A low-field MRI magnetic circuit design method with light weight and controllable magnetic field strength temperature characteristics and a magnet system thereof

Abstract

The application belongs to the technical field of magnetic resonance imaging, and relates to a low-field MRI magnetic circuit design method with light weight and controllable magnetic field strength temperature characteristics and a magnet system thereof. The method sets parallelly arranged permanent magnetic materials and soft magnetic materials in a magnetic pole disc of an MRI magnetic circuit, adjusts the mass ratio and corresponding arrangement mode of the permanent magnetic materials and the soft magnetic materials, matches the temperature change characteristics of the magnetic permeability of the soft magnetic materials and the remanence temperature characteristics of the permanent magnetic materials in a negative correlation, thereby regulating the magnetic field strength temperature characteristics and improving the temperature stability of the magnetic field. The magnet system comprises an upper magnetic pole disc, a lower magnetic pole disc, an iron yoke support and a pole head. The application fully utilizes the super-high magnetic energy product of the permanent magnetic materials to realize the light weight of the equipment, utilizes the soft magnetic materials for composite magnetic circuit design to weaken the rapid change of the magnetic field strength of the permanent magnetic materials with temperature, and constructs a light-weight low-field MRI stable magnet system which is not sensitive to the change of the ambient temperature.

Description

Technical Field

[0001] This invention belongs to the field of magnetic resonance imaging technology, specifically relating to a lightweight low-field MRI magnetic circuit design method and its magnet system with controllable magnetic field strength and temperature characteristics. Background Technology

[0002] Magnetic resonance imaging (MRI), as an advanced medical imaging technology with no radiation and high soft tissue resolution, plays an irreplaceable role in disease diagnosis. However, in my country, high-field MRI systems (usually referring to 1.5T and above) are mainly concentrated in core medical institutions in large and medium-sized cities due to their technical complexity, large size, and high purchase and maintenance costs. This uneven distribution of resources makes it difficult for a large number of patients, especially those in grassroots and remote areas, to obtain timely and convenient MRI diagnostic services, delaying the early detection and treatment of diseases. Therefore, developing low-cost, lightweight, and easily deployable low-field MRI systems has significant social and clinical value in promoting the downward flow of high-quality medical resources and improving the accessibility of public health services.

[0003] To achieve widespread application of MRI equipment, enabling it to move beyond traditional radiology departments and into hospital emergency rooms, outpatient clinics, community health centers, and even sports rehabilitation facilities, shopping mall health stations, and home health management scenarios, lightweight and miniaturized equipment are crucial prerequisites. Lightweight MRI not only means reduced physical size and mass but also lower requirements for power supply, shielding, and installation environment, thus significantly expanding its application boundaries. However, achieving this goal faces two core technological challenges: First, to generate a static magnetic field sufficient for imaging within a limited volume and mass, permanent magnet materials with extremely high magnetic energy products must be used to reduce the amount of magnets used. Second, after leaving the temperature- and humidity-controlled radiology department, the equipment will face complex and variable environmental temperature fluctuations, and its internal gradient coils will also generate heat during operation. Therefore, the permanent magnet material must possess excellent temperature stability to ensure that the main magnetic field strength does not drift with temperature, which is essential for obtaining stable, high-quality images from the MRI imaging system.

[0004] Among numerous permanent magnet materials, sintered neodymium iron boron (NdFeB) magnets stand out due to their unparalleled ultra-high energy product (exceeding 50 MGOe), far surpassing other rare-earth permanent magnet materials such as samarium cobalt (SmCo). From a magnetic circuit design perspective, a higher energy product means that the volume and mass of the magnet required to generate the same magnetic field strength can be significantly reduced, making it an ideal choice for achieving extreme lightweighting of MRI equipment. However, a fundamental drawback limiting its widespread application in this field is its poor temperature stability of magnetic properties. The remanence temperature coefficient of a typical sintered NdFeB magnet is approximately -1100 ppm / ℃, meaning that for every 1℃ change in ambient temperature, the magnetic field strength it provides will change by approximately 1100 ppm. For precision imaging MRI systems, this magnetic field drift is highly detrimental. It can directly lead to image distortion and a decrease in signal-to-noise ratio, seriously affecting diagnostic accuracy. Especially for diseases like ischemic stroke (cerebral infarction) with tight time windows and extremely high diagnostic accuracy requirements, magnetic field instability may directly lead to missed or misdiagnosed subtle lesions, affecting treatment outcomes. Summary of the Invention

