CN115831570A - Shimming method of Halbach-configuration magnet - Google Patents
Shimming method of Halbach-configuration magnet Download PDFInfo
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- CN115831570A CN115831570A CN202111087703.9A CN202111087703A CN115831570A CN 115831570 A CN115831570 A CN 115831570A CN 202111087703 A CN202111087703 A CN 202111087703A CN 115831570 A CN115831570 A CN 115831570A
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Abstract
The invention discloses a shimming method and a shimming device for a Halbach-shaped magnet, wherein the method comprises the following steps: 1) Constructing a magnetic block arrangement model capable of generating corresponding harmonic components; 2) Measuring a main magnetic field of a Halbach-shaped magnet, and performing spherical harmonic expansion on the main magnetic field to obtain uneven components of the main magnetic field; 3) Based on the uneven component of the main magnetic field harmonic wave development, the number of the magnetic blocks needed at the reserved positions of the magnetic blocks is evaluated; 4) Optimizing the number of shimming magnetic blocks in each group by using an optimization algorithm so as to obtain the number of magnetic blocks in each position; 5) And based on the optimization result, shimming is iteratively realized. According to the invention, the arrangement of the magnetic blocks capable of generating corresponding harmonic components is constructed, the shimming strategy is optimized and iterated for multiple times by utilizing an optimization algorithm and a shimming experiment based on the harmonic expansion of an initial field, and a high-precision shimming scheme can be obtained; the shimming device provided by the invention has the advantages of stability, reliability, low cost, convenience in installation and the like.
Description
Technical Field
The invention relates to the field of magnet shimming, in particular to a shimming method of a Halbach-configuration magnet.
Background
The Halbach-shaped magnet is widely applied to magnetic resonance instruments, particle accelerators and refrigeration systems due to high magnetic material utilization rate. The ideal Halbach magnet structure is infinite in length and continuous in magnetizing direction, and based on the proportion of the inner diameter and the outer diameter, a magnetic field with specific field intensity and high uniformity in the direction perpendicular to the axial direction can be generated. However, in actual manufacturing, the manufacturing of Halbach is usually realized by cutting off the length of the magnet and discretizing the magnetization direction, but correspondingly introducing high-order components. In addition, due to the processing errors (size, declination, remanence) and installation errors of the magnetic blocks, the nonuniformity of the magnetic field after installation is seriously influenced, which is very unfavorable for high-resolution magnetic resonance application. Shimming is a main method for improving the uniformity of a magnetic field, and comprises active shimming and passive shimming. Compared with active shimming, passive shimming has enough shimming capability, current constraint is reduced for subsequent active shimming, and on the other hand, high-precision passive shimming can enable the instrument to be directly applied to some relaxation applications.
Passive shimming is achieved by placing a certain number of ferromagnetic plates at specific positions to shim the main magnetic field. In the shimming process, the unevenness of the initial field is often required to be quantified, and the shimming mode or the shimming means is required to be evaluated, wherein the shimming capability and the shimming precision of the shimming small magnetic sheets are determined by the selection of the size and the position of the shimming small magnetic sheets. In addition, the processing, the position and the theory of the ferromagnetic sheet have certain deviation, the shimming result has certain difference compared with the theory, the magnetic field uniformity is further improved through an iteration mode, and the repeatability, the simplicity and the convenience of passive shimming operation are required.
Therefore, it is necessary to provide a high-precision shimming method and apparatus that is easy to operate to solve the above problems.
Disclosure of Invention
The technical problem to be solved by the invention is to provide a shimming method of a Halbach configuration magnet aiming at the defects in the prior art.
In order to solve the technical problems, the invention adopts the technical scheme that: a shimming method of a Halbach configuration magnet comprises the following steps:
1) Constructing a combination arrangement mode of a plurality of magnetic blocks capable of generating harmonic components, determining the harmonic components to be generated and the relative position relationship of the arrangement of the magnetic blocks capable of generating the harmonic components, and obtaining a magnetic block arrangement model for shimming, wherein the magnetic block arrangement model at least comprises a plurality of shimming positions for placing the magnetic blocks and the number of the magnetic blocks placed in each shimming position;
2) Measuring a main magnetic field of a Halbach-shaped magnet, and expanding a harmonic component of the main magnetic field to obtain an uneven component expanded by the harmonic component of the main magnetic field;
3) Evaluating the number of the magnetic blocks in the magnetic block arrangement model obtained in the step 1) based on the uneven component of the main magnetic field harmonic development;
4) Optimizing the number of the magnetic blocks at each shimming position by using an optimization algorithm;
5) And 4) carrying out shimming experiments, and correcting all shimming positions in the magnetic block arrangement model obtained in the step 4) and the number of the magnetic blocks at each shimming position according to the experiment results to obtain the final magnetic block arrangement model for shimming the main magnetic field.
