B0 shimming device for mri

Abstract

A magnetic resonance (MR) apparatus comprises magnet means for generating a main magnetic field in a sample region, encoding means for generating encoding magnetic fields superimposed to the main magnetic field, RF transmitter means for generating MR radiofrequency fields, driver means for operating said encoding means and RF transmitter means to generate superimposed time dependent encoding fields and radiofrequency fields according to an MR sequence for forming images or spectra; and acquisition means for acquiring an MR signal from said object. The magnet means comprise a primary magnetic field source providing a static magnetic field B0 and at least one secondary magnetic field source providing an adjustable magnetic field B′. To provide improved shimming, the secondary magnetic field source comprises at least two spatially distinct portions of a first magnetic material and of a second magnetic material, respectively, said first magnetic material having a first magnetic moment density m1 and said second magnetic material having a second magnetic moment density m2, and means for independently adjusting said second magnetic moment density m2 by variation of an external control parameter.

1 . A magnetic resonance (MR) apparatus comprising:
 a) magnet means for generating a main magnetic field in a sample region;   b) encoding means for generating encoding magnetic fields superimposed to the main magnetic field,   c) RF transmitter means for generating MR radiofrequency fields;   d) driver means for operating said encoding means and RF transmitter means to generate superimposed time dependent encoding fields and radiofrequency fields according to an MR sequence for forming images or spectra; and   e) acquisition means for acquiring an MR signal from said object;   wherein said magnet means comprise a primary magnetic field source providing a static magnetic field B 0  and at least one secondary magnetic field source providing an adjustable magnetic field B′;   wherein said secondary magnetic field source comprises at least two spatially distinct portions of a first magnetic material and of a second magnetic material, respectively, said first magnetic material having a first magnetic moment density m 1  and said second magnetic material having a second magnetic moment density m 2 , and means for independently adjusting said second magnetic moment density m 2  by variation of an external control parameter,   the apparatus further comprising means for determining an instant spatial distribution of the static magnetic field resulting from said static magnetic field B0 and said adjustable magnetic field B′.   
     
     
         2 . The MR apparatus according to  claim 1 , wherein said second magnetic moment density m 2  is adjustable in a range extending from values that are smaller than the first magnetic moment density m 1  to values that are larger than the first magnetic moment density m 1 . 
     
     
         3 . The MR apparatus according to  claim 1 , wherein said external control parameter is selected from the group consisting of temperature, pressure, shear and light illumination prevailing at the secondary magnetic field source and electric current flowing therethrough. 
     
     
         4 . The MR apparatus according to  claim 3 , wherein said external control parameter is the local temperature of said portion of second magnetic material, and wherein said adjusting means are configured to adjust said local temperature in a control temperature range extending from a lowest control temperature T L  to a highest control temperature T H , and wherein said second magnetic material has a Curie temperature T C  lying within said control temperature range. 
     
     
         5 . The MR apparatus according to  claim 3 , wherein said Curie temperature T C  is in the range from 300K to 450K. 
     
     
         6 . The MR apparatus according to  claim 1 , wherein said secondary magnetic field source is formed as an elongated body having a longitudinal axis, and wherein a plurality of spatially distinct portions of said second magnetic material are arranged at distinct positions along said longitudinal axis. 
     
     
         7 . The MR apparatus according to  claim 6 , comprising a plurality of secondary magnetic field sources arranged circumferentially around the sample region. 
     
     
         8 . The MR apparatus according to  claim 1 , wherein each secondary magnetic field source comprises means for determining an instant value of the external control parameter. 
     
     
         9 . The MR apparatus according to  claim 1 , wherein said second magnetic material is formed of an alloy containing Fe, Ni, Ga, Mn, Gd, Sm, Nd, Dy, Eu or Co. 
     
     
         10 . The MR apparatus according to  claim 1 , wherein the adjustable magnetic field B′ of each secondary magnetic field source in an axial direction z of the static magnetic field B 0  can be switched between positive and negative values as a function of the external control parameter. 
     
     
         11 . A method of operating the MR apparatus according to  claim 1 , wherein said external control parameter is adjusted in such manner as to obtain a predetermined main magnetic field B resulting from the superposition of the static magnetic field B 0  and the adjustable magnetic field B′ of each secondary magnetic field source, wherein the adjustment is carried out at the beginning of an MR sequence for forming images or spectra, after a sample or subject has been introduced into the sample region. 
     
     
         12 . The method according to  claim 11 , wherein said second magnetic moment density m 2  is adjustable in a range extending from values that are smaller than the first magnetic moment density m 1  to values that are larger than the first magnetic moment density m 1 . 
     
     
         13 . The MR apparatus according to  claim 3 , wherein said secondary magnetic field source is formed as an elongated body having a longitudinal axis, and wherein a plurality of spatially distinct portions of said second magnetic material are arranged at distinct positions along said longitudinal axis. 
     
     
         14 . The MR apparatus according to  claim 13 , wherein said external control parameter is the local temperature of said portion of second magnetic material, and wherein said adjusting means are configured to adjust said local temperature in a control temperature range extending from a lowest control temperature T L  to a highest control temperature T H , and wherein said second magnetic material has a Curie temperature T C  lying within said control temperature range. 
     
     
         15 . The MR apparatus according to  claim 13 , wherein said Curie temperature T C  is in the range from 300K to 450K. 
     
     
         16 . The MR apparatus according to  claim 13 , wherein said secondary magnetic field source is formed as an elongated body having a longitudinal axis, and wherein a plurality of spatially distinct portions of said second magnetic material are arranged at distinct positions along said longitudinal axis. 
     
     
         17 . The MR apparatus according to  claim 16 , comprising a plurality of secondary magnetic field sources arranged circumferentially around the sample region. 
     
     
         18 . A method of operating the MR apparatus according to  claim 3 , wherein said external control parameter is adjusted in such manner as to obtain a predetermined main magnetic field B resulting from the superposition of the static magnetic field B 0  and the adjustable magnetic field B′ of each secondary magnetic field source, wherein the adjustment is carried out at the beginning of an MR sequence for forming images or spectra, after a sample or subject has been introduced into the sample region. 
     
     
         19 . The method according to  claim 18 , wherein said second magnetic moment density m 2  is adjustable in a range extending from values that are smaller than the first magnetic moment density m 1  to values that are larger than the first magnetic moment density m 1 .