Patent Publication Number: US-2011057655-A1

Title: Software for adjusting magnetic homogeneity, method for adjusting magnetic homogeneity, magnet device, and magnetic resonance imaging apparatus

Description:
TECHNICAL FIELD  
     The present invention relates to software for adjusting magnetic homogeneity, a method for adjusting magnetic homogeneity, a magnetic device, and a magnetic resonance imaging apparatus. 
     BACKGROUND ART  
     Using a nuclear magnetic resonance phenomenon that occurs when a specimen placed in a homogeneous static magnetic field is irradiated with high frequency pulses, a magnetic resonance imaging (MRI) apparatus can obtain an image representing the physical and chemical properties of the specimen, and is particularly used for medical purposes. The MRI apparatus mainly includes a magnetic field generation source for applying a homogeneous static magnetic field in an imaging region into which the specimen is carried, an RF coil for irradiating the imaging region with high frequency pulses, a receiving coil for receiving a response from the imaging region, and a gradient magnetic field coil for applying a gradient magnetic field to provide the imaging region with position information of a resonance phenomenon. 
     In the MRI apparatus, one of the factors for improving the image quality is an improvement in the static magnetic field homogeneity in the imaging region. In designing and manufacturing of a magnetic device used for the MRI apparatus, magnetic field homogeneity adjustment is performed at the respective stages of designing, assembling, and installing in order to make a static magnetic field, which is generated in the imaging region by a magnetic field generation source, homogeneous. 
     Among these, the magnetic field homogeneity adjustment performed at the installing stage can be realized by adding or removing magnetic field homogeneity adjusting pieces (magnetic shims) of a magnetic material to/from a magnetic device, for example, when a magnetic field inhomogeneity component has been caused by a manufacturing error or the surrounding environment. For example, in a magnetic device of a type that forms an imaging region and a homogeneous magnetic field space thereof between magnetic field generation sources (magnetic poles) vertically facing each other, a structure is formed, in general, by providing and disposing a magnetic field homogeneity adjustment mechanism (means), which is called a shim tray, in a tray shape of a non-magnetic material in each of the spaces sandwiched by the respective magnetic poles and respective gradient magnetic field coils disposed inside with respect to the magnetic poles (namely, on the imaging region side) (for example, refer to Patent Document 1). 
     On the other hand, in a magnetic device of a type that incorporates a plurality of superconducting coils to be a magnetic field generation source in a double cylindrical container and forms an imaging region inside thereof and a homogeneous magnetic filed space along the axial direction of the cylinder, a structure is formed, in general, by providing a shim tray (magnetic field homogeneity adjuster) in the space sandwiched by a gradient magnetic field coil disposed on the inner circumferential side of the container and the inner circumferential surface of the container, or by incorporating a shim tray in the gradient magnetic field coil (for example, refer to Patent Document 2). 
     Consideration on where and how many magnetic shims are to be disposed on these shim trays is, in general, an optimization problem having an objective function for the magnetic field homogeneity in the imaging region, and disposition of magnetic shims is often determined by a linear optimization method or a method modified therefrom, using a given magnetic field distribution (for example, refer to Patent Document 3). 
     PRIOR ART DOCUMENTS 
     Patent Documents 
     
         
         Patent Document 1: JP 3733441 B2 
         Patent Document 2: JP 2007-202900 A 
         Patent Document 3: JP 2003-167941 A 
       
    
     Non-Patent Document 
     
         
         Non-patent Document 1: authors Haruo Yanai, Kei Takeuchi ‘Projection Matrix, Generalized Inverse Matrix and Singular Value Decomposition’, UP Applied Mathematics Library, University of Tokyo Press, 1983 
       
