneity magnetic field by correcting the components of poor homogeneity resulting the production error and ambient environment. This requires final adjustment of the magnetic field to be made in the phase of assembling and installation.  
         [0006]     The magnetic field in the phase of assembling is adjusted by correcting the components of poor homogeneity resulting the production error and ambient environment for example, by additionally distributing the magnetic field homogeneity adjusting members composed of a magnetic substance and a permanent magnet, to the magnet apparatus or by removing the magnetic field homogeneity adjusting member. The magnetic field homogeneity adjusting member is commonly arranged in the space sandwiched between the opposing magnetic field generation sources and the gradient coil arranged inside (in the imaging area). Based on this arrangement, Japanese Application Patent Laid-open Publication No. 2001-78984 discloses the method of arranging the magnetic field adjusting members at an interval smaller than the magnetic field adjusting member, thereby ensuring high-precision adjustment of the magnetic field.  
         [0007]     Magnetic field adjustment in the phase of installation is the same as that in the phase of assembling. A design is preferably worked out to ensure that each adjustment of magnetic field can be made by one and the same magnetic field adjusting mechanism. Another adjusting mechanism is provided in some cases for improvement of workability. Japanese Application Patent Laid-open Publication No. 2002-238874 discloses a method of using a superconducting coil as a magnetic field generation source. A magnetic field homogeneity adjusting mechanism is provided in the space adjacent to the coil container for storing this superconducting coil, thereby enhancing the efficiency of the magnetic field adjustment. Japanese Application Patent Laid-open Publication No. 2001-224570 introduces a method of forming a radiant groove on a piece of magnetic pole and inserting into this groove a non-magnetic rod with a magnetic field homogeneity adjusting member made of a magnetic substance arranged thereon, whereby the efficiency of magnetic field adjacent is improved. Japanese Application Patent Laid-open Publication No. 2003-153879 describes the method of forming a hole on the gradient coil, which is used as a magnetic field homogeneity adjusting mechanism.  
         [0008]     Japanese Application Patent Laid-open Publication No. 2002-165773 describes an MRI apparatus wherein multiple shim holes of different sizes are formed on a shim plate, and a plurality of different shim materials are combined and inserted into the shim holes.  
       SUMMARY OF THE INVENTION  
       [0009]     When the static magnetic field generated by the magnet apparatus exhibits a high degree of intensity, the impact of the surrounding environment upon magnetic field homogeneity is so high that it cannot be ignored. Adjustment of magnetic field in the phase of assembling and installation requires use of a great number of magnetic field homogeneity adjusting members in many cases. According to the prior art described in the Japanese Application Patent Laid-open Publication No. 2001-78984, the Japanese Application Patent Laid-open Publication No. 2002-238874, the Japanese Application Patent Laid-open Publication No. 2001-224570, and the Japanese Application Patent Laid-open Publication No. 2003-153879, magnetic field homogeneity in each of the phases of assembling and installation must be adjusted by using one and the same mechanism for major magnetic field adjustment work with a great number of magnetic field homogeneity adjusting members, and minor magnetic field adjustment work with a small number of magnetic field homogeneity adjusting members; or by using separate mechanisms installed in different spaces.  
         [0010]     When one and the same mechanism is used or major magnetic field adjustment work and minor magnetic field adjustment work, it is necessary to prepare a great number of magnetic field homogeneity adjusting members, ranging from the magnetic field homogeneity adjusting members having a very small volume to those having a large volume. In the meantime, when separate mechanisms are installed in different spaces, the magnet apparatus is required to have a new space for the magnetic field homogeneity adjusting mechanism. This has raised the problem of an increased size of the apparatus, hence increased costs.  
         [0011]     According to the prior art described in the Japanese Application Patent Laid-open Publication No. 2002-165773, it is necessary to prepare a great number of magnetic field homogeneity adjusting members, ranging from the magnetic field homogeneity adjusting members having a very small volume to those having a large volume.  
         [0012]     The first object of the present invention is to provide a magnetic resonance imaging apparatus and the magnet apparatus thereof capable of covering a wide scope of magnetic field homogeneity adjusting work ranging from major adjusting work to minor adjusting work, using a smaller number of types of magnetic field homogeneity adjusting members in a limited space.  
         [0013]     The second object of the present invention is to provide a magnetic resonance imaging apparatus and the magnet apparatus thereof capable of separately performing major magnetic field adjusting work and minor magnetic field adjusting work, using a pair of magnetic field homogeneity adjusting members arranged face to face with each other.  
