A Halbach magnet for head magnetic resonance imaging and its optimization method

Abstract

The present invention relates to a Halbach magnet and an optimization method for head magnetic resonance imaging, belonging to the field of magnetic resonance technology. The magnet comprises a main magnet, an end rotation adjustment unit, two shim rings, a main magnet substrate, and an end rotation substrate; the main magnet is composed of an array of M magnetic rings of the same diameter stacked axially to form a hollow cylinder; the end rotation adjustment unit is arranged on the periphery of one end of the main magnet and rotates at any position along the circumference of the magnetic ring, and comprises several layers of axial magnetic rings and several radial magnetic rings. Based on the traditional cylinder, the present invention uses a genetic algorithm to adjust the size of the magnetic blocks in each layer of the magnetic rings of the end rotation adjustment unit, and adds shim rings at both ends of the magnet to increase the compensation for the magnetic field in the target area of ​​the magnet, homogenize the magnetic field distribution in the target area, and effectively solve the end effect caused by the height reduction of traditional magnets. This structure does not require an external magnetic field compensation device, and the overall structure is simple, which greatly reduces the difficulty of assembling the magnet.

Description

Technical Field

[0001] The invention belongs to the technical field of magnetic resonance and relates to a Halbach magnet for head magnetic resonance imaging and an optimization method. Background Art

[0002] Magnetic resonance imaging (MRI) is an imaging technique widely used in clinical diagnosis and medical research. An MRI system places the human body in a strong static magnetic field and transmits radiofrequency pulses to excite atomic nuclei in certain areas of tissue. After the radiofrequency field is removed, these excited nuclei radiate radiofrequency signals, which are received by an antenna. Because a gradient magnetic field is incorporated into this process, the radiofrequency signals can be used to obtain spatial distribution information about the human body, thereby reconstructing a two-dimensional or three-dimensional image of the body.

[0003] As one of the core components of magnetic resonance imaging systems, magnets have long been a key research topic. MRI equipment requires a highly uniform magnetic field within the imaging area. This uniform magnetic field is spherical. The area to be imaged is placed within this spherical area, and scanning captures the image of the area. Therefore, the performance of the magnet is directly related to the magnetic field strength and uniformity of the magnetic resonance system, which, to a certain extent, determines the clarity of the image.

[0004] Traditional magnetic resonance systems are heavy and cannot meet the needs of being used in any department on any floor of a hospital. In order to meet the needs of magnetic resonance systems entering ordinary wards in major departments, the first problem that needs to be solved is the lightweighting of the main magnet. Compared with other permanent magnets, the Halbach magnet configuration has the advantages of a large field strength to mass ratio (the magnetic field strength that can be generated per unit mass compared with other configurations), a small stray magnetic field, no need for an iron yoke, and light weight. However, when the volume of the magnet is further reduced, the uniformity of the magnetic field becomes very poor. Therefore, how to design and optimize the magnet so that it can be small in size while maintaining a high uniformity of the magnet is a technical problem that the Halbach magnet urgently needs to solve. Summary of the Invention

[0005] In view of this, the object of the present invention is to provide a Halbach magnet and optimization method for head magnetic resonance imaging, which maximizes the magnetic flux density of the magnet by optimizing the magnet shape to form a highly uniform magnetic field in the target area, and can eliminate the need for an iron yoke to reduce the weight of the magnetic resonance imaging equipment.

[0006] In order to achieve the above object, the present invention provides the following technical solutions:

[0007] A Halbach magnet for head magnetic resonance imaging comprises: a main magnet, an end rotation adjustment unit, two shim rings, a main magnet base plate and an end rotation base plate;

[0008] The main magnet is composed of M magnetic ring arrays of the same diameter stacked axially to form a hollow cylinder, where M ≥ 2. Each layer of magnetic ring arrays is composed of N identical magnetic blocks arranged in a circular array on the main magnet substrate. Adjacent layers of magnetic ring arrays are coaxially connected by non-magnetic screws; the non-magnetic screws are also provided with gaskets of different thicknesses for adjusting the spacing between adjacent magnetic rings.

