Design method of ultra-low field magnetic resonance imaging elliptic cylindrical magnet structure

Abstract

The invention relates to a design method of an ultra-low field magnetic resonance imaging elliptic cylinder type magnet structure, and belongs to the technical field of medical electromagnetic detection. According to the method, a shape-dependent method or a virtual magnet method is adopted to design a magnet structure, a magnet is rotated in a gear transmission mode, and a large gear on the magnet is tightly matched with a small gear driven by a motor to realize rotation phase control, so that sliding of the magnet when the magnet starts to rotate and stops rotating during friction force transmission can be overcome; therefore, high-resolution two-dimensional radio frequency phase coding imaging is realized. The magnet design method is novel, the design difficulty of the lightweight magnet can be reduced, the high uniformity of the main magnetic field of the magnet is ensured, a gradient coil system is not needed, two-dimensional radio frequency phase coding imaging is realized by rotating the magnet, and the complexity of equipment is reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of medical electromagnetic detection and relates to a design method of an ultra-low field magnetic resonance imaging elliptical cylindrical magnet structure. Background Art

[0002] The development of lightweight ultra-low field MRI technology has brought revolutionary changes to many fields. The mobility of this technology allows it to be easily used in different environments, such as emergency diagnosis and analysis of cerebral hemorrhage or cerebral infarction in the early diagnosis of stroke, or basic medical imaging diagnosis in developing countries or remote rural areas. In addition, its low cost makes ultra-low field MRI technology more popular and no longer limited to well-equipped laboratories or hospitals. Its structure is lightweight, small in size, easy to move, and it uses permanent magnets to provide the main magnetic field, which is inexpensive and has low energy consumption.

[0003] Traditional ultra-low-field nuclear magnetic resonance imaging methods require the use of gradient coils and gradient power amplifiers, and also need to take into account the uniformity of the main magnetic field of the ultra-low-field magnetic resonance imaging magnet and the lightweight of the magnet. The design is difficult, resulting in a decrease in the signal-to-noise ratio, which will affect the imaging details and contrast.

[0004] Therefore, it is necessary to adopt a new design method to improve the main magnetic field uniformity of ultra-low field MRI magnets. Under the premise of ensuring that the amount of magnet material used remains unchanged, the magnetic induction intensity and main magnetic field uniformity can be increased as much as possible to improve the performance of these magnet structure systems while maintaining their lightness and cost-effectiveness. Summary of the invention

[0005] In view of this, the purpose of the present invention is to provide a design method for an ultra-low field magnetic resonance imaging elliptical cylindrical magnet structure, reduce the difficulty of lightweight magnet design, ensure high uniformity of the main magnetic field of the magnet, do not require a gradient coil system, and realize two-dimensional radio frequency phase-encoded imaging by rotating the magnet, thereby reducing the complexity of the equipment.

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

[0007] A design method for an ultra-low field magnetic resonance imaging elliptical cylindrical magnet structure, which only relies on permanent magnets to construct a main magnetic field B0, adopts a shape-dependent method to design the magnet structure, and adjusts the magnetic induction intensity of the main magnetic field B0 by changing the ellipticity ε; or adopts a virtual magnet method to design the magnet structure, and uses a virtual permanent magnet method to determine the magnetization direction of each magnet position; and also adopts a gear transmission method to rotate the magnet to overcome the sliding of the magnet when it starts to rotate and stops rotating during friction transmission, so as to realize two-dimensional radio frequency phase encoding imaging.

[0008] The magnet structure designed by the shape-dependent method is: the magnet group is a magnetic ring composed of N magnet blocks of the same specifications and materials, the magnet blocks are arranged in an elliptical column structure with equal arc lengths, and the magnetization direction of each magnet block in the magnet group is related to the coordinates and ellipticity of its position.

[0009] The magnet structure is designed by using a virtual magnet method as follows: a magnet group is composed of N magnet blocks of the same specification and the same material to form a magnetic ring, and the magnet group is arranged in an elliptical cylindrical structure in a manner of equal arc length. It is assumed that a virtual elliptical cylindrical permanent magnet is placed in the magnet group, and the virtual elliptical cylindrical permanent magnet generates a closed virtual magnetic field around itself. Then, the magnetization direction of the virtual magnetic field at the geometric center position of each magnet block of the magnet group is used as the actual magnetization direction of each magnet block.

