An analytical approach towards passive ferromagnetic shimming design for a high-resolution NMR magnet (original) (raw)
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A Theoretical Design Approach for Passive Shimming of a Magic-Angle-Spinning NMR Magnet
IEEE Transactions on Applied Superconductivity, 2016
This paper presents a passive shimming design approach for a magic-angle-spinning (MAS) NMR magnet. In order to achieve a 1.5-T magic-angle field in NMR samples, we created two independent orthogonal magnetic vector fields by two separate coils: the dipole and solenoid. These two coils create a combined 1.5-T magnetic field vector directed at the magic angle (54.74 o from the spinning axis). Additionally, the stringent magnetic field homogeneity requirement of the MAS magnet is the same as that of a solenoidal NMR magnet. The challenge for the magic-angle passive shimming design is to correct both the dipole and solenoid magnetic field spherical harmonics with one set of iron pieces, the so-called ferromagnetic shimming. Furthermore, the magnetization of the iron pieces is produced by both the dipole and solenoid coils. In our design approach, a matrix of 2 mm by 5 mm iron pieces with different thicknesses was attached to a thin-walled tube, 90-mm diameter and 40-mm high. Two sets of spherical harmonic coefficients were calculated for both the dipole and solenoid coil windings. By using the multiple-objective linear programming optimization technique and coordinate transformations, we have designed a passive shimming set that can theoretically reduce 22 lower-order spherical harmonics and improve the homogeneity of our MAS NMR magnet.
A New Method for Shimming a Magnetic Field in NMR System
PIERS Online, 2007
A new approach has been presented in the paper to gain a uniform magnetic field in NMR system. First, the adopted shimming piece is modeled as a magnetic dipole moment to calculate its magnetization effect on the background field within region of interest. Then, sequential quadratic programming method is utilized to determine the ideal solution for shimming work. Finally, the ideal solution is discretized, and quantization error control technique is prepared for special cases. This new method helps to reduce the inhomogeneity in the region of interest from 56.8 ppm to 14.21 ppm, within one hour in practical shimming work.
NMR SHIM coil design utilising a rapid spherical harmonic calculation method
The ANZIAM Journal
A rapid spherical harmonic calculation method is used for the design of Nuclear Magnetic Resonance shim coils. The aim is to design each shim such that it generates a field described purely by a single spherical harmonic. By applying simulated annealing techniques, coil arrangements are produced through the optimal positioning of current-carrying circular arc conductors of rectangular cross-section. This involves minimizing the undesirable harmonics in relation to a target harmonic. The design method is flexible enough to be applied for the production of coil arrangements that generate fields consisting significantly of either zonal or tesseral harmonics. Results are presented for several coil designs which generate tesseral harmonics of degree one.
Simulation and optimisation of homogeneous permanent magnet for portable NMR applications
International Journal on Smart Sensing and Intelligent Systems
This paper focuses on the magneto static simulation of magnet arrangement used for portable NMR (Nuclear Magnetic Resonance) apparatus. In NMR experiments, a very homogeneous magnetic field is required in a large sample volume. In our case, NdFeB magnets are used to generate a homogeneous field, with a Halbach arrangement. The homogeneity of the magnetic field B 0 can be improved by dividing a long configuration of magnets into several rings. The size of useful volume is dependent on both the gap between each ring and some others shim magnets. Our aim is to enhance the sensitive volume size while maintain the highest magnetic field. In this goal, we optimized the gap between the two rings and put two shim magnets rings in the bore of the structure. Optimizing the position of the shim magnets allow drastic improvement. The homogeneity of the magnetic field B 0 is optimized with the help of CAD and mathematical software. Our results were confirmed with a Finite Element Method. As a result the method described here achieves a significant improvement of the homogeneity in the particular case of Halbach arrangement.
IEEE Transactions on Magnetics, 2000
This paper presents a new passive shim design method and a novel shimming procedure to correct the magnetic field inhomogeneities generated by C-shape permanent biplanar magnetic resonance imaging magnets. The method expresses the shim distribution as a sum of orthogonal functions multiplied by unknown amplitudes. The oscillating modes of the shim magnetization-thickness function are normalized within a finite disk. By minimizing the shim set weight and constraining the magnetization-thickness function, the method produces a continuous map of the required shim contribution. The map defines the shim shape and a discrete process then determines the regions where no shim contributions are needed. With this methodology, passive shims capable of generating magnetic field harmonics with minimal impurities and ferro-shim pieces can be generated. The paper reports a study of magnetic coupling among the iron pieces and its influence over the magnetic field harmonics of linear and nonlinear iron, and demonstrates that the exclusion of the magnetic coupling in the shimming process produces an unacceptable error in the final shimmed field homogeneity. The proper selection and arrangement of individual shim sizes produces a better conditioned field source matrix and hence improves the design. A number of examples show that the new method can effectively cancel target impurity harmonics while controlling high-order harmonics.
