Magnetic assembly and method for defining a magnetic field for an imaging volume

Abstract

Disclosed herein is a magnet assembly that includes at least two magnets arranged in a fixed spaced relationship with one another thereby to define a space between the magnets that encompasses an imaging volume. Each of the magnets produces a variety of magnetic field strengths across inward-facing surfaces thereof that, in combination, produce an acceptably homogeneous magnetic field in the imaging volume. Also disclosed is a method of defining a magnetic field for an imaging volume. The method comprises generating an initial model of a magnet assembly; estimating a magnetic field for the imaging volume based on the model; calculating deviation between the estimated magnetic field and a target magnetic field for the imaging volume; and updating the model to reduce the deviation by modifying the magnet assembly to produce a variety of magnetic field strengths that, in combination, produce substantially the target magnetic field in the imaging volume.

1 . A magnet assembly comprising:
 at least two magnets arranged in a fixed spaced relationship with one another thereby to define a space therebetween, each of the magnets producing a variety of magnetic field strengths across inward-facing surfaces thereof that, in combination, produce a target magnetic field in a target volume.   
     
     
         2 . The magnet assembly of  claim 1 , wherein the target magnetic field is a desired homogeneous magnetic field, preferably comprising about 10 ppm or less of magnetic field inhomogeneity. 
     
     
         3 . (canceled) 
     
     
         4 . The magnet assembly of  claim 1 , wherein each magnet is shaped to have a respective variety of thicknesses of magnetic material thereby to produce the variety of magnetic field strengths. 
     
     
         5 . The magnet assembly of  claim 1 , wherein each magnet comprises coil magnets, each coil magnet having respective properties thereby to produce the variety of magnetic field strengths, wherein the properties preferably comprise at least one of: number of coil turns, coil wire gauge, coil shape, coil position, and coil current. 
     
     
         6 . (canceled) 
     
     
         7 . A method of defining a magnetic field for a target volume, the method comprising:
 generating an initial model of a magnet assembly;   estimating a magnetic field for the target volume based on the model;   calculating deviation between the estimated magnetic field and a target magnetic field for the target volume; and   updating the model to reduce the deviation by modifying the magnet assembly to produce a variety of magnetic field strengths that, in combination, produce substantially the target magnetic field in the imaging volume.   
     
     
         8 - 9 . (canceled) 
     
     
         10 . The method of  claim 7 , wherein the magnet assembly comprises at least two magnets arranged in a fixed spaced relationship with one another thereby to define a space between the at least two magnets. 
     
     
         11 . The method of  claim 10 , wherein the at least two magnets are maintained in the fixed spaced relationship by a yoke assembly and the yoke assembly is part of the initial model. 
     
     
         12 - 13 . (canceled) 
     
     
         14 . The method of  claim 10 , wherein the magnets each comprise an inward-facing surface adjacent the space between the two magnets, and the updating comprises modifying one or more parameters representing the surface geometry of one or both of the inward-facing surfaces. 
     
     
         15 . The method of  claim 14 , wherein variable parameters representing the surface geometry of one or both of the inward-facing surfaces comprise positions along a longitudinal axis of the magnet assembly of respective points on the inward-facing surfaces. 
     
     
         16 . The method of  claim 14 , wherein the variable parameters are modified to modify the surface geometry axisymmetrically. 
     
     
         17 . The method of  claim 14 , wherein the variable parameters are modified to modify the surface geometry non-axisymmetrically. 
     
     
         18 . The method of  claim 7 , wherein the initial model is based on parameters specifying the magnet assembly and the updating comprises modifying one or more parameters, wherein the parameters preferably comprise at least one of: magnet shape, magnet dimensions, magnet material, and a range of permissible magnet assembly sizes. 
     
     
         19 . The method of  claim 18 , wherein at least the magnet shape and magnet dimensions are modifiable during the modifying. 
     
     
         20 . The method of  claim 18 , wherein at least the range of permissible magnet assembly sizes remains constant during the modifying. 
     
     
         21 . The method of claim  13 , wherein in the initial model the parameters specify that the inward-facing surfaces of the magnets are planar, and during the modifying the parameters are modified to specify that at least one of the inward-facing surfaces of the magnets is not planar. 
     
     
         22 . The method of  claim 7 , wherein the target magnetic field is an acceptably homogeneous magnetic field, and the method further comprises:
 generating an updated model incorporating, in addition to the magnet assembly, one or more objects capable of disturbing the magnetic field in the imaging volume in a manner that would cause the magnetic field in the imaging volume to be unacceptably inhomogeneous; and   performing the estimating, calculating and updating based on the updated model.   
     
     
         23 . The method of  claim 7 , wherein the target magnetic field is an acceptably homogeneous magnetic field, and the method further comprises comprising:
 generating an updated model incorporating, in addition to the magnet assembly, one or more devices capable of disturbing, and/or being disturbed by, the magnetic field in the imaging volume in a manner that would cause the one or more devices to operate unacceptably and/or cause the magnetic field in the imaging volume to be unacceptably inhomogeneous;   modifying the target magnetic field based on the updated model to reduce interference of the magnetic field with the one or more devices; and   performing the estimating, calculating and updating based on the updated model.   
     
     
         24 . The method of  claim 23 , wherein modifying the imaging volume comprises modifying the shape of the imaging volume. 
     
     
         25 . The method of  claim 23 , wherein modifying the target magnetic field comprises modifying an acceptability threshold of inhomogeneity. 
     
     
         26 - 33 . (canceled) 
     
     
         34 . The method of  claim 7 , wherein the estimating, calculating and updating are performed iteratively either a threshold number of times, and/or iteratively until the magnitude of the deviation falls below a threshold level, and/or iteratively until the magnitude of the deviation fails to change by more than a threshold amount between successive iterations. 
     
     
         35 - 38 . (canceled) 
     
     
         39 . The magnetic assembly of  claim 1 , wherein the target magnetic field is a desired non-homogeneous magnetic field, having a magnetic field gradient with a predefined slope and direction. 
     
     
         40 - 41 . (canceled) 
     
     
         42 . The magnet assembly of  claim 1 , wherein the at least two magnets are dissimilar. 
     
     
         43 - 49 . (canceled) 
     
     
         50 . The method of  claim 10 , wherein the at least two magnets in the initial model are alike and the at least two magnets in the updated model are alike. 
     
     
         51 . (canceled) 
     
     
         52 . The method of  claim 10 , wherein the at least two magnets in the initial model are alike and the at least two magnets in the updated model are dissimilar. 
     
     
         53 - 57 . (canceled) 
     
     
         58 . A non-transitory computer readable medium embodying a computer program for defining a magnetic field for a target volume, the computer program comprising:
 computer program code generating an initial model of a magnet assembly;   computer program code estimating a magnetic field for the target volume based on the model;   computer program code calculating deviation between the estimated magnetic field and a target magnetic field for the target volume; and   computer program code updating the model to reduce the deviation by modifying the magnet assembly to produce a variety of magnetic field strengths that, in combination, produce substantially the target magnetic field in the target volume.