[0005] To address the contradiction between the difficulty of achieving both high magnetic energy product and high temperature stability in a single material, this invention proposes an innovative lightweight low-field MRI magnetic circuit design method and its magnet system, which features adjustable magnetic field strength temperature characteristics by coupling permanent magnet materials and soft magnetic materials. The core objective of this invention is to achieve lightweight equipment by fully utilizing the ultra-high magnetic energy product of permanent magnet materials, such as sintered NdFeB magnets, while simultaneously using a clever composite magnetic circuit design and a special soft magnetic alloy to fundamentally reduce the rapid temperature changes in the magnetic field strength of NdFeB magnets. This results in a lightweight, low-field, stable MRI magnet system that is insensitive to changes in ambient temperature (i.e., possesses excellent temperature characteristics of the magnetic field strength).

[0006] The technical solution adopted in this invention is as follows: A lightweight and temperature-controlled low-field MRI magnetic circuit design method is proposed. This method involves placing permanent magnet materials and soft magnetic materials arranged in parallel in the magnetic pole disk of the MRI magnetic circuit. By adjusting the mass ratio and arrangement of the permanent magnet materials and soft magnetic materials, the temperature change characteristics of the permeability of the soft magnetic materials are negatively correlated with the temperature characteristics of the remanence of the permanent magnet materials, thereby controlling the temperature characteristics of the magnetic field strength and improving the temperature stability of the magnetic field.

[0007] Furthermore, the permanent magnet material is a sintered NdFeB magnet, a sintered Samarium Cobalt magnet, a bonded NdFeB magnet, a bonded Samarium Iron Nitrogen magnet, or a bonded NdFeB magnet; the soft magnetic material is an iron-nickel alloy.

[0008] Furthermore, the permanent magnet material is located in the central region of the upper and lower magnetic pole disks, while the soft magnetic material is located in the edge region of the upper and lower magnetic pole disks.

[0009] Furthermore, the upper and lower magnetic pole disks comprise multiple permanent magnet material blocks and multiple soft magnetic material blocks, with each permanent magnet material block and each soft magnetic material block being closely adjacent to each other.

[0010] Furthermore, the upper and lower magnetic pole disks comprise multiple concentric permanent magnet material rings, with soft magnetic material rings on the inner and outer sides of each permanent magnet material ring.

[0011] Furthermore, the mass ratio of the permanent magnet material to the soft magnetic material is 99:1 to 50:50.

[0012] A lightweight low-field MRI magnet system with controllable magnetic field strength and temperature characteristics includes an upper magnetic pole disk, a lower magnetic pole disk, and a yoke support; 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 upper and lower magnetic pole disks; the upper and lower magnetic pole disks contain permanent magnet materials and soft magnetic materials arranged in parallel.

[0013] Further, the permanent magnet material is a sintered NdFeB magnet, a sintered Samarium Cobalt magnet, a bonded NdFeB magnet, a bonded Samarium Iron Nitrogen magnet, or a bonded NdFeB magnet; the soft magnetic material is an iron-nickel alloy; and the mass ratio of the permanent magnet material to the soft magnetic material is 99:1 to 50:50.

[0014] Furthermore, the permanent magnet material and the soft magnetic material are arranged in one of the following ways: The permanent magnet material is located in the central region of the upper and lower magnetic pole disks, and the soft magnetic material is located in the edge region of the upper and lower magnetic pole disks. The upper and lower magnetic pole disks comprise multiple permanent magnet material blocks and multiple soft magnetic material blocks, with each permanent magnet material block and each soft magnetic material block being closely adjacent to each other. The upper and lower magnetic pole disks contain multiple concentric permanent magnet material rings, and each permanent magnet material ring has a soft magnetic material ring on its inner and outer sides.

[0015] Furthermore, the aforementioned magnet system also includes pole heads, which are mounted on permanent magnet materials in the upper and lower magnetic pole disks to improve the uniformity of the magnetic field and reduce eddy currents.

[0016] The main innovation and beneficial effects of this invention lie in the development of a unique temperature-compensated magnetic circuit structure, based on the fundamental reduction of the rapid temperature change in the magnetic field strength of permanent magnets (such as NdFeB magnets) by selecting special soft magnetic alloys. Specifically, this invention does not seek a completely new "perfect" material that combines high magnetic energy product and high stability, but rather achieves performance optimization at the magnet system level through material combination and magnetic circuit design. In terms of magnetic circuit design, this invention integrates high-energy-product permanent magnets (such as sintered NdFeB magnets) with specially selected soft magnetic materials in a parallel manner. The core feature of this soft magnetic material is that its permeability (μ) has a significant temperature sensitivity, that is, its permeability changes at a large rate (dμ / dT) with temperature. Furthermore, based on careful calculations, by adjusting the mass ratio and arrangement of the permanent magnet material and the soft magnetic material, the temperature change characteristics of the permeability of the soft magnetic material can be precisely negatively correlated with the remanence temperature characteristics of the sintered NdFeB magnet, ultimately realizing a magnet system with adjustable temperature characteristics of MRI magnetic field strength. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the principle of a composite magnetic circuit of sintered NdFeB magnets and soft magnetic materials.

[0018] Figure 2 This is a schematic diagram of the magnet system after adding soft magnetic material.

[0019] Figure 3 This is a schematic diagram of the first distribution of permanent magnet material (green) and soft magnetic material (red) in a magnetic pole disk.

[0020] Figure 4 This is a schematic diagram of the second distribution of permanent magnet material (green) and soft magnetic material (red) in the magnetic pole disk.

[0021] Figure 5 This is a schematic diagram of a third distribution of permanent magnet material (green) and soft magnetic material (red) in a magnetic pole disk. Detailed Implementation

[0022] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to specific embodiments and accompanying drawings.

[0023] The working principle of this invention is as follows: In a closed magnetic circuit composed of permanent magnet materials (such as sintered NdFeB magnets), soft magnetic materials, an iron yoke support, and a working air gap, the total magnetomotive force and magnetic flux distribution are jointly determined by the magnetic reluctance of each branch (the permanent magnet material branch, the soft magnetic material branch, and the air gap branch). The working air gap refers to the gap between the upper and lower magnetic pole disks.

[0024] Taking sintered NdFeB magnets as an example of permanent magnet materials, when the ambient temperature rises, the remanence (Br) of the sintered NdFeB magnet decreases, tending to weaken the magnetic field in the working air gap. Simultaneously, the permeability of the soft magnetic material in the parallel branch decreases significantly with increasing temperature. This decrease in permeability directly leads to a sharp increase in the magnetic reluctance of the soft magnetic material branch. According to the principle of magnetic circuit shunting, the change in magnetic reluctance redistributes the magnetic flux throughout the entire magnetic circuit. Specifically, a larger proportion of the magnetic flux is "forced" to flow through the air gap branch with relatively smaller changes in magnetic reluctance, thus compensating for the flux loss in the working air gap caused by the decrease in the remanence of the sintered NdFeB magnet. Figure 1 This diagram illustrates the change in magnetic flux in the composite magnetic circuit composed of sintered NdFeB magnets and soft magnetic materials as the temperature increases. When the ambient temperature decreases, the process reverses: the remanence (Br) of the sintered NdFeB magnet increases, tending to strengthen the magnetic field in the working air gap. Simultaneously, the permeability of the soft magnetic material increases significantly with decreasing temperature, leading to a sharp decrease in the magnetic reluctance of the soft magnetic material branch. This, in turn, reduces the magnetic flux flowing through the air gap branch, thus balancing the increase in magnetic flux in the working air gap caused by the increased remanence of the sintered NdFeB magnet.

