Novel magnet structure suitable for ultra-low field magnetic resonance imaging system

Abstract

The invention relates to the technical field of low-field magnetic resonance imaging equipment, in particular to a novel magnet structure suitable for an ultralow-field magnetic resonance imaging system, which comprises a dissipation control array arranged on the outer side of an annular magnetic steel array and comprising at least two groups of dissipation control magnetic steel arranged along the vertical direction; and the magnetic directions of the adjacent dissipation control magnetic steels are opposite to form a plurality of groups of inward magnetic lines to restrain the main magnetic field. In order to solve the problem that eddy current is generated between a shimming ring and a pole plate due to rapid switching of gradient coils in the magnet structure design in the prior art, the shimming ring, the pole plate and other structures in the traditional design are cancelled, and an annular magnetic steel array is adopted to generate a more concentrated magnetic field; the dissipation control array arranged on the outer side of the annular magnetic steel array is adopted to generate an inward magnetic field, the dissipation phenomenon of a main magnetic field is reduced, good imaging can still be achieved under the condition that a shimming ring and a pole plate are omitted, and the eddy current problem caused by the structure is eliminated.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of low-field magnetic resonance imaging equipment, and particularly relates to a novel magnet structure suitable for an ultralow-field magnetic resonance imaging system. BACKGROUND

[0002] MRI is a commonly used medical tomography method, which obtains electromagnetic signals from a human body by using a magnetic resonance phenomenon and reconstructs information of the human body. Ultralow-field magnetic resonance generally refers to magnetic resonance imaging with a field strength lower than 0.1 T. Compared with high-field magnetic resonance, the ultralow-field magnetic resonance has the advantages of low examination cost and easy popularization. Moreover, because the field strength is relatively low, a shielding room does not need to be arranged, and a relatively safe examination process can be realized. Open-type radio frequency shielding magnetic resonance can provide a comfortable scanning experience for patients. Domestic research and development in the field of ultralow-field magnetic resonance (ULF-MRI) are mainly concentrated in enterprises (such as Beiji Wang, Wuxi Mingshi Junqi) and colleges and universities (such as Shanghai Jiaotong University and Chongqing University of Posts and Telecommunications). The ultralow-field HEAD-500 craniocerebral magnetic resonance developed by Beiji Wang adopts a traditional samarium-cobalt permanent magnetic material, the magnetic field strength is 0.55 T, and the overall weight is about 630 kg. The SPEN (spatiotemporal encoding) acquisition method suitable for a multi-channel parallel acceleration-free portable magnetic resonance device is developed by the team of Zhang Zhiyong of Shanghai Jiaotong University. Compared with the traditional EPI (echo planar imaging) technology, the undersampling image of the method exhibits smaller geometric distortion. The Electrical Engineering College of Chongqing University and the Southwest Hospital of the Army Medical University successfully developed a prototype machine of a head-specialized bedside MRI system. The field strength of the system is 0.05 T, the total weight is less than 500 kg, and the system is powered by a 220V / 50Hz standard power supply. The device can be directly placed in an intensive care unit, a general ward and an operating room, and is used as a brain disease diagnosis and monitoring device, especially for cerebral apoplexy, to realize bedside imaging examination.

[0003] In the prior art, the configuration of a low-field and ultralow-field magnetic resonance system generally comprises a magnetic steel array, a magnet yoke and a gradient coil.

[0004] For example, the patent document CN202210725616.X discloses an ultra-low field special magnetic resonance imaging system for spine, which comprises a support assembly, a magnet assembly and an equipment cabinet. The support assembly comprises a support plate, a support, a rotating mechanism and a wire groove. The outer side of the support is provided with the rotating mechanism, and the other outer side of the support is provided with the wire groove. The outer end of the rotating mechanism is provided with the magnet assembly. The magnet assembly comprises a C-shaped magnet, a gradient coil, a flat plate type transmitting coil and an openable receiving coil. The system is powered by a rechargeable battery, solving the signal interference problem caused by mains power supply. The magnet can be set to horizontal and vertical states. The system can perform standing and lying scanning on patients. In standing scanning mode, the patient's spine is in a daily life state, which can provide magnetic resonance imaging images of the actual use state of the spine for clinical diagnosis. In lying scanning mode, magnetic resonance imaging can be performed on patients who cannot stand and postoperative patients.

[0005] For example, the patent document CN202411432238.1 discloses an ultra-low field nuclear magnetic resonance measuring device with a large range of adjustable background fields, and an ultra-low field nuclear magnetic resonance measuring method with a large range of adjustable background fields. A flat solenoid is used to achieve high background magnetic field requirements and ultra-low environmental magnetic fields. The flat solenoid can accommodate more coils and obtain more turns, achieving a wider range of adjustable background fields. The ends of the flat solenoid extend outside the magnetic shielding module, which ensures that the magnetic flux lines at the ends of the flat solenoid are connected outside the shielding area, eliminating the problem of background field inhomogeneity caused by ordinary solenoid coils. The interference of the environmental magnetic field is eliminated by passive magnetic shielding and active magnetic field compensation. First, the environmental magnetic field is passively shielded by multiple electromagnetic shielding cylinders. Then, the active magnetic field compensation is performed by generating a magnetic field opposite to the environmental magnetic field through the shim coil, which offsets the interference of the environmental magnetic field. The shim coil can also improve the uniformity of the central magnetic field.

