Pole piece for MR imager
This application is a continuation, of application Ser. No. 08/297,981, filed Aug. 30, 1994, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a pole piece design to generate very homogeneous magnetic field even with an asymmetric, open C-arm magnet to be used in medical magnetic resonance imaging equipment or MRI equipment. 
     In magnetic imagers with pole pieces it has been quite expensive to generate homogeneous magnetic flux inside a large enough imaging volume. It has normally required very symmetric structure for the magnet. U.S. Pat. No. 4,672,346 describes this kind of symmetric structures for a permanent magnet imager where the realization of homogeneity requires the movement of very large masses, at least the movement of the whole pole piece. U.S. Pat. No. 5,003,276 describes this kind of symmetric structure wherein a complex mechanism for shimming is done with tens of movable tuning pieces attached to both pole pieces. The inhomogeneity of an asymmetric structure is essentially larger than that of a symmetric structure and no working solution has been presented to achieve the required homogeneity of 20 ppm. 
     SUMMARY OF THE INVENTION 
     It is a characteristic of the invention that the pole piece is formed in such a way that there are annular parts, movable with respect of each other. The inner part of the pole piece can be transferred and tilted in such a way that a homogeneous magnetic field can be achieved in case of an asymmetric structure. The separate structure of the inner part makes lateral movement and tilting quick and exact so that better homogeneity is achieved over traditional solutions. The weight of the inner part can be considerably smaller than the total weight of the pole. This makes possible the optimization of the performance and cost during the selection of materials and the method of production. 
     An advantage of the present invention is that good homogeneity can be achieved effectively and economically. 
     Still other advantages will become apparent to others upon reading and understanding the following Detailed Description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a side view of the MRI equipment with the pole pieces according to this invention. 
     FIG. 1B is a view of the MRI equipment as viewed along cut line A--A in FIG. 1A. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1A shows the equipment for patient imaging, in which the patient 1 is lying between pole pieces 23 (the inner part 24) in the magnetic field produced by current flowing in the coils 2. The necks 4 of the coils connect the pole pieces to the C-shaped body 3 of the magnet. These parts are made of iron and their function is to form the path for the return flux of the magnet. The invention is also suitable for magnet designs based on permanent magnet material. These differ only in the sense that in between body 3 and pole pieces 23 instead of coils 2 and the necks 4 permanent magnet material blocks are used. 
     To maximize the homogeneity of the magnetic field, shim rings 6, 7, 8 are concentrically arranged in an as-symmetric way, on the pole piece. Theoretical field calculations for a symmetric magnet indicates that extremely good, about 20 ppm (parts per million), homogeneity is achievable inside the imaging volume, when a pole piece having shim rings, as shown in FIGS. 1A and 1B, are used. The largest one of these rings 6, the first shim, forms the outer rim. The next one 7 is typically roughly one half radius of the former and the innermost one 8, i.e., center pole, is a bump on the pole. The calculations give numerous optimized results for the dimensions of these rings depending on the ratio of the outer diameter and the distance of the poles, with the optimized ratio being about two (2). In practice, the dimensions of the pole piece should be realized with accuracy of almost one hundredth of a millimeter. This accuracy of pole dimension is possible, however, it is difficult to align the big parts weighing almost one ton with the same accuracy. According to the present invention the pole piece is divided into two or more parts so that the outer part 23 forms a heavy cup inside which there is lighter and adjustable inner pole 24 defining the specific imaging properties. The outer part 23 carries the main part of the flux including the stray flux which is closing to the C-arm. The inner pole 24 can be tilted with screws 12 and can be moved in the direction of its principal plane with the screws 11. There can be more pairs of screws, but the desired effect is achievable with three pair of screws, 11 and 12. Measurements indicate that a sufficient range for adjustment is reached with less than a one millimeter air gap between the outer part and inner part. 
     The shape and the dimensions of the pole are the same as one constructed from a single piece. The ratio of the thickness of the outer part 23 to the inner part 24 is, for example, 2:1 so that the weight of the inner part will be about one-third of the total weight of the pole. The thickness of the inner part, however, can be considerably different, most suitably within the range of 10-40% from the total thickness of the pole. A reserve for half millimeter movement in the pole plane is typically sufficient to cancel the axial uncertainties in mounting the poles. 
     The unsymmetry of the magnetic flux density mused by the C-arm, because of forming a leakage path for the flux, is on the order of 1000 ppm. This, however, was compensated below 100 ppm by tilting the inner part 24 so that the pole gap or the distance to the opposite pole near the C-arm decreased about one millimeter. The homogeneity can then be compensated below a required level by attaching small iron pieces to the circumference of outer part 23. Additional fine tuning is possible by turning the tightening screws 13, used for attaching the inner part, so that the pole plate is stressed to form an arched surface. The number of tightening and adjusting screws is at least three of both, but it can be considerably larger. Other solutions for adjustment can also be applied. According to the invention, the inner part can be divided into two or more inner parts which are movable with respect of each other and are to be locked with screws after adjustment. Further, according to the invention, the one pole piece can be fixed or formed from a single part and the other adjustable. According to another embodiment of the invention the ratio between the performance and costs of the magnet can be optimized by selecting for the material of the inner part of the pole 24 more expensive iron alloy, for example with cobalt, nickel, or silicon alloyed iron in which the hysteresis and eddy current losses due to operation of the gradient coils located near the pole can be reduced. In addition, the inner part can be a laminated structure whose surface is comprised of an electrically weakly conducting ferromagnetic, formed plate 25 (shown In phantom in FIG. 1A). This surface part is preferable to insulate electrically or to form from a strip which is wound into a spiral in order to minimize the eddy currents. 
     The function of the outer cover of the pole by screws 11, as a massive piece, is to form the path for the advantageous flux and the stray flux over the coil and the C-arm. The part of this stray flux exceeds the flux through the inner pole, but the requirement for accuracy is smaller, meaning that the outer pole 23 can be manufactured from cheap iron by machining or economically by casting. 
     According to still another aspect of the invention, the center pole 8 contains an aperture 26a for receiving an inner part 26b. The position of the inner part is adjustable, relative to the aperture, in a direction along the thickness of the inner element 24. Magnetic field homogeneity is affected by adjusting the position of the inner part relative to the aperture. 
     The above invention has been described with reference to the preferred embodiments. Obvious modifications and combinations will occur to others upon reading and understanding the preceding description. It is intended that the invention be construed as including all such modifications, combinations and alterations insofar as they come within the scope of the appended claims or the equivalents hereof.