CN101281239B - Permanent magnet for magnetic resonance imaging and manufacturing method thereof - Google Patents

Abstract

A permanent magnet used for magnetic resonance imaging, the main magnet (2) of which is formed by superimposing a plurality of magnet rings (21) arranged in a stepped shape along the central axis of the magnet ring. The magnet ring (21) is divided into a plurality of magnetic lobes (211) in the radial direction, and the magnetization direction of the magnetic lobes (211) on any cross-section is a 360 ° stepwise change around the central axis of the magnet ring; each magnetic lobes ( 211) The magnetization direction on any half of the longitudinal section changes stepwise along the central axis of the magnet ring, and the tangent to the magnetization direction of each magnetic lobe (211) is continuously distributed. The magnetization direction of the main magnetic field of the magnet ring (21) at the central axis of the magnet ring is axial. The method for manufacturing this permanent magnet is that the main magnetic pole (213) is superimposed on the left and right in the radial direction to form a magnetic lobe (211); The sequence of the step-by-step change forms the magnet ring (21); the magnet ring (21) is stacked up and down along the central axis of the magnet ring to form the main magnet (2).

Description

Permanent magnet for magnetic resonance imaging and manufacturing method thereof

technical field

The invention relates to a magnetic resonance imaging system, in particular to a permanent magnet for magnetic resonance imaging and a manufacturing method thereof.

Background technique

Magnetic resonance imaging can provide high spatial resolution, unlimited penetration depth, very good soft tissue contrast, and excellent spatial anatomical positioning, so magnetic resonance imaging has incomparable advantages over other imaging techniques. The new magnetic resonance imaging requires higher magnetic field strength and higher spatial resolution. For increasing the magnetic field strength, superconducting magnets are undoubtedly an option. However, superconducting magnets are expensive to manufacture, operate and maintain. Permanent magnets are getting more and more attention because of their reliable work, low price, no need for auxiliary equipment, simple operation and maintenance, and energy saving.

We know that conventional permanent magnets can be divided into two categories: open and closed. Among them, the typical representative of open magnets is C-type magnets; closed magnets include many types, such as "mouth" type, column type, "I" type, spherical type, cylindrical type and so on. Compared with open magnets, closed magnets have the advantages of good magnetic field stability, high uniformity, small magnetic flux leakage, and simple installation and fabrication. The principles followed in magnet design are: the field strength should be as high as possible; the magnetic field should be as uniform as possible; no obvious flux leakage is allowed; and the weight should be as light as possible. Therefore, we first consider designing the magnet with a closed structure. The traditional frame structure magnet is composed of yoke iron, main magnetic block and pole surface. Due to the edge effect, only the central region of the air gap can obtain a highly uniform magnetic field, while the magnetic field in the edge region of the air gap is inhomogeneous. In order to obtain a highly uniform magnetic field with an inhomogeneity less than 10-4 within a small area of the air gap central area, the ratio of the pole face width to the air gap height needs to reach 4-8, that is, the magnet needs to be made very large. In order to reduce the cost of magnets, in 1984, American Field Effect Company developed an improved permanent magnet. It is composed of several trapezoidal magnetic blocks with different magnetization directions to form a polygonal ring magnet, and the inside of the ring is an air gap area. Because no yoke iron is used, the magnet is lighter in weight, and the amount of magnetic block material is also lower than that of the above-mentioned common permanent magnet. However, due to the different magnetization directions in each trapezoidal magnetic block, it is difficult to process and manufacture. Moreover, the working points in the magnetic block are inconsistent, and many magnetic blocks are at the working point with a small magnetic energy level, so the amount of magnetic block material is still quite large. Chinese patent CN85103498 is an improved structure of a common iron yoke frame magnet. The structure adopts the polygonal annular magnet structure of Field Effect Company in the United States, which retains the simple shape of the magnetic block (mostly square and triangular) in the ordinary permanent magnet structure, which is convenient for processing and manufacturing, and the frame-shaped yoke forms a closed magnetic circuit so that the magnetic flux Advantages such as no leakage to the outside of the yoke. Compared with the ordinary permanent magnet structure, the plugging magnetic block and the side magnetic block are added to improve the uniformity of the magnetic field at the air gap. The earliest permanent magnet used in magnetic resonance imaging was the QEDBeta 3000 designed and manufactured by Fonar Corporation of the United States. Its working magnetic density is 3000 Gauss, and the permanent magnetic ferrite material is used as the magnetic source. The total weight reaches almost 100 tons. According to the relevant laws of magnetism, if the Chinese patent CN85103498 is used to manufacture magnets with technical indicators similar to those of Fonar's QED Beta3000, the estimated total weight will exceed 60 tons, and the absolute weight is still very large. The processing and installation of magnetic blocks and magnets are more complicated. Chinese patent CN1042795 is another improved iron-less yoke structure, which uses strontium calcium ferrite as the magnetic source. When the magnetic density is 1500 gauss, the weight reaches 9 tons, but the leakage field range is relatively large. Another magnet developed by New York University School of Medicine (see IEEE Transaction on Magnetics, Vol.Mag-25, No.5, P3904-3906) is assembled from permanent magnet blocks of different shapes and magnetization directions, processed and installed are very complicated. Although Chinese patents 200420009046.1 and 200420047676.8 are both closed structures, it is difficult for the field strength to reach a relatively high level.