[0006] However, in actual implementation, the inventors found that such magnetic resonance systems usually adopt traditional magnet designs, including magnetic steel, pole plate, anti-eddy current components and shim ring structures. In order to meet certain imaging space, magnetic field uniformity and reduce the influence of eddy current on images, each component needs to meet certain shape size and thickness requirements in addition to meeting the material property requirements, which increases the weight of the magnet itself. With the increase of magnet field strength and opening, the weight of the magnet increases geometrically. At the same time, due to the existence of the pole plate and the shim ring in the traditional structure of the permanent magnet, the rapid switching of the gradient coil will generate eddy current in them. The eddy current in the image will form eddy current artifacts, which will affect the imaging quality. SUMMARY

[0007] In view of the above problems in the prior art, the present application provides a kind of.

[0008] The specific technical solutions are as follows: a novel magnet structure suitable for an ultra-low field magnetic resonance imaging system, comprising a ring-shaped magnet steel array and a diffusion control array; the ring-shaped magnet steel array comprises an upper ring-shaped array and a lower ring-shaped array; the upper ring-shaped array and the lower ring-shaped array are oppositely arranged; the diffusion control array is arranged outside the ring-shaped magnet steel array; the diffusion control array comprises 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 form multiple groups of inward magnetic force lines and form a continuous "magnetic potential barrier" to constrain the main magnetic field formed between the upper ring-shaped array and the lower ring-shaped array; the diffusion control magnet steel adopts a high-performance permanent magnet material, preferably a neodymium iron boron (such as N52, N54 grade) or samarium cobalt (such as Sm2Co17) material; the residual magnetism (Br) thereof is not less than 1.45 T, the coercive force (Hcb) is not less than 12 kOe, and the maximum magnetic energy product ((BH)max) is not less than 52 MGOe, so as to meet the requirements of the ultra-low field magnetic resonance imaging on the stability and strength of the magnetic field; the size of a single diffusion control magnet steel is as follows: a height of 20-40 mm, a width of 30-50 mm, and a thickness of 10-30 mm; the number of diffusion control magnet steels arranged on each side column is 6-12.

[0009] A non-magnetic insulating gasket (such as an epoxy plate) is arranged between the diffusion control magnet steel and the steel column, so as to avoid magnetic short circuit and improve the magnetic field control efficiency.

[0010] On the other hand, the ultra-low field magnetic resonance imaging system comprises a whole yoke; the whole yoke is in the shape of a mouth in the front view direction; the inner surfaces of the upper and lower sides of the whole yoke are respectively used for arranging the upper ring-shaped array and the lower ring-shaped array; the left and right sides of the whole yoke are yoke columns; and the diffusion control array is arranged on the inner side of the yoke column.

[0011] On the other hand, the lower side of the whole yoke is sequentially provided with a lower anti-eddy current assembly, a lower magnet steel fixing plate, and a lower gradient coil; the lower anti-eddy current assembly is arranged on the inner surface of the lower side of the whole yoke; the lower magnet steel fixing plate is fixed on the upper surface of the lower anti-eddy current assembly; the lower ring-shaped array is arranged above the lower magnet steel fixing plate; a first fixing ring is arranged on the outer periphery of the lower magnet steel fixing plate; and the lower gradient coil is fixedly installed above the first fixing ring and keeps a certain gap between the lower gradient coil and the lower ring-shaped array.

[0012] On the other hand, the upper side of the whole yoke is sequentially provided with: an upper anti eddy current assembly, which is arranged below the inner surface of the upper side of the whole yoke; an upper magnetic steel fixing plate, which is fixed to the lower surface of the upper anti eddy current assembly; the upper annular array is arranged below the upper magnetic steel fixing plate; the outer periphery of the upper magnetic steel fixing plate is provided with a second fixing ring; the lower side of the second fixing ring is fixedly installed with an upper gradient coil, and a certain gap is maintained between the upper gradient coil and the lower annular array.

[0013] On the other hand, the annular magnetic steel array includes a plurality of concentric circularly arranged annular magnetic steel 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 magnetic steel block is composed of a plurality of fan ring magnetic steels; the radius of each fan ring magnetic steel is determined according to the width of the annular magnetic steel block; the inner arc and the outer arc of the fan ring magnetic steel are determined according to the inner diameter and the outer diameter of the annular magnetic steel block, respectively.

[0015] On the other hand, the heights of the plurality of annular magnetic steel arrays from inside to outside are sequentially increased.