    
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     As a magnetic field homogeneity adjuster, a structure that allows disposition of magnetic shims (with a large volume) as many as possible per unit area makes a high magnetic field homogeneity adjustment capacity. This is because a large change in the magnetic field strength can be generated in an imaging region by a large volume of magnetic shims. In order to easily realize this, a method was considered that makes a large number of screw holes through a shim tray and screws magnetic material shims called shim bolts into these screw holes. In this method, by finely disposing screw holes, it is possible to dispose shim bolts as many as the number of the screw holes, and as a result, a large number of magnetic shims can be disposed. Further, by disposing screw holes at fine relative positions to each other, formation of a spatially fine magnetic filed distribution can be expected. 
     However, in order to realize a homogeneous magnetic field, in a case of individually managing a large number of screw holes and thus applying an optimization method such as a linear programming method to determine disposition of shim bolts, an elaborate work of screwing optimum shim bolts into the respective holes without an error is required. Assuming that, for example, several thousands of screw holes have been formed through one shim tray, a person to be engaged in the magnetic field homogeneity adjustment work needs to accurately array screws (shim bolts) necessary for the respective screw holes, which makes the work efficiency extremely low. 
     In order to improve this work efficiency, for example, a method of setting the number to a required minimum and thus decreasing the number of screw holes to be managed was considered. In this situation, it was also considered to make the diameter of screw holes to be provided through a shim tray large, however, the size of a shim bolt, in other words, the size of a screw hole cannot be made quite large, taking into account handling shim bolts for magnetic field homogeneity adjustment in a strong magnetic field generated by a magnetic device. Consequently, there is a problem that a sufficient magnetic field homogeneity adjustment capacity cannot be obtained. 
     Therefore, a method was considered, as a method for improvement, that divides a shim tray into several regions in advance such that each region includes a plurality of screw holes through the shim tray without a change in the diameter nor the number of holes, adds the volumes of shim bolts to be disposed at the screw holes in each region, and then displays the individual total volumes in the respective regions together. Herein, the above-described regions can be designed to attain a spatial accuracy sufficient for magnetic field homogeneity adjustment. It is necessary to set the regions to have a size matching a magnetic field homogeneity adjustment that is the finest adjustment expected at the time of adjusting the installation of a magnetic device. In such a manner, the work efficiency can be made higher than that for a case of individually managing a large number of screw holes. 
     However, because the size of these regions is normalized merely to ensure a sufficient spatial accuracy of the magnetic field distribution as described above, the size inevitably becomes small, in other words, the number of regions still remains large. If such region dividing is performed, even in a case of performing a sort of general (in other words, not-detailed) magnetic field homogeneity adjustment that does not require a high spatial accuracy of magnetic field distribution, it is necessary to finely dispose shim bolts in many regions, which causes a problem of a low work efficiency. 
     An object of the present invention is to provide software for adjusting magnetic homogeneity, a method for adjusting magnetic homogeneity, a magnetic device, and an MRI apparatus that contribute to improvement in the work efficiency in performing general magnetic field homogeneity adjustment which does not require, as described above, a significantly fine spatial accuracy of the magnetic field distribution. 
     Means for Solving the Problems 
     To solve the above-described problems, in accordance with the present invention, the positions and volumes of magnetic members (for example, shim bolts) to be disposed on a shim tray are calculated first on a computational mesh, based on the magnetic field strength distribution in a magnetic field space, to make the magnetic field strength distribution homogeneous. Subsequently, from the distribution of the calculated positions and volumes of the magnetic members, the local maximum values and the local minimum values thereof are extracted; the volume distribution regions of the magnetic members, with the centers thereof respectively at the positions of the extracted local maximum and minimum values, are extracted; and the volumes of the magnetic members distributed in these distribution regions are added in the respective regions. Finally, results of these calculations are displayed together with the corresponding local maximum value positions or the corresponding local minimum value positions. 
     Preferably, this method of extraction of distribution regions is desired to be a method that establishes the regions while checking the relationship with respect to the mass between adjacent nodes on the computational mesh and sequentially expanding the regions. 
     Further, the mass display method, described above, preferably displays the positions and the mass to be visually recognizable on a screen that displays the shape of the shim tray. 
     Advantages of the Invention 
     According to the present invention, it is possible to provide software for adjusting magnetic homogeneity, a method for adjusting magnetic homogeneity, a magnetic device, and an MRI apparatus that contribute to improvement in the work efficiency in performing general magnetic field homogeneity adjustment which does not require a significantly fine spatial accuracy of the magnetic field distribution. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a perspective view showing an example of a magnetic device as an object for magnetic field homogeneity adjustment in accordance with the present invention; 
         FIG. 1B  is a longitudinal sectional view of the magnetic device shown in  FIG. 1A ; 
         FIG. 2  is an enlarged longitudinal sectional view showing the details of the upper magnetic pole of the magnetic device shown in  FIG. 1 ; 
         FIG. 3A  is an enlarged perspective view showing a shim tray (magnetic field homogeneity adjuster) of the magnetic device shown in  FIG. 1 ; 
         FIG. 3B  is a longitudinal cross-sectional schematic view of the shim tray; 
         FIG. 4  is a schematic view showing an example of computational mesh for calculating the volume distribution of shim bolts corresponding to the magnetic device shown in  FIG. 1 ; 
         FIG. 5A  is a distribution diagram showing, by contour lines, the distribution of magnetic moments of shim bolts calculated on a computational mesh shown in  FIG. 4 ; 
         FIG. 5B  is a graph showing the change in the value of the magnetic moments along the radial direction; 
         FIG. 6A  and  FIG. 6B  are examples of a display of the volume distribution of the shim bolts calculated on the nodes of the computational mesh in a case where the volumes of the shim bolts are added in the regions of the orthogonal grid; 
         FIG. 7A  and  FIG. 7B  are schematic views showing the concept of an algorithm for calculating the volume distribution regions with the initial points thereof at the respective peak positions, wherein the calculation is made from the volume distribution, of the shim bolts calculated on the nodes of the computational mesh; 
         FIG. 8A  and  FIG. 8B  are diagrams showing an example of a display, wherein the volume distribution regions with the initial points thereof at the respective peak positions are calculated from the volume distribution of the shim bolts, the volume distribution having been calculated on the nodes of the computational mesh, and results of adding the mass in the respective regions are displayed; 
         FIG. 9  is a flow chart showing the procedure of magnetic field homogeneity adjustment using a method for adjusting magnetic homogeneity in accordance with the present invention; 
         FIG. 10  is an illustration showing a magnetic field distribution measurement device and a computer for magnetic field homogeneity adjustment; 
         FIG. 11  is a block diagram illustrating the operation of software for adjusting magnetic homogeneity; 
         FIG. 12  is a table used in display methods of shim bolt volume distribution, in a second embodiment in accordance with the present invention, corresponding to  FIG. 6  in the first embodiment; 
         FIG. 13  is a table used in display methods of shim bolts volume distribution, in the second embodiment in accordance with the present invention, corresponding to  FIG. 8  in the first embodiment; 
         FIG. 14  is a longitudinal sectional view showing the outline of a magnetic device of an MRI apparatus as an object for magnetic field homogeneity adjustment work as a third embodiment in accordance with the present invention; and 
         FIG. 15  is a schematic diagram showing an example of computational mesh for calculating the volume distribution of shim bolts corresponding to the magnetic device shown in  FIG. 14 . 
     