从公式中可以看出，无论圆柱型或球型，理论上这种磁体的场强可以达到无限大。但是实际上，由于永磁材料工作于深度磁饱和和充磁后磁滞回线的第二象限退磁部分，当场强达到一定高度时，磁体会遭受磁材高矫顽力的强退磁作用，最终把场强限制在约2.5T左右，甚至达不到2T。显然这种传统的永磁磁体设计是难以进一步提高磁场强度的。In this type of magnet, the more classic magnet structure is Halbach. In the Halbach structure, the magnetic field direction of each magnet theoretically follows distributed. This structure is very practical and economical, and basically replaces low-field electromagnets (<1.8T) routinely, and is especially suitable for designing small-scale magnets in laboratories. The formula for calculating the magnetic field strength of a Halbach cylindrical magnet is B _Z ＝μ ₀ ·M·log(R _ext /R _int ), and the formula for calculating the magnetic field strength of a spherical magnet is

It can be seen from the formula that, regardless of the cylindrical or spherical shape, the field strength of this magnet can theoretically reach infinity. But in fact, since the permanent magnet material works in the second quadrant demagnetization part of the hysteresis loop after deep magnetic saturation and magnetization, when the field strength reaches a certain height, the magnet will suffer from the strong demagnetization effect of the high coercive force of the magnetic material, and finally Limit the field strength to about 2.5T, or even less than 2T. Obviously, this traditional permanent magnet design is difficult to further increase the magnetic field strength.

Contents of the invention

In order to overcome the shortcomings of the prior art, the present invention proposes an improved permanent magnet for magnetic resonance imaging and a manufacturing method thereof.

The permanent magnet for magnetic resonance imaging provided by the present invention includes a yoke, a main magnet, and upper and lower pole pieces. It is characterized in that the main magnet is composed of multiple magnet rings; the magnet ring is a ring structure, which is divided into multiple magnetic lobes in the radial direction, and the magnetization direction of the magnetic lobes on any cross section is 360° around the central axis of the magnet ring. The magnetization direction of each magnetic lobe on any half of the longitudinal section changes stepwise along the central axis of the magnet ring, and the tangent of the magnetization direction of each magnetic lobe is continuously distributed; the magnet ring is in the magnet ring. The magnetization direction of the main magnetic field at the axis is axial.

The main magnet is a closed ring structure, and the magnetization direction of the main magnetic field at the central axis of the magnet ring is axial; the main magnet is formed by superimposing multiple magnet rings in a stepped distribution along the central axis of the magnet ring; the superimposed main magnet is in the The magnetization direction on any cross-section is 360° stepwise change around the central axis of the magnet ring, and the magnetization direction on any half longitudinal section is 360° stepwise change along the central axis of the magnet ring, and in The angle between the tangent line of the magnetization direction of each order magnet ring on the same half longitudinal section and the central axis of the magnet ring is continuously distributed; the axial length of the main magnet is greater than the maximum radial length of the magnet ring; the yoke is wrapped in The outer periphery of the entire main magnet; the upper and lower pole pieces are located at the two end faces of the imaging area of the main magnet, providing a uniform magnetic field for the imaging area.