         [0014]     To achieve the aforementioned objects, a magnetic resonance imaging apparatus and the magnet apparatus thereof according to the present invention comprise plural pairs of magnetic field generation sources, arranged face to face with each other, for forming a magnetic space; and a magnetic field homogeneity adjusting apparatus for providing homogeneity adjustment of the magnetic field formed in the magnetic field. A plate-formed tray formed of a non-magnetic substance is provided with a plurality of through-holes for arranging a magnetic field homogeneity adjusting member of small volume; and a plurality of mounts for arranging a magnetic field homogeneity adjusting member of large volume, wherein these mounts are formed over the area containing several through-holes. Thus, the magnetic field homogeneity adjusting member is configured to separately perform major magnetic field adjusting work and minor magnetic field adjusting work.  
         [0015]     The plate-formed tray is equipped with a plurality of tapped holes as through-holes, and a counterbore for installing the magnetic field homogeneity adjusting member of large volume.  
         [0016]     The present invention allows major magnetic field adjusting work and minor magnetic field adjusting work to be performed separately, using a pair of magnetic field homogeneity adjusting members arranged face to face with each other. The present invention also makes it possible to handle a wide scope of magnetic field homogeneity adjusting work ranging from major adjusting work to minor adjusting work, using a few number of types of magnetic field homogeneity adjusting members in a limited space. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     The present invention will be understood more fully from the detailed description given hereinafter and from the accompanying drawings of the preferred embodiment of the present invention, which, however, should not be taken to be limitative to the invention, but are for explanation and understanding only.  
         [0018]     In the drawings:  
         [0019]      FIG. 1  is an perspective view of a magnet apparatus as an embodiment of the present invention;  
         [0020]      FIG. 2  is a vertical sectional view representing the magnet apparatus shown in  FIG. 1 ;  
         [0021]      FIG. 3  is a vertical cross sectional view of a coil container;  
         [0022]      FIG. 4  is a perspective view of a magnetic field homogeneity adjusting apparatus;  
         [0023]      FIG. 5  is a vertical sectional view representing the magnetic field homogeneity adjusting apparatus shown in  FIG. 4 ;  
         [0024]      FIG. 6  is a vertical sectional view representing the minor magnetic field adjustment work of the magnetic field homogeneity adjusting apparatus;  
         [0025]      FIG. 7  is a vertical sectional view representing the major magnetic field adjustment work of the magnetic field homogeneity adjusting apparatus;  
         [0026]      FIG. 8  is a vertical sectional view representing an example of a variation of the magnetic field homogeneity adjusting apparatus;  
         [0027]      FIG. 9  is a vertical sectional view representing an example of another variation of the magnetic field homogeneity adjusting apparatus;  
         [0028]      FIG. 10  is a plan view representing an example of a further variation of the magnetic field homogeneity adjusting apparatus; and  
         [0029]      FIG. 11  is a vertical sectional view representing an example of still further variation of the magnetic field homogeneity adjusting apparatus. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]     The following describes an embodiment of the present invention with reference to drawings:  FIG. 1  is a perspective view of the magnet apparatus of a magnetic resonance imaging apparatus as the present invention.  FIG. 2  is a vertical sectional view representing the magnet apparatus shown in  FIG. 1 .  
         [0031]     As shown in  FIG. 1 , in the magnet apparatus as a magnetic field generation source for a magnetic resonance imaging apparatus, a pair of coil containers  1  and  2  are arranged face to face with each other through connection columns  4  and  5  to ensure that a pair of coil containers  1  and  2  form a magnetic field space  3 . As shown in  FIG. 2 , the upper coil container  1  accommodates a superconducting coil designed in an annular form, and the lower coil container  2  also accommodates a superconducting coil designed in an annular form.  
         [0032]     As shown in  FIG. 3 , the upper coil container  1  comprises a vacuum container  12  formed in an approximately cylindrical form, and a radiation shield  13  accommodated in the vacuum container  12 , and a helium container  14  stored in the radiation shield  13 . The helium container  14  incorporates a main coil  6  as an annular superconducting coil, correction coils  7  and  8 , and liquid helium as a superconducting refrigerant.  FIG. 3  shows the case of the upper coil container  1 . The lower coil container  2  is the same as the upper coil container  1 .  