[0016] On the other hand, each fan ring magnetic steel has the same gap with the adjacent fan ring magnetic steel.

[0017] On the other hand, the number of annular magnetic steel blocks is between 3-15; each annular magnetic steel block has 8-24 fan ring magnetic steels.

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

[0019] The technical scheme has the following advantages or beneficial effects: in the prior art, the magnet structure design needs to add components such as a shimming ring and a pole plate, which leads to the problem that eddy current is generated in the shimming ring and the pole plate when the gradient coil is rapidly switched; in the scheme, the shimming ring and the pole plate in the traditional design are cancelled, a ring-shaped magnetic steel array is adopted to generate a more concentrated magnetic field, and a dissipation control array arranged outside the ring-shaped magnetic steel array is adopted to generate a magnetic field inward to reduce the dissipation of the main magnetic field; in the case of cancelling the shimming ring and the pole plate, good imaging can still be achieved, and the problem of eddy current caused by the above structure is eliminated. BRIEF DESCRIPTION OF DRAWINGS

[0020] Reference will be made to the accompanying drawings to more fully describe embodiments of the present application. However, the accompanying drawings are only used for illustration and explanation, and do not constitute a limitation on the scope of the present application.

[0021] Figure 1 is a schematic diagram of the whole embodiment of the present application; Figure 2 is a schematic diagram of the magnet structure in the prior art; Figure 3 is a schematic diagram of the ring-shaped magnetic steel array in the embodiment of the present application; Figure 4 is a schematic diagram of the stepped height of the ring-shaped magnetic steel array in the embodiment of the present application. DETAILED DESCRIPTION

[0022] The technical scheme in the embodiments of the present application will be described clearly and completely in combination with the drawings of the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0023] It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict.

[0024] The present application will be further described in combination with the drawings and specific embodiments, but is not limited by the present application.

[0025] The application comprises a new magnet structure suitable for an ultra-low field magnetic resonance imaging system, including a ring-shaped magnet steel array 1 and a dispersion control array 2; the ring-shaped magnet steel array 1 comprises an upper ring-shaped array 11 and a lower ring-shaped array 12; the upper ring-shaped array 11 and the lower ring-shaped array 12 are oppositely arranged; the dispersion control array 2 is arranged outside the ring-shaped magnet steel array 1; the dispersion control array 2 comprises at least two groups of dispersion control magnet steels 21 arranged in a vertical direction; the magnetism directions of adjacent dispersion control magnet steels 21 are opposite to each other to form multiple groups of inward magnetic force lines to constrain the main magnetic field formed between the upper ring-shaped array 11 and the lower ring-shaped array 12.

[0026] Specifically, for the magnet structure design in the prior art, the addition of a shim ring and a pole plate and the like components causes the problem that the eddy current is generated in both of them when the gradient coil is rapidly switched, in the present scheme, the shim ring and the pole plate and the like structures in the traditional design are cancelled, a ring-shaped magnet steel array is adopted to generate a more concentrated magnetic field, and a dispersion control array arranged outside the ring-shaped magnet steel array is adopted to generate an inward magnetic field to reduce the dispersion phenomenon of the main magnetic field, and good imaging can still be realized under the condition that the shim ring and the pole plate are cancelled, and the problem of eddy current caused by the above structures is eliminated.

[0027] Specifically, as shown in the figure, Figure 2 the traditional structure of the low-field magnetic resonance magnet steel array comprises the following parts: a ring-shaped yoke iron providing a guiding path for the magnetic field; a lower ring-shaped array 102 arranged on the bottom surface of the ring-shaped yoke iron, the array comprising a plurality of magnet steel blocks for forming a specific magnetic field direction; above the lower ring-shaped array 102 is a lower pole plate 103, and above the lower pole plate 103 is a lower anti-eddy current assembly 104 for isolating the lower pole plate and the magnet steel from the lower gradient coil above; above the lower anti-eddy current assembly 104 is a lower gradient coil 105 for generating a uniform and variable magnetic field to examine the patient in the cavity above; the lower anti-eddy current assembly 104 suppresses the magnetic field to reduce the eddy current generated on the pole plate and the magnet steel during the process that the lower gradient coil 105 generates a variable magnetic field; the outer side of the lower anti-eddy current assembly 104 and the lower gradient coil 105 is sleeved with a shim ring to realize the convergence of the magnetic field and achieve good imaging effect at a lower field strength; the upper magnet structure is similar to the above-mentioned lower magnet structure, but the direction and sequence are reversed. The magnetic force lines are generated from the lower part, pass through the examination area, reach the upper part, then enter the ring-shaped yoke iron and go downward, forming a complete magnetic field.

[0028] In the above process, because the pole plate and the shim ring are arranged in the structure, the eddy current is generated in both of them when the gradient coil is rapidly switched, the eddy current forms an eddy current artifact in the image, and then affects the imaging quality.

