Abstract:
A method for positioning permanent magnetic blocks includes providing a plurality of magnetized blocks, positioning the magnetized blocks on a yoke in a row by applying a mechanical force at a first end of the row with a first clamping member and at a second end of the row with a second clamping member, and repositioning the blocks by reducing the mechanical force at the first end by moving the first clamping member away from the first end, and moving the second clamping member toward the second end.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to magnetic resonance image (MRI) imaging, and, more particularly, to methods and apparatus for assembling magnetized permanent magnetic blocks used for an MRI magnetic field generator.  
           [0002]    A high uniform magnetic field is useful for using magnetic resonance image (MRI) and nuclear magnetic resonance (NMR) systems as a medical device or a chemical/biological device. Popular and low maintenance cost MRI systems currently available use a permanent magnet system that creates a middle range uniform field (0.2 to 0.5 Tesla) in a pre-determined space (imaging volume). A permanent magnet system usually uses multiple permanent magnet blocks such as NdFeB to form a single magnetic object and to achieve desire high uniform magnetic field in the imaging volume.  
           [0003]    For a magnetic field generator for an MRI that uses permanent magnets, the magnets used in such an apparatus are often formulated from a plurality of magnetized blocks. However, it is difficult to place un-magnetized material blocks on a yoke plate first and then magnetize each block. Therefore, in actual manufacturing, the blocks are fabricated and then magnetized before being placed on the yoke. The magnetized blocks are then arranged on the yoke plate so that each of the magnet blocks has a same magnetic pole facing upward. A pole piece is then placed on the top of the magnetized blocks.  
           [0004]    However, it is difficult to position the permanent magnetic blocks on the yoke for an MRI system with high uniformity and accuracy due to the nature of very large magnetic force acting upon with each other and with other magnetic objects (such as the pole piece). Obviously, the magnetic field uniformity for the assembled magnetic system is very much depended on the positioning of the assembled permanent magnetic blocks. Hence, the positioning of the magnetic block dimensions is important to the quality of the magnetic field, and thus to the quality of a magnetic field generator for an MRI device. Due to the nature of multiple factors, such as material physical tolerance, magnetic interaction force, assembly tolerance, process variation, etc., the assembled magnetic blocks will not be at perfect location as intended. Frequently, the location of the magnetic blocks needs to be adjusted to make better alignment in order to achieve a desired magnetic field.  
         BRIEF DESCRIPTION OF THE INVENTION  
         [0005]    In one aspect, a method for positioning permanent magnetic blocks is provided. The method includes providing a plurality of magnetized blocks, positioning the magnetized blocks on a yoke in a row by applying a mechanical force at a first end of the row with a first clamping member and at a second end of the row with a second clamping member, and repositioning the blocks by reducing the mechanical force at the first end by moving the first clamping member away from the first end, and moving the second clamping member toward the second end.  
           [0006]    In another aspect, a magnetic block positioning apparatus is provided. The apparatus includes a yoke having a first end and a second end distal from the first end, a first clamping member support attached to the yoke at the first end, a first clamping member movably coupled to the first clamping member support, a second clamping member support attached to said yoke at said second end, and a second clamping member movably coupled to said second clamping member support.  
           [0007]    In yet another aspect, a magnetic block positioning apparatus includes a plate including a linearly extending middle section having a first end and a second end, a first end section extending obliquely from the first end, and a second end section extending obliquely from the second end, a first clamping member holder extending at least over the middle section, and a second clamping member holder mounted to the first end section, the second clamping member holder nonlinear with the first clamping member holder.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is a block schematic diagram of an imaging system.  
         [0009]    [0009]FIG. 2 is a block schematic diagram of an imaging system.  
         [0010]    [0010]FIG. 3 illustrates an overview of a positioning apparatus.  
         [0011]    [0011]FIG. 4 is a perspective view of one embodiment of the positioning apparatus shown in FIG. 3.  
         [0012]    [0012]FIG. 5 is a perspective view of one embodiment of the positioning apparatus shown in FIG. 3.  
         [0013]    [0013]FIG. 6 is a top view of one embodiment of the positioning apparatus shown in FIG. 3.  
         [0014]    [0014]FIG. 7 illustrates one embodiment of the positioning apparatus shown in FIG. 3 in use.  
         [0015]    [0015]FIG. 8 illustrates one embodiment of the positioning apparatus shown in FIG. 3 in use. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    As used herein, an element or step recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural said elements or steps, unless such exclusion is explicitly recited. Furthermore, references to “one embodiment” of the present invention are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features.  
         [0017]    [0017]FIG. 1 is a block schematic diagram of an imaging system  10  such as an MRI system  10  including two plate yokes  12  and a plurality of columnar yokes  14  extending between plate yokes  12 . Alternatively, an MRI system  10  with a single C shaped yoke  16  may be used as shown in FIG. 2. System  10  includes magnets  18  secured to yoke surfaces, pole piece bases  20  and support rings  22  secured to magnets  18  and a pole piece  24  is secured to each pole piece base  20  and support ring  22 . A gap  25  is formed between pole pieces  24 . A body part to be imaged is inserted into gap  25 . MRI system  10  also may contain electronics  26  and a display  28 . Electronics  26  may include a control system such as a computer, a transmitter, a receiver, an imager, and/or a memory.  