         [0033]     As shown in  FIGS. 1 and 2 , the concave portions  15  and  16  of the cylindrical coil containers are formed on the surface, of coil containers  1  and  2 , opposing to the magnetic field space  3 . Magnetic field homogeneity adjusting apparatuses  17  and  18  are incorporated in the concave portions  15  and  16 , and gradient coils  19  and  20  are arranged on the side of the magnetic field homogeneity adjusting apparatuses  17  and  18  where the magnetic field space  3  is located. RF coils  21  and  22  are arranged on the side of the gradient coils  19  and  20  where the magnetic field space  3  is located. The magnetic field homogeneity adjusting apparatuses  17  and  18 , gradient coils  19  and  20 , and RF coils  21  and  22  are mounted removably on the coil containers  1  and  2 .  
         [0034]     The main coil  6 , correction coils  7  and  8  accommodated in the upper coil container  1 , and the main coil  9  and correction coils  10  and  11  accommodated in the lower coil container  2  operate in such a way as to form a homogeneity magnetic field area in the imaging area  23  as part of the magnetic field space  3 . The main coils  6  and  9  are arranged on the outer periphery of the coil containers  1  and  2 , where the magnetic field exhibits the highest intensity. A static magnetic field is formed in the perpendicular direction in the magnetic field space. The correction coils  7  and  8  of the upper coil container  1  and the correction coils  10  and  11  of the lower coil container  2  are provided to correct the non-homogeneity components in the magnetic field formed by the main coils  6  and  9 , whereby the imaging area  23  is formed into a homogeneity magnetic field.  
         [0035]     The direction of the currents of the correction coils  7 ,  8 ,  10  and  11  is determined by the non-homogeneity component of the magnetic field occurring in the imaging area  23 . For example, the currents of the correction coils  7  and  10  are determined as positive, and those of the correction coils  8  and  11  are determined as negative. Further, gradient coils  19  and  20  are the coils for forming a gradient magnetic field in the imaging area  23 , wherein this gradient magnetic field provides information on the position of nuclear magnetic resonance. The RF coils  21  and  22  serve as the transmission coils for applying electromagnetic wave to the imaging area  23 .  
         [0036]     As described above, the main coils  6  and  9  and correction coils  7 ,  8 ,  10  and  11  are arranged to create a homogeneity magnetic field for the imaging  10  area  23 . An incorrect magnetic field will be produced in the imaging area due to an assembling error or some factor related to the installation environment. To remove such an incorrect magnetic field, the magnetic field homogeneity adjusting apparatuses  17  and  18  are provided.  
         [0037]     As shown in  FIG. 4 , the magnetic field homogeneity adjusting apparatuses  17  and  18  are disk-formed trays, and are composed of a non-magnetic substance such as FRP and aluminum. As shown in  FIG. 5 , many tapped holes  24  penetrating from the surface of the magnetic field space  3  (hereinafter referred to as “front surface”) to the surface of the coil containers  1  and  2  hereinafter referred to as “back surface”) are provided. Rectangular counterbores  25  as mounts are formed on the back surface of the magnetic field homogeneity adjusting apparatuses  17  and  18  so as to incorporate a plurality of tapped holes. To be more specific, the counterbores  25  are arranged in the area covering a plurality of tapped holes. A magnetic field homogeneity adjusting member  26  made of a magnetic substance such as thread-formed iron and permanent magnet is screwed in the tapped holes  24  of the front surface. A plate-formed magnetic field homogeneity adjusting member  27  made of a magnetic substance such as rectangular iron and permanent magnet is arranged in the counterbores  25  of the back surface. The volume of the counterbore  25  is determined to be greater than that of the tapped holes  24  arranged on the front surface of the magnetic field homogeneity adjusting apparatuses  17  and  18 . The volume of the plate-formed magnetic field homogeneity adjusting member  27  is determined to be greater than that of the thread-formed magnetic field homogeneity adjusting member  26 . Further, the thread-formed magnetic field homogeneity adjusting member  26  is suited for adjusting the magnetic field distribution of low-order error and magnetic field of smaller error. The plate-formed magnetic field homogeneity adjusting member  27  is suited for adjusting the magnetic field distribution of high-order error and magnetic field of greater error.  
         [0038]     When an incorrect magnetic field resulting from an assembling error or installation error—a non-homogeneity magnetic field—is small, homogeneity can be reached by minor magnetic field adjustment. While magnetic field homogeneity adjusting apparatuses  17  and  18  are fixed on the concave portions  15  and  16  of the coil containers  1  and  2 , the thread-formed magnetic field homogeneity adjusting member  26  is screwed into the magnetic field homogeneity adjusting apparatuses  17  and  18  from the front surface. In this case, the quantity, volume and position of the thread-formed magnetic field homogeneity adjusting members  26  to be arranged are determined by the direction and magnitude of the incorrect magnetic field component in the imaging area  23 . In the minor magnetic field adjustment, the plate-formed magnetic field homogeneity adjusting member  27  is not arranged in the counterbores  25  arranged on the back surfaces of the magnetic field homogeneity adjusting apparatuses  17  and  18 . Accordingly, the counterbores  25  remain hollow and the tapped holes  24  are through-holes; therefore, magnetic field can be adjusted using the long thread-formed magnetic field homogeneity adjusting member  26  that reaches the counterbores  25 , as shown in  FIG. 6 .  