[0029] To solve the above problems, the magnet structure in this embodiment removes the components of the pole plate and the field-shaping ring, thereby eliminating the cause of eddy current. To achieve clear imaging of the ultra-low field system without the pole plate and the field-shaping ring, the following improvements are made to the layout of the magnetic steel in this solution: first, the main magnetic steel array is replaced by a ring-shaped magnetic steel array, which includes a plurality of circular ring-shaped magnetic steel blocks arranged in a circular ring shape. The ring-shaped magnetic steel array provides a natural circularly converging main magnetic field, thereby improving the uniformity of the magnetic field. Second, to address the problem of possible escape of the main magnetic field, an escape control array 2 is arranged on both sides of the yoke, which provides inward magnetic field to converge the main magnetic field.

[0030] As shown in Figure 3 The escape control array 2 includes at least one group of escape control magnetic steels 21 arranged in the vertical direction. The magnetic directions of adjacent escape control magnetic steels 21 arranged from high to low are opposite, and the magnetic field lines of adjacent escape control magnetic steels 21 will pass in and out in the shape of an 8, thereby forming a constraint magnetic field distributed along the height direction on the whole escape control array 2. The magnetic field lines of the main magnetic field will be repelled by the constraint magnetic field, so they cannot pass through the escape control array 2 to the outside, which can block the outward escape of the main magnetic field and reduce the 5 Gs line range.

[0031] In a preferred embodiment, the escape control magnetic steels 21 are made of N52 grade neodymium iron boron material, which has typical performance parameters of: residual magnetization Br ≥ 1.48 T, coercive force Hcb ≥ 12.5 kOe. The size of the single magnetic steel is designed to be 30 mm in height, 40 mm in width, and 20 mm in thickness. Eight such magnetic steels are installed on the inner side of each side yoke column 31 in the vertical direction, and the magnetization directions of adjacent magnetic steels are opposite (i.e., N-S-N-S… alternate arrangement). An epoxy resin plate with a thickness of 2 mm is used as a non-magnetic insulating gasket between the escape control magnetic steels 21 and the mounting surface of the yoke column 31.

[0032] After the above arrangement, the escape control array 2 can generate a significant horizontal inward constraint magnetic field. Test results show that in an imaging system with a central field strength of 0.1 T, this design can effectively reduce the 5 Gs line range of the main magnetic field from about 1.2 meters in the traditional structure to within 0.8 meters, significantly suppressing the escape of the magnetic field, and indirectly improving the uniformity of the magnetic field in the central imaging area. Since the array is composed of discrete high-resistivity permanent magnets and does not contain continuous metal pole plates or field-shaping rings, the problem of eddy current generated by the rapid switching of the gradient coil is fundamentally avoided.

[0033] The reason why the existing traditional structure does not remove the pole plate and the field-shaping ring is that, after removal, the magnetic field uniformity stability is poor and the magnetic field at the edge escapes outward. The root cause of the poor magnetic field uniformity stability is that the main magnetic steel is directly exposed to the air, resulting in too fast heat loss and poor stability; this point can be solved in the application by applying a heat preservation layer on the surface of the magnetic steel; the root cause of the outward escape of the edge magnetic field is that, after the traditional magnet removes the field-shaping ring, it is not conducive to the focusing of the magnetic field to the center, affecting the stability of the uniform field; for this point, in the application, the vertical arrangement of the magnetic steel array on the two side columns is mainly used to suppress the outward dispersion of the magnetic field. The gradient coil in the traditional structure is directly covered on the surface of the magnetic pole, and the rapid switching of the gradient coil will generate eddy current around the pole plate and the field-shaping ring, thereby affecting the image quality. In the application, the gradient coil is located above the magnetic steel and maintains a certain gap with the magnetic steel. Due to the material properties of the magnetic steel, which is a high-resistance material, it is not sensitive to eddy current, and the magnet in the application does not contain a pole plate and a field-shaping ring. The eddy current generated by the rapid switching of the gradient coil is most likely in the yoke, and the distance between the gradient coil and the yoke is much greater than the propagation distance of the eddy current, so the influence of the eddy current is very small, and no software compensation is needed or only a small compensation is needed to offset the influence of the eddy current.

[0034] The magnet in patent No. CN 118501785 B adopts a C-shaped structure, which has the advantages of better openness and relatively low cost compared to the symmetrical double-column structure in the present application. However, the field strength is relatively low, the uniformity is poor, the volume is relatively large, and the temperature stability is poor. The C-shaped structure magnet has a long magnetic circuit and exists magnetic leakage, resulting in low magnetic energy utilization rate. The open design leads to a limited magnetic field concentration area. The double-column symmetrical design shortens the magnetic circuit and reduces magnetic leakage, resulting in higher magnetic energy utilization rate and a magnetic field concentration area closer to the center between the two columns. The C-shaped structure magnet has poor magnetic field uniformity due to the asymmetric magnetic circuit, resulting in significant magnetic field gradient changes. It is suitable for applications with low uniformity requirements (such as limb imaging). The double-column magnet has better magnetic field uniformity (symmetry can reduce high-order magnetic field distortion), making it easy to shim and suitable for whole-body imaging or high-resolution applications. The C-shaped structure magnet has a long magnetic circuit, which is significantly affected by temperature. The double-column magnet has better temperature stability due to the symmetrical design of heat stress dispersion.

