Abstract:
Disclosed herein is a method for assembling a magnetic field generator for a magnetic resonance imaging system. The method comprises: establishing an arrangement for a permanent magnet of a magnet assembly comprising a ferromagnetic yoke plate and a permanent magnet, wherein the arrangement includes a portion of a cavity formed from placement of a portion of a plurality of retainers attached at substantially the perimeter of the yoke plate. The method also includes populating the first portion of the cavity with a set of rails attached to the yoke plate and affixing a plurality of gliders to a plurality of magnet blocks and magnetizing the gliders and magnet blocks to form a plurality of block assemblies. Finally, the method includes sliding each block assembly of the plurality of block assemblies along a rail of the set of rails; initiating with an outermost rail and concluding with an innermost, securing each successively filled rail with a retainer.

Description:
BACKGROUND OF INVENTION 
     This invention relates to a magnetic field generator for an MRI, a method for assembling the same, and a method for assembling a magnet unit for the same. More specifically, this invention relates to a magnetic field generator for MRI incorporating permanent magnets, a method for assembling the same, and a method for assembling a magnet unit for the same. It will be appreciated, however, that the-invention is also amenable to other like applications for complex assembly of components exhibiting large interaction forces between the members to be assembled. 
     A magnetic field generator for MRI uses permanent magnets. The magnets used in such an apparatus are often formulated from a plurality of magnet blocks. It is very difficult to place material blocks first and then magnetize each block. Therefore, in actual manufacturing, the blocks are fabricated and then magnetized. The magnetized blocks are then arranged on a yoke plate so that each of the magnet blocks has a same magnetic pole facing upward. A pole pieces is then placed on the top of the magnetized blocks. Such arrangement on a yoke plate is difficult due to the interaction of the large magnetic forces between each of the magnet blocks and between the blocks, pole piece and the yoke plate. 
     Conventionally, when placing the magnet blocks on the yoke plate, a surface of the yoke plate is first applied with adhesive, and then magnet blocks are bonded or attached to the surface, as disclosed in the Japanese Patent No. 2,699,250 for example. According to such a bonding method, upper surfaces of respective magnet blocks bonded to the yoke plate surface are not flush with each other, making an uneven surface. A magnetic field generator incorporating the permanent magnets made of such magnet blocks is apt to produce non-uniform magnetic field between a pair of pole piece opposed to each other. Further, pole pieces for correcting the non-uniformity of the magnetic field may be tilted to produce non-uniformity in the magnetic field. Generally, after a step of mounting a pair of permanent magnets to oppose each other, a step of adjustment for uniformly distributing the magnetic field is indispensable. However, if the magnet blocks are mounted according to the above method, the non-uniformity of the magnetic field is so large that the adjustment becomes very time consuming. 
     Further, according to the above method of bonding the magnet blocks, the magnet blocks each exhibiting very large magnetic forces is placed from above, onto the upper surface of the yoke plate, making it extremely difficult to fit each of the magnet blocks snugly to adjacent magnet blocks. More specifically, when mounting, each magnet block is held with a face of predetermined magnetic pole facing upward. When the magnet block is brought above the other magnet block, which is already fixed onto the yoke plate, a pulling force is generated between the two. Further, when the two magnet blocks are brought in adjacency, a repelling force is generated between the two. Since the magnet block to be placed is under such intense forces, the magnet block must be firmly held for safety while being transported. For a conventional holding mechanism, it is very difficult to fit the magnet block snugly to the place of bonding efficiently against these strong forces. 
     The pair of magnet units thus assembled as described above is then opposed to each other so the permanent magnets are faced at a predetermined distance. This process is achieved by first assembling one magnet unit, then connecting one or more posts or a yoke column to the magnet unit, and finally connecting the other magnet unit to the post(s). 
