Patent Publication Number: US-8108987-B2

Title: Method of manufacturing a pole face for a permanent magnet MRI system with laminated structure

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of application Ser. No. 10/248,170 now U.S. Pat. No. 6,825,666, entitled “POLE FACE FOR PERMANENT MAGNET MRI WITH LAMINATED STRUCTURE”, filed on Dec. 23, 2002. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to magnetic resonance imaging (MRI) devices, and more particularly to MRI devices with a removable keeper plate. 
     MRI devices are widely used in the medical community as a diagnostic tool for imaging items such as tissue and bone structures. As described, for example, in U.S. Pat. No. 5,680,086 (which is incorporated by reference herein in its entirety), a conventional MRI device may include opposing pole pieces which define between them an imaging volume for the item to be imaged, the pole pieces having pole plates which are fabricated from wound high permeability soft magnetic material, and/or laminated members. One conventional MRI manufacturing technique is described in European Patent Application EP 0 978 727 A2 (“EP &#39;727” hereafter), filed on Jul. 22, 1999, which is incorporated by reference herein in its entirety. 
     In conventional MRI device manufacturing techniques, a yoke is assembled between two ends of the MRI device, and two pole pieces are positioned thereon with a gap provided underneath the pole piece at each end for the MRI magnet. The MRI magnet is then formed by pushing individual pre-magnetized permanent magnet blocks in place under the pole pieces on each end of the MRI device. Once all of the pre-magnetized permanent magnet blocks are positioned and secured in place, the pole pieces are lowered to their ultimate position and secured in place. 
     An MRI device manufactured by the aforementioned manufacturing technique, however, suffers from inclusion of a backing plate used to reinforce the pole pieces during the manufacturing process. Specifically, in the course of pushing the pre-magnetized permanent magnet blocks into place, the pole pieces must endure large bending forces due to the moving pre-magnetized permanent magnet blocks. A backing plate is used to overcome the bending forces by stiffening the pole pieces. The backing plate, however, affects the magnetic field generated by the permanent magnet block array. In highly precise MRI devices, this can lead to a degradation in the detection properties of the MRI device. 
     Thus, a need exists for a method of manufacturing an MRI device free of a backing plate in a pole face of the pole piece, and/or a method of manufacturing an MRI device with the pole pieces being subjected to a weaker and/or channeled magnetic fields during assembly. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is directed at reducing or eliminating one or more of the problems set forth above, and other problems found within the prior art. 
     According to one aspect of the present invention, a method of manufacturing a magnetic resonance imaging (MRI) device is provided comprising the steps of providing at least one magnet positioned between a keeper device and a yoke, the keeper device being positioned at a pole region of the at least one magnet, positioning at least one pole device at the pole region of the at least one magnet, and removing the keeper device from the pole region to allow the at least one pole device to be positioned at the pole region of the at least one magnet. 
     According to another aspect of the present invention, a magnetic resonance imaging (MRI) device including a yoke and at least one magnet aligned with the yoke is provided, the at least one magnet generating a magnetic field within the MRI device, the MRI device comprising at least one pole device for shaping the magnetic field generated by the at least one magnet, the at least one pole device being positioned at a pole region of the at least one magnet and being free of a solid metal backing plate in a pole face of the at least one pole device. 
     According to another aspect of the present invention, a magnetic resonance imaging (MRI) device is provided comprising means for generating a magnetic field, means for shaping the magnetic field, and means for positioning the means for generating a magnetic field at a pole region of the means for shaping the magnetic field. 
     According to another aspect of the present invention, a method of reconfiguring a magnetic resonance imaging (MRI) device is provided comprising the steps of positioning a keeper device at a pole region of at least one magnet, moving the at least one magnet from an initial position to a reconfigured position, and removing the keeper device from the pole region of the at least one magnet to allow at least one pole device to be positioned at the pole region of the at least one magnet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a MRI device undergoing an assembly step according to a preferred embodiment of the present invention. 
         FIG. 2  is a block diagram of the MRI device of  FIG. 1  undergoing a different assembly step according to a preferred embodiment of the present invention. 
         FIG. 3  is a block diagram of an assembled MRI device according to a preferred embodiment of the present invention. 
         FIG. 4  is a flow chart of a method of manufacturing a MRI device according to a preferred embodiment of the present invention. 
         FIG. 5  is a flow chart of a method of reconfiguring a MRI device according to a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to presently preferred embodiments of the present invention. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
     A MRI device  100  undergoing assembly at one end thereof according to a first embodiment of the present invention is shown in the block diagram of  FIG. 1 . For purposes of illustration only, the following description and the accompanying Figures only describe the manufacture of one end of the MRI device  100 . It should be appreciated, however, that the opposite end of the MRI device  100  can be manufactured from the same (or similar) technique, simultaneously with, before, or after the end depicted is assembled. 
     The MRI device  100  includes an iron yoke  110  and a keeper plate  150 . Preferably, the iron yoke  110  is first assembled on the MRI device  100 , and then the keeper plate  150  is temporarily positioned thereon. Once the keeper plate  150  is secured in place, a plurality of permanent magnet blocks  120  are slid into place beneath the keeper plate  150  (collectively the assembled “MRI magnet” 125 in  FIG. 2 ) as indicated by the arrows pointing from right to left. The assembled MRI magnet  125  can be made of any one of a number of conventional magnetic materials, and formed in a conventional manufacturing process as is well known in the art (see, for example, U.S. Pat. No. 4,770,723, which is incorporated herein by reference in its entirety). The assembled MRI magnet  125  is arranged on (or near) iron yoke  110  in such a way as to generate a magnetic field within the MRI device  100  for imaging an item. One exemplary arrangement is described in U.S. application Ser. No. 09/824,245 filed on Apr. 3, 2001, which is incorporated by reference herein in its entirety. 
