Homogenization method for Halbach configuration magnets

Abstract

This invention discloses a shimming method and apparatus for a Halbach-configured magnet. The method includes the following steps: 1) constructing a magnetic block arrangement model that generates corresponding harmonic components; 2) measuring the main magnetic field of the Halbach-configured magnet and obtaining the non-uniform components of the main magnetic field by performing spherical harmonic expansion; 3) evaluating the number of magnetic blocks required at the reserved positions based on the non-uniform components of the main magnetic field harmonic expansion; 4) optimizing the number of shimming magnetic blocks in each group using an optimization algorithm to obtain the number of magnetic blocks at each position; 5) iteratively implementing shimming based on the optimization results. This invention, by constructing a magnetic block arrangement that generates corresponding harmonic components, and by using optimization algorithms and shimming experiments to repeatedly optimize and iterate the shimming strategy based on the harmonic expansion of the initial field, can obtain a high-precision shimming scheme. The shimming apparatus provided by this invention has advantages such as stability, reliability, low cost, and convenient installation.

Description

Technical Field

[0001] This invention relates to the field of magnet shimming, and particularly to a shimming method for a Halbach-configured magnet. Background Technology

[0002] Halbach magnets, due to their high magnetic material utilization, are widely used in magnetic resonance instruments, particle accelerators, and refrigeration systems. An ideal Halbach magnet structure is infinitely long with a continuous magnetization direction. Based on the ratio of its inner to outer diameter, it can generate a specific field strength and highly homogeneous magnetic field with the magnetic field direction perpendicular to the axial direction. However, in actual fabrication, Halbach magnets are usually achieved by truncating the magnet length and discretizing the magnetization direction, which introduces higher-order components. Furthermore, errors in the manufacturing of the magnet blocks (dimensions, magnetic declination, remanence) and installation errors will severely affect the inhomogeneity of the magnetic field after installation, which is highly detrimental to high-resolution magnetic resonance applications. Homogenization, as the main method to improve magnetic field homogeneity, includes active shimming and passive shimming. Compared to active shimming, passive shimming has a sufficiently large shimming capability, reducing current constraints for subsequent active shimming. On the other hand, high-precision passive shimming allows the instrument to be directly applied to some relaxation applications.

[0003] Passive shimming involves placing a certain number of ferromagnetic sheets at specific locations to homogenize the main magnetic field. During shimming, it is often necessary to quantify the initial field's inhomogeneity and evaluate the shimming method or technique. The size and placement of the shimming sheets determine the shimming capability and accuracy. Furthermore, the fabrication and placement of the ferromagnetic sheets may deviate from theoretical specifications, leading to discrepancies between the shimming results and the theoretical values. Iterative methods are often used to further improve magnetic field uniformity, necessitating repeatable and convenient passive shimming operations.

[0004] Therefore, it is necessary to provide a high-precision, easy-to-operate shimming method and apparatus to solve the above problems. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a shimming method for Halbach configuration magnets, which addresses the shortcomings of the prior art.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: a method for homogenizing a Halbach-configured magnet, comprising the following steps:

[0007] 1) Construct a combination arrangement of multiple magnetic blocks that can generate each harmonic component, determine the relative positional relationship between the harmonic component to be generated and the arrangement of magnetic blocks that can generate the harmonic component, and obtain a magnetic block arrangement model for shimming. The magnetic block arrangement model includes at least multiple shimming positions for placing magnetic blocks and the number of magnetic blocks placed at each shimming position.

[0008] 2) Measure the main magnetic field of the Halbach configuration magnet and expand the harmonic components of the main magnetic field to obtain the non-uniform components of the main magnetic field harmonic expansion.

[0009] 3) Based on the non-uniform components of the main magnetic field harmonic expansion, evaluate the number of magnetic blocks in the magnetic block arrangement model obtained in step 1).

[0010] 4) Optimize the number of magnetic blocks at each uniform field position using an optimization algorithm;

[0011] 5) Conduct a shimming experiment. Based on the experimental results, correct all shimming positions and the number of magnetic blocks at each shimming position in the magnetic block arrangement model obtained in step 4) to obtain the final magnetic block arrangement model for shimming the main magnetic field.