         [0039]     When an incorrect magnetic field resulting from an assembling error or installation error is great, major magnetic field adjustment is required. The magnetic field homogeneity adjusting apparatuses  17  and  18  are removed from the coil containers  1  and  2 , and the plate-formed magnetic field homogeneity adjusting member  27  is placed into the rectangular counterbores  25  on the back surface. In this case, the quantity, volume and position of the magnetic field homogeneity adjusting members  27  to be arranged are determined by the direction and magnitude of the incorrect magnetic field component in the imaging area  23 . In the phase of placing the plate-formed magnetic field homogeneity adjusting member  27  in the counterbores  25 . It is sufficient only if greater incorrect magnetic field components can be removed. Fine adjustment is not required, and therefore it is not necessary to prepare many types of plate-formed magnetic field homogeneity adjusting members  27 .  
         [0040]     After the magnetic field homogeneity adjusting apparatuses  17  and  18  with the magnetic field homogeneity adjusting member  27  placed into counterbores  25  have been connected with the coil containers  1  and  2 , the thread-formed magnetic field homogeneity adjusting member  26  is screwed into the tapped holes  24  from the front surface. Then minor adjustment of the magnetic field is performed to correct the incorrect magnetic field, whereby a homogeneity magnetic field is obtained. In this case, as shown in  FIG. 7 , the plate-formed magnetic field homogeneity adjusting members  27  are placed in the counterbores  25  provided on the back surface of the magnetic field homogeneity adjusting apparatuses  17  and  18 , so shorter thread-formed magnetic field homogeneity adjusting members  26  are screwed into the tapped holes  24 . The magnetic field adjustment range is reduced when shorter magnetic field homogeneity adjusting members are used, as compared to the case where the longer magnetic field homogeneity adjusting members are utilized. Accordingly, finer adjustment of the magnetic field can be performed, when compared to the case where the longer magnetic field homogeneity adjusting members are utilized.  
         [0041]     As described above, upon completion of the adjustment of the magnetic field using the magnetic field homogeneity adjusting apparatuses  17  and  18 , the gradient coils  19  and  20 , and RF coils  21  and  22  are fitted to the coil containers  1  and  2 , whereby a magnet apparatus is constructed.  
         [0042]     The MRI apparatus comprises a receiving coil for collecting nuclear magnetic resonance signals from a test object when a patient as the test object placed on the bed is brought into the magnetic field space of a magnet apparatus; and an analysis apparatus for analyzing the nuclear magnetic resonance signal having been received.  
         [0043]     The present embodiment is so structured that the magnetic field homogeneity adjusting members  26  and  27  to be installed on both the front and back surfaces of the magnetic field homogeneity adjusting apparatuses  17  and  18 . This arrangement allows the front surface to be used for major magnetic field homogeneity adjusting work, and the front surface to be used for minor magnetic field homogeneity adjusting work. This arrangement provides a magnet apparatus capable of covering a wide scope of magnetic field homogeneity adjusting work ranging from major adjusting work to minor adjusting work in a limited space.  
         [0044]     Large-sized counterbores  25  are formed on the back surfaces of the magnetic field homogeneity adjusting apparatuses  17  and  18 . This arrangement ensures minor magnetic field adjusting work to be performed, with the magnetic field homogeneity adjusting apparatuses  17  and  18  installed in position.  
         [0045]     The aforementioned arrangement eliminates the need for fine adjustment in the major magnetic field adjusting work, and hence reduces the number of the types of the magnetic field homogeneity adjusting members to be prepared in advance.  
         [0046]      FIG. 8  is a drawing representing an example of a variant of magnetic field homogeneity adjusting apparatuses  17  and  18 . The magnetic field homogeneity adjusting member  27  is provided with the tapped holes  28  formed at the same position as that of the tapped holes  24  formed on the magnetic field homogeneity adjusting apparatuses  17  and  18 . In the minor magnetic field adjustment using the magnetic field homogeneity adjusting member  26 , a long magnetic field homogeneity adjusting member  26  can be used even if the magnetic field homogeneity adjusting member  27  is installed. This expands the scope of adjusting the magnetic field in the minor magnetic field adjustment work.  