    
    
     BEST MODES FOR CARRYING OUT THE INVENTION  
     Embodiments in accordance with the present invention will be described in detail, with reference to the accompanying drawings. 
     First Embodiment 
       FIG. 1A  is a perspective view showing an example of a magnetic device  50  as an object for magnetic field homogeneity adjustment in accordance with the present invention.  FIG. 1B  is a longitudinal sectional view of the magnetic device  50  shown in  FIG. 1A . 
     As shown in  FIG. 1A , as a magnetic field generation source of an MRI apparatus, the magnetic device  50  has a structure where an upper coil container  1  and a lower coil container  2  in a pair are disposed facing each other with coupling poles  4 ,  5  therebetween such as to form a magnetic field space  3 . As shown in  FIG. 1B , superconducting coils  8 ,  11  formed in a ring shape are housed in the upper coil container  1 , and superconducting coils  9 ,  10  formed in a ring shape are housed in the lower coil container  2 . 
       FIG. 2  is an enlarged longitudinal sectional view showing the details of the upper magnetic pole of the magnetic device  50  shown in  FIG. 1 . 
     As shown in  FIG. 2 , the upper coil container  1  includes, for example, a vacuum container  12  formed substantially in a cylindrical shape, a radiation shield  13  housed in the vacuum container  12 , and a helium container  14  housed in the radiation shield  13 . In the helium container  14 , the superconducting coil (primary coil)  8  and the shield coil  11  formed in a ring shape are housed together with liquid helium (not shown) as superconducting refrigerant.  FIG. 2  shows the upper coil container  1 , and likewise the lower coil container  2  also has the same internal structure symmetrical to that of the upper coil container  1  with respect to the magnetic field space  3  (refer to  FIGS. 1A and 1B ). 
     Returning to  FIG. 1B , the upper coil container  1  is formed with a cylindrical recess  15  on the surface thereof facing the magnetic field space  3 . A shim tray  17  of a non-magnetic material, such as plastic or aluminum, is housed in the recess  15 . A gradient magnetic field coil  19  is disposed on the magnetic field space  3  side of the shim tray  17 , and an RF transmitting/receiving coil  21  is disposed between the magnetic field space  3  and the gradient magnetic field coil  19 . 
     Likewise, the lower coil container  2  is formed with a cylindrical recess  16  on the surface thereof facing the magnetic field space  3 . A shim tray  18  of a non-magnetic material is housed in the recess  16 . A gradient magnetic field coil  20  is disposed on the magnetic field space  3  side of the shim tray  18 , and an RF transmitting/receiving coil  22  is disposed between the magnetic field space  3  and the gradient magnetic field coil  20 . 
     The superconducting coils  8 ,  9 ,  10 , and  11  form an imaging region  23 , which is a part of the magnetic field space  3 , as a homogeneous magnetic field space. The superconducting coils (primary coils)  8 ,  9  generate the strongest magnetic field and form a static magnetic field along the vertical direction in the magnetic filed space  3 . The shield coils  10 ,  11  are provided to prevent the magnetic field formed by the superconducting coils (primary coils)  8 ,  9  from leaking outside. Further, the gradient magnetic field coils  19 ,  20  form a dynamic magnetic field in the imaging region  23 . The RF transmitting/receiving coils  21 ,  22  irradiate the imaging region  23  with an electromagnetic wave (radio wave) and receive the electromagnetic wave. 
     The superconducting coils  8 ,  9 ,  10 , and  11  are disposed such as to generate a homogeneous magnetic field in the imaging region  23 , as described above. If the superconducting coils  8 ,  9 ,  10 , and  11  are insufficient to obtain necessary strength or homogeneity of the magnetic field, then ferromagnetic members (not shown), such as iron pieces (including iron alloy, the same hereinafter) or permanent magnets, are disposed (or removed from), for example, inside or outside the vacuum container  12 , inside the radiation shield  13 , or inside the helium container  14  to increase (or attenuate) the magnetic field strength or to improve the homogeneity. The above description has been made for a case of disposing four superconducting coils  8 ,  9 ,  10 , and  11 , however, more or fewer superconducting coils may be disposed. 
     In such a manner, the magnetic device  50  is designed such as to generate a homogeneous magnetic field, using the superconducting coils  8 ,  9 ,  10 , and  11  and iron pieces or the like (not shown), however, in reality, an error magnetic field is generated in the imaging region  23  by an assembling error, effects by the installation environment, or the like. The shim trays  17 ,  18  are provided in order to remove this error magnetic field component. 
