A novel magnet structure suitable for use in ultra-low field magnetic resonance imaging systems

Abstract

The application relates to the technical field of low-field magnetic resonance imaging equipment, in particular to a novel magnet structure suitable for an ultra-low-field magnetic resonance imaging system, which comprises: a diffusion control array arranged outside a ring-shaped magnet steel array, the diffusion control array comprising at least two groups of diffusion control magnet steels arranged in a vertical direction; the magnetic directions of adjacent diffusion control magnet steels are opposite, so as to constitute multiple groups of inward magnetic force lines and constrain the main magnetic field. In view of the problem that the magnet structure design in the prior art needs to add components such as a shimming ring and a pole plate, so that eddy current is generated in the two components when the gradient coil is rapidly switched, the shimming ring and the pole plate and other structures in the traditional design are cancelled, the ring-shaped magnet steel array is adopted to generate a more concentrated magnetic field, and the diffusion control array arranged outside the ring-shaped magnet steel array is adopted to generate an inward magnetic field, so that the diffusion phenomenon of the main magnetic field is reduced, good imaging can still be realized under the condition that the shimming ring and the pole plate are cancelled, and the eddy current problem caused by the above structure is eliminated.

Description

Technical Field

[0001] This invention relates to the field of low-field magnetic resonance imaging equipment technology, and specifically to a novel magnet structure suitable for ultra-low field magnetic resonance imaging systems. Background Technology

[0002] MRI is a commonly used medical computed tomography method that uses the magnetic resonance phenomenon to obtain electromagnetic signals from the human body and reconstruct human information. Ultra-low field MRI generally refers to MRI with a field strength below 0.1T. Compared with high field MRI, it has the advantages of lower examination cost and easier accessibility. Furthermore, due to the lower field strength, a relatively safe examination process can be achieved without the need for a shielded room. Open, radiofrequency-free MRI can provide patients with a comfortable scanning experience. Domestic research and development in the field of ultra-low field magnetic resonance imaging (ULF-MRI) is mainly concentrated in enterprises (such as Arctic King and Wuxi Mingshi Junzhi) and universities (such as Shanghai Jiao Tong University and Chongqing University of Posts and Telecommunications). Among them, Arctic King's ultra-low field HEAD-500 cranial MRI uses traditional cobalt-tungsten permanent magnet material with a magnetic field strength of 0.55T and an overall weight of about 630kg. Zhang Zhiyong's team at Shanghai Jiao Tong University developed the SPEN (spatiotemporal coding) acquisition method suitable for portable MRI equipment without multi-channel parallel acceleration. Compared with the traditional EPI (echo planar imaging) technology, the undersampled images of this method show less geometric distortion. The School of Electrical Engineering of Chongqing University, in conjunction with the Southwest Hospital of Army Medical University, successfully developed a prototype of a bedside MRI system for the head. This system has a field strength of 0.05T, a total weight of less than 500kg, and is powered by a 220V / 50Hz standard power supply. This device can be directly placed in intensive care units, general wards, and operating rooms as a diagnostic and monitoring device for brain diseases, especially stroke, to achieve bedside imaging examinations.

[0003] In the prior art, the configuration of low-field and ultra-low-field magnetic resonance systems typically includes a magnetic steel array, a magnetic yoke, and gradient coils.

[0004] For example, patent document CN202210725616.X discloses an ultra-low field spinal magnetic resonance imaging system, including a support assembly, a magnet assembly, and an equipment cabinet. The support assembly includes a support plate, a bracket, a rotating mechanism, and a wire channel. The rotating mechanism is located on one external side of the bracket, and the wire channel is located on the other external side. The magnet assembly is located at one external end of the rotating mechanism. The magnet assembly includes a C-shaped magnet, a gradient coil, a planar transmitting coil, and an openable receiving coil. It is powered by a rechargeable battery, solving the signal interference problem caused by mains power. The magnet can be set to horizontal or vertical positions. The system can perform standing and lying scans on patients. In standing scan mode, the patient's spine is in an everyday living state, providing magnetic resonance imaging images of the spine in its actual use state for clinical diagnosis. In lying scan mode, magnetic resonance imaging can be performed on patients who cannot stand or postoperatively.