[0010] The gear transmission method of rotating the magnet is: the rotation phase control is achieved by close cooperation between the large gear on the magnet structure and the small gear driven by the motor, which can overcome the slippage of the magnet when it starts to rotate and stops rotating during friction transmission, thereby achieving high-resolution two-dimensional radio frequency phase-encoded imaging.

[0011] Further, the shape-dependent method for designing a magnet structure is specifically as follows: a magnet group is composed of a number (N) of magnet blocks of the same specification and material (needle iron boron or samarium cobalt) to form a magnetic ring, and the magnet blocks are arranged in an elliptical column structure in an equal arc length manner, and its cross section is elliptical, with a major semi-axis of b and a minor semi-axis of a; the ratio of the major axis to the minor axis (aspect ratio) is between 1:1 and 2:1; the magnetization direction of each magnet block in the magnet group is related to the coordinates of the position of each magnet and the ellipticity, that is, the condition is satisfied: α m =2arctan(k·ε·y m / x m ), where α m is the magnetization angle of the position of the mth magnet block, m = 1, 2, 3, ..., N-1, N, k is an optimization parameter in the range of 0.2 to 1.5; ε is the ellipticity of the minor semi-axis a of the ellipse divided by the major semi-axis b (ε = a / b), x m ,y m are the horizontal and vertical coordinates of the geometric center of the mth magnet block when the ellipticity ε is constant, arctan is the inverse tangent operation, or k·ε is regarded as an optimization parameter ε', so x m ,y m It is necessary to continuously adjust the calculation according to the ε' optimization parameter.

[0012] Furthermore, the virtual magnet method for designing a magnet structure specifically comprises the following steps:

[0013] (1) A virtual elliptical cylindrical permanent magnet is placed in a blank area. Theoretically, the length of the virtual elliptical cylindrical permanent magnet is required to be long enough. In actual operation, the aspect ratio is set between 5 and 10 so that the magnetic field in the central target area is not affected by the limited length of the magnet. The magnetization direction of the virtual elliptical cylindrical permanent magnet is along the radial direction of the minor semi-axis of the ellipse. The electromagnetic field finite element method is used to calculate the closed virtual magnetic field generated by the virtual elliptical cylindrical permanent magnet around itself.

[0014] (2) a magnet group is placed outside and around the virtual elliptical cylindrical permanent magnet, the cross section of the magnet group is elliptical, and the ellipticity of the magnet group is required to be the same as the ellipticity of the virtual elliptical cylindrical permanent magnet; the magnet group is a magnetic ring composed of a number (N) of magnet blocks of the same specification and material (needle iron boron or samarium cobalt), the magnet blocks are arranged in an elliptical cylindrical structure in an equal arc length manner, the major semi-axis is b, and the minor semi-axis is a; the elliptical magnet structure composed of the magnet group requires the ratio of the major axis to the minor axis (aspect ratio) to be between 1:1 and 2:1, so as to determine the position of the geometric center of each magnet block in the magnet group;

[0015] (3) The virtual magnetic field magnetization direction of each magnet block in the magnet group at the geometric center position is taken as the actual magnetization direction of each magnet block, that is, α n =arctan(By n / Bx n ), α n is the magnetization angle of the nth magnet block, 1, 2, 3, ..., N-1, N, By n , Bx n They are the geometric center positions of the nth magnet block (the horizontal and vertical coordinates are x n ,y n )'s virtual magnetic field y-direction magnetic field component and x-direction magnetic field component;

[0016] (4) By removing the virtual elliptical cylindrical permanent magnet, the elliptical cylindrical magnet structure composed of the magnet group can generate a main magnetic field with higher uniformity.

[0017] Furthermore, the housing, large gear and small gear of the fixed magnet group are processed by computerized numerical control (CNC) with high processing accuracy; the large gear and the housing of the fixed magnet group are fixed by slot connection or copper screws.

