Method and apparatus for producing homogeneous magnetic fields

Abstract

A method for shimming a magnetic field is disclosed. The method uses a single shim current to contribute to suppression of more than one geometrical component of an inhomogeneity in the magnetic field without changing the geometry of the shim path. Apparatuses to implement the method are also disclosed. In embodiments the apparatuses comprise substantially commonly oriented shim paths.

What is claimed is: 
     
         1 . A method for shimming a magnetic field in which there is an inhomogeneity having more than one geometrical component, the magnetic field being produced by a magnet assembly, the method comprising:
 applying a plurality of coordinately modulated single shim currents along corresponding ones of a plurality of substantially commonly oriented shim paths, a portion of each shim path located within the magnet assembly; and   suppressing the more than one geometrical component of the inhomogeneity in the magnetic field using the plurality of shim currents.   
     
     
         2 . The method according to  claim 1 , wherein the plurality of shim currents are disposed in a common plane 
     
     
         3 . The method according to  claim 1  wherein individual ones of said substantially commonly oriented shim paths:
 (a) are substantially straight; 
 (b) comprise two substantially straight portions separated by an abrupt change of direction; 
 (c) comprise three substantially straight portions separated by abrupt changes of direction; or 
 (d) comprise a region having a zig-zag configuration. 
 
     
     
         4 . The method according to  claim 1 , wherein a sample is inserted into the magnetic field along an axis and wherein the orientation of the shim paths is substantially parallel to said axis. 
     
     
         5 . The method according to  claim 1 , wherein each individual one of the plurality of shim currents flows in a respective one of the plurality of shim paths and wherein the individual ones of the currents are determined by:
 estimating a magnetic field produced by application of a known current to each of the plurality of shim paths;   computing a scalar product of functions representative of the geometrical components of the estimated magnetic field to obtain geometrical component values;   arranging the geometrical component values into a matrix;   determining a pseudoinverse of the matrix to obtain pseudoinverse matrix values; and   choosing the individual ones of the currents according to the pseudoinverse matrix values.   
     
     
         6 . The method according to  claim 1 , wherein said applying comprises coordinately adjusting the magnitudes of said shim currents. 
     
     
         7 . The method according to  claim 1 , wherein a said single shim current induces an image current in a magnetically permeable material in the vicinity of the shim path. 
     
     
         8 . A detector for detecting magnetic resonance in a sample exposed to a main magnetic field, the detector comprising:
 a substantially flat shim panel having two ends; and   a plurality of shim paths having a substantially common orientation extending between the two ends, the shim paths for applying shim currents thereon.   
     
     
         9 . The detector according to  claim 8 , further comprising a longitudinal space having an axis, the longitudinal space for insertion of a sample probe containing the sample along the axis and wherein the orientation is substantially parallel to the axis. 
     
     
         10 . The detector according to  claim 9 , wherein the flat shim panel comprises two flat panels. 
     
     
         11 . The detector according to  claim 10 , further comprising pole pieces on which are mounted the two flat shim panels, the pole pieces extending within the longitudinal space. 
     
     
         12 . The detector according to  claim 11 , wherein one of the shim currents induces an image current within a said pole piece. 
     
     
         13 . The detector according to  claim 8 , wherein at least a subset of the plurality of shim paths are disposed on substantially parallel planes. 
     
     
         14 . The detector according to  claim 8 , further comprising a printed circuit board on which is comprised the flat shim panel. 
     
     
         15 . The detector according to  claim 8  wherein said shim paths either:
 a) comprise substantially straight regions separated by an abrupt change of direction; or 
 b) are substantially parallel over at least a portion of their lengths. 
 
     
     
         16 . A shim panel having first and second substantially distanced ends and a substantially flat portion comprising a plurality of shim paths each substantially extending between said ends, wherein individual ones of the shim paths:
 a) comprise substantially straight regions separated by an abrupt change of direction of the path; or   b) are substantially parallel over at least a portion of their length; or   c) have a substantially common orientation.   
     
     
         17 . The shim panel according to  claim 16  wherein said paths are in a common plane. 
     
     
         18 . The detector according to  claim 16  wherein the detector accepts insertion of a sample along an axis and wherein said ends of said panel are substantially oriented along said axis. 
     
     
         19 . The detector according to  claim 19  wherein the shim panel comprises a printed circuit board.