Preferably, the step 2) specifically includes: measuring a main magnetic field, and performing spherical harmonic expansion on the main magnetic field, wherein the expansion is as follows:
wherein the content of the first and second substances,
represents the main magnetic field, B 00 And
uniform and non-uniform components developed for the main magnetic field harmonics, respectively, C nm Coefficient of a term of degree m in order n, o (r) 4 ) Abbreviations representing higher order components;
calculating coefficients C of each term from the measurement of the main magnetic field nm 。
Preferably, the step 4) specifically includes: measuring step 2) to obtain an initial magnetic field, performing 3-order internal harmonic expansion, calculating harmonic components generated by the magnetic blocks in the magnetic block arrangement model by adopting a genetic algorithm, restricting the number of the magnetic blocks at each shimming position, optimizing by using the minimization of the harmonic components after the main magnetic field is superposed with the magnetic fields of the magnetic block arrangement model as a target, solving the number of the magnetic blocks in each group, and deducing the number of the magnetic blocks at each shimming position to obtain a primarily optimized magnetic block arrangement model.
Preferably, step 4) further comprises: and measuring the magnetic field of the primarily optimized magnetic block arrangement model, carrying out harmonic expansion, and optimizing the number of the magnetic blocks at each shimming position again according to the measurement result until the magnetic field reaches the set precision to obtain the again optimized magnetic block arrangement model.
Preferably, the step 5) specifically includes: the main magnetic field is actively shimmed by adopting an active shimming device, the full width at half maximum of an FID (Free Induction Decay) signal is minimized by optimizing the current value of each component in the active shimming, then a magnetic block arrangement scheme which can generate the magnetic block closest to the harmonic component is deduced based on the harmonic component of the magnetic field generated by the active shimming device under the optimal current value, and the secondary optimized arrangement model is finally optimized according to the magnetic block arrangement scheme to obtain a final magnetic block arrangement model.
The invention also provides a shimming device of the Halbach-shaped magnet, which adopts the method to shim the magnetic field of the Halbach-shaped magnet.
Preferably, the shimming device is arranged in the aperture of the magnet with the Halbach configuration, and comprises a plurality of magnetic blocks which are arranged according to the final magnetic block arrangement model obtained in the step 5).
Preferably, the shimming apparatus further comprises a non-magnetic mounting member on which a plurality of magnet blocks are arranged, the mounting member being movably arranged within the bore of the main magnet in a Halbach configuration.
Preferably, the mounting member is a cylinder body which can be inserted into the bore of the main magnet in a Halbach configuration, and the peripheral array of the cylinder body is provided with a plurality of mounting grooves for mounting the magnetic blocks.
Preferably, the magnetic block is rectangular or cylindrical.
The invention has the beneficial effects that: according to the shimming method of the Halbach configuration magnet, the arrangement of the magnetic blocks capable of generating each harmonic component in 3 orders is constructed, the arrangement of the magnetic blocks is optimized for multiple times by utilizing an optimization algorithm and a shimming experiment based on the harmonic expansion of an initial field, a high-precision shimming scheme can be obtained, and the operation is convenient; the shimming device further provided by the invention has the advantages of stability, reliability, low cost, convenience in installation and the like.
Drawings
FIG. 1 is a flow chart of the shimming method of the present invention;
2a-2d are the magnetic block distribution capable of generating 2 nd order inner harmonic component constructed by the invention and the magnetic field distribution generated on the spherical surface with the diameter of 5mm at the center;
FIGS. 3a-3b are the magnetic block distribution constructed by the present invention to generate 3 rd order harmonic component independent term and the magnetic field distribution generated on the spherical surface with 5mm diameter at the center;
FIG. 4 shows the magnetic block distribution that can generate 3 rd order harmonic component independent term and the magnetic field distribution generated on the spherical surface with a diameter of 5mm at the center by combining and overlapping several groups of magnetic blocks constructed by the present invention;
FIG. 5 is a schematic view of the structure of an apparatus according to embodiment 2 of the present invention;
FIG. 6 is a comparison of the results of the initial main magnetic field and the magnetic field after initial shimming in example 2 of the present invention;
fig. 7 final shimming results in embodiment 2 of the invention.
Description of reference numerals:
1-a magnet of Halbach configuration; 2-a mounting member; 3, mounting a groove; 4-active shimming device; 5-test tube for sample to be detected.