     The post(s) magnetically connect the pair of magnet units, and therefore must be made of a magnetic material. Thus, when the post is connected to the magnet unit, the post is brought under the pulling force from the magnet unit. This large force makes it difficult to connect the two yoke plates with high accuracy. Likewise, when the second magnetic unit is connected to the post already connected to the first magnet unit, it is also difficult to connect the two at a high accuracy. 
     Another method to assemble a magnetic field generator is disclosed by European Patent No. EP0978727A2 and U.S. Pat. No. 6,336,989. In these patents, a non-magnetic fixed projection is placed at the center of the yoke, with two orthogonal guide rails. The magnetic blocks are then slid into place and bonded to each other along the non-magnetic fixed projection and guide rails. This approach while adequate for its intended purposes is still cumbersome and requires additional special tooling. What is desired is a method for assembling the magnetic field generator to desired tolerances with a minimum of specialized tooling and assembly steps. 
     SUMMARY OF INVENTION 
     The above discussed and other drawbacks and deficiencies are overcome or alleviated by a method for assembling a magnetic field generator for a magnetic resonance imaging system. The method comprises: establishing an arrangement for a permanent magnet of a magnet assembly comprising a ferromagnetic yoke plate and a permanent magnet, wherein the arrangement includes a portion of a cavity formed from placement of a portion of a plurality of retainers attached at substantially the perimeter of the yoke plate. The method also includes populating the first portion of the cavity with a set of rails attached to the yoke plate and affixing a plurality of gliders to a plurality of magnet blocks and magnetizing the gliders and magnet blocks to form a plurality of block assemblies. Finally, the method includes sliding each block assembly of the plurality of block assemblies along a rail of the set of rails; initiating with an outermost rail and concluding with an innermost, securing each successively filled rail with a retainer. 
     Also disclosed herein is a magnetic field generator for a magnetic resonance imaging system. The magnetic field generator comprises: an arrangement for a permanent magnet of a magnet assembly comprising a ferromagnetic yoke plate and a permanent magnet, wherein the arrangement includes a portion of a cavity formed from placement of a portion of a plurality of retainers attached at substantially the perimeter of the yoke plate. The magnetic field generator has the first portion of the cavity populated with a set of rails attached to the yoke plate and a plurality of gliders affixed to a plurality of magnet blocks and magnetized to form a plurality of block assemblies. Each block assembly of the plurality of block assemblies is slid along a rail of the set of rails; initiating with an outermost rail and concluding with an innermost, securing each successively filled rail with a retainer. 
     Also disclosed herein is a re-workable magnetic field generator for a magnetic resonance imaging system comprising: a means for establishing an arrangement for a permanent magnet of a magnet assembly comprising a ferromagnetic yoke plate and a permanent magnet, wherein the arrangement includes a portion of a cavity formed from placement of a portion of a plurality of retainers attached at substantially the perimeter of the yoke plate. The re-workable magnetic field generator also includes a means for populating the first portion of the cavity with a set of rails attached to the yoke plate and a means for affixing a plurality of gliders to a plurality of magnet blocks and magnetizing the gliders and magnet blocks to form a plurality of block assemblies. Finally, the re-workable magnetic field generator further includes a means for sliding each block assembly of the plurality of block assemblies along a rail of the set of rails; initiating with an outermost rail and concluding with an innermost, securing each successively filled rail with a retainer and a means for removing one or more retainers of the plurality of retainers and sliding each block assembly of the plurality of block assemblies along a rail of the set of rails off the rail and the yoke plate. 
    
    
     The above discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings. 