[0018] Furthermore, the gear transmission structure is as follows: two small gears are connected in series through two copper tubes respectively, and the four small gears are on the same horizontal plane. The long tube connecting the two small gears in series is connected to the stepper motor through a coupling and driven by the stepper motor. The distance between the two small gears is d1 and they serve as driving wheels. In addition, the short tube connecting the two small gears in series is placed in parallel with the long tube and the distance between them is d2. The distance between the two small gears is also d1 and they serve as driven wheels. Let the vertical distance from the geometric center of the large gear (i.e. the geometric center of the magnet) to the plane of the center points of the four small gears be d3, and the vertical distance from the center of the large gear to the long tube connecting the two driving wheel small gears in series be d4. In order to make the large and small gears work closely together, (d4) must be satisfied. 2 =(d2) 2 +(d3) 2 .

[0019] Furthermore, the stepper motor may be a closed-loop stepper motor, the holding torque is determined by the total weight of the magnet structure and the outer diameter of the large gear that wraps the magnet, and the step angle is 1.8°±5%.

[0020] The beneficial effects of the present invention are as follows: the design method of the ultra-low field magnetic resonance imaging elliptical cylindrical magnet structure provided by the present invention is simple, and the magnetization direction of each magnet position can be easily determined, while improving the magnetic field uniformity of the main magnetic field B0 in the target area. The ultra-low field magnetic resonance imaging elliptical cylindrical magnet structure designed by the method is small in size, does not require gradient coils and gradient power amplifiers for gradient magnetic field encoding, and has a simplified structure, light weight, and reliable performance. Under the same magnet block material conditions, a higher main magnetic field uniformity and a greater magnetic induction intensity can be generated in the cavity, and signal measurement of nuclear magnetic resonance imaging can be realized, and it is suitable for ultra-low field magnetic resonance imaging.

[0021] The design method of the ultra-low field magnetic resonance imaging elliptical cylindrical magnet structure provided by the present invention can be used in a low-cost portable MRI system. It can be used for stroke detection, image monitoring of patients in intensive care units, detection of joint bone age, and analysis of sample components. It can be used to carry out health checks in poor and remote areas, and realize lightweight, reliable, non-invasive and continuous monitoring, which has broad application prospects.

[0022] Other advantages, objectives and features of the present invention will be described in the following description to some extent, and to some extent, will be obvious to those skilled in the art based on the following examination and study, or can be taught from the practice of the present invention. The objectives and other advantages of the present invention can be realized and obtained through the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] 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 in conjunction with the accompanying drawings, wherein:

[0024] Figure 1 An overall schematic diagram of the ultra-low field magnetic resonance imaging elliptical cylindrical magnet structure designed for the present invention;

[0025] Figure 2 Schematic diagram of the magnet structure designed using the shape-dependent method;

[0026] Figure 3 The process of determining the magnetization direction of the magnetic block for the magnetic structure designed for the virtual magnet method;

[0027] Figure 4 It is a schematic diagram of rotating magnets in gear transmission mode;

[0028] Figure numerals: 1-large gear with elliptical cylindrical magnet structure; 2-driving pinion connected to the motor through a coupling; 3-driven pinion; 4-magnet block unit in the magnet group; 5-housing for fixing the magnet block unit; 6-target area. DETAILED DESCRIPTION

[0029] The following describes the embodiments of the present invention by 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 only illustrate the basic concept of the present invention in a schematic manner, and the following embodiments and features in the embodiments can be combined with each other without conflict.

[0030] Among them, the drawings are only used for illustrative explanations, and they only represent schematic diagrams rather than actual pictures, and should not be understood as limitations on the present invention. In order to better illustrate the embodiments of the present invention, some parts of the drawings may be omitted, enlarged or reduced, and do not represent the size of actual products. For those skilled in the art, it is understandable that some well-known structures and their descriptions in the drawings may be omitted.

[0031] 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 the terms "upper", "lower", "left", "right", "front", "rear", etc. indicate the orientation or position relationship, they are based on the orientation or position relationship shown in the drawings, which is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operate in a specific orientation. Therefore, the terms describing the position 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.