Detailed Description
The present invention is further described in detail below with reference to examples so that those skilled in the art can practice the invention with reference to the description.
It will be understood that terms such as "having," "including," and "comprising," as used herein, do not preclude the presence or addition of one or more other elements or groups thereof.
Referring to fig. 1, the shimming method for the Halbach configuration magnet of the embodiment includes the following steps:
1) The method includes the steps of constructing a combination arrangement mode of multiple magnetic blocks capable of generating each harmonic component (in the embodiment, each harmonic component in order 3), determining a harmonic component to be generated and a relative position relation of arrangement of the magnetic blocks capable of generating the harmonic component, and obtaining a magnetic block arrangement model for shimming, wherein the magnetic block arrangement model at least comprises a plurality of shimming positions for placing the magnetic blocks and the number of the magnetic blocks placed in each shimming position. 2) And measuring the main magnetic field of the Halbach-shaped magnet, and expanding the harmonic component of the main magnetic field to obtain the uneven component expanded by the harmonic component of the main magnetic field.
The method specifically comprises the following steps: the main magnetic field is measured using a gaussmeter, and the non-uniformity of the main magnetic field is quantified by developing harmonic components of the main magnetic field. The magnetic field is a vector field, follows Maxwell equation, can be decomposed into spherical harmonic expansion, and for a Halbach-shaped magnet, the main magnetic field
The direction is vertical to the axial direction, and the expansion formula of the main magnetic field for spherical harmonic expansion is as follows:
wherein, the first and the second end of the pipe are connected with each other,
represents the main magnetic field, B 00 And
uniform and non-uniform components developed for the main magnetic field harmonics, respectively, C nm Is the coefficient of the term of degree m in order n, which in this example is the spherical harmonic expansion of order 3, o (r) 4 ) Abbreviations representing components above order 3;
calculating coefficients C of each term from the measurement of the main magnetic field nm 。
3) Based on uneven components of main magnetic field harmonic wave development, the number of the magnetic blocks in the magnetic block arrangement model obtained in the step 1) is evaluated, and the shimming capability and shimming precision of the magnetic block arrangement model are guaranteed.
4) And optimizing the number of the magnetic blocks at each shimming position by utilizing an optimization algorithm.
The method specifically comprises the following steps:
measuring step 2) to obtain an initial magnetic field, performing 3-order internal harmonic expansion, calculating harmonic components generated by the magnetic blocks in the magnetic block arrangement model by adopting a genetic algorithm (or other methods such as nonlinear optimization), restricting the number of the magnetic blocks at each shimming position, optimizing by using the minimization of the harmonic components after the main magnetic field is superposed with the magnetic fields of the magnetic block arrangement model as a target, solving the number of shimming in each group, and deducing the number of the magnetic blocks at each shimming position to obtain a primarily optimized magnetic block arrangement model; each harmonic component after superposition in the objective function can have different weights so as to obtain an optimal result;
and measuring the magnetic field of the primarily optimized magnetic block arrangement model by using a gaussmeter, carrying out harmonic expansion, and optimizing the number of the magnetic blocks at each shimming position again according to the measurement result until the magnetic field reaches the set precision to obtain the again optimized magnetic block arrangement model.
5) And 4) carrying out shimming experiments, and correcting all shimming positions in the magnetic block arrangement model obtained in the step 4) and the number of the magnetic blocks at each shimming position according to the experiment results to obtain the final magnetic block arrangement model for shimming the main magnetic field.
The main magnetic field is actively shimmed by adopting an active shimming device, the full width at half maximum of an FID (Free Induction Decay) signal is minimum (the smaller the full width at half maximum is, the higher the uniformity is) by optimizing the current value of each component in the active shimming, then a magnetic block arrangement scheme which can generate the magnetic block arrangement model closest to the harmonic component is deduced based on the harmonic component of the magnetic field generated by the active shimming device under the optimal current value, and the magnetic block arrangement model which is optimized again is finally optimized according to the magnetic block arrangement scheme to obtain the final magnetic block arrangement model.
The magnetizing directions of the shimming magnetic blocks can be two directions from inside to outside or other directions.
The magnetic blocks at the same position and in opposite directions can be overlapped and offset.
