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EP0219013A2
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Method for homogenizing a static magnetic field over an arbitrary volume
Assignee: General Electric Co
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Filing: 1986-10-03
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Granted: —
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Expires: 2006-10-03
Abstract
Abstract A method for minimizing the inhomogeneity of a static magnetic field, produced by a main magnet, over a selected arbitrary volume, by operation of a plurality N of shimming coils each independently producing an associated shimming magnetic field having an associated inhomogeneity which is a function of spatial location over that same volume, uses the steps of: measuring the magnitude of the main magnet field, in the absence of all shimming fields, at a plurality of locations X 1 upon the surface of an imaginary sphere enclosing the arbitrary volume; measuring the polarity and magnitude of each of the N shimming fields, with respect to the polarity and magnitude of a D.C. current flowing throught the associated shimming coil, at the same plurality of locations X i upon the imaginary sphere surface; determining the weighted mean-square variation of the total field at each of another plurality of points Ywithin the volume of said imaginary sphere; and - (d) selecting the current magnitudes and polarities for each of the N shimming coils to provide each of the N associated shimming fields with magnitude and polarity to minimize the inhomogeneity of the main magnetic field in the arbitrary volume. The set of weighting coefficients may be selected to all be equal, or to emphasize at least one desired region of the arbitrary volume.
Inventors
- 👤 Matthew O'donnell
- 👤 Steven George Karr
- 👤 William Daniel Barber
- 👤 Jish Min Wang
- 👤 William Alan Edelstein
Bibliographic Details
Application Number
—
Jurisdiction
EP
Data Source
google_patents
Last Synchronized
2026-11-08 14:47
CPC Classifications
G; G01; G01N; G01N24/00; G01R; G01R33/00; G01R33/20; G01R33/28; G01R33/38; G01R33/387; G01R33/3875; G01R33/24
Claims Analysis
Total Claims: 15
Claim 1
Independent Claim
215 words
A method for minimizing the inhomogeneity of a static magnetic field, produced by a main magnet means, over a selected arbitrary volume, by operation of a plurality N of shimming coil means each independently producing an associated shimming magnetic field over that same volume, comprising the steps of: (a) measuring the magnitude of the main magnet static field, in the absence of all shimming fields, at a plurality of locations X i upon the surface of an imaginary sphere defined to enclose the arbitrary volume; (b) measuring the magnitude and polarity of each of the N shimming fields, with respect to the magnitude and polarity of a DC current flowing through the associated shimming coil means, at the same plurality of locations X i upon the imaginary sphere surface; (c) determining the weighted mean-square error E of the total field, to the homogeneous field, at each of another plurality of points Y ; within the volume of said imaginary sphere; and (d) selecting the magnitude and polarity of each current flowing through an associated one of the N shimming coils means to provide each of the N associated shimming fields with a magnitude and a polarity acting to substantially minimize the error E and the inhomogeneity of the main magnet field in the arbitrary volume.
Claim 2
Independent Claim
215 words
A method for minimizing the inhomogeneity of a static magnetic field, produced by a main magnet means, over a selected arbitrary volume, by operation of a plurality N of shimming coil means each independently producing an associated shimming magnetic field over that same volume, comprising the steps of: (a) measuring the magnitude of the main magnet static field, in the absence of all shimming fields, at a plurality of locations X i upon the surface of an imaginary sphere defined to enclose the arbitrary volume; (b) measuring the magnitude and polarity of each of the N shimming fields, with respect to the magnitude and polarity of a DC current flowing through the associated shimming coil means, at the same plurality of locations X i upon the imaginary sphere surface; (c) determining the weighted mean-square error E of the total field, to the homogeneous field, at each of another plurality of points Y ; within the volume of said imaginary sphere; and (d) selecting the magnitude and polarity of each current flowing through an associated one of the N shimming coils means to provide each of the N associated shimming fields with a magnitude and a polarity acting to substantially minimize the error E and the inhomogeneity of the main magnet field in the arbitrary volume.
Claim 3
Dependent Claim
32 words
The method of claim 1, wherein step (c) includes the step of selecting all of the weighting coefficients, utilized in determining the weighted mean-square error of the total field, to be equal.
Claim 4
Dependent Claim
32 words
The method of claim 1, wherein step (a) further comprises the step of selecting the arbitrary volume to be a cylindrical volume having a preselected radius R' and a preselected length L'.
Claim 5
Dependent Claim
25 words
The method of claim 3, wherein step (a) further comprises the step of selecting the imaginary sphere to have a diameter D greater than 2R'.
Claim 6
Dependent Claim
43 words
The method of claim 1, wherein step (b) further comprises the step of locating each point X i , where X i =(η, θ i , Φ i ), at a zero of an associated Legendre function + 1 (cos θ i ).
Claim 7
Dependent Claim
33 words
The method of claim 5, wherein step (b) further comprises the step of setting the associated Legendre function indices m and n to retain all non-negligible components produced by the N shimming coils.
Claim 8
Dependent Claim
30 words
The method of claim 6, wherein step (b) includes the step of locating each point X i at a zero of the associated Legendre function +1 (cos θ i ).
Claim 9
Dependent Claim
39 words
The method of claim 1, wherein step (b) includes the step of characterizing each shimming field B j , where 1 ≦ j ≦ n, at a plurality of magnitudes of the associated shimming coil current I j .
Claim 10
Dependent Claim
46 words
The method of claim 8, wherein step (b) further comprises the step of locating each point X i , where X i =(r i , θ i , Φ i ), at a zero of an associated Legendre function P Q +1 (cos θ i ).
Claim 11
Dependent Claim
33 words
The method of claim 9, wherein step (b) further comprises the step of setting the associated Legendre function indices m and n to retain all non-negligible components produced by the N shimming coils.
Claim 12
Dependent Claim
30 words
The method of claim 10, wherein step (b) includes the step of locating each point X i at a zero of the associated Legendre function +1 (cos θ i ).
Claim 13
Dependent Claim
28 words
The method of claim 1, wherein step (d) includes the step of solving N equations, by matrix inversion, to determine a vector of the required N shimming-field-forming currents.
Claim 14
Dependent Claim
84 words
The method of claim 12, further comprising the steps of: determining the shimming currents for minimization of errors in each of a plurality of different arbitrary volumes; storing the shimming current vectors for each of the plurality of arbitrary volumes; retrieving the shimming current vectors for a selected one of the plurality of arbitrary volumes; and causing each of the shimming currents to assume the value thereof in the seiected matrix to provide the minimized inhomogeneity main magnet field in the associated selected volume.
Claim 15
Dependent Claim
63 words
The method of claim 13, further comprising the step of providing automatic means, normally associated with the main magnet and the plurality of shimming coil means in a system, for automatically establishing the required magnitude and polarity of each of the N shimming current flows to substantially homogenize the main magnet field for any selected one of the plurality of different arbitrary volumes.
References Cited (Backward Citations)
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