[0035] In summary: compared to the traditional structure of the permanent magnet, the application solves the technical difficulties of poor magnetic field uniformity stability, outward escape of the edge magnetic field, and the influence of eddy current on the image after removing the pole plate and the field-shaping ring in the traditional structure through the new layout design of the magnet internal structure, while effectively reducing the weight of the magnet. Compared to the C-shaped structure mentioned in CN 118501785 B, the new symmetrical double-column magnet structure in the present application has better overall performance.

[0036] In one embodiment, the ultra-low field magnetic resonance imaging system comprises a whole yoke 3; the whole yoke 3 is in the shape of a mouth in the front view; the inner surfaces of the upper and lower sides of the whole yoke 3 are respectively used to arrange an upper annular array 11 and a lower annular array 12; the left and right sides of the whole yoke 3 are yoke columns 31; and the escape control array 2 is arranged on the inner side of the yoke columns 31.

[0037] Specifically, in order to achieve a better escape control effect, when the above-mentioned magnet structure is applied in the ultra-low field magnetic resonance imaging system, the upper annular array 11 and the lower annular array 12 are arranged on the inner surfaces of the upper and lower sides of the whole yoke 3 respectively, wherein the upper annular array 11 is located on the inner surface of the upper side, and the lower annular array 12 is located on the inner surface of the lower side.

[0038] The whole yoke 3 is arranged outside the annular array in the shape of a mouth to provide a complete magnetic induction path. The part of the cavity serves as an examination area so that at least part of the limbs of the patient, such as the arm, spine, head, etc., can be inserted to apply an alternating magnetic field to the patient.

[0039] The left and right sides of the whole yoke 3 are yoke columns 31, the escape control array 2 is arranged on the inner side of the yoke columns 31, a plurality of groups of magnetic steels are sequentially arranged along the height direction of the yoke columns, and the magnetic steels adjacent in the height direction are reversely arranged during the arrangement of the magnetic steels, so as to form a specific magnetic field distribution on the inner side of the yoke columns 31 to block the path of the main magnetic field escaping to the outside.

[0040] According to the different widths of the magnetic steel blocks in the magnetic steel array, a plurality of groups of magnetic steel arrays may need to be arranged along the vertical direction, but the magnetic steel array generally needs to completely cover the height of the middle shim area and cover the width of the yoke column 31 in the lateral direction.

[0041] In one embodiment, the escape control array 2 is provided with a plurality of escape control magnetic steels 21 along the height direction of the yoke column; the magnetic directions of the adjacent escape control magnetic steels 21 distributed along the height direction are opposite; and the escape control array 2 completely covers the spacing between the annular magnetic steel arrays 1.

[0042] The escape control magnetic steels adopt high-performance permanent magnetic materials, such as neodymium iron boron (e.g. N52, N54 grade) or samarium cobalt (e.g. Sm2Co17) materials, whose residual magnetism (Br) is not less than 1.45 T and coercive force (Hcb) is not less than 12 kOe. The typical size range of a single magnetic steel block is: height 20-40 mm, width 30-50 mm, and thickness 10-30 mm. The number of escape control magnetic steels arranged on each side column is generally 6-12.

[0043] Specifically, to achieve better control of the escape range of the main magnetic field, in the embodiment, a plurality of escape control magnetic steels 21 are arranged along the height direction of the yoke stand. The magnetic directions of adjacent escape control magnetic steels 21 arranged from high to low are opposite, and the magnetic field lines of adjacent escape control magnetic steels 21 will sequentially pass in and out in the shape of an 8, thereby forming a constraint magnetic field distributed along the height direction on the overall escape control array 2. The magnetic field lines of the main magnetic field will be repelled by the constraint magnetic field and thus cannot pass through the escape control array 2 to the outside, which can block the outward escape of the main magnetic field and reduce the 5Gs line range.

[0044] By controlling that the escape control array 2 completely covers the spacing between the annular magnetic steel arrays 1, effective control of the main magnetic field can be achieved.

[0045] In one embodiment, the lower side of the overall yoke 3 is sequentially provided with: a lower anti eddy current assembly 42, which is arranged on the inner surface of the lower side of the overall yoke 3; a lower magnetic steel fixing plate 52, which is fixed to the upper surface of the lower anti eddy current assembly; a lower annular array 12, which is arranged above the lower magnetic steel fixing plate 52; a first fixing ring 62, which is arranged on the outer periphery of the lower magnetic steel fixing plate 52; and a lower gradient coil 72, which is installed and fixed above the first fixing ring 62 and maintains a certain gap between the lower gradient coil 72 and the lower annular array 12.