[0009] The end rotation adjustment unit is arranged on the periphery of one end of the main magnet and can rotate at any position along the circumferential direction of the magnetic ring; the end rotation adjustment unit includes several layers of axial magnetic rings and several radial magnetic rings, wherein the axial magnetic rings are coaxial with the main magnet and are stacked along the axial direction of the main magnet. The radius of each layer of axial magnetic rings is the same, and the total length of the axial magnetic rings is about 20 cm (the approximate distance from the shoulder to the top of the head of an adult is 20 cm). The radial magnetic rings are concentric with the main magnet ring and are stacked outward in the radial direction of the main magnet ring. The radius of the radial magnetic rings increases from the inside to the outside. The axial magnetic rings and the radial magnetic rings can slide at any position along the circular ring. The axial magnetic rings and the radial magnetic rings are used to adjust the magnetic field uniformity of the imaging area;

[0010] Two shim rings are pre-installed at both ends of the main magnet, closely attached to the inner wall within the cavity and coaxial with the main magnet. Their specific positions are adjusted through optimization methods. The shim rings generate a specific magnetic field in the target area to offset the inhomogeneous components of the static magnetic field.

[0011] Each layer of magnetic rings of the main magnet is correspondingly configured with a main magnet substrate, which is used for positioning the magnetic blocks assembled in the main magnet;

[0012] The end rotation substrate is used to fix the magnetic block in the end rotation adjustment unit.

[0013] Preferably, a slide rail and a positioning rod are provided on the end rotating substrate. The shape and thickness of the magnetic blocks on each end rotating substrate are different. The total length of each layer of magnetic blocks is pre-set to one-third of the circumference of its corresponding magnetic ring. The position of the magnetic block can be moved according to the examination part of the human body. The number, number of layers and positions of the magnetic blocks arranged on the actual radial magnetic ring are optimally arranged according to the genetic algorithm.

[0014] Preferably, the constituent magnetic blocks of the magnetic ring array are made of permanent magnet material, and the cross-sectional shape thereof can be a triangle, a trapezoid, a rectangle, a circle, an ellipse or a regular polygon.

[0015] Preferably, the gaps between the magnetic blocks and magnetic rings of the magnetic ring array are filled with non-magnetic conductive materials, and the non-magnetic conductive materials are used to fix the magnetic blocks.

[0016] Preferably, the main magnet substrate and the end rotating substrate are made of non-magnetic materials.

[0017] Preferably, the magnetization directions of the magnetic blocks of the magnetic ring array rotate regularly with the positions of the magnetic blocks, and the magnetization directions of every two adjacent magnetic blocks are deflected in sequence along the ring and symmetrically distributed. The position deflection angle θ of the i-th magnetic block is 2(i-1)·π / N, and the corresponding magnetization rotation angle ω of the magnetic block is i =2θ,i=0,1,…,N.

[0018] Preferably, the optimization method is to use a genetic algorithm to optimize the magnetic ring structure and the shim ring of the end adjustment unit, and the specific steps are:

[0019] S1: First adjust the axial magnetic ring of the end rotation adjustment unit. Assume that there are x axial magnetic rings and the total number of magnetic blocks on the axial magnetic ring is set to n. x From the outermost ring to the innermost ring, they are numbered n1, n2, n3, ..., n x , the magnetic blocks on the first circle of magnetic ring are numbered 1, 2, 3, ..., n1 in clockwise or counterclockwise direction; the magnetic blocks on the second circle of magnetic ring are numbered n1+1, n1+2, n1+3, ..., n2 in clockwise or counterclockwise direction, and so on. The magnetic blocks on the xth circle of magnetic ring are numbered n in clockwise or counterclockwise direction. x-1 +1,n x-1 +2,n x-1 +3,n x-1 +4,…,n x , where n1 <n2<n3<n4<…<n x ;