[0032] See also Figure 1 to Figure 4 , Figure 1 The overall schematic diagram of the ultra-low field magnetic resonance imaging elliptical cylindrical magnet structure designed for the present invention is as follows: Figure 1 As shown, the method for designing the magnet structure includes: using a shape-dependent method or a virtual magnet method to design the magnet structure, and using a gear transmission method to rotate the magnet to achieve two-dimensional radio frequency phase-encoded imaging.

[0033] 1) Designing the magnet structure by shape-dependent method, specifically: the magnet group is composed of a number (N) of magnet blocks of the same specification and the same material (NdFeB or SmCo) to form a magnetic ring, and the magnet group is arranged in an elliptical column structure with equal arc length, and its cross section is elliptical, with the major semi-axis b and the minor semi-axis a; the ratio of the major axis to the height (aspect ratio) is between 1:1 and 2:1; the magnetization direction of each magnet in the magnet group is related to the coordinates and ellipticity of the location of each magnet, that is, the condition is satisfied: α m =2arctan(k·ε·y m / x m ), α m is the magnetization angle of the position of the mth magnet, m = 1, 2, 3, ..., N-1, N, k is an optimization parameter ranging from 0.2 to 1.5; ε is the ellipticity of the minor semi-axis of the ellipse divided by the major semi-axis (ε = a / b), x m ,y m are the horizontal and vertical coordinates of the geometric center of the mth magnet when the ellipticity ε is constant. arctan is the inverse tangent operation. k·ε can also be regarded as an optimization parameter ε', so x m ,y m The calculation should be adjusted continuously according to the ε' optimization parameter. The shape-dependent method is used to design the elliptical cylindrical Halbach magnet structure. Figure 2 shown.

[0034] 2) Virtual magnet method is used to design the magnet structure. The design is divided into the following four steps:

[0035] (a) A virtual elliptical cylindrical permanent magnet is placed in a blank area. Theoretically, the length of the virtual permanent magnet is required to be long enough. In actual operation, the aspect ratio is set between 5 and 10 so that the magnetic field in the central target area can avoid the influence of the limited length of the magnet. The magnetization direction of the virtual permanent magnet is along the radial direction of the minor semi-axis of the ellipse. The electromagnetic field finite element method is used to calculate the closed virtual magnetic field generated by the virtual permanent magnet around itself.

[0036] (b) A magnet group of the same specifications is placed around the virtual permanent magnet with equal arc length. The ellipticity of the placed magnet group is required to be the same as the ellipticity of the virtual permanent magnet. The magnet group is composed of a number (N) of magnet blocks of the same specifications and the same material (NdFeB or SmCo) to form a magnetic ring. The cross-section of the magnet group is elliptical, with the major semi-axis b and the minor semi-axis a. The elliptical magnet structure composed of the magnet group requires the ratio of the major axis to the minor axis (aspect ratio) to be between 1:1 and 2:1. The position of the geometric center of each magnet in the magnet group can be determined.

[0037] (c) The virtual magnetic field magnetization direction of each magnet in the magnet group at the geometric center is taken as the actual magnetization direction of each magnet, that is, α n =arctan(By n / Bx n ), α n is the magnetization angle of the nth magnet, n = 1, 2, 3, ..., N-1, N, By n , Bx n is the location of the geometric center of the nth magnet (the horizontal and vertical coordinates are x n ,y n )’s virtual magnetic field includes the y-direction magnetic field component and the x-direction magnetic field component.

[0038] (d) Remove the virtual elliptical permanent magnet, and the elliptical magnet structure composed of the magnet group can generate a main magnetic field with high uniformity. The process of determining the magnetization direction of the magnetic block by the virtual permanent magnet method is as follows: Figure 3 shown.