     As shown in  FIG. 2 , once the MRI magnet  125  is assembled, the keeper plate  150  is “swapped” with pole face laminate  130 . The keeper plate  150  is adapted to channel at least a portion of the magnetic field generated by the assembled MRI magnet  125  while the pole face laminate  130  is positioned. In  FIGS. 1 and 2 , the keeper plate  150  is shown positioned at the pole regions near the sides of the assembled MRI magnet  125 . Hence, the pole face laminate  130  is subjected to a weaker magnetic field, which makes it easier to properly align and position the pole face laminate  130 , and also eliminates the need for a solid metal backing plate used to position and protect (e.g., via sliding) the pole face laminate  130  during assembly. Thus, in the present invention, the pole face laminate  130  can be used without a backing plate. Alternatively and less preferably, a solid metal backing plate may be used in conjunction with the keeper plate  150 . 
     It should be appreciated that the pole face laminate  130  is positioned in such a way as to shape and focus the magnetic field generated by the assembled MRI magnet  125  in the item to be imaged (e.g., by drawing magnetic flux generated by the assembled MRI magnet  125 ). Preferably, the pole face laminate  130  is thus positioned between the assembled MRI magnet  125  and the “hypothetical” item to be imaged. The pole face laminate  130  can be made as described, for example, in U.S. application Ser. No. 09/198,507 filed on Nov. 24, 1998 which is incorporated by reference herein in its entirety. 
     As noted above, the keeper plate  150  is preferably removable, such that the keeper plate  150  can be gradually removed during simultaneous positioning of the pole face laminate  130 . Swapping the keeper plate  150  with the pole face laminate  130  provides for easier removal of the keeper plate  150  and placement of the pole face laminate  130 , as the pole face laminate  130  and the keeper plate  150  are only partially subjected to the magnetic field generated by the assembled MRI magnet  125  by sharing the magnetic flux load with the other. An MRI device  100  manufactured by the aforementioned process is shown in the block diagram of  FIG. 3 . As shown, the MRI device  100  includes yoke  110 , two pole face laminates  130 , and two assembled MRI magnets  125 . The MRI device  100  is free, however, from a backing plate attached to each of the pole face laminates  130 . By eliminating the backing plate, the cost and complexity of the MRI device  100  and machinery necessary to assemble it can be reduced. Furthermore, elimination of the backing plate eliminates any magnetic interference previously caused by the backing plate, resulting in a more uniform (and predictable) magnetic field for imaging the item to be imaged. 
     A method of manufacturing an MRI device  100  according to an embodiment of the present invention will now be described in reference to the flowchart of  FIG. 4 . It should be appreciated that the order of the method steps may vary and/or be combined, as would be readily apparent to one of ordinary skill in the art after reading this disclosure. Furthermore, additional method steps may be provided as needed, such as cleaning steps, gluing steps, etc. In other words, the following description is not an exhaustive list of every method step or order thereof within the scope of the present invention. 
     In step  410 , an iron yoke is positioned in an MRI device. A magnetic keeper plate is then positioned substantially above the iron yoke in step  420 , providing a gap between the magnetic keeper plate and the iron yoke for a MRI magnet. A plurality of permanent magnet blocks (collectively the assembled “MRI magnet”) are then arranged in the gap between the magnetic keeper plate and the iron yoke in step  430  in such a way as to generate a magnetic field in an item to be imaged. The permanent magnet blocks may be formed in a conventional manner as previously described. Preferably, the magnetic keeper plate is positioned in step  420  prior to assembling the MRI magnet on the iron yoke in step  430  in order to channel the magnetic field generated by the MRI magnet as it is being assembled. 
     After the MRI magnet is assembled on the iron yoke in step  430 , the keeper plate will be positioned at the pole region of the magnet. The pole face laminates are then positioned in step  440 . Preferably, the pole face laminates are positioned in step  440  simultaneously while removing the keeper plate in step  450  (e.g., by sliding the keeper plate out of the device), thereby “swapping” the pole face laminates for the keeper plate. This allows the pole face laminates to be easily positioned due to the magnetic field being partially channeled by the keeper plate as it is removed. Once the pole face laminates are positioned at the pole regions in step  440 , the keeper plate is then completely removed from the device in step  450 , and can be used to assemble another MRI device (if desired). 
     One advantage of the aforementioned removable keeper plate is the ability to reconfigure an MRI device previously assembled using the aforementioned manufacturing process (see  FIG. 5 ). To reconfigure the MRI device, the keeper plate is moved back into a pole region position in step  510 . Preferably, the keeper plate is moved back into position in step  510  while simultaneously removing the pole face laminates in step  520 . The pole face laminates can then be exchanged for new or reconfigured pole face laminates, the MRI magnet can be adjusted (e.g., by adding, removing, or relocating permanent magnet blocks), etc. and then repositioned in step  530 . The keeper plate is then removed in step  540 , preferably while simultaneously performing step  530 . Hence, the present invention provides for reconfigurable MRI devices, which could not be easily performed with conventional MRI devices. 
     The foregoing description of preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents.