Low-field permanent magnet type MRI magnetic circuit design method and magnet system thereof

Abstract

The invention discloses a low-field permanent magnet type MRI magnetic circuit design method and a magnet system thereof, and belongs to the technical field of magnetic resonance imaging. A sintered neodymium-iron-boron magnet with a high magnetic energy product and a sintered samarium-cobalt magnet with an excellent temperature characteristic are utilized to form the annular hybrid magnetic pole disc, the sintered samarium-cobalt magnet is located in the center area of the hybrid magnetic pole disc, and the sintered neodymium-iron-boron magnet is located in the edge area of the hybrid magnetic pole disc. And finally, the low-field MRI magnet system with light weight and excellent uniformity and temperature stability of the magnetic field is realized. The total weight of the magnet system is less than 400kg, and the central magnetic field intensity of the magnet system reaches 0.07 T; meanwhile, in a sphere space with the diameter of 200 mm and with the center of the magnet system as the sphere center, the magnetic field non-uniformity at the boundary is smaller than 100 ppm, and the fluctuation of the magnetic field non-uniformity at the boundary along with the temperature is also smaller than 100 ppm / DEG C.

Description

Technical Field

[0001] This invention relates to the magnetic circuit design and manufacture of permanent magnet magnetic resonance imaging (MRI) magnet systems, specifically to a lightweight, low-field permanent magnet MRI magnetic circuit design and magnet system with excellent magnetic field uniformity and temperature stability. Background Technology

[0002] Magnetic resonance imaging (MRI) is one of the most successful devices currently available for the diagnosis and treatment of human diseases. Its greatest advantage lies in the fact that it is one of the few safe, rapid, and accurate clinical diagnostic methods that causes no harm to the human body.

[0003] The main magnetic field B0 of an MRI machine, also called the static magnetic field, is a uniform magnetic field generated within the working space of the magnet aperture (usually ≤70cm) for scanning and imaging. The magnetic field strength of the MRI machine is the magnitude of this magnetic field; the higher the magnetic field strength, the higher the image signal-to-noise ratio and the better the image quality.

[0004] Superconducting high-field MRI is widely used due to its high magnetic field strength, resulting in high image resolution. However, it is costly to maintain, immobile, and time-consuming. Patients undergoing MRI scans must spend nearly half an hour alone in a dark, noisy, and confined environment, which can cause discomfort. Therefore, low-cost, short-time, lightweight (or portable), and open-type MRI is currently a hot research topic.

[0005] Permanent magnet MRI offers advantages such as low cost (including low maintenance costs, low failure rate, and low energy consumption). However, traditional mid-field (greater than 0.3 T) or high-field (greater than 1 T) permanent magnet MRI devices remain bulky (weighing tens of tons), immobile, and require long examination times. Low-field (less than 0.3 T) permanent magnet MRI has limited applications due to its low image resolution. In recent years, the development of artificial intelligence technology has greatly improved the imaging quality of low-field permanent magnet MRI, making it possible to use low-field MRI for disease prevention and diagnosis.

[0006] Since only low-field permanent magnet MRI can achieve lightweight (less than 500 kg) and portability, thus moving it out of the traditional radiology department with its constant temperature environment and into hospital emergency rooms, other departments, remote areas, sports fields, and daily living spaces, solving problems such as long patient waiting times and high testing costs, the research and development of lightweight low-field permanent magnet MRI with excellent temperature stability is one of the important future development directions.

[0007] We believe that this new type of low-field permanent magnet MRI magnet system should have the following characteristics: (1) lightweight (less than 500 kg) and easy to move; (2) the magnetic field should not only have excellent uniformity, but also good temperature stability, so that it can be applied to everyday life scenarios.