[0012] Preferably, step 2) specifically includes: measuring the main magnetic field and performing a spherical harmonic expansion of the main magnetic field, as shown in the following expansion:

[0013]

[0014] in, B represents the main magnetic field. 00 and These represent the uniform and non-uniform components of the main magnetic field harmonic expansion, respectively, C. nm Let r be the coefficient of the term of degree m in order n, o(r 4 ) represents an abbreviation for higher-order components;

[0015] Based on the measurement results of the main magnetic field, the coefficient C of each term is calculated. nm .

[0016] Preferably, step 4) specifically includes: measuring the initial magnetic field obtained in step 2), performing third-order inner harmonic expansion, using a genetic algorithm to calculate the harmonic components generated by each magnetic block in the magnetic block arrangement model, constraining the number of magnetic blocks at each shimming position, optimizing with the goal of minimizing the harmonic components after the superposition of the main magnetic field and the magnetic field of the magnetic block arrangement model, solving for the number of shimming magnetic blocks in each group, and thus deducing the number of magnetic blocks at each shimming position to obtain a preliminary optimized magnetic block arrangement model.

[0017] Preferably, step 4) further includes: measuring the magnetic field of the initially optimized magnetic block arrangement model and performing harmonic expansion; based on the measurement results, optimizing the number of magnetic blocks at each uniform field position again until the magnetic field reaches the set accuracy, thus obtaining a further optimized magnetic block arrangement model.

[0018] Preferably, step 5) specifically includes: using an active shimming device to actively shim the main magnetic field, optimizing the current value of each component in the active shimming to minimize the full width at half maximum (FWHM) of the FID (Free Induction Decay) signal, and then, based on the harmonic components of the magnetic field generated by the active shimming device under the optimal current value, deduce the magnetic block arrangement scheme that can generate the closest harmonic component, and then perform a final optimization of the further optimized arrangement model according to the magnetic block arrangement scheme to obtain the final magnetic block arrangement model.

[0019] The present invention also provides a field homogenizing device for a Halbach-type magnet, which uses the method described above to homogenize the magnetic field of the main magnet of the Halbach configuration.

[0020] Preferably, the shimming device is disposed inside the aperture of the Halbach configuration magnet. The shimming device includes several magnetic blocks, which are arranged according to the final magnetic block arrangement model obtained in step 5) above.

[0021] Preferably, the shimming device further includes a non-magnetic mounting component, on which a plurality of magnetic blocks are disposed, and the mounting component is movably disposed within the aperture of the main magnet of the Halbach configuration.

[0022] Preferably, the mounting component is a cylindrical body that can be inserted into the aperture of the main magnet of the Halbach configuration, and the outer periphery of the cylindrical body is provided with a plurality of mounting slots for mounting the magnet.

[0023] Preferably, the magnetic block is in the shape of a cuboid or a cylinder.

[0024] The beneficial effects of this invention are as follows: The shimming method for Halbach-configured magnets provided by this invention can obtain a high-precision shimming scheme by constructing a magnetic block arrangement that can generate harmonic components within the third order, and by optimizing the magnetic block arrangement multiple times based on the harmonic expansion of the initial field and using optimization algorithms and shimming experiments. It is also easy to operate. Furthermore, the shimming device provided by this invention has the advantages of being stable and reliable, low in cost, and easy to install. Attached Figure Description

[0025] Figure 1 The flowchart of the shimming method of the present invention is shown below;

[0026] Figures 2a-2dThe magnetic block distribution that can generate second-order inner harmonic components and the magnetic field distribution generated on a sphere with a central diameter of 5 mm are constructed for this invention.

[0027] Figures 3a-3b The magnetic block distribution that generates independent terms of third-order harmonic components and the magnetic field distribution generated on a sphere with a central diameter of 5 mm are constructed for this invention.

[0028] Figure 4 The present invention constructs a magnetic block distribution that generates independent terms of third-order harmonic components and a magnetic field distribution on a sphere with a central diameter of 5 mm by combining and superimposing several sets of magnetic blocks.