[0005] For example, patent document CN202411432238.1 discloses an ultra-low field nuclear magnetic resonance measurement device with a wide range of adjustable background field, and also discloses an ultra-low field nuclear magnetic resonance measurement method with a wide range of adjustable background field. It uses a flat solenoid to achieve the requirements of high background magnetic field and ultra-low ambient magnetic field. The flat solenoid can also accommodate more coils and obtain more turns, achieving a wider range of adjustable background field. Both ends of the flat solenoid extend outside the magnetic shielding module, ensuring that the magnetic flux lines appearing at the ends of the flat solenoid connect to outside the shielding area, eliminating the problem of background field inhomogeneity generated by ordinary solenoid coils. Interference from the ambient magnetic field is mitigated through passive magnetic shielding and active magnetic field compensation. First, multiple electromagnetic shielding cylinders passively shield the ambient magnetic field, and then a shimming coil generates a magnetic field opposite to the ambient magnetic field for active magnetic field compensation, canceling out the interference from the ambient magnetic field. The shimming coil can also improve the uniformity of the central magnetic field.

[0006] However, in actual implementation, the inventors discovered that such magnetic resonance systems typically employ traditional magnet designs, including magnets, pole plates, anti-eddy current components, and shimming rings. To meet certain imaging space requirements, magnetic field uniformity, and reduce the impact of eddy currents on the image, each component, in addition to meeting its own material property requirements, must also meet certain size and thickness requirements, which increases the weight of the magnet itself. As the magnet field strength and opening increase, the magnet weight increases exponentially. At the same time, due to the presence of pole plates and shimming rings in traditional permanent magnet structures, rapid switching of gradient coils will generate eddy currents between them. These eddy currents manifest as eddy current artifacts in the image, thus affecting the image quality. Summary of the Invention

[0007] In view of the above-mentioned problems existing in the prior art, a solution is now provided.

[0008] The specific technical solution is as follows: A novel magnet structure suitable for ultra-low field magnetic resonance imaging systems includes a ring-shaped magnet array and a dissipation control array; the ring-shaped magnet array includes an upper ring array and a lower ring array; the upper ring array and the lower ring array are arranged opposite to each other; the dissipation control array is arranged outside the ring-shaped magnet array; the dissipation control array includes at least two sets of dissipation control magnets arranged vertically; the magnetic directions of adjacent dissipation control magnets are opposite to form multiple sets of inward magnetic field lines, forming a continuous "magnetic potential barrier" to constrain the main magnetic field formed between the upper ring array and the lower ring array; the dissipation control magnets use high-performance permanent magnet materials, preferably neodymium iron boron (such as N52, N54 grade) or samarium cobalt (such as Sm2Co17) materials; its remanence (Br) is not less than 1.45 T, its coercivity (Hcb) is not less than 12 kOe, and its maximum magnetic energy product ((BH)max) is not less than 52. MGOe is used to meet the requirements of magnetic field stability and strength for ultra-low field magnetic resonance imaging; the individual dimensions of the escaping control magnet are: height 20-40 mm, width 30-50 mm, and thickness 10-30 mm; the number of escaping control magnets set on each side column is 6-12.

[0009] A non-magnetic insulating pad (such as an epoxy board) is provided between the escaping control magnet and the steel column to avoid magnetic short circuits and improve magnetic field control efficiency.

[0010] On the other hand, the ultra-low field magnetic resonance imaging system includes an integral yoke; the integral yoke is U-shaped in the frontal view; the inner surfaces of the upper and lower sides of the integral yoke are respectively used to set the upper annular array and the lower annular array; the left and right sides of the integral yoke are yoke pillars; the dispersion control array is set inside the yoke pillars.

[0011] On the other hand, the lower side of the integral yoke is provided with the following components in sequence: a lower anti-eddy current assembly, which is disposed on the inner surface of the lower side of the integral yoke; a lower magnet fixing plate, which is fixed to the upper surface of the lower anti-eddy current assembly; a lower annular array disposed above the lower magnet fixing plate; a first fixing ring disposed on the outer periphery of the lower magnet fixing plate; and a lower gradient coil installed and fixed above the first fixing ring, maintaining a certain gap between the lower gradient coil and the lower annular array.

[0012] On the other hand, the upper side of the integral yoke is provided with the following components in sequence: an upper anti-eddy current assembly, which is located below the inner surface of the upper side of the integral yoke; an upper magnet fixing plate, which is fixed to the lower surface of the upper anti-eddy current assembly; an upper annular array located below the upper magnet fixing plate; a second fixing ring located on the outer periphery of the upper magnet fixing plate; and an upper gradient coil installed and fixed below the second fixing ring, maintaining a certain gap between the upper gradient coil and the lower annular array.

[0013] On the other hand, the annular magnet array includes multiple concentric annular magnet blocks; the first magnetic direction of the upper annular array is upward and the second magnetic direction is downward; the first magnetic direction of the lower annular array is upward and the second magnetic direction is downward, so that the second magnetic direction of the upper annular array matches the first magnetic direction of the lower annular array.

[0014] On the other hand, the annular magnet block is composed of multiple fan-shaped magnets; the radius of each fan-shaped magnet is determined according to the width of the annular magnet block; the inner and outer arcs of the fan-shaped magnets are determined according to the inner and outer diameters of the annular magnet block, respectively.