[0008] Currently, the main magnetic field of low-field permanent magnet MRI systems under development primarily consists of multi-ring magnetic pole disks made of a single material: sintered NdFeB magnets or sintered Samarium Cobalt magnets. Each ring of the pole disk is composed of multiple magnets, with gaps between the magnets and between the rings themselves. These gaps are to prevent the alternating magnetic field generated by the gradient coils from inducing large eddy currents in the sintered magnets. While sintered NdFeB magnets have a high energy product, their temperature stability is poor. Therefore, low-field permanent magnet MRI systems constructed with NdFeB magnets need to consider the impact of environmental factors or alternating magnetic fields on magnet temperature variations and their effect on image quality. Sintered Samarium Cobalt magnets have excellent temperature characteristics but a low energy product, making magnet systems constructed with them relatively bulky and several times more expensive than those made with NdFeB magnets. Summary of the Invention

[0009] The purpose of this invention is to provide a magnetic circuit design method and magnet system for a low-field permanent magnet MRI device that is lightweight and has excellent magnetic field uniformity and temperature stability.

[0010] The design concept of this invention is to combine the advantages of sintered NdFeB magnets and sintered Samarium Cobalt magnets to research and develop a permanent magnet low-field MRI magnet system hybridized with sintered NdFeB and Samarium Cobalt magnets. The use of sintered NdFeB magnets primarily leverages their high energy product to reduce the weight of the magnet system; while the use of sintered Samarium Cobalt magnets mainly utilizes their excellent temperature characteristics, ultimately ensuring the development of a low-field permanent magnet MRI system that combines lightweight construction with excellent magnetic field temperature characteristics. The core focus of this invention is how to fully utilize the advantages of both sintered NdFeB and Samarium Cobalt magnets while avoiding their disadvantages, forming an organically unified whole.

[0011] To achieve the above-mentioned technical objectives, in a first aspect of the present invention, a method for designing a low-field permanent magnet MRI magnetic circuit is provided. This method involves a circular magnetic pole disk composed of a hybrid of two materials: a high-energy-product sintered NdFeB magnet and a high-temperature-stability sintered Samarium Cobalt magnet. The Samarium Cobalt magnet is located in the central region of the magnetic pole disk, while the NdFeB magnet is located in the edge region. Then, based on the specifications, dimensions, and field strength requirements of the MRI magnet system, a low-field permanent magnet MRI magnet system is designed and constructed through electromagnetic field simulation and modeling.

[0012] Furthermore, the magnetic pole disk is composed of multiple ring-shaped magnetic circuits, each ring consisting of multiple magnets with gaps between them, and also with gaps between magnets in adjacent rings. Preferably, the central region of the magnetic pole disk consists of at least one ring-shaped magnetic circuit composed of multiple sintered samarium cobalt magnets; the edge region of the magnetic pole disk consists of at least one ring-shaped magnetic circuit composed of multiple sintered neodymium iron boron magnets.

[0013] The gaps between the magnets are left to avoid generating large eddy currents in the sintered magnets. Preferably, the gap size between the magnets is 2-5 mm.

[0014] The magnetic circuit is designed based on the technical parameters of the MRI magnet system used for examination of specific areas. The main technical parameters include: the diameter of the magnetic pole disk, the distance between the two magnetic pole disks, the magnetic field strength, and the magnetic properties of the sintered NdFeB and SmCo magnets.

[0015] The magnetic circuit design can be obtained by simulating the magnetic field using finite element method (FEM) calculations.

[0016] In some designs of this invention, the central region of the magnetic pole disk is composed of at least one (e.g., 1-6) annular magnetic circuits made of sintered samarium-cobalt magnets; the edge region of the magnetic pole disk is composed of at least one (e.g., 1-6) annular magnetic circuits made of sintered neodymium-iron-boron magnets. Each annular magnetic circuit consists of multiple sector magnets. To achieve high uniformity of the central magnetic field, the outermost magnetic circuit adopts a design strategy where the left and right sides are higher than the front and back sides, i.e., the magnets on the left and right sides are higher than the magnets on the front and back sides.