[0025] The permeability of soft magnetic materials decreases upon heating. For the present invention, it is preferable to use soft magnetic materials whose permeability changes rapidly with temperature near room temperature (10-40℃). The faster the permeability of the soft magnetic material changes with temperature, the more sensitively the temperature characteristics of the sintered NdFeB magnet can be adjusted, and the purpose of controlling the temperature characteristics of the NdFeB magnet can be achieved using a small amount of soft magnetic material.

[0026] By precisely designing the weight, geometry, and position of the soft magnetic material in the magnetic circuit, the temperature variation characteristics of its magnetic permeability can be accurately negatively correlated with the remanence temperature characteristics of the sintered NdFeB magnet. This results in a superior temperature-dependent magnetic field strength variation trend in the working air gap (i.e., the imaging area) compared to a magnet system composed of a single NdFeB magnet within a set operating temperature range. In other words, the magnet system with the soft magnetic material will exhibit higher temperature stability. The geometry refers to the shape and size of the soft magnetic material, including its length, thickness, and width.

[0027] Based on the above working principle, this invention designs a low-field (less than 0.5 T) lightweight MRI magnet system composed of sintered NdFeB magnets and soft magnetic materials. Figure 2This illustrates the MRI magnet system described in the above embodiments, which consists of a single sintered NdFeB magnet and a sintered NdFeB magnet combined with soft magnetic materials. The magnet system mainly includes pole disks (upper and lower), a yoke support, and pole heads. The yoke support is primarily used to achieve a magnetic field converging effect, fixing the positions of the upper and lower pole disks and connecting the magnetic lines of force between them. The pole heads are mainly used to improve the uniformity of the magnetic field and reduce eddy currents, and are mounted on the NdFeB permanent magnet material in the upper and lower pole disks. The pole heads are made of high-permeability, low-coercivity soft magnetic materials (e.g., electrical pure iron, laminated silicon steel, iron-cobalt soft magnetic alloy), installed inside the upper and lower pole disks facing the working air gap. They are used to receive the magnetic flux of the entire permanent magnet block, gather and homogenize the magnetic lines of force, form a uniform and stable static magnetic field in the scanning area, and suppress eddy currents caused by gradient alternating fields, ensuring MRI imaging quality and long-term magnetic field stability. Figure 2 As shown, the magnet system also includes a column and a positioning column. The positioning column is used to fix the pole head and position the pole disk during installation to ensure the coaxiality of the upper and lower pole disks with the pole head. The column is used to position the upper and lower pole disks and connect the magnetic lines of force of the two pole disks and the corresponding yoke supports.

[0028] The specific structure of the upper or lower magnetic pole disk is as follows: Figure 3 As shown. By Figure 3 As can be seen, the upper or lower magnetic pole disk of the magnet system is composed of sintered NdFeB magnets and soft magnetic materials. The sintered NdFeB magnets are located in the central region of the pole disk, and can be composed of either block-shaped or tile-shaped NdFeB magnets. They can be packed tightly together or have gaps between them and adjacent magnets. The soft magnetic material is located at the edge of the pole disk, arranged side-by-side with the sintered NdFeB magnets in the center. The mass ratio of sintered NdFeB magnets to soft magnetic materials ranges from 99:1 to 50:50. By adjusting the mass ratio of sintered NdFeB magnets to soft magnetic materials, the temperature characteristics of the magnetic field strength can be controlled. This results in a significant improvement in the temperature characteristics of the magnetic field strength of the MRI magnet system after adding soft magnetic materials.

[0029] In one embodiment of the present invention, a specially selected soft magnetic material is added around the upper and lower magnetic pole disks of an MRI magnet system composed of a single sintered NdFeB magnet. The soft magnetic material used is an iron alloy, preferably an iron-nickel alloy. The ratio of the amount of soft magnetic material used to the mass of the sintered NdFeB magnet is 10:90, 15:85, and 20:80, respectively.