[0015] On the other hand, the height of the plurality of said annular magnet arrays increases sequentially from the inside to the outside.

[0016] On the other hand, each of the fan-shaped magnets has the same gap as the adjacent fan-shaped magnets.

[0017] On the other hand, the number of the annular magnet blocks is between 3 and 15; each annular magnet block has 8 to 24 equal parts of the fan-shaped magnets.

[0018] On the other hand, the evaporation control array is provided with a plurality of evaporation control magnets along the height direction of the yoke column; the magnetic directions of adjacent evaporation control magnets distributed along the height direction are opposite; the evaporation control array completely covers the spacing between the annular magnet array.

[0019] The above technical solution has the following advantages or beneficial effects: Addressing the problem that existing magnet structure designs require the addition of components such as shimming rings and pole plates, leading to eddy currents generated during rapid switching of gradient coils, this solution eliminates the shimming rings and pole plates found in traditional designs. Instead, a ring-shaped magnet array is used to generate a more concentrated magnetic field. Furthermore, an inward-facing magnetic field generated by a dissipation control array located outside the ring-shaped magnet array reduces the dissipation of the main magnetic field. Even without the shimming rings and pole plates, good imaging can still be achieved, and the eddy current problem caused by the aforementioned structures is eliminated. Attached Figure Description

[0020] Embodiments of the invention will be described more fully with reference to the accompanying drawings. However, the drawings are for illustration and explanation only and do not constitute a limitation on the scope of the invention.

[0021] Figure 1 This is an overall schematic diagram of an embodiment of the present invention;

[0022] Figure 2 This is a schematic diagram of a magnet structure in the prior art;

[0023] Figure 3 This is a schematic diagram of a ring magnet array in an embodiment of the present invention;

[0024] Figure 4 This is a schematic diagram of the stepped height of the annular magnet array in an embodiment of the present invention. Detailed Implementation

[0025] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0026] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0027] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but this is not intended to limit the scope of the invention.

[0028] This invention includes: a novel magnet structure suitable for ultra-low field magnetic resonance imaging systems, comprising a ring magnet array 1 and a dissipation control array 2; the ring magnet array 1 includes an upper ring array 11 and a lower ring array 12; the upper ring array 11 and the lower ring array 12 are arranged in the same direction; the dissipation control array 2 is disposed outside the ring magnet array 1; the dissipation control array 2 includes at least two sets of dissipation control magnets 21 arranged in the vertical direction; the magnetic directions of adjacent dissipation control magnets 21 are opposite to form multiple sets of inward magnetic field lines to constrain the main magnetic field formed between the upper ring array 11 and the lower ring array 12.

[0029] Specifically, in addressing the issue that existing magnet structure designs require the addition of components such as shimming rings and poles, which can cause eddy currents to form between the gradient coils during rapid switching, this solution eliminates the shimming rings and poles found in traditional designs. Instead, it employs a ring-shaped magnet array to generate a more concentrated magnetic field. Furthermore, an inward-facing magnetic field is generated by a dissipation control array positioned outside the ring-shaped magnet array, reducing the dissipation of the main magnetic field. This solution achieves good imaging even without the shimming rings and poles, and eliminates the eddy current problem caused by the aforementioned structures.

[0030] Specifically, such as Figure 2 As shown, a conventional low-field magnetic resonance magnet array includes the following components: a ring yoke, providing a guiding path for the magnetic field; a lower ring array 102, disposed on the bottom surface of the ring yoke, comprising multiple magnet blocks to form a specific magnetic field direction; a lower pole plate 103 above the lower ring array 102, and a lower anti-eddy current assembly 104 above the lower pole plate 103 to isolate the lower pole plate and magnets from the upper lower gradient coil; and a lower gradient coil above the lower anti-eddy current assembly 104. Coil 105 generates a uniformly varying magnetic field to examine the patient in the upper cavity. The lower anti-eddy current assembly 104 suppresses the magnetic field generated by the lower gradient coil 105 during the changing magnetic field process, reducing eddy currents on the poles and magnets. A shimming ring is fitted around the lower anti-eddy current assembly 104 and the lower gradient coil 105 to concentrate the magnetic field, achieving good imaging results at lower field strengths. The upper magnet structure is similar to the lower magnet structure, but in the opposite direction and order. Magnetic field lines are generated from the lower part, pass through the examination area, reach the upper part, enter the annular yoke, and then descend, forming a complete magnetic field.

[0031] During the above process, due to the presence of pole plates and shimming rings in the structure, rapid switching of the gradient coils will generate eddy currents in both. These eddy currents will manifest as eddy current artifacts in the image, thus affecting the image quality.