     BRIEF DESCRIPTION OF DRAWINGS 
     Referring to the exemplary drawings wherein like elements are numbered alike in the several Figures: 
     FIG. 1 depicts a cutaway view of an exemplary MRI magnetic field generator assembly; 
     FIG. 2 depicts a yoke plate configuration; 
     FIG. 3 depicts a diagrammatic representation of the assembled stoppers of an exemplary embodiment; 
     FIG. 4 depicts a diagrammatic representation of stoppers attached to the yoke plate to facilitate assembly; 
     FIG. 5 depicts a diagrammatic representation of a rails set for an exemplary embodiment; 
     FIG. 6 depicts a diagrammatic representation of rails attached to the yoke plate surface; 
     FIG. 7 depicts a diagrammatic representation if attaching un-magnetized permanent magnet block to the glider, and then magnetizing them as an assembly; 
     FIG. 8 depicts a diagrammatic representation of an assembly process of an exemplary embodiment for a first magnet block onto the yoke plate; 
     FIG. 9 depicts a diagrammatic representation of an assembly process of an exemplary embodiment for a second magnet block onto the yoke plate; 
     FIG. 10 depicts a diagrammatic representation of adding a first finishing stopper for the first row of magnet blocks; 
     FIG. 11 depicts a diagrammatic representation of assembling a second row of magnet blocks; 
     FIG. 12 depicts a diagrammatic representation of adding a finishing stopper for a 2nd row of magnet blocks; 
     FIG. 13 depicts a diagrammatic representation of a partial assembly before introducing the last row of magnet blocks; 
     FIG. 14 depicts a diagrammatic representation of a final assembly of magnet blocks installed on a yoke plate; 
     FIG. 15 depicts a diagrammatic representation of installation of a pole piece to the final assembly; 
     FIG. 16 depicts a diagrammatic representation of multiple configurations for the rail and glider configuration of an exemplary embodiment; and 
     FIG. 17 depicts an exemplary apparatus for insertion of the block assemblies  60 . 
    
    
     DETAILED DESCRIPTION 
     Disclosed herein is another method and system for assembly of a permanent magnet such as employed in a magnetic field generator for MRI. The method and system employs a series of gliders and rails to guide a plurality of magnet blocks into a desired position on a yoke plate. It should be noted that although the disclosed embodiments are described by way of reference to assembly of a magnetic field generator for MRI applications, it will be appreciated that such references are illustrative only and the disclosed embodiments may be applied to any instance of assembly where there are large interaction forces between the elements to be assembled. Moreover, the references and descriptions herein may apply to many forms of assembly beyond magnets and magnetic blocks including, but not limited to, hybrid permanent/electrical magnet system and the like. 
     Referring first to FIG. 1, a magnetic field generator for MRI  10  as an embodiment of this invention comprises an upper magnet unit  11  and lower magnet unit  12 . Each of the magnet units  11  and  12 , includes, but is not limited to, a yoke plate  14 , a permanent magnet  16 , and a pole piece  18 . Each of the yoke plate  14  has a surface opposed to the other yoke plate, and this surface is provided with a permanent magnet  16 , on which a pole piece  18  is provided. Each of the permanent magnets  16  includes a plurality of magnet blocks  20 . Each of the magnet blocks  20  of the magnet unit  12  is fitted with adjacent ones, with the same magnetic pole facing upward. On the other hand, each of the magnet blocks  20  of the magnet unit  11  is fitted with adjacent ones, with the other magnetic pole facing downward. In other words, the permanent magnet  16  of the magnet unit  12  and the permanent magnet  16  of the magnet unit  11  are faced to each other so that different magnetic poles are opposed to each other. 
     The magnet blocks  20  may be a magnet made from a ternary system compound Nd—Fe—B composed mainly of neodynium (Nd), iron (Fe) and boron (B). Alternatively, part of Nd of the Nd—Fe—B may be replaced by dysprosium (Dy) while part of the Fe may be replaced by cobalt (Co). The Nd—Fe—B is known as a strong neodynium magnetic material with a maximum energy product of over 320 kj/m 3. It should be noted here that a method for making a rare earth magnet is disclosed in detail, for example, in the U.S. Pat. No. 4,770,723. 
     The pair of opposed magnet units  11  and  12 , are supported and magnetically connected by one or more posts  22 , with a selected space in between, for example 40 cm to 60 cm. With such a structure, the magnetic field generator  10  is configured to form a uniform magnetic field in a space between the pair of pole pieces  18 . 