     From the surface of the upper coil container  1 , the shim tray  17 , the gradient magnetic field coil  19 , and the RF transmitting/receiving coil  21  are disposed in this order. Likewise, from the surface of the lower coil container  2 , the shim tray  18 , the gradient magnetic field coil  20 , and the RF transmitting/receiving coil  22  are disposed in this order. The gradient magnetic field coils  19 ,  20 , and the RF transmitting/receiving coils  21 ,  22  are installed to be removable. The shim trays  17 ,  18  may be or may not be removable. 
       FIG. 3A  is an enlarged perspective view showing a shim tray (magnetic field homogeneity adjuster)  17  ( 18 ) of the magnetic device  50  shown in  FIG. 1 , and  FIG. 3B  is a longitudinal cross-sectional schematic view of the shim tray  17  ( 18 ). 
     The shim trays  17 ,  18  have a shape of a disk or the like and are provided with a number of screw holes (female screw)  26  therethrough. In magnetic field homogeneity adjustment, when shim bolts  27 , which are magnetic shims in a screw shape (male screw), are screwed into the screw holes  26 , the shim trays  17 ,  18  are added with a magnetic material by the shim bolts  27 . Shim bolts  27  are prepared in advance, having various volumes and shapes depending on the length and the machining method, and a worker for magnetic field homogeneity adjustment selects and uses appropriate shim bolts  27  with required volumes and shapes. 
     As shown in  FIG. 3A , the surfaces of the shim trays  17 ,  18  are partitioned by grid lines  28  into small regions. The grid lines  28  are drawn such that plural screw holes  26  are included inside the respective grid sections. Although the shim bolts  27  has been described about a case of using magnetic shims in a screw shape, magnetic shims not in a screw shape may be used. Magnetic shims in various shapes, for example, a cylindrical shape, a prismatic shape, a conical shape, a pyramid shape, a plate shape, a rivet shape, and other shapes, can be suitably used, depending on the conditions. 
     The magnetic field homogeneity adjustment work means disposing shim bolts  27 , which are necessary to make the magnetic field distribution in the imaging region  23  homogeneous, in the screw holes  26  provided through the shim trays  17 ,  18 . Based on the measurement values of the magnetic field strength distribution in the imaging region (homogeneous magnetic field space)  23 , software (software for adjusting magnetic homogeneity), which is installed on a computer, calculates at which positions and in what approximate volumes shim bolts  27  are to be disposed on the shim trays  17 ,  18  in order to obtain a desirable homogeneous magnetic field. 
     The algorithm of the software determining the disposition may be based on, for example, a numerical programming method such as a linear programming method, other optimization methods, and may be based on a method that solves an inverse problem. In the present embodiment, an algorithm according to the inverse problem solution method will be described as an example. 
       FIG. 4  is a schematic diagram showing an example of computational mesh for calculating the volume distribution of shim bolts  27  corresponding to the magnetic device  50  shown in  FIG. 1 . 
     First, the initial measurement is, as described later, performed in a state that shim bolts  27  are not disposed on the shim trays  17 ,  18 , then measurement is repeated while shim bolts  27  are sequentially added, and thereby a predetermined magnetic field homogeneity is obtained. As shown in  FIG. 4 , the shim trays  17 ,  18 , and the imaging region (the homogeneous magnetic field space)  23  are expressed by computational mesh. The nodes of the computational mesh of the shim trays  17 ,  18 , for example, may match and may not match the positions of the screw holes  26  provided through the shim tray  17 . On the other hand, the nodes of the computational mesh of the imaging region (the homogeneous magnetic field space)  23  are set in advance to match the positions for actual measurement of the magnetic field strength through magnetic field homogeneity adjustment work, or to match positions for calculating the magnetic field strength. 
     When a shim bolt  27  with a volume V i  and a magnetic charge M is disposed at a certain node i on the computational mesh of the shim tray  17 ,  18 , the shim bolt  27  causes a magnetic field strength B (i, j) at a certain adjacent node j in the imaging region (homogeneous magnetic field space)  23 , the magnetic field strength B being proportional to the volume V i  and the magnetic charge M. 
       Expression 1 
         B ( i, j )∝ V   i   M=m   i    (1)
 