[0017] In a second aspect of the present invention, based on the above-described magnetic circuit design method, a low-field permanent magnet MRI magnet system that combines lightweight design with excellent magnetic field uniformity and temperature stability is realized, mainly comprising: Hybrid magnetic pole disk: Used to generate the main magnetic field, it consists of two completely mirror-symmetrical magnetic pole disks. Each magnetic pole disk consists of a sintered samarium cobalt magnet in the central region and a sintered neodymium iron boron magnet in the edge region. The samarium cobalt magnet and neodymium iron boron magnet in the two magnetic pole disks are installed in the same way. They are assembled in the vertical direction to form the N pole and S pole respectively.

[0018] Yoke support: used to achieve magnetic field converging effect. The yoke support is used to fix the position of the upper and lower magnetic pole disks and connect the magnetic lines of force of the two magnetic pole disks through the yoke support. Anti-eddy current disk: Used to resist eddy currents generated in the yoke when the gradient coil is working, and is set between the yoke support and the magnetic pole disk.

[0019] Furthermore, the yoke support is made of high-purity iron with a purity greater than 99.99%, and its dimensions are designed according to the size of the magnetic pole disk and the strength of the central magnetic field; the anti-eddy current disk is made of a magnetically conductive but non-conductive material, and its dimensions are also designed according to the size of the magnetic pole disk and the strength of the central magnetic field.

[0020] In some specific embodiments of the present invention, anisotropic sintered NdFeB magnets with a magnetic energy product greater than 52 MGOe and anisotropic sintered SmCo magnets with excellent temperature characteristics (remanence temperature coefficient better than -0.03 % / ℃ and Curie temperature higher than 800℃) are machined according to the specifications and dimensions of the magnets and yokes obtained by the magnetic circuit design, and then assembled into an MRI main magnetic field system that can be used for examination of specific parts.

[0021] The low-field permanent magnet MRI magnet system designed and assembled in this invention provides the main magnetic field of the magnet by a ring-shaped magnetic pole disk composed of sintered NdFeB magnets and sintered SmCo magnets, achieving lightweight design and excellent magnetic field uniformity and temperature stability. The remanence temperature coefficient of the magnet system is better than -0.08 % / ℃. Attached Figure Description

[0022] Figure 1 A schematic diagram of a U-shaped low-field MRI magnet system designed in this embodiment of the invention, consisting of a hybrid annular magnetic pole disk composed of a high-energy-product sintered NdFeB magnet and a high-temperature-stability sintered SmCo magnet.

[0023] Figure 2 This embodiment of the invention describes the specifications and dimensions of two rings of sintered NdFeB magnets in the outer region of the hybrid annular magnetic pole disk, obtained through electromagnetic field simulation. Three types of sector magnets, A-1, A-2, and B, were used. The upper figure is a top view, and the lower figure is a front view. The figures indicate the specific dimensions of each sector magnet. Length units in the figures are in mm.

[0024] Figure 3 This embodiment of the invention presents the specifications and dimensions of five sintered samarium-cobalt magnets in the central region of a hybrid annular magnetic pole disk, obtained through electromagnetic field simulation. Five types of sector magnets, with specifications C, D, E, F, and G, were used. The upper figure is a top view, and the lower figure is a front view, with the specific dimensions of each sector magnet labeled in the figures. The unit of length in the figures is mm.

[0025] Figure 4 The image shows a low-field MRI magnet system with a magnetic field strength of 0.07 T, which is composed of a hybrid annular magnetic pole disk of sintered NdFeB magnets and sintered SmCo magnets, constructed in this embodiment of the invention. The left image shows the entire system structure, and the right image shows the magnetic pole disk structure.

[0026] Figure 5The design drawing of the yoke support in this embodiment of the invention is shown in the top view and the bottom view. The length unit in the drawings is mm. Detailed Implementation

[0027] The present invention will be further described below with reference to the accompanying drawings and embodiments to help those skilled in the art to further understand the invention, but without limiting the scope of the invention in any way. It should be noted that those skilled in the art can make several changes and improvements without departing from the concept of the present invention.