Referring to fig. 2a-2d, the magnetic block distribution capable of generating 2-order internal harmonic component constructed for the present invention and the magnetic field distribution generated on the spherical surface with a diameter of 5mm at the center; FIGS. 3a-3b are the magnetic block distribution constructed by the present invention to generate 3 rd order harmonic component independent term and the magnetic field distribution generated on the spherical surface with 5mm diameter at the center; fig. 4 shows the magnetic block distribution which is constructed by combining and overlapping several groups of magnetic blocks and can generate independent terms of harmonic component of order 3 and the magnetic field distribution generated on a spherical surface with the diameter of 5mm at the center, and in fig. 1-3, the magnetization directions of the magnets of two colors are outward (1 #) and inward (2 #).
Example 2
A shimming device of a magnet in Halbach configuration adopts the method of embodiment 1 to shim the magnetic field of the main magnet in Halbach configuration. The shimming device is arranged in the aperture of the Halbach-shaped magnet and comprises a plurality of magnets which are arranged according to the final magnet arrangement model obtained in the step 5) of the embodiment 1.
In this embodiment, the shimming apparatus further includes a non-magnetic mounting member, the plurality of magnetic blocks are disposed on the mounting member, and the mounting member is movably disposed in the bore of the main magnet in a Halbach configuration. Wherein, the shimming device is printed by non-metallic material 3D, for example, materials such as resin, nylon. The shimming magnetic blocks can be arranged on a plurality of inserting bars and inserted into the magnetic field in a drawer mode, or the shimming magnetic blocks are directly arranged on a passive shimming device and are placed into the main magnetic field after all the shimming magnetic blocks are arranged.
In a preferred embodiment, the mounting member is a cylindrical body insertable into the bore of a main magnet of Halbach configuration, the peripheral array of the cylindrical body being provided with a number of mounting slots for mounting the magnet blocks. In this embodiment, the cylinder is printed by nylon materials 3D and is formed.
Referring to fig. 5, which is a schematic diagram of the arrangement involved in this embodiment, the shimming arrangement is located within the bore in the middle of the magnet in the Halbach configuration. The shimming target region is a spherical region with a diameter of 5 mm. The device comprises a Halbach-shaped magnet 1, a passive shimming device 2, a reserved mounting groove 3, an active shimming device for shimming experiments 4, a sample test tube to be detected 5 and a shimming target region in 5.
In a preferred embodiment, the magnetic block is in the shape of a cuboid or cylinder or other shape. In this embodiment, the magnetic block is rectangular, is made of rubidium, iron and boron, and has a size of 5 × 2 × 1mm 3 N38 (Br =1.23T, coercivity 955 kA/m).
In this example, the initial homogeneity of the main magnetic field was 811ppm as measured by a gauss meter; according to the method of embodiment 1, arranging magnetic blocks on the cylinder according to the magnetic block arrangement model optimized in step 4), inserting the magnetic blocks into the aperture of the main magnet to shim the main magnetic field, and then increasing the uniformity of the main magnetic field to 132ppm, referring to fig. 6; shimming the main magnetic field according to the final magnetic block arrangement model obtained in the step 5), and then improving the uniformity of the main magnetic field to 4.7ppm, referring to fig. 7.
While embodiments of the invention have been disclosed above, it is not limited to the applications listed in the description and the embodiments, which are fully applicable in all kinds of fields of application of the invention, and further modifications may readily be effected by those skilled in the art, so that the invention is not limited to the specific details without departing from the general concept defined by the claims and the scope of equivalents.
Claims (10)
1. A shimming method of a Halbach configuration magnet is characterized by comprising the following steps:
1) Constructing a combination arrangement mode of a plurality of magnetic blocks capable of generating harmonic components, determining the harmonic components to be generated and the relative position relationship of the arrangement of the magnetic blocks capable of generating the harmonic components, and obtaining a magnetic block arrangement model for shimming, wherein the magnetic block arrangement model at least comprises a plurality of shimming positions for placing the magnetic blocks and the number of the magnetic blocks placed in each shimming position;
2) Measuring a main magnetic field of a Halbach-shaped magnet, and expanding harmonic components of the main magnetic field to obtain uneven components expanded by the harmonic components of the main magnetic field;
3) Evaluating the number of the magnetic blocks in the magnetic block arrangement model obtained in the step 1) based on the uneven component of the main magnetic field harmonic wave development;
4) Optimizing the number of the magnetic blocks at each shimming position by using an optimization algorithm;
5) And 4) carrying out shimming experiments, and correcting all shimming positions in the magnetic block arrangement model obtained in the step 4) and the number of the magnetic blocks at each shimming position according to the experiment results to obtain the final magnetic block arrangement model for shimming the main magnetic field.