     The symbol m i  represents the magnetic dipole moment of the shim bolt  27 . Herein, the magnetic charge M is assumed to be constant. Accordingly, the distribution of the magnetic moments of the shim bolts  27  disposed at the respective nodes on the computational mesh of the shim tray  17 ,  18  is expressed by the following expression. 
     
       
         
           
             
               
                 
                   Expression 
                    
                   
                       
                   
                    
                   2 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     m 
                     -&gt; 
                   
                   = 
                   
                     ( 
                     
                       
                         
                           
                             m 
                             1 
                           
                         
                       
                       
                         
                           
                             m 
                             2 
                           
                         
                       
                       
                         
                           ⋮ 
                         
                       
                       
                         
                           
                             m 
                             n 
                           
                         
                       
                     
                     ) 
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     The distribution of the magnetic field strengths created, by these, at the respective nodes on the computational mesh of the imaging region (the homogeneous magnetic field space)  23  is expressed by the following expression. 
     
       
         
           
             
               
                 
                   Expression 
                    
                   
                       
                   
                    
                   3 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     b 
                     -&gt; 
                   
                   = 
                   
                     ( 
                     
                       
                         
                           
                             b 
                             1 
                           
                         
                       
                       
                         
                           
                             b 
                             2 
                           
                         
                       
                       
                         
                           ⋮ 
                         
                       
                       
                         
                           
                             b 
                             l 
                           
                         
                       
                     
                     ) 
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     Then, the relationship between the magnetic field distribution and the magnetic moment distribution is expressed by the following expression, representing the coefficient matrix by matrix A. 
       Expression 4 
       {right arrow over (b)}=A{right arrow over (m)}  (4)
 
     Applying a singular value decomposition method to the matrix A, a generalized inverse matrix A′ of the matrix A can be obtained. As a result, the following expression is obtained. The singular value decomposition method is, for example, the method described in the above-described “Non-patent Document 1”. 
       Expression 5 
       {right arrow over (m)}=A′{right arrow over (b)}  (5)
 
     That is, once the magnetic field distribution (to be generated) as a target is determined, required magnetic moment distribution can be calculated by calculating the matrix product between itself and the generalized inverse matrix A′ as shown in Expression (5). As described in the present invention, for a magnetic field homogeneity adjustment work, namely, a work for making the magnetic field distribution of the imaging region (homogeneous magnetic field space)  23  homogeneous, the target homogeneous magnetic field distribution is expressed by the following. 
       Expression 6 
       {right arrow over (b)} u    (6)
 
     Further, the measured values (or the calculated values) of the magnetic field distribution on the current imaging region (homogeneous magnetic field space)  23  are expressed by the following. 
       Expression 7 
       {right arrow over (b)} m    (7)
 
     Then, the magnetic field distribution to be generated can be calculated by the following expression. 
       Expression 8 
         {right arrow over (b)}={right arrow over (b)}   u   −{right arrow over (b)}   m    (8)
 
     If the magnetic moment distribution is obtained, then the volumes V i  of shim bolts  27  corresponding to respective magnetic moments m i  can be simply calculated by the following expression from Expression (1). 
       Expression 9 
         V   i   =m   i   /M    (9)
 
       FIG. 5A  is a distribution diagram showing, by contour lines, the distribution of magnetic moments of shim bolts  27  calculated on the computational mesh shown in  FIG. 4 , and  FIG. 5B  is a graph showing the change in the value of the magnetic moments along the radial direction. 
     More concretely, these show an example of the distribution of the magnetic moments obtained by Expression (5), wherein  FIG. 5A  shows an example that represents, by contour lines, the distribution of the magnetic moments m i  calculated on the shim tray  17  (or the shim tray  18 ), and  FIG. 5B  is a diagram schematically showing this distribution by a graph taking into account only one dimensional direction (radial direction). 
     In such a manner, the magnetic moments m i  are the amounts distributed at the respective nodes. Herein, a positive magnetic moment refers to a magnetic moment in the same direction as that of the magnetic field caused by the magnetic device  50 , and a negative magnetic moment refers to a magnetic moment in the opposite direction to that of the magnetic field caused by the magnetic device  50 . 
     As described above, reproducing the distribution of the magnetic moments (in other words, the volumes of the shim bolts  27 ) distributed at the respective nodes (or the respective screw holes  26 ) as it is on the shim trays  17 ,  18  makes the work efficiency significantly low due to a large number of nodes. In this situation, in order to increase the work efficiency, it is considered to define regions in a state of the grid lines (orthogonal grid)  28  such as to be a background shown in  FIG. 5A , and to add the magnetic moments, at the nodes, that are present in the regions (for example, a region A) inside the respective grid sections. Herein, the total value m A  is expressed by the following expression. 
     Expression 10 
     
       
         
           
             
               
                 
                   
                     m 
                     A 
                   
                   = 
                   
                     
                       ∑ 
                       
                         i 
                         ∈ 
                         A 
                       
                     
                      
                     
                       m 
                       i 
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
     Further, the volume V A  of the shim bolts  27  is obtained by the following expression. 
     
       
         
           
             
               
                 
                   Expression 
                    
                   
                       
                   
                    
                   11 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     V 
                     A 
                   