     Now, for the above magnetic field generator  10 , description will be made as to a method for assembling the permanent magnet  16  by placing a plurality of magnet blocks  20  in a generally disc pattern on an upper surface of the yoke plate  14 . Each of the magnet blocks  20  may include a plurality (eight, for example) of magnet members. The magnet member is made by pressing and sintering magnetic powder into a general cube. Then the plurality of magnet members are bonded with each other to form a magnet block  20  (the magnet block  20  is affixed to a glider first, then magnetized as will be described at a later point herein. 
     Referring to FIGS. 2-5, depicted is an exemplary layout of a yoke plate  14  of a magnet unit  11 , and  12 . Fastened to the yoke plate  14 , on one side, is plurality of stoppers or retainers  28 . The plurality of retainers  28  is affixed to the bottom the yoke plate  14  for the upper magnet unit  11  and the top for the lower magnet unit  12 . In their completed layout, the plurality of retainers  28 , form effectively, a perimeter substantially similar to that of the yoke plate  14  and a cavity  24 , which is to be populated with the magnet blocks  20 . A retainer  28  may include, but not be limited to, a block or clamp apparatus. The retainer  28  may, but need not be, constructed of a ferromagnetic material preferably, but not necessarily, the same as the yoke plate  14 . Common non-magnetic materials may include but not be limited to aluminum, stainless steel, plastic G-10, and the like, as well as combinations including at least one of the foregoing. FIG. 3 provides a depiction of an exemplary arrangement for the plurality of retainers  28 . 
     Referring now to FIG. 4, a first set  30  of the plurality of retainers  28  are arranged substantially about a portion of perimeter of the yoke plate  14  of magnet unit ( 11  or  12 ) and fixed to the yoke plate  14  in a manner that facilitates assembly such as with a fastener  26 , keeper, or adhesive. Each of the retainers  28  may be detachably affixed to the yoke plate  14  employing a fastener  26  such as screw, bolt, and the like. The fastener  26  may be constructed of a ferromagnetic material preferably, but not necessarily, the same as the yoke plate  14 . The first set  30  of the plurality of retainers  28  are arranged and fixed to the yoke plate  14  in an approximately semicircular configuration about the perimeter of one side of the yoke plate  14 . The first set  30  of the plurality of retainers  28  forming a substantially semicircular, C, U, or V-shape portion of the cavity  24 , which is to be populated with the magnet blocks  20 . 
     Referring now to FIGS. 5 and 6, an exemplary set of rails  40  is depicted. The set of rails  40  in an exemplary embodiment comprise a series of bars exhibiting a cross section configured to facilitate magnet blocks  20  being slid along their length, yet impede motion laterally. The set of rails  40  are arranged and fixed to the yoke plate  14  in a manner that facilitates assembly such as with a fastener  26 , keeper, or adhesive such as glue or epoxy. The set of rails  40  may be detachably affixed to the yoke plate  14  employing a fastener  26  such as screw, bolt, and the like. The set of rails  40  and fastener  26  (if utilized) may, but need not be, constructed of a ferromagnetic material preferably, but not necessarily, the same as the yoke plate  14 . In the figure, the set of rails  40  are depicted to be substantially bars of substantially trapezoidal cross section with, in this instance the shorter base of the trapezoidal cross section proximate to the yoke plate  14  and the larger base of the trapezoidal cross section of the set of rails  40  directed away from the yoke plate. The set of rails  40  are arranged in the cavity  24  formed by the retainers  28  on the yoke plate  14  extending substantially parallel to the opening of the semicircular, C, U, or V-shape portion of the cavity  24 . The set of rails  40  are arranged substantially parallel to one another, with various lengths extending substantially side to side within the cavity  24  formed by the retainers  28 . Additionally, each of the rails the set of rails  40  are spaced substantially equidistant from one another. FIG.6 depicts the yoke plate  14  with the first set of retainers  28  and rails  40  in the cavity  24  formed therefrom. 