[0032] To address the aforementioned issues, this embodiment removes the pole plates and shimming rings from the magnet structure, thereby eliminating the cause of eddy currents. To ensure clear imaging of the ultra-low field system even after removing the pole plates and shimming rings, the magnet layout is improved as follows: First, the main magnet array is replaced with a ring-shaped magnet array, which includes multiple sets of ring-shaped magnet blocks arranged in a circular pattern. The ring-shaped magnet arrangement provides a naturally circular and converged main magnetic field, thereby improving the uniformity of the magnetic field. Second, to address the potential for main magnetic field dissipation, a dissipation control array 2 is installed on both sides of the yoke to provide an inward magnetic field to converge the main magnetic field.

[0033] like Figure 3 As shown, the evaporation control array 2 includes at least one set of evaporation control magnets 21 arranged sequentially along the vertical direction. The magnetic directions of adjacent evaporation control magnets 21 arranged from high to low are opposite, so the magnetic field lines of adjacent evaporation control magnets 21 will pass through in a figure-eight pattern, thereby forming a confinement magnetic field distributed along the height direction on the entire evaporation control array 2. The magnetic field lines of the main magnetic field will be repelled by this confinement magnetic field, thus preventing them from passing through the evaporation control array 2 to reach the outside, which can block the main magnetic field from diverging outward and reduce the range of the 5Gs line.

[0034] In a preferred embodiment, the dissipation control magnet 21 is made of N52 grade neodymium iron boron material, with typical performance parameters of: remanence Br ≥ 1.48 T, coercivity Hcb ≥ 12.5 kOe. The dimensions of a single magnet are designed to be 30 mm in height, 40 mm in width, and 20 mm in thickness. A total of eight of these magnets are installed vertically on the inner side of each yoke post 31, with adjacent magnetization directions opposite (i.e., alternating N-S-N-S…). A 2 mm thick epoxy resin board is placed between the mounting surfaces of the dissipation control magnet 21 and the yoke post 31 as a non-magnetic insulating pad.

[0035] After the above arrangement, the evaporation control array 2 can generate a significant horizontally inward confined magnetic field. Experimental results show that in an imaging system with a central field strength of 0.1T, this design effectively reduces the 5 Gs line range of the main magnetic field from approximately 1.2 meters in the traditional structure to within 0.8 meters, significantly suppressing magnetic field evaporation and indirectly improving the magnetic field uniformity of the central imaging region. Because this array is composed of discrete high-resistivity permanent magnets and does not contain continuous metal poles or shimming rings, it fundamentally avoids the problem of eddy currents generated during rapid switching of gradient coils.

[0036] The reason why existing traditional structures do not remove the pole plates and shimming rings is that their removal would lead to a decrease in the stability of magnetic field uniformity and an outward dissipation of the magnetic field at the edges. The root cause of the decreased magnetic field uniformity and stability is that the main magnet is directly exposed to the air, resulting in excessive heat loss and decreased stability. This can be solved in this invention by applying a heat insulation layer to the surface of the magnet. The root cause of the outward dissipation of the magnetic field at the edges is that after removing the shimming rings from the traditional magnet, it is not conducive to the magnetic field focusing towards the center, affecting the stability of the magnetic field in the uniform region. To address this, this invention mainly uses vertically arranged arrays of magnets on both side columns to suppress the outward divergence of the magnetic field. In traditional structures, the gradient coils directly cover the surface of the magnetic poles. The rapid switching of the gradient coils generates eddy currents around the pole plates and shimming rings, which in turn affects image quality. In this invention, the gradient coil is located above the magnet and maintains a certain gap with it. Since the magnet is a high-resistivity material, it is not sensitive to eddy currents. Furthermore, the magnet in this invention does not contain pole plates or shimming rings. The eddy currents generated by the rapid switching of the gradient coil are most likely to be in the yoke. The distance between the gradient coil and the yoke is much greater than the propagation distance of the eddy currents, so the influence of the eddy currents is very small. No software compensation is required, or only a small compensation is needed to offset the influence of the eddy currents.

[0037] The magnet in patent CN 118501785 B adopts a C-shaped structure. Compared with the symmetrical double-column structure in this invention patent, its advantages are better openness and relatively lower cost. However, it has relatively lower field strength, poorer uniformity, larger volume, and poorer temperature stability. The C-shaped magnet has a long magnetic circuit and leakage, resulting in lower magnetic energy utilization. The open design also leads to a limited magnetic field concentration area. In contrast, the symmetrical double-column design shortens the magnetic circuit, reduces leakage, has higher magnetic energy utilization, and the magnetic field concentration area is closer to the center between the two columns. The C-shaped structure has poor magnetic field uniformity due to the asymmetrical magnetic circuit, which causes significant changes in the magnetic field gradient. It is suitable for applications with lower uniformity requirements (such as limb imaging). The double-column magnet has better magnetic field uniformity (symmetry can reduce higher-order magnetic field distortion), is easier to homogenize, and is suitable for whole-body imaging or high-resolution applications. The C-shaped magnet has a long magnetic circuit and is significantly affected by temperature. The symmetrical design of the double-column magnet disperses thermal stress and has better temperature stability.