[0046] Specifically, to solve the problem that the addition of the shim ring and the pole plate in the magnetic body structure design of the prior art causes eddy current in the two when the gradient coil rapidly switches, in the embodiment, the magnetic steel array is optimized, and the coil layout in the magnetic resonance system is also adjusted, including removing the lower anti eddy current assembly 42 from between the magnetic steel array and the gradient coil and replacing it below the lower magnetic steel fixing plate 52, thereby avoiding the interference problem caused by the anti eddy current assembly itself.

[0047] The lower magnetic steel fixing plate 52 is a fixing plate for installing the magnetic steel blocks in the magnetic steel array and the gradient coil in the magnetic resonance system, which needs to fix the magnetic steel blocks, and the fixing structure has a certain thickness and further attenuates the eddy current.

[0048] The plurality of magnetic steels in the lower annular array 12 are arranged and fixed above the lower magnetic steel fixing plate 52 according to a specific arrangement and form a ring-shaped arrangement to provide a uniform magnetic field.

[0049] The upper surface of the lower magnetic steel fixing plate 52 is provided with the first fixing ring 62 along the outer periphery 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 the lower gradient coil 72 is lifted to a certain height, so that a certain gap is maintained between the lower gradient coil 72 and the lower annular array 12, thereby reducing the eddy current in the magnetic steel blocks.

[0050] In one embodiment, the upper side of the whole yoke iron 3 is sequentially provided with: an upper anti eddy current assembly 41, which is arranged below the inner surface of the upper side of the whole yoke iron 3; an upper magnetic steel fixing plate 51, which is fixed to the lower surface of the upper anti eddy current assembly 41; an upper annular array 11, which is arranged below the upper magnetic steel fixing plate 51; a second fixing ring 61, which is arranged on the outer periphery of the upper magnetic steel fixing plate 51; and an upper gradient coil 71, which is fixed below the second fixing ring 61 and has a certain gap with the upper annular array 11.

[0051] Specifically, in the prior art, the addition of the field uniformity ring and the pole plate and other components to the magnet structure design causes the problem that the rapid switching of the gradient coil generates eddy currents in the two components. In the embodiment, the magnetic steel array is optimized, and the coil layout in the magnetic resonance system is adjusted, including removing the upper anti eddy current assembly 41 from between the magnetic steel array and the gradient coil and replacing it above the upper magnetic steel fixing plate 51, thereby avoiding the interference problem caused by the anti eddy current assembly itself.

[0052] The upper magnetic steel fixing plate 51 is a fixing plate for fixing the magnetic steel blocks in the magnetic steel array and the gradient coil in the magnetic resonance system. The magnetic steel needs to be fixed, and the fixing structure has a certain thickness and further attenuates the eddy current.

[0053] The plurality of magnetic steels in the upper annular array 11 are fixed above the upper magnetic steel fixing plate 51 according to a specific arrangement and form a ring-shaped arrangement to provide a uniform magnetic field.

[0054] The upper surface of the upper magnetic steel fixing plate 51 is provided with the second fixing ring 61 along the outer periphery 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 the upper gradient coil 71 is lowered by a certain height, so that a certain gap is maintained between the upper gradient coil 71 and the upper annular array 11, thereby reducing the eddy current in the magnetic steel block and dissipating heat.

[0055] The common gap should be controlled to be greater than 3 mm.

[0056] In one embodiment, the annular magnetic steel array 1 includes a plurality of circular ring-shaped magnetic steel blocks arranged in concentric circles; 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.

[0057] In one embodiment, the first magnetic direction is N-pole and the second magnetic direction is S-pole, but the reverse substitution is also possible according to actual needs.

[0058] Specifically, to achieve a more concentrated magnetic field effect, in the embodiment, the annular magnetic steel array 1 is replaced by a circular ring-shaped magnetic steel block including a plurality of concentric circular arrangements. The circular ring-shaped magnetic steel block with concentric circular arrangements is used to achieve a more concentrated magnetic field distribution. Meanwhile, the second magnetic direction of the upper annular array 11 is matched with the first magnetic direction of the lower annular array 12 to achieve the direction of the magnetic field lines upward or downward.

[0059] It should be noted that if the circular ring-shaped magnetic steel structure is selected, a certain number of air gaps should be provided on the circular ring to destroy the continuity and avoid the problem of eddy current caused by the complete conductor formed by the magnetic steel structure.

[0060] Generally, according to the requirements of field strength and magnetic field area, the concentric circular arrangement of the circular ring structure is about 3-15, and the more common parameter range is 5-8.

[0061] In one embodiment, the circular ring-shaped magnetic steel block is composed of a plurality of fan ring-shaped magnetic steels 13. The radius of each fan ring-shaped magnetic steel 13 is determined according to the width of the circular ring-shaped magnetic steel block. The inner and outer arcs of the fan ring-shaped magnetic steel 13 are determined according to the inner and outer diameters of the circular ring-shaped magnetic steel block, respectively.