                   = 
                   
                     
                       m 
                       A 
                     
                     M 
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
       FIG. 6  is an example of a display of the volume distribution of the shim bolts calculated on the nodes of the computational mesh in a case where the volumes of the shim bolts are added in the respective regions of the orthogonal grid.  FIG. 6(   a ) shows a case of displaying only numbers, and  FIG. 6(   b ) shows a case of displaying the numbers together with contour lines shown in  FIG. 5A . 
     The unit (dimension) of the respective numbers is, for example, the cubic centimeter, for representation of the volume of shim bolts  27 . A positive number means that a magnetic moment whose value corresponds to the value of this number is to be given in the direction reinforcing the static magnetic field caused in this region by the magnetic device  50 . Concretely, shim bolts  27  of ferromagnetic pieces (iron or the like) corresponding to the volume represented by this number are to be given. A negative number means that a magnetic moment corresponding to the value of this number is to be given in the direction attenuating the static magnetic field caused in this region by the magnetic device  50 . Concretely, shim bolts  27  of permanent magnets with a strength corresponding to this number are to be disposed in the opposite direction to that of the static magnetic field caused by the magnetic device  50 , or, if shim bolts  27  of ferromagnetic pieces are already disposed, these are to be removed. The size (area, shape) of each section partitioned by the grid lines (orthogonal grid)  28  is appropriately determined in advance such as to have a sufficient performance for magnetic field adjustment work, and is, for example, 50 millimeters square. By this method, the workability is improved compared with disposing a shim bolt/bolts  27  at each node. 
       FIG. 7  is a schematic view showing the concept of an algorithm for calculating the volume distribution regions with the initial points thereof at the respective peak positions, wherein the calculation of the volume distribution regions is made from the volume distribution, of the shim bolts  27 , calculated on the nodes of the computational mesh. 
     As shown in  FIG. 6 , by the above-described method, as it is necessary to dispose shim bolts  27 , corresponding to the number of orthogonal grid sections, it is desired to decrease the number of man-hour of disposing shim bolts  27 . 
     In this situation, as shown in  FIG. 7 , a region An where magnetic moments are distributed with the center at the peak position Pn is extracted, and the magnetic moments in the region An are added by Expression (10) and Expression (11). Although, two dimensional computation processing is performed in reality, the procedure is schematically viewed by a one-dimensional schematic view in  FIG. 7  for simplifying the principle. In  FIG. 7 , the length in the radial direction is roughly divided into regions A 1  to A 7  with boundaries at the positions where the sign reverses or at the positions where the gradient reverses, and the corresponding peaks P 1  to P 7  are extracted. Accordingly, it is found that it is only required to dispose shim bolts  27  at the few peaks P 1  to P 7 . In the present description, peaks and bottoms are not distinguished from each other, and both are expressed as peaks. That is, peaks include bottoms. 
     Expansion of this principle to a real two-dimensional computational mesh will be as follows. 
     First, all peak positions Pn are extracted from the distribution of magnetic moments. The magnetic moment at a certain node i will be represented by m i , and the magnetic moment at each adjacent node j will be represented by m j . If the following expression is satisfied for every adjacent node j, the node i is the peak position. 
       Expression 12 
       m i &gt;m j &gt;0 or m i &lt;m j &lt;0   (12)
 
     Next, while checking the values of the magnetic moments at the adjacent nodes with initial points at the respective peak positions Pn, the boundaries of the regions An are determined as follows.
     (1) The node corresponding to a peak position Pn is defined to be on the 0 th  layer.   (2) Out of all nodes adjacent to a certain node k belonging to the n th  layer, the group of nodes except nodes having already been defined to be on the n th  or another layer is represented by C.   (3) If the peak position Pn has a positive magnetic moment with respect to all the nodes o belonging to the node group C, and the following expression is satisfied, then the nodes o are defined to be on the (n+1) th  layer.   

       Expression 13 
       0&lt;m k &lt;m o  or m o &lt;0   (13)
 
     Further, if the peak position Pn has a negative magnetic moment, and the following expression is satisfied, then the nodes o are defined to be on the (n+1) th  layer. 
       Expression 14 
       m o &lt;m k &lt;0 or m o &gt;0   (14)
 
     If any one of nodes o do not satisfy these expressions, then redefinition is made such that the node k instead of the nodes o is defined to be on the (n+1) th  layer.
     (4) The expressions (2) and (3) are repeated until all the nodes belonging to the n th  layer are redefined to be on the (n+1) th  layer.   (5) The group of nodes finally forming the outermost layer forms the boundary of the region An.   