[0038] In summary, compared to traditional permanent magnet structures, this invention addresses the technical challenges of reduced magnetic field uniformity and stability, outward magnetic field dissipation from edges, and the impact of eddy currents on images caused by the removal of poles and shimming rings in traditional structures through a novel internal structure layout design. Simultaneously, it effectively reduces the magnet's weight. Compared to the C-type structure mentioned in CN 118501785 B, the novel symmetrical double-column magnet structure in this invention exhibits superior overall performance.

[0039] In one embodiment, the ultra-low field magnetic resonance imaging system includes an integral yoke 3; the integral yoke 3 is shaped like a square in the frontal view; the inner surfaces of the upper and lower sides of the integral yoke 3 are respectively used to set the upper annular array 11 and the lower annular array 12; the left and right sides of the integral yoke 3 are yoke pillars 31; and the evaporation control array 2 is set inside the yoke pillars 31.

[0040] Specifically, in order to achieve better dissipation control, in this embodiment, when the above-mentioned magnet structure is applied to an ultra-low field magnetic resonance imaging system, an upper annular array 11 and a lower annular array 12 are respectively set on the inner surfaces of the upper and lower sides of the overall yoke 3, wherein the upper annular array 11 is located on the upper inner surface and the lower annular array 12 is located on the lower inner surface.

[0041] The integral yoke 3 is arranged in a U-shape on the outside of the ring array to provide a complete magnetic path. The cavity serves as the examination area, allowing at least part of the patient's limbs, such as the arm, spine, or head, to be inserted to apply an alternating magnetic field to the patient.

[0042] The left and right sides of the overall yoke 3 are yoke pillars 31. The escaping control array 2 is set inside the yoke pillars 31. Multiple sets of magnets are installed sequentially along the height direction of the yoke pillars. During the installation of the magnets, the adjacent magnets in the height direction are reversed, thereby forming a specific magnetic field distribution inside the yoke pillars 31 and blocking the path of the main magnetic field to escape to the outside.

[0043] Depending on the width of the magnet blocks in the magnet array, multiple sets of magnet arrays may need to be set up in the vertical direction. However, the magnet arrays usually need to completely cover the height of the middle uniform field area and the width of the yoke column 31 in the horizontal direction.

[0044] In one embodiment, the evaporation control array 2 is provided with a plurality of evaporation control magnets 21 along the height direction of the yoke column; the magnetic directions of adjacent evaporation control magnets 21 distributed along the height direction are opposite; the evaporation control array 2 completely covers the spacing between the annular magnet array 1.

[0045] The evaporation control magnets are made of high-performance permanent magnet materials, such as neodymium iron boron (N52, N54 grade) or samarium cobalt (Sm2Co17), with a remanence (Br) of not less than 1.45 T and a coercivity (Hcb) of not less than 12 kOe. Typical dimensions of a single magnet are: height 20-40 mm, width 30-50 mm, and thickness 10-30 mm. The number of evaporation control magnets installed on each side column is typically 6-12.

[0046] Specifically, to achieve better control over the dispersion range of the main magnetic field, in this embodiment, multiple dispersion control magnets 21 are arranged along the height direction of the yoke column. The magnetic directions of adjacent dispersion control magnets 21 arranged from high to low are opposite, so the magnetic field lines of adjacent dispersion control magnets 21 will pass through in a figure-eight pattern, thereby forming a constraint magnetic field distributed along the height direction on the overall dispersion control array 2. The magnetic field lines of the main magnetic field will be repelled by this constraint magnetic field, thus preventing them from passing through the dispersion control array 2 to reach the outside, which can block the main magnetic field from diverging outward and reduce the 5Gs line range.

[0047] By controlling the spacing between the ring magnet array 1 to completely cover the entire area of ​​the escaping control array 2, effective control of the main magnetic field can be achieved.

[0048] In one embodiment, the lower side of the integral yoke 3 is provided with the following components in sequence: a lower anti-eddy current assembly 42, which is disposed on the inner surface of the lower side of the integral yoke 3; a lower magnet fixing plate 52, which is fixed to the upper surface of the lower anti-eddy current assembly; a lower annular array 12, which is disposed above the lower magnet fixing plate 52; a first fixing ring 62 is disposed on the outer periphery of the lower magnet fixing plate 52; a lower gradient coil 72 is installed and fixed above the first fixing ring 62, and a certain gap is maintained between the lower gradient coil 72 and the lower annular array 12.