[0039] 3) Gear transmission method to rotate the magnet, specifically: the rotation phase control is achieved by closely matching the large gear on the magnet with the small gear driven by the motor, which can overcome the slippage of the magnet when it starts to rotate and stops rotating during friction transmission, thereby achieving high-resolution two-dimensional radio frequency phase encoding imaging. The shell and large gear of the fixed magnet group are processed by computerized numerical control (CNC) with high processing accuracy, and the material is polyoxymethylene (Polyformaldehyde), POM. Considering the wear resistance and high precision requirements of the small gear, the small gear is processed by computerized numerical control (CNC) with high processing accuracy, and the material is a new type of polyamide engineering plastic MC nylon (Monomer casting nylon, MC). The large gear and the shell of the fixed magnet group are fixed by slot socket or copper screws.

[0040] The gear transmission structure is: two small gears are connected in series through two aluminum tubes or copper tubes respectively, and the four small gears are on the same horizontal plane. The long aluminum or copper tube connecting the two small gears in series is connected to the stepper motor through a coupling and driven by the stepper motor. The distance between the two small gears is d1 and they serve as driving wheels. In addition, the short aluminum or copper tube connecting the two small gears in series is placed in parallel with the long aluminum or copper tube and the distance is d2. The distance between the two small gears is also d1 and they serve as driven wheels. Let the vertical distance from the geometric center of the large gear (i.e. the geometric center of the magnet) to the plane of the center points of the four small gears be d3, and the vertical distance from the center of the large gear to the long aluminum or copper tube connecting the two driving wheel small gears in series be d4. In order to make the large and small gears work closely together, (d4) must be satisfied. 2 =(d2) 2 +(d3) 2 The stepper motor can be a closed-loop stepper motor. The holding torque is determined by the total weight of the magnet structure and the outer diameter of the large gear that wraps the magnet. The step angle is 1.8°±5%. The gear transmission mode rotates the magnet with large and small gears as follows: Figure 4 shown.

[0041] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention and are not limiting. The virtual magnet design method can design an elliptical magnet or a magnet of any other shape. Although the present invention is described in detail with reference to the preferred embodiments, a person of ordinary skill in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solution, which should be included in the scope of the claims of the present invention.

Claims

1. A method for designing an elliptical cylindrical magnet structure for ultra-low field magnetic resonance imaging, characterized in that: The method includes designing a magnet structure by using a shape-dependent method, and rotating the magnet by a gear transmission method to realize two-dimensional radio frequency phase-encoded imaging; The magnet structure designed by the shape-dependent method is: the magnet group is a magnetic ring composed of N magnet blocks of the same specification and material, the magnet blocks are arranged in an elliptical column structure in a manner of equal arc length, and the magnetization direction of each magnet block in the magnet group is related to the coordinates and ellipticity of the position where it is located; The gear transmission method of rotating the magnet is to achieve rotation phase control through close cooperation between the large gear on the magnet structure and the small gear driven by the motor.

2. The method for designing an ultra-low field magnetic resonance imaging elliptical cylindrical magnet structure according to claim 1, characterized in that: The shape-dependent method for designing a magnet structure is specifically as follows: a magnet group is composed of N magnet blocks of the same specification and material to form a magnetic ring, and the magnet blocks are arranged in an elliptical column structure in an equal arc length manner, and its cross section is an ellipse, with a major semi-axis of b and a minor semi-axis of a; the ratio of the major axis to the minor axis is between 1:1 and 2:1; the magnetization direction of each magnet block in the magnet group is related to the coordinates and ellipticity of the position of each magnet block, that is, the condition is satisfied: α m =2arctan(k·ε·y m / x m ), where α m is the magnetization angle of the position of the mth magnet block, m = 1, 2, 3, ..., N-1, N, k is the optimization parameter; ε is the ellipticity of the ellipse's minor axis a divided by its major axis b, x m ,y m are the horizontal and vertical coordinates of the geometric center of the mth magnet block when the ellipticity ε is constant, arctan is the inverse tangent operation, or k·ε is regarded as an optimization parameter ε', so x m ,y m It is necessary to continuously adjust the calculation according to the ε' optimization parameter.