     Turning now to FIG. 7, attention may now be directed to the magnets blocks  20  and gliders  34 . In an exemplary embodiment, the gliders  34  comprise a block of substantially the same footprint as the magnet blocks  20 . The glider  34 , like the other elements of the assembly, may be constructed of a ferromagnetic material, preferably, but not necessarily, the same as the yoke plate  14 . The glider  34  includes a slot  36  on side nearest the yoke plate  14  of matched geometry and configured to mate with the shape of the rails  40 . For example, as depicted in FIG. 7, the rails  40  have a substantially trapezoidal cross section and the slot  36  in the glider  34  is substantially of trapezoidal cross section. It will be appreciated that numerous variations for rail  40  and glider combinations may be conceived. For example, FIG. 16 depicts a few exemplary configurations of rails  40  and gliders  34 . 
     The magnet blocks  20 , while un-magnetized are affixed to the side opposite the slot  36  of the gliders  34 . In an exemplary embodiment, to facilitate assembly, the magnet blocks  20  are affixed to the gliders  34  with adhesive, for example glue or epoxy. It should be appreciated however, that numerous variations for attaching the magnet block  20  to the glider  34  are possible. Advantageously, once the magnet blocks  20  are affixed to the gliders  34 , they may be magnetized as an assembly, thereby forming a block assembly  60  in preparation for assembly of the magnet units  11  and  12 . Another advantage of configuring the magnet block  20  and glider  34  as described above, is that it yields a single or common block assembly  60  for all the magnet blocks  20 , gliders  34 , and the entirety of the magnet units  11  and  12 . 
     Turning now to FIG. 8, a diagram depicting the assembly of the permanent magnet  16  for each of the magnet units  11  and  12  is provided. A first block assembly  62  may now be slid along a first rail  42  substantially until the first block assembly encounters the retainer  28  at the distal end of the first rail  42 . Thereafter, as depicted in FIG. 9, a second block assembly  64  (and subsequent block assemblies, if required) may be slid along the first rail  42  until the area provided by the first rail  42  as a first portion of the cavity  24  is filled with block assemblies  60  to completion. The completed first row  71  of block assemblies  60  along the length of the first rail  42 . Turning to FIG. 10, a retainer  28  is installed to contain and hold the first block assembly  62  and second block assembly  64  (and subsequent block assemblies if any). 
     Turning now to FIG. 11, a diagram depicting the continued assembly of the permanent magnet  16  is provided. A third block assembly  66  may now be slid along a second rail  44  substantially until the third block assembly encounters the retainer  28  at the distal end of the second rail  44 . Thereafter, a fourth block assembly  68  (and subsequent block assemblies, if required) may be slid along the second rail  44  until the area provided by the second rail  44  as a second portion of the cavity  24  is filled with block assemblies  60  to completion. The completed second row  72  of block assemblies  60  along the length of the second rail  44  as depicted in the figure. Turning to FIG. 12, a retainer  28  is thereafter installed to contain and hold the block assemblies  60  (e.g. block assemblies  66  and subsequent block assemblies) installed on the second rail  44 . Once again, the retainer  28  may be installed and attached with a fastener  26  such as a screw or bolt. 
     Turning now to FIG. 13, a diagram depicting the nearly completed assembly of the permanent magnet  16  is provided. As depicted in the figure, a third and fourth row of block assemblies  74  and  76  respectively, have been completed and secured with retainers  28 . Additionally, the figure depicts the assembled and secured eighth, seventh, and sixth rows,  78 ,  77 , and  76  respectively. It should be noted at this time, that it should be evident from the figures that the retainers  28  may overlap more than one rail  40  and therefore more than one row e.g.,  71 - 78  from the outermost to the innermost. Therefore, to facilitate assembly and installation, filling the rails  40  with the components is most easily accomplished by starting with the outermost rails  40  and moving toward the center. It should be appreciated that disclosed herein is just an illustrative assembly sequence. Other sequences are possible, and likely, depending upon the selected orientation for the rails  40  and retainers  28  on the yoke plate  14 . 