2. The shimming device for a Halbach configuration magnet according to claim 1, wherein the step 2) specifically comprises: measuring a main magnetic field, and performing spherical harmonic expansion on the main magnetic field, wherein the expansion is as follows:
wherein the content of the first and second substances,
represents the main magnetic field, B 00 And
homogeneous and inhomogeneous components, C, developed for the main magnetic field harmonics, respectively nm Coefficient of a term of degree m in order n, o (r) 4 ) Abbreviations representing higher order components;
calculating coefficients C of each term from the measurement of the main magnetic field nm 。
3. The shimming device for a Halbach configuration magnet according to claim 2, wherein the step 4) comprises in particular: measuring step 2) to obtain an initial magnetic field, performing harmonic expansion, calculating harmonic components generated by the magnetic blocks in the magnetic block arrangement model by adopting a genetic algorithm, restricting the number of the magnetic blocks in each shimming position, optimizing by taking minimization of the harmonic components after superposition of the main magnetic field and the magnetic fields of the magnetic block arrangement model as a target, solving the shimming number of each group, and deducing the number of the magnetic blocks in each shimming position to obtain an initially optimized magnetic block arrangement model.
4. The shimming apparatus for a Halbach configuration magnet according to claim 3, wherein step 4) further comprises: and measuring the magnetic field of the primarily optimized magnetic block arrangement model, carrying out harmonic expansion, and optimizing the number of the magnetic blocks at each shimming position again according to the measurement result until the magnetic field reaches the set precision to obtain the again optimized magnetic block arrangement model.
5. The shimming device for a Halbach configuration magnet according to claim 4, wherein the step 5) comprises in particular: the method comprises the steps of actively shimming a main magnetic field by adopting an active shimming device, enabling the full width at half maximum of an FID signal to be minimum by optimizing the current value of each component in the active shimming, and deducing a magnetic block arrangement scheme which can generate the magnetic block arrangement model closest to the harmonic component based on the harmonic component of the magnetic field generated by the active shimming device under the optimal current value, so that the re-optimized magnetic block arrangement model is finally optimized according to the magnetic block arrangement scheme, and the final magnetic block arrangement model is obtained.
6. Shimming device for a magnet of Halbach configuration, characterized in that it is used for shimming the magnetic field of a main magnet of Halbach configuration by a method according to any one of claims 1-5.
7. The shimming method of the Halbach-shaped magnet according to claim 6, wherein the shimming device is arranged in the aperture of the Halbach-shaped magnet, the shimming device comprises a plurality of magnetic blocks, and the plurality of magnetic blocks are arranged according to the final magnetic block arrangement model obtained in the step 5) in claim 1.
8. The method of shimming a Halbach configured magnet according to claim 7, wherein the shimming apparatus further comprises a non-magnetic mount on which a plurality of magnet blocks are disposed, the mount being movably disposed within the bore of the Halbach configured main magnet.
9. The method of shimming a Halbach configured magnet according to claim 8, wherein the mounting member is a cylinder insertable into the bore of a Halbach configured main magnet, the peripheral array of the cylinder being provided with a plurality of mounting slots for mounting the magnet blocks.
10. The method for shimming a Halbach configuration magnet according to claim 9, wherein the magnet is cuboid or cylindrical.
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| CN202111087703.9A CN115831570A (en) | 2021-09-16 | 2021-09-16 | Shimming method of Halbach-configuration magnet |
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Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN116256680A (en) * | 2023-05-16 | 2023-06-13 | 中国科学院精密测量科学与技术创新研究院 | Passive shimming structure and method of Halbach magnet for magnetic resonance |
| CN116702568A (en) * | 2023-08-04 | 2023-09-05 | 天津天达图治科技有限公司 | Magnetic resonance imaging permanent magnet design method, system, equipment and medium |
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2021
- 2021-09-16 CN CN202111087703.9A patent/CN115831570A/en active Pending
Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN116256680A (en) * | 2023-05-16 | 2023-06-13 | 中国科学院精密测量科学与技术创新研究院 | Passive shimming structure and method of Halbach magnet for magnetic resonance |
| CN116256680B (en) * | 2023-05-16 | 2023-08-04 | 中国科学院精密测量科学与技术创新研究院 | Passive shimming structure and method of Halbach magnet for magnetic resonance |
| CN116702568A (en) * | 2023-08-04 | 2023-09-05 | 天津天达图治科技有限公司 | Magnetic resonance imaging permanent magnet design method, system, equipment and medium |
| CN116702568B (en) * | 2023-08-04 | 2023-11-10 | 天津天达图治科技有限公司 | Magnetic resonance imaging permanent magnet design method, system, equipment and medium |
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