         [0047]      FIG. 9  is a drawing representing an example of another variant of magnetic field homogeneity adjusting apparatuses  17  and  18 . The counterbores  25  are formed on the front surface of the magnetic field homogeneity adjusting apparatuses  17  and  18 . When the magnetic field homogeneity adjusting apparatuses  17  and  18  are formed in the structure shown in  FIG. 9 , the magnetic field homogeneity adjusting apparatuses  17  and  18  need not be removed from the coil containers  1  and  2 , also in the case of major magnetic field adjustment. This arrangement improves the working efficiency. The tapped holes  28  may be formed on the magnetic field homogeneity adjusting member  27 . The magnetic field homogeneity adjusting member  26  can be installed even after the magnetic field homogeneity adjusting member  27  has been installed. This means improved working efficiency.  
         [0048]      FIG. 10  is a drawing representing the another embodiment of the magnetic field homogeneity adjusting apparatuses  17  and  18 . As illustrated, a plurality of annular grooves  30  are formed in the circumferential direction of the magnetic field homogeneity adjusting apparatuses  17  and  18 , and can be used as counterbores  25 . In this case, the magnetic field homogeneity adjusting member  27  is fan-shaped, and can be placed at a desired position of the disk-formed magnetic field homogeneity adjusting apparatuses  17  and  18 , whereby the degree of freedom in magnetic field homogeneity adjustment is enhanced.  
         [0049]     Further,  FIG. 11  shows still another embodiment of the magnetic field homogeneity adjusting apparatuses  17  and  18 .  FIG. 11  is a vertical cross sectional view showing an upper coil container  1 . The lower coil container is configured in the same way as the upper coil container  1 . The plate-formed magnetic field homogeneity adjusting member  17  is supported by a support member  33  secured on the coil container  1 . The support members  33  may be arranged like scattered dots at appropriate intervals on the surface of the magnetic field homogeneity adjusting apparatuses  17  and  18  where the upper coil container  1  is located. Alternatively, the supports member  33  may be arranged in a ring-form, as shown in  FIG. 10 .  
         [0050]     A space  34  is formed between the coil container  1  and magnetic field homogeneity adjusting apparatus  17  by the support member  33  provided in the aforementioned manner. The magnetic field homogeneity adjusting members  27  for major magnetic field adjustment can be arranged in this space  34 . Further, the magnetic field homogeneity adjusting apparatus  17  is provided with the tapped holes  24 , and the magnetic field homogeneity adjusting members  26  can be screwed in the tapped holes  24 .  
         [0051]     For the aforementioned structure, the magnetic field homogeneity adjusting apparatus  17  is removed from the support member  33 , and the magnetic field homogeneity adjusting member  27  is placed at a desired position on the back surface to carried out major magnetic field adjustment, according to the example given in  FIG. 11 . The magnetic field homogeneity adjusting member  27  can be installed by using screws or adhesives. After the magnetic field homogeneity adjusting member  27  has been installed, the magnetic field homogeneity adjusting apparatus  17  is mounted on the support member  33  and the magnetic field homogeneity adjusting member  26  is installed at a desired position from the front surface of the magnetic field homogeneity adjusting apparatus  17 , whereby minor magnetic field adjustment is carried out. The present embodiment allows both major and minor magnetic field adjustment work to be performed using one and the same magnetic field adjusting apparatus, without providing a counterbore.  
         [0052]     In the aforementioned embodiments, tapped holes  24  are formed for minor magnetic field adjustment work, whereas the counterbores  25  or space  34  is provided for major magnetic field adjustment work. Without being restricted to such an arrangement, through-holes may be formed minor magnetic field adjustment work. Instead of forming the counterbores  25  or space  34 , a steel plate, for example, of the magnetic field homogeneity adjusting member  27  may be attached directly on the front surface or back surface of the magnetic field homogeneity adjusting apparatuses  17  and  18 . Further, a magnet apparatus using a superconducting coil as a magnetic field generation source has been described in the aforementioned embodiments. Without being restricted thereto, a non-superconducting coil or a permanent magnet may be used as a magnetic field generation source.  
         [0053]     Although the present invention has been illustrated and described with respect to exemplary embodiment thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omission and additions may be made therein and thereto, without departing from the spirit and scope of the present invention. Therefore, the present invention should not be understood as limited to the specific embodiment set out above but to include all possible embodiments which can be embodied within a scope encompassed and equivalent thereof with respect to the feature set out in the appended claims.