[0030] This invention compares and examines the relationship between the magnetic field strength of a magnet system and temperature before and after the addition of soft magnetic material. Before the addition of soft magnetic material, the magnetic field strength changed by temperature at 939 ppm / ℃; after adding 10%, 15%, and 20% soft magnetic material, the magnetic field strength decreased to 768 ppm / ℃, 683 ppm / ℃, and 587 ppm / ℃, respectively. These results demonstrate that using a specially selected iron alloy as the soft magnetic material can indeed significantly improve the temperature stability of the magnetic field strength in a permanent magnet MRI magnet system. Furthermore, by adjusting the mass ratio of soft magnetic material to permanent magnet material, an MRI magnet system with adjustable temperature characteristics of the magnetic field strength can be achieved.

[0031] Furthermore, this magnetic circuit design method offers high flexibility and scalability. It can be applied not only to simple U-shaped or C-shaped MRI magnet systems, but also to more complex multipole, toroidal, or Halbach array MRI magnet systems to optimize magnetic field homogeneity.

[0032] This invention can also be implemented in other ways. For example, the sintered NdFeB magnet can be replaced with sintered SmCo magnets, bonded NdFeB magnets, bonded SmFeNi magnets, or bonded NdFeNi magnets and other permanent magnet materials. Besides iron alloys, the soft magnetic material can also be other materials whose permeability changes rapidly with temperature. The magnetic pole disk can be circular, elliptical, square, or rectangular, etc.

[0033] Regarding the position of soft magnetic materials in the magnetic circuit and the arrangement of permanent magnet materials and soft magnetic materials. Figure 3 A soft magnetic material is placed on the outermost layer of all permanent magnet materials; this arrangement can be called global control. Global control treats all the permanent magnet materials in the pole disk as a whole, and then controls the magnetic flux change in the entire magnetic circuit through the outermost soft magnetic material. This control method is particularly suitable for magnet systems with small pole disks, and the corresponding machining and assembly are simple. Except... Figure 3 Besides the arrangement shown, other arrangements are also possible, such as... Figure 4 , Figure 5 The arrangement shown, etc. Figure 4 This involves placing a soft magnetic material block alongside each permanent magnet block, a process known as local control. Local control allows each permanent magnet block in the magnetic pole disk to independently control the magnetic flux change in the magnetic circuit through the adjacent soft magnetic material, thereby controlling the magnetic flux change in the entire magnetic circuit. For example... Figure 4As shown, the overall structure is a multi-layered concentric circular radial array, comprising multiple concentric rings. Each ring is radially divided into multiple fan-shaped arc-shaped units along its circumference. Each permanent magnet material block (green fan-shaped structure) is closely adjacent to (fitted with) a soft magnetic material block (red dividing strip). Compared to global control, local control is applicable to almost all magnet systems (both smaller and larger magnetic pole disks), but the corresponding machining and assembly are more complex. Figure 5 This involves incorporating multiple rings of soft magnetic material within a permanent magnet, which also falls under the category of localized control. For example... Figure 5 As shown, its overall structure is a multi-layered concentric circular radiating disk structure, containing multiple concentric permanent magnet material rings (green), with soft magnetic material rings (red) on the inner and outer sides of the permanent magnet material rings. Figure 5 The scheme using multiple soft magnetic material rings allows for localized control, which is applicable to all magnet systems. However, the corresponding machining and assembly are relatively complex.

[0034] In summary, this invention achieves self-compensation for magnetic field drift in MRI magnet systems by intelligently combining high-energy-product sintered NdFeB magnets with a specially formulated soft magnetic material whose permeability is highly temperature-sensitive through an innovative composite magnetic circuit configuration. This solution does not require pursuing extreme breakthroughs in any single material performance aspect; instead, through ingenious engineering, it solves the problem of poor magnetic field temperature stability in low-field, lightweight MRI systems for broader applications at a relatively low cost and with a controllable temperature-compensated magnetic circuit. This lays a solid technical foundation for developing next-generation, widely applicable MRI devices suitable for primary healthcare, mobile healthcare, specialist clinics, and even future home health monitoring, and has broad market prospects and social benefits.