[0020] S2: The structural parameters of several magnetic blocks of the multiple magnetic rings of the end rotation adjustment unit are R1, T1, b1, h1, R2, T2, b2, h2, R3, T3, b3, h3, ..., R x 、T x 、b x 、h x Represented, several sets of magnetic block structural parameters of several magnetic rings are obtained, where R is the radius of the inner arc of the magnetic block, b is the thickness, h is the height, T is the angle of the arc, and the distances between the two shim rings and the center of the end rotation adjustment unit cylinder are recorded as r1, H1, r2, H2, r3, H3, ..., r x 、H x ;

[0021] S3: Encode each structural parameter variable by 3, 4 or 5-bit binary coding, dividing a variable into several equal parts;

[0022] S4: All structural parameter variables are grouped into a gene code (the number of characters in the gene code increases with the number of bits in the code), the number of characters (3, 4 or 5) * n x *6, where the first (3, 4 or 5) *6 fields are the variables of block 1 (R1, T1, b1, h1, r1, H1), and the last field is the variables of block n. x Variables (Rx, T X , b X , h X , r x 、H x );

[0023] S5: Genetically encode the radial sub-magnetic ring of the end rotation adjustment unit according to steps S1-S4;

[0024] S6: Select several samples and randomly generate several groups of structural parameter samples as the initial values ​​for optimization. Then, the computer calculates the magnetic field generated by each group of samples and the uniformity of the magnetic field in the target area. The samples are sorted according to the best uniformity, and two adjacent sample individuals are crossed to form a new sample individual.

[0025] S7: A certain field of the newly generated sample individual mutates with a certain probability to form a new individual group. The computer then calculates the magnetic field uniformity of the target area under each structural parameter.

[0026] S8: Repeat steps S6 to S7 several times until a magnetic block structure with optimal uniformity is obtained, thereby obtaining the structural parameters of each magnetic block and the specific position of the shim ring.

[0027] Furthermore, in step S1, when the plurality of magnetic rings of the end rotation adjustment unit are preliminarily optimized, the number of magnetic blocks on each magnetic ring may be equal or unequal.

[0028] The beneficial effects of the present invention are as follows: the Halbach magnet for head magnetic resonance imaging provided by the present invention adjusts the size of the magnetic blocks in each layer of the magnetic ring of the end adjustment unit on the basis of the traditional cylinder, and adds shim rings at both ends of the magnet to increase the compensation for the magnetic field in the target area of ​​the magnet, homogenize the magnetic field distribution in the target area, and effectively solve the end effect caused by the height reduction of traditional magnets. This structure does not require an external magnetic field compensation device, has a simple overall structure, and greatly reduces the difficulty of assembling the magnet. Therefore, the permanent magnet provided by the present invention can generate a magnetic field with high strength and uniformity in a small volume structure, and is suitable for portable nuclear magnetic resonance detection systems.

[0029] Other advantages, objects, and features of the present invention will be described in part in the following description and, in part, will be apparent to those skilled in the art upon examination of the following description or may be learned from practice of the present invention. The objects and other advantages of the present invention may be realized and obtained through the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention will be described in detail below with reference to the accompanying drawings, in which:

[0031] Figure 1 is an overall structural diagram of a magnetic resonance device;

[0032] Figure 2 This is the main magnet structure diagram;

[0033] Figure 3 It is a top view of the end rotation adjustment unit;

[0034] Figure 4 is a polarization direction diagram of the magnetic block in an embodiment of the present invention;

[0035] Figure 5 1 is a parameter structure diagram of a magnetic resonance magnet block in an embodiment of the present invention;

[0036] Figure 6 Schematic diagram of magnetic block encoding in an embodiment of the present invention.

[0037] Reference numerals: 1 - main magnet, 2 - end rotation adjustment unit. DETAILED DESCRIPTION

[0038] The following describes the embodiments of the present invention by means of specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments are only schematic illustrations of the basic concept of the present invention, and the following embodiments and features in the embodiments can be combined with each other without conflict.