[0062] Specifically, to achieve better production, in the embodiment, the circular ring-shaped magnetic steel block is achieved by assembling the fan ring-shaped magnetic steels 13.

[0063] The fan ring-shaped magnetic steel 13 in the top view direction is composed of two arcs and two diameters. The inner and outer arcs are determined according to the inner and outer diameters of the circular ring-shaped magnetic steel block, respectively. The radius is determined according to the width of the circular ring-shaped magnetic steel block, and has a certain height. The fan ring-shaped magnetic steels in each circular ring-shaped magnetic steel block are usually consistent, which are designed by equal division of the circular ring.

[0064] In the traditional magnet, the arrangement of the magnetic steel is usually flatly laid in the entire disc according to the same size, and a special-shaped support structure of stainless steel needs to be reserved at the center position of the disc to support the anti-eddy current assembly. Since this structure occupies the original filling of the magnetic steel, the magnetic field at the center is very low, causing the discontinuity of the imaging space magnetic field and increasing the difficulty of the later shimming.

[0065] By applying the fan ring-shaped fan ring-shaped magnetic steel 13, better magnetic field uniformity can be achieved in the splicing process, and the amount of magnetic steel is reduced.

[0066] In one embodiment, the heights of the plurality of circular ring-shaped magnetic steel blocks increase successively from inside to outside.

[0067] Specifically, since the magnetic field generated by the permanent magnet itself has the characteristic of dissipation, which leads to the field strength near the center of the magnet being higher than that at the edge of the magnet, the field strength in the range of the central shimming ball is basically uniform, and the closer to the edge, the smaller the field strength. In view of this problem, the prior art usually considers increasing the total area of the magnetic steel array to ensure that the area with high central strength can meet the inspection requirements. However, this will lead to an overall increase in the volume and weight of the system.

[0068] In view of this problem, in the embodiment, the heights of the plurality of annular magnetic steel blocks are selected to increase successively from inside to outside, thereby increasing the volume per unit area of the annular magnetic steel blocks on the outside. By designing a specific height difference of the magnetic steel, the magnetic field strength of the magnetic steel in each region can be compensated.

[0069] When the fan-shaped annular magnetic steel 13 is spliced to form an annular magnetic steel block, the height of each fan-shaped annular magnetic steel 13 is adjusted respectively, so as to change the magnetic field strength provided by the magnetic steel.

[0070] In one embodiment, each fan-shaped annular magnetic steel has the same gap with the adjacent fan-shaped annular magnetic steel.

[0071] The specific arrangement mode has the following modes: 1) arranged according to the equicentral fan-shaped radiation matrix, the sizes of the magnetic steels are distributed in a stepped manner from outside to inside, each magnetic steel maintains a certain gap to avoid eddy current generated by the rapid switching of the gradient coil in the magnetic steel. 2) The magnetic steels are arranged in a rectangular block and closely arranged with each other to form a disc as a whole, the contact surfaces of each block are coated with liquid oxygen glue for insulation, and the middle part is a non-magnetic stainless steel fixing part, which is fixed on the magnetic steel array fixing plate below.

[0072] Each annular magnetic steel block has 8-24 fan-shaped annular magnetic steels, and the more common parameters are 12-18 divisions in each annular ring.

[0073] In the traditional magnet structure, the magnetic steels are flatly tiled to fill the entire disc, and there is no gap between the magnetic steels, which are insulated by coating epoxy glue on the contact surfaces. Because the center needs to reserve a fixed support structure for the anti-eddy current component, the magnetic field at the center of the imaging space is easily lower than that at the periphery, which can only be adjusted by passive shimming after the imaging, and the adjustment process is complex and difficult. On the other hand, because the traditional structure contains a magnet pole plate, the rapid switching of the gradient coil will generate a large eddy current in the pole plate. Even if the magnet has an anti-eddy current component, it cannot completely eliminate the image of the eddy current, and it needs to be actively compensated by software to eliminate it. Moreover, because the eddy current is dynamic, the software compensation also needs to be adjusted regularly, which increases the complexity and period of debugging.

[0074] The novel magnet structure gradient coil of the present application is directly mounted above the magnetic steel. Since the material of the magnetic steel is high-resistivity material and the surface of the magnetic steel is coated with special insulating paint, the influence of eddy current on the magnetic steel can be reduced. The gradient coil and the magnetic steel maintain a certain gap, which facilitates the timely diffusion of heat generated by the gradient coil during operation and accelerates the decay of eddy current. The magnetic steel is fixed by a magnetic steel fixing plate below the magnetic steel. The fixing structure has a certain thickness, which further attenuates the eddy current. The outer periphery of the magnetic steel fixing plate is a cylindrical ring for fixing the gradient coil. The anti-eddy current assembly is below the magnetic steel fixing plate. In order to prevent the generation of eddy current in the yoke, a longitudinal array of magnetic steel is fixedly installed on the outer surface of the yoke column on both sides. The magnetic steel can generate a horizontal inward magnetic field, which can block the outward dispersion of the main magnetic field and reduce the range of 5Gs.