[0035] The specific embodiments of the present invention disclosed above are intended to help understand the content of the present invention and to implement it accordingly. Those skilled in the art will understand that various substitutions, changes, and modifications are possible without departing from the spirit and scope of the present invention. The present invention should not be limited to the content disclosed in the embodiments of this specification; the scope of protection of the present invention is defined by the claims.

Claims

1. A lightweight low-field MRI magnetic circuit design method with controllable magnetic field strength and temperature characteristics, characterized in that, In the magnetic pole disk of the MRI magnetic circuit, permanent magnet materials and soft magnetic materials are arranged side by side. By adjusting the mass ratio and arrangement of the permanent magnet materials and soft magnetic materials, the temperature change characteristics of the permeability of the soft magnetic materials are negatively correlated with the remanence temperature characteristics of the permanent magnet materials, thereby regulating the temperature characteristics of the magnetic field strength and improving the temperature stability of the magnetic field.

2. The method according to claim 1, characterized in that, The permanent magnet material is a sintered NdFeB magnet, a sintered Samarium Cobalt magnet, a bonded NdFeB magnet, a bonded Samarium Iron Nitrogen magnet, or a bonded NdFeN magnet; the soft magnetic material is an iron-nickel alloy.

3. The method according to claim 1, characterized in that, The permanent magnet material is located in the central region of the upper and lower magnetic pole disks, while the soft magnetic material is located in the edge region of the upper and lower magnetic pole disks.

4. The method according to claim 1, characterized in that, The upper and lower magnetic pole disks contain multiple permanent magnet material blocks and multiple soft magnetic material blocks, with each permanent magnet material block and each soft magnetic material block being closely adjacent to each other.

5. The method according to claim 1, characterized in that, The upper and lower magnetic pole disks contain multiple concentric permanent magnet material rings, and each permanent magnet material ring has a soft magnetic material ring on its inner and outer sides.

6. The method according to claim 1, characterized in that, The mass ratio of the permanent magnet material to the soft magnetic material is 99:1 to 50:

50.

7. A lightweight low-field MRI magnet system with controllable magnetic field strength temperature characteristics, characterized in that, It includes an upper magnetic pole disk, a lower magnetic pole disk, and a yoke support; the yoke support is used to fix the position of the upper and lower magnetic pole disks and connect the magnetic lines of force between the upper and lower magnetic pole disks; The upper and lower magnetic pole disks contain permanent magnet materials and soft magnetic materials arranged side by side.

8. The magnet system according to claim 7, characterized in that, The permanent magnet material is a sintered NdFeB magnet, a sintered SmCo magnet, a bonded NdFeB magnet, a bonded SmCo magnet, or a bonded NdFeB magnet; the soft magnetic material is an iron-nickel alloy; the mass ratio of the permanent magnet material to the soft magnetic material is 99:1 to 50:

50.

9. The magnet system according to claim 7, characterized in that, The permanent magnet material and the soft magnetic material are arranged in one of the following ways: The permanent magnet material is located in the central region of the upper and lower magnetic pole disks, and the soft magnetic material is located in the edge region of the upper and lower magnetic pole disks. The upper and lower magnetic pole disks comprise multiple permanent magnet material blocks and multiple soft magnetic material blocks, with each permanent magnet material block and each soft magnetic material block being closely adjacent to each other. The upper and lower magnetic pole disks contain multiple concentric permanent magnet material rings, and each permanent magnet material ring has a soft magnetic material ring on its inner and outer sides.

10. The magnet system according to claim 7, characterized in that, It also includes pole heads, which are installed on the permanent magnet material in the upper and lower magnetic pole disks to improve the uniformity of the magnetic field and reduce eddy currents.

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