Shimming method for NMR magnet having large magnetic field inhomogeneities
First Claim
1. A method of determining the inhomogeneities in the magnetic field of a magnet which generates a magnetic field in a region containing a sample material, the method comprising:
- A) generating an RF magnetic pulse which is incident on the sample material;
B) generating a phase encoded gradient pulse in a gradient direction which is incident on the sample;
C) detecting a first set of free induction decay signals s(tr)1 corresponding to a plurality of spatial points in the sample at a first time T1 ;
D) detecting a second set of free induction decay signals s(tr)2 corresponding to said plurality of spatial points in the sample at a second time T2 ;
E) transforming s(T1)1 and s(T1)2 from the time domain to image signal sets S(r)1 and S(r)2 in the frequency domain;
F) converting S(r)1 and S(r)2 to polar coordinate signals Pw (r)1 and Pw (r)2, respectively;
G) for each of Pw (r)1 and Pw (r)2, detecting a change in phase at each of a plurality of spatial points relative to a phase of a next previously detected point and, when said change in phase at a particular point exceeds a predetermined threshold indicative of a phase wrap, unwrapping the phase by adjusting the phase at said particular point by 2π
, the phase unwrapping for all the spatial points resulting in phase unwrapped signals P(r)1 and P(r)2 ;
H) subtracting P(r)1 from P(r)2 to obtain a phase difference signal P(r)D ; and
I) acquiring a base frequency distribution, FB, in the gradient direction by calculating FB =(P(r)D /(t2 -t1)) for each of said spatial points.
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Abstract
A method of shimming an NMR magnet uses a plurality of 1D projections through a sample volume to determine the inhomogeneities of the field of the NMR magnet. The frequency distributions obtained are assembled from the phase signals of the various projections. In order to avoid frequency errors due to phase wrapping, the phase of each signal is monitored over time for discontinuities indicative of aliasing. For each phase wrap, the signal is "unwrapped" by adjusting the value of the phase signal by 2π. The same method is used to establish a shim field map for each of the shim coils being used. With one shim coil at a time being driven with a predetermined current, the detection method is repeated to acquire a shim-base frequency map for each shim coil. The base frequency map is then subtracted from the shim-base frequency maps to obtain shim field maps. The proper shim currents are then acquired through matrix operations on the shim field maps and the base frequency map.
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Citations
11 Claims
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1. A method of determining the inhomogeneities in the magnetic field of a magnet which generates a magnetic field in a region containing a sample material, the method comprising:
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A) generating an RF magnetic pulse which is incident on the sample material; B) generating a phase encoded gradient pulse in a gradient direction which is incident on the sample; C) detecting a first set of free induction decay signals s(tr)1 corresponding to a plurality of spatial points in the sample at a first time T1 ; D) detecting a second set of free induction decay signals s(tr)2 corresponding to said plurality of spatial points in the sample at a second time T2 ; E) transforming s(T1)1 and s(T1)2 from the time domain to image signal sets S(r)1 and S(r)2 in the frequency domain; F) converting S(r)1 and S(r)2 to polar coordinate signals Pw (r)1 and Pw (r)2, respectively; G) for each of Pw (r)1 and Pw (r)2, detecting a change in phase at each of a plurality of spatial points relative to a phase of a next previously detected point and, when said change in phase at a particular point exceeds a predetermined threshold indicative of a phase wrap, unwrapping the phase by adjusting the phase at said particular point by 2π
, the phase unwrapping for all the spatial points resulting in phase unwrapped signals P(r)1 and P(r)2 ;H) subtracting P(r)1 from P(r)2 to obtain a phase difference signal P(r)D ; and I) acquiring a base frequency distribution, FB, in the gradient direction by calculating FB =(P(r)D /(t2 -t1)) for each of said spatial points. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8)
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9. A method of determining the frequency signal for a particular spatial point in the sample volume of an NMR apparatus, the method comprising:
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measuring the time-domain free induction decay signal of the spatial point at a plurality times during a decay period; determining the phase of the free induction decay signal of said spatial point as a function of time; and taking the derivative of said phase as a function of time, and averaging said derivative over a predetermined time period. - View Dependent Claims (10, 11)
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Specification