3. The method for designing an ultra-low field magnetic resonance imaging elliptical cylindrical magnet structure according to claim 1, characterized in that: A virtual magnet method is used to replace the shape-dependent method to design the magnet structure. Specifically, the magnet group is composed of N magnet blocks of the same specifications and materials to form a magnetic ring. The magnet group is arranged into an elliptical cylindrical structure with equal arc lengths. It is assumed that a virtual elliptical cylindrical permanent magnet is placed in the magnet group. The virtual elliptical cylindrical permanent magnet generates a closed virtual magnetic field around itself. Then the magnetization direction of the virtual magnetic field at the geometric center position of each magnet block in the magnet group is used as the actual magnetization direction of each magnet block.

4. The method for designing an ultra-low field magnetic resonance imaging elliptical cylindrical magnet structure according to claim 1, characterized in that: The virtual magnet method for designing a magnet structure specifically comprises the following steps: (1) A virtual elliptical cylindrical permanent magnet is placed in a blank area. The length of the virtual elliptical cylindrical permanent magnet is required to be long enough so that the magnetic field in the central target area is not affected by the limited length of the magnet. The magnetization direction of the virtual elliptical cylindrical permanent magnet is along the radial direction of the minor semi-axis of the ellipse. The electromagnetic field finite element method is used to calculate the closed virtual magnetic field generated by the virtual elliptical cylindrical permanent magnet around itself. (2) a magnet group is placed outside and around the virtual elliptical cylindrical permanent magnet, the cross section of the magnet group is elliptical, and the ellipticity of the magnet group is required to be the same as the ellipticity of the virtual elliptical cylindrical permanent magnet; the magnet group is a magnetic ring composed of N magnet blocks of the same specifications and materials, the magnet blocks are arranged in an elliptical cylindrical structure in an equal arc length manner, the major semi-axis is b, and the minor semi-axis is a; the elliptical magnet structure composed of the magnet group requires the ratio of the major axis to the minor axis to be in the range of 1:1 to 2:1, so as to determine the position of the geometric center of each magnet block in the magnet group; (3) The virtual magnetic field magnetization direction of each magnet block in the magnet group at the geometric center position is taken as the actual magnetization direction of each magnet block, that is, α n =arctan(By n / Bx n ), α n is the magnetization angle of the nth magnet block, 1, 2, 3, ..., N-1, N, By n , Bx n They are respectively the y-direction magnetic field component and the x-direction magnetic field component of the virtual magnetic field at the location of the geometric center of the nth magnet block; (4) By removing the virtual elliptical cylindrical permanent magnet, the elliptical cylindrical magnet structure composed of the magnet group can generate a main magnetic field with high uniformity.

5. The method for designing an ultra-low field magnetic resonance imaging elliptical cylindrical magnet structure according to any one of claims 1 to 4, characterized in that: The housing, large gear and small gear of the fixed magnet group are processed by computer digital control with high processing accuracy; the large gear and the housing of the fixed magnet group are fixed by slot connection or copper screws.

6. The method for designing an ultra-low field magnetic resonance imaging elliptical cylindrical magnet structure according to any one of claims 1 to 4, characterized in that: The gear transmission structure is: two small gears are connected in series through two copper tubes respectively, and the four small gears are on the same horizontal plane. The long tube connecting the two small gears in series is connected to the stepper motor through a coupling and driven by the stepper motor. The distance between the two small gears is d1 and they serve as driving wheels. In addition, the short tube connecting the two small gears in series is placed in parallel with the long tube and the distance is d2. The distance between the two small gears is also d1 and they serve as driven wheels. Let the vertical distance from the geometric center of the large gear, that is, the geometric center of the magnet, to the plane of the center points of the four small gears be d3, and the vertical distance from the center of the large gear to the long tube connecting the two driving wheel small gears in series be d4. In order to make the large and small gears work closely together, (d4) must be satisfied. 2 =(d2) 2 +(d3) 2 .

7. The method for designing an ultra-low field magnetic resonance imaging elliptical cylindrical magnet structure according to claim 6, characterized in that: The stepper motor adopts a closed-loop stepper motor. The holding torque is determined by the total weight of the magnet structure and the outer diameter of the large gear that wraps the magnet. The step angle is 1.8°±5%.

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