         [0018]    [0018]FIG. 3 illustrates an overview of a positioning apparatus  30  including a plurality of movably coupled clamping members  32 . Clamping members  32  are movable to adjust the positions of magnetic blocks  18  (shown in FIG. 1) as explained in greater detail below during fabrication of system  10 .  
         [0019]    [0019]FIG. 4 is a perspective view of one embodiment of positioning apparatus  30  (shown in FIG. 3) including a clamping member support  34  mounted on yoke  12 . Clamping member support  34  includes a plurality of threaded sections  36  to which clamping members  32  are threadably coupled. Clamping members  32  extend in a plane substantially parallel to a plane defined by a surface  38  of yoke  12 . Clamping member supports  34  and clamping members  32  are fabricated from a high-strength non-magnetic material such as 316 stainless steel.  
         [0020]    In use, a first apparatus  30  is attached to one side of yoke  12 , and a second apparatus  30  is attached to an opposite side of yoke  12  such that first and second apparatus  30  are positioned with clamping members  32  of first apparatus  30  substantially aligned with clamping members  32  of second apparatus  30 . A plurality of blocks  18  are clamped forming a row between opposing clamping members  32  as explained in more detail below. To move blocks  18 , a clamping member  32  is loosened to reduce the pressure in a first direction, and an opposing clamping member  32  is tightened to shift blocks  18  in a second direction opposite the first direction. In an exemplary embodiment, and as best shown in FIG. 8, four apparatuses  30  are employed to allow for both row positioning and column positioning.  
         [0021]    [0021]FIG. 5 is a perspective view and FIG. 6 is a top view of one embodiment of positioning apparatus  30 . Clamping support member  34  includes a plate  40  including a linearly extending middle section  42  having a first end  44  and a second end  46 . Plate  40  further includes a first end section  48  extending obliquely from first end  44 , and a second end section  50  extending obliquely from second end  44 . In an exemplary embodiment, end sections  48  and  50  extend at approximately sixty degrees such when four positioning apparatuses  30  are positioned in a square, a hexagon opening is formed. A first clamping member holder  52  extends at least over middle section  42 . In the exemplary embodiment, first clamping member holder  52  extends into both end sections  48  and  50 . A second clamping member holder  54  is mounted to first end section  48 . A third clamping member holder  56  is mounted to second end section  50 . Second and third clamping member holders  54  and  56  are linearly aligned with each other and are both nonlinear with first clamping member holder  52 . Clamping members  32  extend through bores  58  in holders  52 ,  54 , and  56 , and are held in place with a plurality of nuts  60 . Bores  58  may or may not be threaded. Clamping members  32  include a threaded body such that at least a portion of clamping members  32  have threads thereon.  
         [0022]    [0022]FIGS. 7 and 8 illustrate positioning apparatus  30  in use to position blocks  18 . In one embodiment, clamping member supports  34  are first attached to yoke  12  by bolts at a plurality of sides (e.g., front side and back side). A plurality of clamping members  32  are attached at four sides (front side, back side, left side, and right side) in a fashion that tips of clamping members  32  will push against a non-magnetic spacer  62  and causing a force (pressure) then against magnetic blocks  18 .  
         [0023]    After attaching clamping members  32  to clamping member supports  34 , opposite nuts  60  (e.g. front and back or right and left) for the same row or column are tightened to add pressure to eliminate gaps between magnetic blocks  18  until all magnetic blocks  18  are being tightened with a good packing factor in both a front-back and a left-right directions.  
         [0024]    An offset of magnetic blocks  18  from a designed magnet center is measured, and block locations are adjusted by slightly releasing one side clamping member  32 , and tightening an opposite clamping member  32  (to add pressure) to push a block  18  to its desired location. Working from a center row gradually toward both sides (right, left), then working from a center column gradually toward both faces (front, back), the above process is repeated on non-center rows and columns if needed until all blocks  18  are in their desired location. After all blocks  18  are in desired locations, permanent clamps (not shown) are applied to maintain blocks  18  fixedly, and clamping members  32  and clamping member supports  34  are removed.  
         [0025]    The herein described methods and apparatus provide a novel approach to positioning magnetized blocks on a yoke for an MRI magnetic field generator. The herein described methods and apparatus also provide for a portable method and apparatus for positioning magnetized blocks.  
         [0026]    Exemplary embodiments of methods and apparatus for positioning magnetized blocks are described above in detail. The methods and apparatus are not limited to the specific embodiments described herein, but rather, components of each method and apparatus may be utilized independently and separately from other components described herein. In addition, each method and apparatus component can also be used in combination with other components described herein.  
         [0027]    While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.