     By the above-described procedure, regions An corresponding to the peak positions Pn are determined, as schematically shown in  FIG. 7 . 
     When the magnetic moment and the volume of the nodes belonging to the region An are calculated by Expression (10) and Expression (11), this volume V A n is the volume of shim bolts  27  to be disposed in the region An. The worker is only required to dispose V A n in the vicinity of the peak position Pn. 
     Concretely, in the vicinity of a peak position to be applied with a positive magnetic moment, shim bolts  27  of ferromagnetic pieces, iron for example, are to be disposed. In the vicinity of a peak position to be applied with a negative magnetic moment, shim bolts of permanent magnets are to be disposed in the direction for applying a negative magnetic moment, or, if there are already existing shim bolts  27 , these are to be removed. 
       FIG. 8  is a diagram showing examples of displays, wherein the volume distribution regions with the initial points thereof at the respective peak positions are calculated from the volume distribution of the shim bolts, the volume distribution having been calculated on the nodes of the computational mesh, and wherein results of adding the mass in the respective regions are displayed. That is, these display examples illustrate the volumes of shim bolts  27  calculated by the above-described method.  FIG. 8(   a ) is a case where only volumes and region boundaries are displayed, and  FIG. 8(   b ) is a case where contour lines shown in  FIG. 5A  are displayed together. 
     The unit (dimension) of the respective numbers is, for example, cubic centimeter for representation of the volumes of shim bolts  27 . In order to decrease work errors by intuitively expressing positive or negative of volumes, the symbols shown on the left side of the numbers represent positive amounts with a mark “Δ” and negative amounts with a mark “∇”. In order to recognizably notify a worker of positions to dispose shim bolts  27 , it is desired that, for example, the shim trays  17 ,  18  are partitioned by the grid lines (orthogonal grid)  28 , and coordinates, as shown in  FIG. 8(   a ), are assigned. 
     When the above-described displays are made by magnetic field adjustment software, the worker is only required to perform disposition, for example, at seven positions in the case of the example shown in  FIG. 8  (This number is no more than an example in  FIG. 8 , and the number is variable depending on the conditions or situations in real magnetic field homogeneity adjustment work.). Thus, the work efficiency is greatly improved compared with disposing the volumes calculated by Expression (9) at the respective nodes or performing disposition according to a volume display in a grid form as shown in  FIG. 6 . Further, a magnetic device  50  that uses such a method or an MRI apparatus using it enables reduction in time taken by installation adjustment, and thereby an inexpensive apparatus can be provided as a result. 
     The above-described method approximately determines regions An and the boundaries thereof. Accordingly, in order to improve the accuracy of magnetic field homogeneity adjustment, this magnetic field homogeneity adjustment is repeated plural times. 
     Next, the flow of magnetic field homogeneity adjustment work in accordance with the present invention will be described. 
       FIG. 9  is a flowchart showing the magnetic field homogeneity adjustment work in accordance with the present invention. 
     First, the magnetic field strength distribution in the imaging region (the homogeneous magnetic field space)  23  is measured (step S 1 ). Concretely, a magnetic field distribution measurement device  60  operates, and a magnetic probe  63  obtains measurement signals (a measurement result). Based on this measurement result, a data obtaining computer  61  generates magnetic field analysis data (magnetic field distribution data  72 ) (described later with reference to  FIG. 10 ). 
     Then, a magnetic field homogeneity adjustment computer  62  (described later with reference to  FIG. 10 )
         calculates the volume distribution of shim bolts  27 , using Expression (5)   determines regions A or regions An   displays a method of disposing shim bolts  27  (step S 2 ).       

     Next, the worker disposes shim bolts  27  on the shim trays  17 ,  18 , while having a view of the display on a display device  65  (refer to  FIG. 10 ) (step S 3 ). 
     Then, similarly to step S 1 , the magnetic field strength distribution of the imaging region (the homogeneous magnetic field space)  23  is measured (step S 4 ). 
     Next, it is determined whether or not the specification of the homogeneous magnetic field is satisfied (step S 5 ). That is, it is determined whether or not the magnetic field homogeneity of the imaging region (the homogeneous magnetic field space)  23  is within a predetermined value. More concretely, the magnetic field homogeneity adjustment computer  62  (refer to  FIG. 10 ) determines whether or not the magnetic strength is within a predetermined range, based on the magnetic analysis data. If the specification of the homogeneous magnetic field is not satisfied (“No” in step S 5 ), the above-described processing on and after step S 2  is repeated. If the specification of the homogeneous magnetic field is satisfied (“Yes” in step S 5 ), the magnetic field homogeneity adjustment work is terminated. 
     In this repeated process, if the display in  FIG. 6  and the display in  FIG. 8  are arbitrarily switchable, the worker can proceed the work, while appropriately changing detailed disposition of shim bolts  27  as shown in  FIG. 6  and a sort of general disposition of shim bolts  27  as shown in  FIG. 8 . 
       FIG. 10  is an illustration of the magnetic field distribution measurement device  60  and the magnetic field homogeneity adjustment computer  62 . 
     The magnetic field distribution measurement device  60  is provided with the magnetic probe  63  that is inserted into the imaging region (the homogeneous magnetic field space)  23  of the magnetic device  50  and detects the magnetic distribution, and the data obtaining computer  61  that is connected to the magnetic probe  63  and has the display device  65  and a data obtaining program installed thereon. 
     The magnetic field homogeneity adjustment computer  62  is a computer that includes the display device  65  and an output device  64 , such as a printer, and has software for adjusting magnetic homogeneity installed thereon. 
       FIG. 11  is a block diagram illustrating the operation of the software for adjusting magnetic homogeneity. 
     The magnetic field homogeneity adjustment computer  62  includes a storage device  66 , a computing device  67 , a display device  65 , and an output device  64 . On the magnetic field homogeneity adjustment computer  62 , the software for adjusting magnetic homogeneity is installed, and forms functions of an input section  73 , a computation section  74 , a display method generation section  75 , and an output section  76 . 
       FIG. 10  shows the data obtaining computer  61  and the magnetic field homogeneity adjustment computer  62  such that they are different computers. However, as it is sufficient if data obtaining software and software for adjusting magnetic homogeneity respectively operate, it is obvious that the computers  61  and  62  may be the same one, in other words, one computer used for the both purposes. 
     The operations from obtaining the magnetic field distribution to displaying shim bolts  27  to be disposed are carried out as follows.
     (1) The magnetic device  50  is excited as rated.   (2) The magnetic field distribution measurement device  60  obtains magnetic field distribution data of the imaging region (homogeneous magnetic field space)  23  of the magnetic device  50 . Concretely, while rotating the magnetic probe  63  having a number of detection sections (refer to  FIG. 10 ), the magnetic field distribution measurement device  60  obtains magnetic field distribution  71  from the imaging region  23  of the magnetic device  50 , and the data obtaining computer  61  generates magnetic field distribution data  72  from the magnetic field distribution  71 .   (3) The magnetic field homogeneity adjustment computer  62  stores the magnetic field distribution data  72  into the storage device  66  by using the software for adjusting magnetic homogeneity.   (4) The input section  73  sequentially receives the magnetic field distribution data  72  from the storage device  66 , and sends the magnetic field distribution data  72  to the computation device  67 .   (5) The computation section  74  computes the volume distribution data of shim bolts  27  based on the magnetic field distribution data  72  having been read.   (6) The display method generation section  75  transmits this volume distribution data to the output section, according to a preset method.   (7) Based on the volume distribution data, the output section  76  displays the volume distribution on the display device  65  or outputs the volume distribution using the output device  64  such as a printer.   (8) The worker carries out magnetic field homogeneity adjustment work (the work of disposing magnetic members (shim bolts  27 )), while having a view of this display or output result.   

