Method for automatically shimming a high resolution NMR magnet
First Claim
1. In an NMR spectrometer having coils for generating a main magnetic field having a direction over a sample volume, means for passing field current through the coils, a gradient coil, means for passing a gradient current through the gradient coil to generate a magnetic field gradient along a predetermined direction within the sample volume, a method for correcting the main magnetic field to a spatially uniform field along the predetermined direction over the sample volume by selectively adjusting the magnitude of the field current, the method comprising the steps of:
- A. inserting a material sample containing NMR-active nuclei into the sample volume;
B. setting the gradient current to a predetermined value;
C. performing an NMR experiment to acquire a frequency spectrum data set of the sample while the gradient current remains at the predetermined value set in step B;
D. adjusting the gradient current to a new value to change the gradient field;
E. repeating steps C and D a predetermined number of times to acquire multiple frequency spectrum data sets with different times and gradient field magnitudes;
F. manipulating the frequency spectrum data sets acquired in step E to produce a new data set relating field strength to spatial position; and
G. using the new data set produced in step F to adjust the magnitude of the field current.
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Abstract
A plurality of measurements are made with a probe containing a material sample with a known frequency spectrum. The probe is positioned in one spot in the magnet bore in the presence of a field gradient along a predetermined axis. Prior to each measurement, the gradient strength is increased by a predetermined constant amount. The resulting measurement values are mathematically manipulated to yield a set of values which can be used to set the shim coil currents based on known equations for the field generated by each shim coil. The method can be used with a material sample with a simple NMR spectrum or a complex NMR spectrum. If a sample with a complex NMR spectrum is employed in the measurement, the spectral complexity can be removed by deconvolution during the manipulation of the measurement values.
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Citations
25 Claims
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1. In an NMR spectrometer having coils for generating a main magnetic field having a direction over a sample volume, means for passing field current through the coils, a gradient coil, means for passing a gradient current through the gradient coil to generate a magnetic field gradient along a predetermined direction within the sample volume, a method for correcting the main magnetic field to a spatially uniform field along the predetermined direction over the sample volume by selectively adjusting the magnitude of the field current, the method comprising the steps of:
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A. inserting a material sample containing NMR-active nuclei into the sample volume; B. setting the gradient current to a predetermined value; C. performing an NMR experiment to acquire a frequency spectrum data set of the sample while the gradient current remains at the predetermined value set in step B; D. adjusting the gradient current to a new value to change the gradient field; E. repeating steps C and D a predetermined number of times to acquire multiple frequency spectrum data sets with different times and gradient field magnitudes; F. manipulating the frequency spectrum data sets acquired in step E to produce a new data set relating field strength to spatial position; and G. using the new data set produced in step F to adjust the magnitude of the field current. - View Dependent Claims (2, 3, 4, 5, 6)
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7. In an NMR spectrometer having coils for generating a main magnetic field having a direction over a sample volume, a shim coil for generating a shim field within the sample volume, and means for passing a shim current with a selected magnitude through the shim coil, a method for correcting the main magnetic field to a spatially uniform field along a predetermined direction over the sample volume by selectively adjusting the magnitude of the shim current, the method comprising the steps of:
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A. inserting a material sample containing NMR-active nuclei into the sample volume; B. setting the shim current to a predetermined value; C. performing an NMR experiment to acquire a frequency spectrum data set of the sample; D. adjusting the shim current to a new value; E. repeating steps C and D a predetermined number of times to acquire multiple frequency spectrum data sets; F. manipulating the frequency spectrum data sets acquired in step E to produce a new data set relating field strength to spatial position; G. using the new data set produced in step F to adjust the magnitude of the shim coil current. - View Dependent Claims (8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19)
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20. In an NMR spectrometer having coils for generating a main magnetic field having a z-axis direction over a sample volume, a z-shim coil for generating an on-axis shim field within the sample volume, and means for passing a first shim current with a selected magnitude through the z-shim coil, a plurality of additional shim coils, means for passing a plurality of second shim currents through the plurality of additional shim coils, a method for correcting the main magnetic field to a spatially uniform field along the z-axis direction over the sample volume by selectively adjusting the magnitude of the first shim current and the plurality of second shim currents, the method comprising the steps of:
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A. inserting a material sample containing NMR-active nuclei into the sample volume; B. setting the first shim current to a predetermined value; C. exciting the sample nuclei at an NMR resonant frequency and detecting a free induction decay signal to acquire a frequency spectrum data set of the sample; D. incrementing the first shim current to a new value; E. repeating steps C and D a predetermined number of times to acquire multiple frequency spectrum data sets; F. Fast Fourier transform the multiple frequency data sets acquired in step E to generate first transformed data; G. interpolating the first transformed data to generate field map data representing magnetic field strength versus position; H. selecting data points in the field map data generated in step G for each position which have the largest field magnitude; I. fitting a curve to the data points selected in step H to obtain coefficients describing the shim coil currents necessary to correct inhomogeneities in the main magnetic field; and J. using the coefficients obtained in step I to adjust the magnitude of the first shim coil current and the magnitude of the plurality of second shim coil currents. - View Dependent Claims (21, 22, 23, 24, 25)
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Specification