[0075] Based on the above arrangement, better magnetic field uniformity and lower inductance increase can be achieved. For a traditional magnet, the inductance increase of the gradient coil after being installed in the magnet is usually between 25% and 30%; the inductance increase of the gradient coil in the present application after being installed in the magnet is about 15% to 20%.

[0076] At the same time, by optimizing the structure of the annular magnetic steel 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 weight of the magnet with the novel structure of the present application is about 1.25t, and the weight of the magnet with the traditional structure is about 2.5t; for a permanent magnet with a central field strength of 0.2T, the weight of the magnet with the novel structure of the present application is about 2.5t, and the weight of the magnet with the traditional structure is about 5t.

[0077] The above is only the preferred embodiment of the present application, and does not limit the implementation and protection scope of the present application. For those skilled in the art, it should be realized that any equivalent replacement and obvious changes made according to the content of the present application should be included in the protection scope of the present application.

Claims

1. A novel magnet structure suitable for use in an ultra-low field magnetic resonance imaging system, characterized in that, The application relates to a novel magnet structure, which comprises a ring-shaped magnet steel array and a diffusion control array; the ring-shaped magnet steel array comprises an upper ring-shaped array and a lower ring-shaped array; the upper ring-shaped array and the lower ring-shaped array are oppositely arranged; the diffusion control array is arranged outside the ring-shaped magnet steel array; the diffusion control array comprises 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 to each other, so as to form multiple groups of inward magnetic force lines, and the main magnetic field formed between the upper ring-shaped array and the lower ring-shaped array is constrained.

2. The novel magnet structure according to claim 1, characterized in that The super-low-field magnetic resonance imaging system comprises a whole yoke; the whole yoke is in the shape of a mouth in the front view direction; the inner surfaces of the upper and lower sides of the whole yoke are respectively used for arranging the upper ring-shaped array and the lower ring-shaped array; the left and right sides of the whole yoke are yoke columns; and the diffusion control array is arranged inside the yoke columns.

3. The novel magnet structure according to claim 1, characterized in that, The lower side of the whole yoke is sequentially provided with a lower anti-eddy current assembly arranged on the inner surface of the lower side of the whole yoke, a lower magnet steel fixing plate fixed on the upper surface of the lower anti-eddy current assembly, the lower ring-shaped array arranged above the lower magnet steel fixing plate, a first fixing ring arranged on the outer periphery of the lower magnet steel fixing plate, and a lower gradient coil fixedly installed above the first fixing ring and kept with a certain gap between the lower gradient coil and the lower ring-shaped array.

4. The novel magnet structure according to claim 1, characterized in that, The upper side of the whole yoke is sequentially provided with an upper anti-eddy current assembly arranged below the inner surface of the upper side of the whole yoke, an upper magnet steel fixing plate fixed on the lower surface of the upper anti-eddy current assembly, the upper ring-shaped array arranged below the upper magnet steel fixing plate, a second fixing ring arranged on the outer periphery of the upper magnet steel fixing plate, and an upper gradient coil fixedly installed below the second fixing ring and kept with a certain gap between the upper gradient coil and the upper ring-shaped array.

5. The novel magnet structure according to claim 1, characterized in that, The ring-shaped magnet steel array comprises a plurality of circular ring-shaped magnet steel blocks arranged in concentric circles; the first magnetic direction of the upper ring-shaped array is upward, and the second magnetic direction is downward; the first magnetic direction of the lower ring-shaped array is upward, and the second magnetic direction is downward, so that the second magnetic direction of the upper ring-shaped array matches the first magnetic direction of the lower ring-shaped array.

6. The novel magnet structure according to claim 5, characterized in that Each circular ring-shaped magnet steel block is composed of a plurality of fan ring-shaped magnet steels; the radius of each fan ring-shaped magnet steel is determined according to the width of the circular ring-shaped magnet steel block; and the inner arc and the outer arc of the fan ring-shaped magnet steel are respectively determined according to the inner diameter and the outer diameter of the circular ring-shaped magnet steel block.

7. The novel magnet structure according to claim 5, characterized in that The heights of the circular ring-shaped magnet steel blocks from inside to outside sequentially increase.

8. The novel magnet structure of claim 6, wherein, Each fan ring-shaped magnet steel has the same gap with the adjacent fan ring-shaped magnet steel.

9. The novel magnet structure of claim 6, wherein, The number of the circular ring-shaped magnet steel blocks is between 3 and 15; and each circular ring-shaped magnet steel block has 8-24 fan ring-shaped magnet steels.

10. An ultra-low field magnetic resonance imaging system, characterized in that The novel magnet structure is applied.

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