[0049] Specifically, in response to the problem that the existing magnet structure design requires the addition of components such as shimming rings and pole plates, which causes eddy currents to be generated between the two when the gradient coils are switched quickly, this embodiment optimizes the magnet array and adjusts the coil layout in the magnetic resonance system. This includes removing the lower anti-eddy current component 42 from between the magnet array and the gradient coils and replacing it below the lower magnet fixing plate 52, thus avoiding the interference problem caused by the anti-eddy current component itself.

[0050] The lower magnet fixing plate 52 is used to fix the magnet blocks in the magnet array and the gradient coils in the magnetic resonance system. It is necessary to fix the magnets. At the same time, the fixing structure has a certain thickness, which further attenuates the eddy currents.

[0051] Multiple magnets in the lower annular array 12 are fixed above the lower magnet fixing plate 52 according to a specific arrangement, and form an annular arrangement to provide a uniform magnetic field.

[0052] On the upper surface of the lower magnet fixing plate 52, a first fixing ring 62 is provided along the outer circumference of the lower annular array 12. The upper surface of the first fixing ring 62 is used to install the lower gradient coil 72 and raise the lower gradient coil 72 to a certain height, so that there is a certain gap between the lower gradient coil 72 and the lower annular array 12, thereby reducing the eddy current in the magnet block.

[0053] In one embodiment, the upper side of the integral yoke 3 is sequentially provided with: an upper anti-eddy current component 41, which is disposed below the inner surface of the upper side of the integral yoke 3; an upper magnet fixing plate 51, which is fixed to the lower surface of the upper anti-eddy current component 41; an upper annular array 11, which is disposed below the upper magnet fixing plate 51; a second fixing ring 61 is disposed on the outer periphery of the upper magnet fixing plate 51; an upper gradient coil 71 is installed and fixed below the second fixing ring 61, and a certain gap is maintained between the upper gradient coil 71 and the upper annular array 11.

[0054] Specifically, in response to the problem that the existing magnet structure design requires the addition of components such as shimming rings and pole plates, which causes eddy currents to be generated between the two when the gradient coils switch rapidly, this embodiment optimizes the magnet array and adjusts the coil layout in the magnetic resonance system. This includes removing the upper anti-eddy current component 41 from between the magnet array and the gradient coils and replacing it above the upper magnet fixing plate 51, thus avoiding the interference problem caused by the anti-eddy current component itself.

[0055] The upper magnet fixing plate 51 is used to fix the magnet blocks in the magnet array and the gradient coils in the magnetic resonance system. It is necessary to fix the magnets. At the same time, the fixing structure has a certain thickness, which further attenuates the eddy currents.

[0056] Multiple magnets in the upper annular array 11 are fixed above the upper magnet fixing plate 51 according to a specific arrangement, and form an annular arrangement to provide a uniform magnetic field.

[0057] On the upper surface of the upper magnet fixing plate 51, a second fixing ring 61 is provided along the outer circumference of the upper annular array 11. The lower surface of the second fixing ring 61 is used to install the upper gradient coil 71 and lower the upper gradient coil 71 to a certain height, so that there is a certain gap between the upper gradient coil 71 and the upper annular array 11, thereby reducing the eddy current in the magnet block and dissipating heat.

[0058] The gap should typically be controlled to be above 3mm.

[0059] In one embodiment, the annular magnet array 1 includes a plurality of concentric annular magnet blocks; the first magnetic direction of the upper annular array 11 is upward and the second magnetic direction is downward; the first magnetic direction of the lower annular array 12 is upward and the second magnetic direction is downward, so that the second magnetic direction of the upper annular array 11 matches the first magnetic direction of the lower annular array 12.

[0060] In one embodiment, the first magnetic direction is the N pole and the second magnetic direction is the S pole, but they can be reversed depending on actual needs.

[0061] Specifically, in order to achieve a more concentrated magnetic field effect, in this embodiment, the annular magnet array 1 is replaced with annular magnet blocks consisting of multiple concentric circles. The annular magnet blocks are used to achieve a more concentrated magnetic field distribution. At the same time, the upward or downward magnetic field lines are controlled by matching the second magnetic direction of the upper annular array 11 with the first magnetic direction of the lower annular array 12.

[0062] It is important to note that if a ring-shaped magnet structure is chosen, a certain number of air gaps should be created on the ring to disrupt the continuity and prevent the magnet structure from forming a complete conductor, which could lead to eddy currents.