     Returning to FIG. 13, additional block assemblies may now be added to fill the remaining rail  40  and complete the fifth row  75  as the final row to be populated. FIG. 14 depicts a permanent magnet  16 , fully populated with block assemblies  60  and secured with retainers  28 . Referring to FIG. 15, to complete the assembly of the magnet units  11  and  12 , the top and bottom pole pieces  18  may be engaged and secured to the permanent magnet  16 . It should be noted at this time, that the pole piece  18  assembly may be at a positioned as described in U.S. Pat. No. 6,336,989 or any position during the entire magnet block assembly  60  insertion process. While the pole piece  18  plays an important role to an easy assembly process, it is not necessarily needed to arrange the pole piece the same way in this disclosure. 
     Turning now to FIG. 17, an exemplary apparatus for insertion of the block assemblies  60  is depicted. Shown in the figure is a magnet block pusher tool  100  configured so that it may be aligned with each of the magnet units  11  and  12  in a manner that facilitates sliding individual block assemblies  60  onto a selected rail  40 . In an exemplary embodiment, magnetized block assemblies  60  are placed on a magnet block pusher  100  for insertion. It will be appreciated that the block assemblies  60  may be configured to readily slid along the rail  40  as part of the assembly and yet fit tightly enough to ensure desirable tolerances for the assembled array of block assemblies of the permanent magnet  16  for each magnet unit  11 ,  12 . The assembly process is therefore relatively easy and conventional. The magnet blocks  20 /block assemblies  60 , rails  40 , and retainers  28 , and other components may be located as accurately as needed with the block assemblies constrained by the rails  40 , gliders  34 , and retainers  28 , to maintain the desired assembly tolerances for the magnet units  11  and  12 . It will be appreciated further that while the layout of the rails in cooperation with the gliders is in illustration a function of a selected footprint for a magnet block  20 , other configurations are readily apparent. The layout could employ a variety of configurations to satisfy the considerations necessary to construct the permanent magnet  16 . 
     Advantageously, because each of the block assemblies  60  is secured by its physical constraints and the rails  40 , gliders  34 , and retainers  28  in the assembly process, no adhesives or bonding is required between the individual block assemblies. This advantage significantly enhances the assembly process as well as facilitates modification, disassembly, system upgrade, rework, or recycle of the magnet members in the future. Other embodiments may be employed perhaps utilizing larger tolerances in the assembly and employing adhesives or epoxies as required to retain the assembly. 
     This feature of an exemplary embodiment eliminating the need for adhesives provides significant advantages in that it facilitates rework of the magnet assembly should it be necessitated. In case of the need for modification, repair, rework, or disassembly, the above disclosed assembly process may be substantially reversed to facilitate the disassembly process since there are no bonding agents or adhesives employed among the magnet members. In other words, should a damaged magnet block  20 /block assembly  60  require removal, because no adhesives are employed between the magnet blocks  20 /block assemblies  60  or between the magnet blocks  20 /block assemblies  60  and the rails  40  or yoke plate  14 , the assembly process may be essentially reversed. The damaged magnet block  20 /block assembly  60  may readily be removed, and replaced with a new one (e.g., raise the pole piece  18 , remove a retainer  28 , remove magnet blocks  60 , and so forth). 
     It will be appreciated that in the above disclosure, numerous examples were provided where an element of the magnet assembly; block retainer  28 , fastener  26 , rail  40 , glider  34  and the like are described as being potentially constructed of a ferromagnetic material preferably, but not necessarily, the same as the yoke plate  14 . There may be instances where non-magnetic construction is beneficial. For example, if non-magnetic material is used, it may be easy to install certain components since there will be no magnetic force interacting upon them. Such a configuration may require the utilization of additional magnet blocks  20  to account for the difference in magnetic materials. Common non-magnetic materials may include but not be limited to aluminum, stainless steel, plastic G-10, and the like, as well as combinations including at least one of the foregoing. 
     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. Moreover, the use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another.