[0028] Example 1 This embodiment designs and constructs a low-field MRI magnet system with a U-shaped magnetic pole disk diameter of 58 cm, a space height of 32 cm between the upper and lower disks, and a magnetic field strength of 0.07 T. The magnet system mainly comprises three parts: sintered NdFeB and sintered SmCo magnets for generating the main magnetic field; a yoke support for magnetic field converging effects; and an anti-eddy current disk to resist eddy currents generated in the yoke during gradient coil operation. The main magnetic field of the magnet system is provided by a ring-shaped magnetic pole disk composed of a hybrid of sintered NdFeB and sintered SmCo magnets, achieving lightweight design and excellent magnetic field uniformity and temperature stability.

[0029] like Figure 1 As shown, a hybrid annular magnetic pole disk is constructed from high-energy-product sintered NdFeB magnets (energy product of 52.7 MGOe) and high-temperature-characteristic sintered Samarium-Cobalt magnets (remanence temperature coefficient better than -0.03 % / ℃ and Curie temperature higher than 800℃). The hybrid multi-ring magnetic pole disk circuit includes an upper group of 7 rings and a lower group of 7 rings, which are mirror-symmetrical. These two ring groups are composed of both sintered NdFeB and sintered Samarium-Cobalt magnets. The five rings in the center of each ring group are sintered Samarium-Cobalt magnets, while the two outer rings are sintered NdFeB magnets. Each ring in the multi-ring magnetic circuit consists of multiple magnets, with gaps between the magnets and between the rings themselves. These gaps between the magnets are to prevent the generation of large eddy currents within the sintered magnets. The upper ring assembly forms the N pole, and the lower ring assembly forms the S pole. The remaining part mainly consists of the yoke support connecting the N and S poles. This support not only achieves the magnetic field converging effect, increasing the magnetic field strength, but also ensures the stability of the mechanical structure of the magnet system. Furthermore, to counteract the temperature rise of the magnet system caused by eddy currents generated in the yoke during gradient coil operation, anti-eddy current disks are designed and installed between the upper yoke and the hybrid ring magnetic pole disk, and between the lower yoke and the hybrid ring magnetic pole disk.

[0030] The magnetic field of a hybrid 7-turn toroidal magnetic circuit composed of sintered NdFeB and sintered SmCo magnets was simulated and designed using finite element method (FEM) calculations. The parameters used in the simulation and design are as follows: the diameter of the magnetic pole disk is 58 cm, the distance between the upper and lower magnetic pole disks is 32 cm, and the magnetic field strength is 0.07 T; the remanence of the sintered NdFeB magnet is 1.474 T, the coercivity is 14.39 kOe, the energy product is 52.7 MGOe, and the remanence temperature coefficient is -0.1 % / ℃; the remanence of the sintered SmCo magnet is 1.200 T, the coercivity is 20.86 kOe, the energy product is 33.06 MGOe, and the remanence temperature coefficient is -0.03 % / ℃.

[0031] Based on the simulation results, the specifications and dimensions of the two outer rings of sintered NdFeB magnets in the hybrid 7-ring annular magnetic pole disk are as follows: Figure 2 As shown. The outermost magnetic ring consists of 50 A-1 type and 34 A-2 type sintered NdFeB magnets of two specifications and sizes. The thicker A-1 type magnets are symmetrically placed on the left and right sides of the outermost ring, while the thinner A-2 type magnets are symmetrically placed on the front and back sides of the outermost ring. The next outermost magnetic ring consists of 66 B type sintered NdFeB magnets of two specifications and sizes.

[0032] Based on simulation results, the specifications and dimensions of the five sintered samarium-cobalt magnets in the central region of the hybrid seven-ring annular magnetic pole disk are as follows: Figure 3 As shown, each concentric ring from the outside in consists of 58 C-type, 50 D-type, 34 E-type, 26 F-type, and 10 G-type sintered samarium cobalt magnets of various specifications and sizes.