[0029] Figure 5 This is a schematic diagram of the device involved in Embodiment 2 of the present invention;

[0030] Figure 6 This is a comparison diagram of the initial main magnetic field and the magnetic field after the first shimming in Embodiment 2 of the present invention;

[0031] Figure 7 The final shimming result in Embodiment 2 of the present invention.

[0032] Explanation of reference numerals in the attached figures:

[0033] 1—Halbach configuration magnet; 2—mounting component; 3—mounting slot; 4—active shimming device; 5—sample tube to be tested. Detailed Implementation

[0034] The present invention will be further described in detail below with reference to embodiments, so that those skilled in the art can implement it based on the description.

[0035] It should be understood that terms such as “having,” “comprising,” and “including” as used herein do not exclude the presence or addition of one or more other elements or combinations thereof.

[0036] Reference Figure 1 The method for homogenizing a Halbach-configured magnet according to this embodiment includes the following steps:

[0037] 1) Construct a combination arrangement of multiple magnetic blocks capable of generating various harmonic components (in this embodiment, various harmonic components within the third order). Determine the relative positional relationship between the harmonic component to be generated and the magnetic block arrangement capable of generating that harmonic component, and obtain a magnetic block arrangement model for shimming. This magnetic block arrangement model includes at least multiple shimming positions for placing magnetic blocks and the number of magnetic blocks placed at each shimming position. 2) Measure the main magnetic field of the Halbach configuration magnet and expand the harmonic components of the main magnetic field to obtain the non-uniform components of the main magnetic field harmonic expansion.

[0038] Specifically, the main magnetic field is measured using a gaussmeter, and its harmonic components are expanded to quantify its inhomogeneity. The magnetic field is a vector field, following Maxwell's equations, and can be decomposed into spherical harmonic expansions. For a Halbach configuration magnet, the main magnetic field... With the direction perpendicular to the axis, the spherical harmonic expansion of the main magnetic field is as follows:

[0039]

[0040] in, B represents the main magnetic field. 00 and These represent the uniform and non-uniform components of the main magnetic field harmonic expansion, respectively, C. nm Let be the coefficient of the term with degree number m in the nth order, which in this embodiment is a 3rd order spherical harmonic expansion, o(r 4 ) represents an abbreviation for components of order 3 and above;

[0041] Based on the measurement results of the main magnetic field, the coefficient C of each term is calculated. nm .

[0042] 3) Based on the non-uniform components of the main magnetic field harmonic expansion, evaluate the number of magnetic blocks in the magnetic block arrangement model obtained in step 1) to ensure its shimming capability and shimming accuracy.

[0043] 4) Optimize the number of magnetic blocks at each shim position using an optimization algorithm.

[0044] Specifically, it includes:

[0045] Step 2) measures the initial magnetic field and performs a third-order inner harmonic expansion. A genetic algorithm (or other methods such as nonlinear optimization) is used to calculate the harmonic components generated by each magnetic block in the magnetic block arrangement model. The number of magnetic blocks at each shim position is constrained. The optimization objective is to minimize the harmonic components resulting from the superposition of the main magnetic field and the magnetic field of the magnetic block arrangement model. The number of shims in each group is solved, and the number of magnetic blocks at each shim position is deduced from this, resulting in a preliminary optimized magnetic block arrangement model. The superimposed harmonic components in the objective function can have different weights to obtain the optimal result.

[0046] The magnetic field of the initially optimized magnetic block arrangement model was measured using a gaussmeter, and harmonic expansion was performed. Based on the measurement results, the number of magnetic blocks at each uniform field position was optimized again until the magnetic field reached the set accuracy, resulting in a further optimized magnetic block arrangement model.

[0047] 5) Conduct a shimming experiment. Based on the experimental results, correct all shimming positions and the number of magnetic blocks at each shimming position in the magnetic block arrangement model obtained in step 4) to obtain the final magnetic block arrangement model for shimming the main magnetic field.