[0039] Among them, the accompanying drawings are only for illustrative purposes and represent only schematic diagrams rather than actual pictures, and should not be understood as limiting the present invention. In order to better illustrate the embodiments of the present invention, some parts of the accompanying drawings may be omitted, enlarged or reduced, and do not represent the dimensions of actual products. For those skilled in the art, it is understandable that some well-known structures and their descriptions may be omitted in the accompanying drawings.

[0040] The same or similar numbers in the drawings of the embodiments of the present invention correspond to the same or similar parts; in the description of the present invention, it should be understood that if there are terms such as "upper", "lower", "left", "right", "front", "back", etc. indicating directions or positional relationships, they are based on the directions or positional relationships shown in the drawings. They are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific direction, be constructed and operate in a specific direction. Therefore, the terms describing the positional relationship in the drawings are only used for illustrative purposes and cannot be understood as limiting the present invention. For ordinary technicians in this field, the specific meanings of the above terms can be understood according to specific circumstances.

[0041] See also Figures 1 to 6 The present invention provides a Halbach magnet for head magnetic resonance imaging, comprising: a main magnet, an end rotation adjustment unit, two shim rings, a main magnet substrate and an end rotation substrate.

[0042] like Figure 1 As shown, the main magnet is composed of M magnetic ring arrays of the same diameter stacked axially to form a hollow cylinder, where M ≥ 2. Each layer of magnetic ring arrays is composed of N identical magnetic blocks arranged in a circular array on the main magnet substrate. Adjacent layers of magnetic ring arrays are coaxially connected by non-magnetic screws; the non-magnetic screws are also provided with gaskets of different thicknesses for adjusting the spacing between adjacent magnetic rings.

[0043] The end rotation adjustment unit is arranged on the periphery of one end of the main magnet and can rotate at any position along the circumferential direction of the magnetic ring. It is composed of several layers of axial magnetic rings and several radial magnetic rings, among which the axial magnetic rings are coaxial with the main magnet and stacked along the axial direction of the main magnet. The radius of each layer of axial magnetic rings is the same, and the total length of the axial magnetic rings is about 20 cm (the approximate distance from the shoulder to the top of the head of an adult is 20 cm). The radial magnetic rings are concentric with the main magnet sub-magnetic rings and are stacked outward in the radial direction of the main magnet magnetic ring. The radius of the radial magnetic rings increases from the inside to the outside. The axial magnetic rings and radial magnetic rings can slide at any position along the circular ring. The axial magnetic rings and radial magnetic rings are used to adjust the magnetic field uniformity in the imaging area.

[0044] Two shim rings are pre-set at both ends of the main magnet, and are close to the inner wall within the cavity and coaxial with the main magnet. The specific positions are optimized and adjusted. The shim rings generate a specific magnetic field in the target area to offset the inhomogeneous components in the static magnetic field.

[0045] The main magnet substrate is made of non-magnetic material. Each layer of magnetic rings of the main magnet is correspondingly configured with a main magnet substrate for positioning the magnetic block assembly.

[0046] The end rotating substrate is made of non-magnetic material and is equipped with a slide rail and a positioning rod. The shape and thickness of the magnetic blocks on each end rotating substrate are different. The total length of each layer of magnetic blocks is pre-set to one-third of the circumference of its corresponding magnetic ring. The position of the magnetic block can be moved according to the examination part of the human body. The number of magnetic blocks arranged on the actual radial magnetic ring, the number of layers and the position are optimized according to the genetic algorithm.

[0047] The gaps between the magnetic blocks and magnetic rings of the magnetic ring array are filled with non-magnetic conductive materials, and the non-magnetic conductive materials are used to fix the magnetic blocks.

[0048] The magnetic blocks that make up the magnetic ring array are made of permanent magnet materials, and their cross-sectional shapes can be triangles, trapezoids, rectangles, circles, ellipses, and regular polygons.