[0033] The completed magnet system consists of a hybrid seven-ring magnetic pole disk made of sintered NdFeB magnets and sintered Samarium Cobalt magnets, as shown below. Figure 4 As shown. The system uses sintered NdFeB magnets weighing approximately 41 kg and sintered SmCo magnets weighing approximately 32.6 kg. The yoke support, anti-eddy current disk, and other accessories weigh approximately 310 kg. The yoke support is made of high-purity iron, and the anti-eddy current disk is made of a magnetically permeable but non-conductive material. Their sizes and dimensions are as follows. Figure 5 As shown, the total weight of the magnet system is 388 kg. Tests on the magnetic field strength and uniformity of the central region of the completed magnet system revealed that the central magnetic field strength reaches 0.07 T. Furthermore, within a 200 mm diameter spherical space centered on the magnet system, the magnetic field non-uniformity at the boundary is less than 100 ppm. The measured change in magnetic field non-uniformity at the boundary with temperature is also less than 100 ppm / ℃. The temperature stability of this magnet system is significantly better than that of a magnet system composed solely of sintered NdFeB magnets.

[0034] Those skilled in the art should understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method of designing a low-field permanent-magnet type MRI magnetic circuit, characterized by, The hybrid ring-shaped magnetic pole disc is composed of two materials of sintered neodymium iron boron magnet and sintered samarium cobalt magnet, wherein the samarium cobalt magnet is in the central region of the magnetic pole disc, and the neodymium iron boron magnet is in the edge region of the magnetic pole disc; then according to the specifications, size and field strength requirements of the MRI magnet system, the magnetic circuit of the low-field permanent magnet type MRI magnet system is designed through electromagnetic field simulation and simulation.

2. The magnetic circuit design method of claim 1, wherein, The magnetic pole disc is composed of multiple ring-shaped magnetic circuits, the central region is at least one ring-shaped magnetic circuit composed of multiple sintered samarium cobalt magnets, and the edge region is at least one ring-shaped magnetic circuit composed of multiple sintered neodymium iron boron magnets; gaps are left between adjacent magnets of each ring-shaped magnetic circuit, and gaps are also left between magnets of adjacent ring-shaped magnetic circuits.

3. The magnetic circuit design method of claim 2, wherein, The size of the gap between the magnets is 2-5 mm.

4. The magnetic circuit design method of claim 1, wherein, The magnetic circuit is designed according to the technical parameters of the required MRI magnet system, including the diameter of the magnetic pole disc, the distance between the two magnetic pole discs, the magnetic field strength, and the magnetic performance parameters of the neodymium iron boron magnet and the samarium cobalt magnet.

5. The magnetic circuit design method of claim 1, wherein, The magnetic field is simulated and simulated through finite element calculation to obtain the magnetic circuit design.

6. A low-field permanent-magnetic MRI magnet system comprising a magnetic pole disc, a yoke support and an anti-eddy disc, the yoke support being used to fix the position of the upper and lower magnetic pole discs and connect the magnetic lines of the two magnetic pole discs through the yoke support, the two magnetic pole discs being completely mirror-symmetrically arranged and respectively constituting N and S poles for generating a main magnetic field; the anti-eddy disc being arranged between the yoke support and the magnetic pole disc; characterized in that, Each magnetic pole disc is a hybrid ring-shaped magnetic pole disc composed of two materials of sintered neodymium iron boron magnet and sintered samarium cobalt magnet, wherein the samarium cobalt magnet is in the central region of the magnetic pole disc, and the neodymium iron boron magnet is in the edge region of the magnetic pole disc.

7. The magnet system of claim 6, wherein, The central region of the magnetic pole disc is at least one ring-shaped magnetic circuit composed of multiple sintered samarium cobalt magnets; the edge region of the magnetic pole disc is at least one ring-shaped magnetic circuit composed of multiple sintered neodymium iron boron magnets.

8. The magnet system of claim 7, wherein, Gaps are left between adjacent magnets of each ring-shaped magnetic circuit, and gaps are also left between magnets of adjacent ring-shaped magnetic circuits.

9. The magnet system of claim 8, wherein, The size of the gap between the magnets is 2-5 mm.

10. The magnet system of claim 8, wherein, Each ring-shaped magnetic circuit is composed of multiple sector-shaped magnets, and the magnets on the left and right sides of the outermost ring-shaped magnetic circuit are higher than the magnets on the front and back sides.

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