[0048] An active shimming device is used to actively shim the main magnetic field. By optimizing the current values ​​of each component in the active shimming, the full width at half maximum (FWHM) of the FID (Free Induction Decay) signal is minimized (the smaller the FWHM, the higher the uniformity). Then, based on the harmonic components of the magnetic field generated by the active shimming device under the optimal current value, the magnetic block arrangement scheme that can produce the closest harmonic component is deduced. Based on this magnetic block arrangement scheme, the further optimized magnetic block arrangement model is finally optimized to obtain the final magnetic block arrangement model.

[0049] The magnetization direction of the uniform magnetic block can be either inward or outward, or it can be any other direction.

[0050] Among them, magnetic blocks in the same position but opposite directions can be superimposed to cancel each other out.

[0051] Reference Figures 2a-2d This invention describes the magnetic block distribution capable of generating second-order internal harmonic components and the magnetic field distribution generated on a sphere with a central diameter of 5 mm. Figures 3a-3b The magnetic block distribution that generates independent terms of third-order harmonic components and the magnetic field distribution generated on a sphere with a central diameter of 5 mm are constructed for this invention. Figure 4 The present invention constructs a magnetic block distribution that, through the superposition of several sets of magnetic blocks, can generate independent terms of the third-order harmonic components and a magnetic field distribution on a spherical surface with a central diameter of 5 mm. Figure 1 In -3, the magnetization directions of the two colors of magnets are one outward (1#) and one inward (2#).

[0052] Example 2

[0053] A field-uniforming device for a Halbach-type magnet is disclosed, which uses the method of Example 1 to uniform the magnetic field of the main Halbach-type magnet. The field-uniforming device is disposed inside the aperture of the Halbach-type magnet and includes several magnetic blocks arranged according to the final magnetic block arrangement model obtained in step 5) of Example 1.

[0054] In this embodiment, the shimming device also includes a non-magnetic mounting component. Several magnetic blocks are disposed on the mounting component, which is movably positioned within the aperture of the main magnet in a Halbach configuration. The shimming device is 3D printed from non-metallic materials, such as resin or nylon. The shimming magnetic blocks can be mounted on multiple inserts and inserted into the magnetic field in a drawer-like manner, or directly mounted on a passive shimming device. After all the shimming magnetic blocks are installed, the device is then placed into the main magnetic field.

[0055] In a preferred embodiment, the mounting component is a cylindrical body that can be inserted into the aperture of the main magnet in a Halbach configuration. The outer periphery of the cylindrical body is provided with a plurality of mounting slots for mounting magnetic blocks. In this embodiment, the cylindrical body is 3D printed from nylon material.

[0056] Reference Figure 5 This is a schematic diagram of the device involved in this embodiment. The shimming device is set in the aperture in the middle of the Halbach-type magnet. The shimming target area is a spherical region with a diameter of 5 mm. 1 is the Halbach-type magnet, 2 is the mounting part of the passive shimming device, 3 is the reserved mounting slot, 4 is the active shimming device for conducting the shimming experiment, and 5 is the test tube of the sample to be tested. The shimming target area is located within 5.

[0057] In a preferred embodiment, the magnetic block is rectangular, cylindrical, or other shapes. In this embodiment, the magnetic block is rectangular, made of neodymium iron boron material, and has dimensions of 5*2*1mm. 3 N38 (Br=1.23T, coercivity 955kA / m).

[0058] In this embodiment, the initial uniformity of the main magnetic field was measured to be 811 ppm using a gaussmeter. Following the method of Embodiment 1, magnetic blocks were arranged on the cylindrical body according to the optimized magnetic block arrangement model (step 4), and then inserted into the aperture of the main magnet to homogenize the main magnetic field. After this, the uniformity of the main magnetic field increased to 132 ppm. Figure 6 After homogenizing the main magnetic field using the final magnetic block arrangement model obtained in step 5), the uniformity of the main magnetic field increased to 4.7 ppm. Figure 7 .

[0059] Although the embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. They can be applied to various fields suitable for the present invention. For those skilled in the art, other modifications can be easily made. Therefore, without departing from the general concept defined by the claims and their equivalents, the present invention is not limited to the specific details.