[0063] Generally, based on requirements such as field strength and magnetic field area, the concentric ring structure is usually between 3 and 15, with a more common parameter range of 5 to 8.

[0064] In one embodiment, the annular magnet block is composed of multiple fan-shaped annular magnets 13; the radius of each fan-shaped annular magnet 13 is determined according to the width of the annular magnet block; the inner and outer arcs of the fan-shaped annular magnet 13 are determined according to the inner and outer diameters of the annular magnet block, respectively.

[0065] Specifically, in order to achieve better manufacturability, in this embodiment, the circular annular magnet block is realized by assembling the fan-shaped annular magnet 13.

[0066] The fan-shaped annular magnet 13, viewed from above, consists of two arcs and two radial lines. The inner and outer arcs are determined according to the inner and outer diameters of the annular magnet block, respectively, and the radius is determined according to the width of the annular magnet block, and it also has a certain height. The fan-shaped annular magnets in each annular magnet block are usually identical, designed by dividing the annulus into equal parts.

[0067] In traditional magnets, the magnets are usually arranged in a flat pattern across the entire disk of the same size. A special-shaped stainless steel support structure is required at the center of the disk to support the anti-eddy current components. Because this structure occupies the original magnet filling space, the magnetic field at the center is very low, resulting in discontinuity of the magnetic field in the imaging space and increasing the difficulty of subsequent shimming.

[0068] By using fan-shaped magnets 13, better magnetic field uniformity can be achieved during the splicing process, and the amount of magnets used can be reduced.

[0069] In one embodiment, the height of multiple annular magnetic blocks increases sequentially from the inside out.

[0070] Specifically, because the magnetic field generated by permanent magnets has the characteristic of dissipation, the field strength near the center of the magnet is higher than that at the edge. The field strength is basically uniform within the central uniform field sphere, decreasing towards the edge. To address this issue, existing technologies typically consider increasing the overall area of ​​the magnet array to ensure that the region with higher central strength can meet inspection requirements. However, this leads to an overall increase in system size and weight.

[0071] To address this issue, this embodiment selects multiple annular magnet blocks with progressively increasing heights from the inside out, thereby increasing the volume per unit area of ​​the outer annular magnet blocks. By designing specific differences in magnet height, compensation for the magnetic field strength of the magnets in each region can be achieved.

[0072] When fan-shaped annular magnets 13 are spliced ​​together to form a circular magnet block, the height of each fan-shaped annular magnet 13 is adjusted to change the magnetic field strength provided by a single magnet block.

[0073] In one embodiment, each sector ring magnet has the same gap as the adjacent sector ring magnet.

[0074] The specific arrangement methods are as follows: 1) Arranged in a fan-shaped radial matrix with equal centers, the magnets are distributed in a stepped manner from the outside to the inside, and each magnet maintains a certain gap to avoid eddy currents generated in the magnets during rapid switching of gradient coils. 2) The magnets are arranged in rectangular blocks, closely arranged to form a disk shape. The contact surfaces of each block are coated with liquid oxygen glue to achieve insulation, and the middle part is a non-magnetic stainless steel fixing component, which is fixed to the magnet array fixing plate below.

[0075] Each ring-shaped magnet block contains 8-24 fan-shaped magnets, with a more common parameter being 12-18 divisions within each ring.

[0076] Traditional magnet structures, where magnets are laid flat and fill the entire disk without gaps, rely on epoxy resin for insulation at the contact surfaces. The central area requires a fixed support structure for anti-eddy current components, which can easily cause the magnetic field at the center of the imaging space to be lower than the surrounding magnetic field. This can only be adjusted passively through post-processing shimming, a complex and difficult process. Furthermore, because traditional structures contain magnet poles, rapid switching of gradient coils generates significant eddy currents within these poles. Even with anti-eddy current components, these effects cannot be completely eliminated, requiring active software compensation. Since eddy currents are dynamic, software compensation also needs periodic adjustments, further increasing the complexity and time required for setup.

[0077] In this invention, the novel magnet structure features a gradient coil directly mounted above a magnet. Because the magnet is made of a high-resistivity material and its surface is coated with a special insulating varnish, the influence of eddy currents on the magnet is comprehensively reduced. Furthermore, a certain gap is maintained between the gradient coil and the magnet, facilitating the timely dissipation of heat generated during operation and accelerating the attenuation of eddy currents. Below the magnet is a magnet fixing plate for securing it. This fixing structure, due to its thickness, further attenuates eddy currents. The outer circumference of the magnet fixing plate is a cylindrical ring used to fix the gradient coil. Below the magnet fixing plate is an anti-eddy current assembly. To prevent eddy currents from being generated in the yoke, a longitudinally arranged array of magnets is fixedly installed on the outer surface of the columns on both sides of the yoke. These magnets generate a horizontally inward magnetic field, which can block the main magnetic field from diverging outward, reducing the range to 5Gs.