[0049] The present invention aims to optimize a magnet structure, and relates to a method for optimizing a magnet structure for magnetic resonance imaging.

[0050] The method for optimizing the magnetic resonance imaging magnet structure of the present invention is to optimize the magnetic ring structure and the shim ring of the end adjustment unit using a genetic algorithm, and specifically comprises the following steps:

[0051] S1. See Figures 1 to 4 , arrange several magnetic blocks into magnetic rings along the circumferential direction, and arrange several magnetic rings with the same diameter at intervals along the axial direction. When several magnetic rings are initially optimized, the number of magnetic blocks on each magnetic ring can be equal or different.

[0052] In this embodiment, five magnetic rings with different diameters are arranged radially at intervals, and the number of magnetic blocks on each magnetic ring is equal to form a magnet.

[0053] S2. Arrange the upper and lower magnetic rings in circumferential order, numbering the five magnetic rings from outermost to innermost as n, 2n, 3n, ..., 5n. The n magnetic blocks on the nth magnetic ring are numbered 1, 2, 3, ..., n in a clockwise or counterclockwise direction. The n magnetic blocks on the 2nth magnetic ring are numbered n+1, n+2, n+3, ..., n+n in a clockwise or counterclockwise direction. Similarly, the n magnetic blocks on the 5nth magnetic ring are numbered 4n+1, 4n+2, 4n+3, ..., 5n in a clockwise or counterclockwise direction. The magnetic blocks can be fan-shaped, rectangular, or trapezoidal.

[0054] S3, the structural parameters of the 5n magnetic blocks of the five magnetic rings are R1, T1, b1, h1, R2, T2, b2, h2, R3, T3, b3, h3, ..., R 5n 、T 5n 、b 5n 、h 5nRepresentation, five sets of magnetic block structural parameters of several magnetic rings are obtained, where R is the radius of the inner arc of the magnetic block, b is the thickness, h is the height, and T is the angle of the arc. The distances between the two shim rings and the center of the end adjustment unit cylinder are recorded as r1, H1, r2, H2, r3, H3, …, r5, H5 respectively.

[0055] S4. Encode each structural parameter variable using a 3- to 5-bit binary code, and divide the value range of a variable into several equal parts in millimeters. In this embodiment, the value range of a variable is divided into eight parts, for example, 000, 001, 010, ..., 111. In this example, if the range of h1 is 1mm to 15mm, it is evenly divided into 8 equally spaced components: 1 3 5 7 911 13 15. When the code value of h1 is 000, it means that the value of h1 is 1mm. When the value of h1 is 111, it means that the value of h1 is 15mm. Other variables are similar.

[0056] S5. All structural parameter variables are grouped into a gene code with 3*5n*6 characters. The first 3*6 fields are the variables of block 1 (R1, T1, b1, h1, r1, H1), and the last field is the variables of block 5n (R 5n , T 5n , b 5n , h 5n , r 5n , H 5n ).

[0057] S6. Select a number of samples. The number is not limited. The more samples, the more accurate the data. In this embodiment, 20 samples are selected. 20 groups of structural parameter samples are randomly generated as the initial values ​​for optimization:

[0058] 1 001 010 101 110 010 100 101 101……101 110 111 101

[0059] 2 001 011 101 100 010 000 011 001……101 010 111 100

[0060] 3 001 110 111 101 010 001 001 101……011 011 101 110

[0061] 4 101 010 111 110 001 011 101 001……011 110 111 101

[0062] …

[0063] 20 100 011 101 101 100 101 011 101……101 010 111 110

[0064] The computer then calculates the magnetic field generated by each group of samples, calculates the uniformity of the magnetic field in the target area, and then sorts the samples according to the best uniformity. It also crosses two adjacent sample individuals and sorts them into 1 and 2, 3 and 4, ... 19 and 20 to form new sample individuals. For example:

[0065] The original individual is:

[0066] 1 001 010 101 110 010 100 101 101……101 110 111 101

[0067] 2 001 011 101 100 010 000 011 001……101 010 111 100

[0068] After crossing the box position, two new sample individuals are formed:

[0069] 1 001 010 101 110 010 100 111 101……101 110 111 101

[0070] 2 001 011 101 100 010 000 001 001……101 010 111 100

[0071] S7. A certain field of the newly generated sample individual mutates with a certain probability, that is, changes from 1 to 0, or 0 to 0. For example, the 0 in the first new individual mentioned above changes to 1 (represented by the box):

[0072] 1 001 010 111 110 010 100 111 101……101 110 111 101

[0073] After forming a new group of individuals, the computer calculates the magnetic field uniformity of the target area under each structural parameter.

[0074] S8. Repeat steps S6 and S7 several times until a magnetic block structure with optimal uniformity is obtained, thereby obtaining the structural parameters of each magnetic block.

[0075] The above optimization results make the uniformity of the magnetic field in the target area optimal.

[0076] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not limiting. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solutions, which should all be included in the scope of the claims of the present invention.

Claims

1. A Halbach magnet for magnetic resonance imaging of the head, characterized in that The magnet includes a main magnet, an end rotation adjustment unit, two shim rings, a main magnet base plate and an end rotation base plate; The main magnet is composed of M magnetic ring arrays of the same diameter stacked axially to form a hollow cylinder, where M ≥ 2. Each layer of magnetic ring arrays is composed of N identical magnetic blocks arranged in a circumferential array on the main magnet substrate, and adjacent layers of magnetic ring arrays are coaxially connected by non-magnetic screws. The end rotation adjustment unit is arranged on the periphery of one end of the main magnet and can rotate at any position along the circumferential direction of the magnetic ring; the end rotation adjustment unit includes several layers of axial magnetic rings and several radial magnetic rings, wherein the axial magnetic rings are coaxial with the main magnet and are stacked along the axial direction of the main magnet, and the radius of each layer of axial magnetic rings is the same; the radial magnetic rings are concentric with the main magnet magnetic ring and are stacked outward in the radial direction of the main magnet magnetic ring, and the radius of the radial magnetic rings increases from the inside to the outside. The axial magnetic rings and the radial magnetic rings can slide at any position along the circular ring, and the axial magnetic rings and the radial magnetic rings are used to adjust the magnetic field uniformity of the imaging area; Two shim rings are pre-installed at both ends of the main magnet, closely attached to the inner wall within the cavity and coaxial with the main magnet. Their specific positions are adjusted through optimization methods. The shim rings generate a specific magnetic field in the target area to offset the inhomogeneous components of the static magnetic field. Each layer of magnetic rings of the main magnet is correspondingly configured with a main magnet substrate, which is used for positioning the magnetic blocks assembled in the main magnet; The end rotation substrate is used to fix the magnetic block in the end rotation adjustment unit.

2. The Halbach magnet for head magnetic resonance imaging according to claim 1, wherein The end rotating base plate is provided with a slide rail and a positioning rod. The shape and thickness of the magnetic blocks on each end rotating base plate are different. The total length of each layer of magnetic blocks is pre-set to one-third of the circumference of its corresponding magnetic ring. The position of the magnetic block moves according to the examination part of the human body. The number, number of layers and positions of the magnetic blocks arranged on the actual radial magnetic ring are optimally arranged according to the genetic algorithm.

3. The Halbach magnet for head magnetic resonance imaging according to claim 1, wherein The non-magnetic screw is also provided with gaskets of different thicknesses for adjusting the distance between adjacent magnetic rings.

4. The Halbach magnet for head magnetic resonance imaging according to claim 1, wherein The cross-sectional shape of the magnetic blocks constituting the magnetic ring array is a triangle, a trapezoid, a rectangle, a circle, an ellipse or a regular polygon.