Claims

1. A method of shimming a Halbach configuration magnet, characterized by, The method comprises the following steps: 1) constructing a combined arrangement mode of multiple magnetic blocks capable of generating harmonic components, determining harmonic components to be generated and relative position relationships of magnetic block arrangements capable of generating the harmonic components, and obtaining a magnetic block arrangement model for shimming, which comprises at least multiple shimming positions for placing magnetic blocks and the number of magnetic blocks placed at each shimming position; 2) measuring a main magnetic field of a Halbach configuration magnet, and expanding harmonic components of the main magnetic field to obtain non-uniform components of the main magnetic field harmonic expansion; 3) evaluating the number of magnetic blocks in the magnetic block arrangement model obtained in step 1) based on the non-uniform components of the main magnetic field harmonic expansion; 4) optimizing the number of magnetic blocks at each shimming position by using an optimization algorithm; 5) performing a shimming experiment, and correcting all shimming positions and the number of magnetic blocks at each shimming position in the magnetic block arrangement model obtained in step 4) according to the experimental results to obtain a final magnetic block arrangement model for shimming of the main magnetic field; The step 2) specifically comprises: measuring the main magnetic field, performing spherical harmonic expansion on the main magnetic field, and the expansion formula is as follows: wherein B denotes the main magnetic field, 00 and C and C denote the uniform and non-uniform components of the harmonic expansion of the main magnetic field, respectively, nm Cnmare the coefficients of the terms of order n and degree m in the expansion o(r 4 ) denotes the abbreviation for the high order components; The coefficient C of each term is calculated from the measurement of the main magnetic field nm ; The step 4) specifically comprises: measuring an initial magnetic field obtained in step 2), performing harmonic expansion, using a genetic algorithm, calculating harmonic components generated by each magnetic block in the magnetic block arrangement model, constraining the number of magnetic blocks at each shimming position, optimizing to minimize harmonic components of the main magnetic field and the magnetic field of the magnetic block arrangement model after superposition, solving each shimming number, and deducing the number of magnetic blocks at each shimming position to obtain a preliminarily optimized magnetic block arrangement model; The step 4) further comprises: measuring the magnetic field of the preliminarily optimized magnetic block arrangement model, performing harmonic expansion, and optimizing the number of magnetic blocks at each shimming position again according to the measurement results until the magnetic field reaches a set accuracy to obtain a re-optimized magnetic block arrangement model; The step 5) specifically comprises: using an active shimming device to actively shim the main magnetic field, minimizing the half-width of the FID signal by optimizing current values of each component in the active shimming, deducing a magnetic block arrangement scheme capable of generating harmonic components closest to the harmonic components based on the harmonic components of the magnetic field generated by the active shimming device under the optimal current values, and finally optimizing the re-optimized magnetic block arrangement model according to the magnetic block arrangement scheme to obtain a final magnetic block arrangement model.

2. A shimming device for a Halbach configuration magnet, characterized in that The method is used for shimming the magnetic field of a Halbach configuration main magnet.

3. A shimming arrangement for a Halbach configuration magnet according to claim 2, c h a r a c t e r i z e d in that The shimming device is arranged inside the aperture of the Halbach configuration magnet, and the shimming device comprises multiple magnetic blocks arranged according to the final magnetic block arrangement model obtained in step 5) in claim 1.

4. A shimming arrangement for a Halbach configuration magnet according to claim 3, wherein, The shimming device further comprises a non-magnetic mounting member, and the multiple magnetic blocks are arranged on the mounting member, and the mounting member is movably arranged in the aperture of the Halbach configuration main magnet.

5. A shimming arrangement for a Halbach configuration magnet according to claim 4, wherein, The mounting member is a cylindrical body that can be inserted into the aperture of the Halbach configuration main magnet, and the outer periphery of the cylindrical body is arrayed with multiple mounting grooves for mounting the magnetic blocks.

6. A shimming arrangement for a Halbach configuration magnet according to claim 5, wherein, The magnetic blocks are cuboids or cylinders.

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