     Second Embodiment 
     With reference to  FIGS. 12 and 13 , a display method by the software for adjusting magnetic homogeneity in a second embodiment in accordance with the present invention will be described. 
       FIG. 12  is a display example corresponding to  FIG. 6  in the first embodiment, and  FIG. 13  is a display example corresponding to  FIG. 8 . In such a manner, in the present embodiment, instead of displaying a distribution image of the magnetic field as shown in  FIG. 6  or  FIG. 8 , the coordinates and the volumes of the disposition points are displayed in a form of a list. The present embodiment is the same as the first embodiment except display, and accordingly duplicate explanation will be omitted. 
     In such a manner, as the correspondence between the positions (coordinates) to dispose shim bolts  27  and the volumes thereof is clear even without drawing the positions in the magnetic field distribution, the workability is significantly improved while keeping the principle of adding the distributed volumes on the shim tray  17  ( 18 ). 
     Third Embodiment 
     With reference to  FIGS. 14 and 15 , as a third embodiment in accordance with the present invention, another example of a magnetic device  51  and a magnetic field homogeneity adjuster of an MRI apparatus as an object will be described below. 
       FIG. 14  is a longitudinal sectional view showing the outline of the magnetic device  51  in the third embodiment in accordance with the present invention. 
     While the magnetic device  50  (refer to  FIG. 1 ) as an object in the respective foregoing embodiments is a magnetic device  50  that generates a magnetic field in the perpendicular direction to the imaging region (the homogeneous magnetic field space)  23  by magnetic poles which are disposed facing vertically each other, the magnetic device  51  (refer to  FIG. 14 ) in the present embodiment generates a magnetic field in the horizontal direction in an imaging region (a homogeneous magnetic field space)  23  by a group of superconducting coils incorporated inside a double cylindrical shape vacuum container  12 , a radiation shield  13 , and a helium container  14 . The same symbols are assigned to configuration elements that can be substantially the same as those in the respective foregoing embodiments, and overlapping description will be omitted. 
     As shown in  FIG. 14 , in the magnetic device  51 , a gradient magnetic field coil  19  is disposed on the inner circumference of the vacuum container  12  in the double cylindrical shape, and a shim tray  17  ( 18 ) is incorporated in the gradient magnetic field coil  19  ( 20 ). This structure is an example, and the shim tray  17  ( 18 ) may be disposed on the inner circumferential side of the gradient magnetic field coil  19  ( 20 ) and may be disposed on the outer circumferential side. 
       FIG. 15  is a conceptual diagram showing an example of computational mesh, corresponding to those in  FIG. 4 , for such a magnetic device  51 . 
     By performing calculation which is similar to that conducted in the first embodiment, using such a computational mesh, magnetic field homogeneity adjustment work is all the same possible also on the magnetic device  51  with the structure shown in  FIG. 14 , and the work efficiency can also be improved. 
     In respective embodiments in accordance with the present invention, a worker for magnetic field homogeneity adjustment is only required to dispose shim bolts (magnetic shims)  27  in a minimum quantity at positions of a minimum requirement in respective stages of a magnetic field homogeneity adjustment work, which eliminates the necessity of managing all the positions of shim trays  17 ,  18  and disposing magnetic shims precisely at the respective positions, and thus the efficiency of the magnetic field homogeneity adjustment work can be significantly increased. Further, because a magnetic device  50  or the like using such a method or an MRI apparatus using such a apparatus can reduce the time for installation adjustment, it is possible to provide an inexpensive apparatus as a result. 
     REFERENCE SYMBOLS  
     
         
           1  upper coil container 
           2  lower coil container 
           3  magnetic field space 
           4 ,  5  coupling pole 
           8 ,  9  primary coil 
           10 ,  11  shield coil 
           12  vacuum container 
           13  radiation shield 
           14  helium container 
           15 ,  16  vacuum container recess 
           17 ,  18  shim tray 
           19 ,  20  gradient magnetic field coil 
           21 ,  22  RF transmitting/receiving coil 
           23  imaging space (homogeneous magnetic field space) 
           26  screw hole 
           27  shim bolt (shim member, magnetic shim) 
           28  grid line (grid line of an orthogonal grid)