Paramagnetic shimming for wide-range variable-field NMR
Journal of magnetic resonance (San Diego, Calif. : 1997), 2014
We propose a new passive shimming strategy for variable-field NMR experiments, in which the magnetic field produced by paramagnetic shim pieces placed inside the magnet bore compensates the inhomogeneity of a variable-field magnet for a wide range of magnet currents. Paramagnetic shimming is demonstrated in (7)Li, (87)Rb, and (45)Sc NMR of a liquid solution sample in magnetic fields of 3.4, 4.0, and 5.4T at a fixed carrier frequency of 56.0MHz. Since both the main-field inhomogeneity and the paramagnetic magnetization are proportional to the main-magnet current, the resonance lines are equally narrowed by the improved field homogeneity with an identical configuration of the paramagnetic shim pieces. Paramagnetic shimming presented in this work opens the possibility of high-resolution variable-field NMR experiments.
Active ferromagnetic shimming of the permanent magnet for magnetic resonance imaging scanner
Chinese Physics B, 2010
This paper presents an approach of active ferromagnetic shimming for C-type permanent magnetic resonance imaging magnet. It is designed to reduce inhomogeneity of magnetostatic field of C-type permanent magnet to meet the stringent requirement for magnetic resonance imaging applications. An optimal configuration (locations and thicknesses) of active ferromagnetic pieces is generated through calculation according to the initial field map and the demanded final homogeneity specifications. This approach uses a minimisation technique which makes the sum of squared magnetic moment minimum to restrict the amount of the active ferromagnetic material used and the maximal thickness of pieces stacked at each hole location in the shimming boards. Simulation and experimental results verify that the method is valid and efficient.
A SQP optimization method for shimming a permanent MRI magnet
Progress in Natural Science, 2009
Based on the sequential quadratic programming (SQP) method, a new approach is presented in this paper to gain a uniform magnetic field for a permanent MRI magnet with biplanar poles. First, the adopted shimming piece is modeled as a magnetic dipole moment to calculate its effect on the background field over the imaging region of interest. Then, the SQP method is utilized to determine the ideal solution for the shimming equation. Finally, the ideal solution is discrete, and the quantization error control technique is used for special cases. This new method helps to reduce the inhomogeneity from 1234.5 ppm to 21.4 ppm over a 36 cm diameter spherical volume (DSV), within hours in practical shimming work.
Magnetic field shimming by fourier analysis
Magnetic Resonance in Medicine, 1988
The homogeneity of the magnetic field of an Oxford 85/310HR magnet was optimized with the use of passive shims (i.e., pieces of steel) and room-temperature electronic shims. Both the characteristics of the passive shims and the current settings for the room-temperature electronic shims were calculated with the goal of minimizing field inhomogeneities as assessed by Fourier analysis of data from circular field plots. The measurement techniques, the methods of data analysis, and the techniques employed in correcting the magnetic field are presented. Difficulties and constructive approaches are suggested in these descriptions. The final results show a field of excellent homogeneity over a 10-cm-diameter spherical volume, with a peak-to-peak variation of 3.2 ppm, which is better than the manufacturer's expected specification of 5 ppm. A water-filled sphere of 10 cm diameter gave a half-height linewidth of 0.14 ppm after a small amount of additional touch-up shimming. The field homogeneity as corrected is much improved over its original state, allowing the acquisition of improved spectra from samples of a size several centimeters in each linear dimension. Thus the field homogeneity is more useful for in vivo spectroscopy, the purpose for which the magnet was intended.
2002
The paper demonstrates a least square method for computation of feeding currents of correcting coils for stationary magnetic field used in NMR imaging. The method needs to perform a magnetic field measurement in selected points of an assigned volume twice: when shim coils are switched off and afterward the measurement of magnetic field changes caused by switching on the feeding current of particular shim coil in each of selected points. The proposed method because of its simplicity and speed of computation is convenient for basic adjustment of the magnetic field homogeneity by first magnet installation or in the case when the magnetic properties of the magnet surroundings are changed.