[0078] Based on the above settings, good magnetic field uniformity and low inductance increase can be achieved. For conventional magnets, the inductance increase after the gradient coil is installed in the magnet is usually between 25% and 30%; the inductance increase of the gradient coil in this invention is approximately 15% to 20%.

[0079] Meanwhile, by optimizing the structure of the ring-shaped magnet array, the weight of the magnet can be effectively reduced. For example, for a permanent magnet with a central field strength of 0.095T, the magnet using the novel structure of this invention weighs about 1.25t, while the magnet using the traditional structure weighs about 2.5 tons; for a permanent magnet with a central field strength of 0.2T, the magnet using the novel structure of this invention weighs about 2.5t, while the magnet using the traditional structure weighs about 5 tons.

[0080] The above are merely preferred embodiments of the present invention and are not intended to limit the implementation methods and protection scope of the present invention. Those skilled in the art should recognize that any equivalent substitutions and obvious changes made based on the description and illustrations of the present invention should be included within the protection scope of the present invention.

Claims

1. A magnet structure suitable for ultra-low field magnetic resonance imaging systems, characterized in that, Including ring magnet arrays and evaporative control arrays; The annular magnet array includes an upper annular array and a lower annular array; The upper ring array and the lower ring array are arranged opposite to each other; The evaporation control array is located outside the annular magnet array; The escaping control array includes at least two sets of escaping control magnets arranged in a vertical direction; The adjacent escaping control magnets have opposite magnetic directions to form multiple sets of inward magnetic lines of force, which constrain the main magnetic field formed between the upper ring array and the lower ring array. The ultra-low field magnetic resonance imaging system includes an integral yoke; The lower side of the integral yoke is provided with the following in sequence: The lower anti-eddy current assembly is disposed on the inner surface of the lower side of the integral yoke; The lower magnet fixing plate is fixed to the upper surface of the lower anti-eddy current component; The lower annular array is positioned above the lower magnet fixing plate; A first fixing ring is provided on the outer periphery of the lower magnetic steel fixing plate; A lower gradient coil is installed and fixed above the first fixed ring, and a certain gap is maintained between the lower gradient coil and the lower ring array; The upper side of the integral yoke is provided with the following in sequence: Upper anti-eddy current assembly, wherein the upper anti-eddy current assembly is disposed below the inner surface of the upper side of the integral yoke; An upper magnet fixing plate is fixed to the lower surface of the upper anti-eddy current assembly; The upper annular array is located below the upper magnet fixing plate; A second fixing ring is provided on the outer periphery of the upper magnetic steel fixing plate; An upper gradient coil is installed and fixed below the second fixing ring, and a certain gap is maintained between the upper gradient coil and the upper annular array. The ring-shaped magnet array is suitable for ultra-low field magnetic resonance imaging systems; The annular magnet array does not include pole plates and shimming rings, and the magnetic field convergence of the annular magnet array is achieved solely by the dissipation control array.

2. The magnet structure according to claim 1, characterized in that, The integral yoke is square-shaped when viewed from the front. The inner surfaces of the upper and lower sides of the integral yoke are respectively used to set the upper annular array and the lower annular array; The left and right sides of the integral yoke are yoke pillars; The evaporation control array is located inside the yoke column.

3. The magnet structure according to claim 1, characterized in that, The annular magnet array includes multiple concentric annular magnet blocks. The first magnetic direction of the upper annular array is upward, and the second magnetic direction is downward. The first magnetic direction of the lower annular array is upward, and the second magnetic direction is downward, so that the second magnetic direction of the upper annular array matches the first magnetic direction of the lower annular array.

4. The magnet structure according to claim 3, characterized in that, The circular magnetic steel block is composed of multiple fan-shaped magnetic steels; The radius of each of the said sector-shaped annular magnets is determined according to the width of the said annular magnet block; The inner and outer arcs of the fan-shaped annular magnet are determined according to the inner and outer diameters of the annular magnet block, respectively.

5. The magnet structure according to claim 3, characterized in that, The height of the multiple annular magnetic steel blocks increases sequentially from the inside to the outside.

6. The magnet structure according to claim 4, characterized in that, Each of the fan-shaped magnets has the same gap as the adjacent fan-shaped magnets.

7. The magnet structure according to claim 4, characterized in that, The number of the annular magnetic steel blocks is between 3 and 15; Each of the said annular magnet blocks has 8-24 equal parts of the said fan-shaped annular magnets.

8. An ultra-low field magnetic resonance imaging system, characterized in that, The magnet structure described in any one of claims 1-7 is applied.

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