5. The Halbach magnet for head magnetic resonance imaging according to claim 1 or 4, characterized in that The magnetic blocks of the magnetic ring array are made of permanent magnet material; The gaps between the magnetic blocks and magnetic rings of the magnetic ring array are filled with non-magnetic conductive materials, and the non-magnetic conductive materials are used to fix the magnetic blocks; The main magnet substrate and the end rotating substrate are made of non-magnetic conductive materials.

6. The Halbach magnet for head magnetic resonance imaging according to claim 1, wherein The total length of the axial magnetic ring is the distance from an adult's shoulder to the top of the head.

7. The Halbach magnet for head magnetic resonance imaging according to claim 1, wherein The magnetization directions of the magnetic blocks of the magnetic ring array rotate with the position of the magnetic blocks. The magnetization directions of the two adjacent magnetic blocks are deflected in sequence along the ring and are symmetrically distributed. The position deflection angle of the i-th magnetic block is θ = 2(i-1)·π / N, and the corresponding magnetization rotation angle ω of the magnetic block is i =2θ,i=0,1,…,N.

8. The Halbach magnet for head magnetic resonance imaging according to claim 1 or 2, characterized in that The optimization method is to optimize the magnetic ring structure and the shim ring of the end adjustment unit using a genetic algorithm, and the specific steps are as follows: S1: First adjust the axial magnetic ring of the end rotation adjustment unit. Assume that there are x axial magnetic rings and the total number of magnetic blocks on the axial magnetic ring is set to n. x From the outermost ring to the innermost ring, they are numbered n1, n2, n3, ..., n x , the magnetic blocks on the first circle of magnetic ring are numbered 1, 2, 3, ..., n1 in clockwise or counterclockwise direction; the magnetic blocks on the second circle of magnetic ring are numbered n1+1, n1+2, n1+3, ..., n2 in clockwise or counterclockwise direction, and so on. The magnetic blocks on the xth circle of magnetic ring are numbered n in clockwise or counterclockwise direction. x-1 +1,n x-1 +2,n x-1 +3,n x-1 +4,…,n x , where n1 <n2<n3<n4<…<n x ; S2: The structural parameters of the magnetic blocks of the multiple magnetic rings of the end rotation adjustment unit are R1, T1, b1, h1, R2, T2, b2, h2, R3, T3, b3, h3, ..., R x 、T x 、b x 、h x Represented, several sets of magnetic block structural parameters of several magnetic rings are obtained, where R is the radius of the inner arc of the magnetic block, b is the thickness, h is the height, T is the angle of the arc, and the distances between the two shim rings and the center of the end rotation adjustment unit cylinder are recorded as r1, H1, r2, H2, r3, H3, ..., r x 、H x ; S3: Encode each structural parameter variable by 3, 4 or 5-bit binary coding, dividing a variable into several equal parts; S4: All structural parameter variables are grouped into a gene code, the number of characters (3, 4 or 5) * n x *6, where the first (3, 4 or 5) *6 fields are the variables of block 1 (R1, T1, b1, h1, r1, H1), and the last field is the variables of block n. x Variables (Rx, T X , b X , h X , r x 、H x ); S5: Genetically encode the radial sub-magnetic ring of the end rotation adjustment unit according to steps S1-S4; S6: Select several samples and randomly generate several groups of structural parameter samples as the initial values ​​for optimization. Then, the computer calculates the magnetic field generated by each group of samples and the uniformity of the magnetic field in the target area. The samples are sorted according to the best uniformity, and two adjacent sample individuals are crossed to form a new sample individual. S7: A certain field of the newly generated sample individual mutates with a certain probability to form a new individual group. The computer then calculates the magnetic field uniformity of the target area under each structural parameter. S8: Repeat steps S6 to S7 several times until a magnetic block structure with optimal uniformity is obtained, thereby obtaining the structural parameters of each magnetic block and the specific position of the shim ring.

9. The Halbach magnet for head magnetic resonance imaging according to claim 8, characterized in that In step S1, when a plurality of magnetic rings of the end rotation adjustment unit are preliminarily optimized, the number of magnetic blocks on each magnetic ring is equal or unequal.

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