The invention provides a method for manufacturing a permanent magnet for magnetic resonance imaging, which is characterized in that, firstly, the permanent magnet material is processed into a magnetic block by wire cutting, and then magnetized, spliced and glued in sequence according to the magnetization direction of the magnetic block to form a magnetic block. Main magnetic pole: one or more main magnetic poles whose magnetization direction gradually changes are superimposed on the left and right in the radial direction, forming a stepwise change in the magnetization direction along the central axis of the magnet ring on any half of the longitudinal section, and the tangent of the magnetization direction is in line with the magnet The included angle of the central axis of the ring is a magnetic lobe that is continuously distributed; then according to the uniqueness of the magnetization direction of the magnetic lobe in the radial direction, it is formed on the central axis of the magnet ring in the order of 360° stepwise change around the central axis of the magnet ring. The magnetization direction of the main magnetic field at is axial to the magnet ring.

According to the sequence of 360° step-wise change of the magnetization direction along the central axis of the magnet ring on any half of the longitudinal section, the magnet rings of various stages are superimposed up and down along the central axis of the magnet ring to form a closed ring-shaped main magnet; upper and lower The pole piece is fixed on the imaging end face of the main magnet; and the soft magnetic yoke iron whose inner surface is distributed in a step shape and whose steps match the steps of the magnet ring is fixed on the outer periphery of the main magnet.

Through the above-mentioned technical scheme of the present invention, compared with the existing permanent magnets, the permanent magnet provided by the present invention has a field strength of several tesla with substantially the same size and shape without obvious leakage. Magnetic, less consumables, light weight, maximize the potential of permanent magnets. Therefore, this permanent magnet can be used not only for magnetic resonance imaging, but also for any other situation that requires a small, high-field magnet that is easy to move and maintain.

Description of drawings

Fig. 1 is the schematic diagram of the longitudinal section structure of the magnet ring of the present invention;

Fig. 2a and Fig. 2b are schematic diagrams of axially stepping magnetization directions in the longitudinal section of the magnet ring of the present invention;

Fig. 3a and Fig. 3b are schematic diagrams of the radial section of the magnet ring of the present invention;

4a to 4j are schematic diagrams of the shape of the radial section of the magnet ring magnetic lobe of the present invention;

Fig. 5 is a structural schematic diagram of the longitudinal section of the main magnet of the present invention;

Fig. 6 is a schematic diagram of various stages of magnet rings in the longitudinal section of the main magnet of the present invention;

Fig. 7 is a schematic diagram of the longitudinal section of the main magnet of the present invention and the axial stepping magnetization direction;

Fig. 8a and Fig. 8b are schematic longitudinal sectional structural diagrams of the permanent magnet device of the present invention.

In the figure: 1 yoke, 2 main magnet, 3 upper and lower pole shoes, 4 imaging area, 5 entrance and exit channel of imaging object, 21 magnet ring, 211 magnetic lobe, 212 magnetic hole, 213 main magnetic pole.

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments.

According to the first content of the present invention, as shown in FIGS. 1 to 4 , the present invention provides a ring-shaped magnet ring 21, which is divided into a plurality of magnetic lobes 211 in the radial direction, so that The magnetization direction of the magnetic lobes 211 on any cross-section is 360° stepwise change around the central axis of the magnet ring; The central axis changes stepwise, and the angles between the tangents of each magnetization direction and the central axis of the magnet ring are continuously distributed; the magnetization direction of the main magnetic field of the annular structure magnet ring 21 at the central axis of the magnet ring is axial.

Fig. 2a and Fig. 2b are schematic diagrams of the axially stepping magnetization direction of the longitudinal section of the magnet ring 21 of the present invention. As shown in Figure 2a and Figure 2b, the magnetization direction on any half of the longitudinal section of the magnet ring 21 of the present invention is a stepwise change from the magnetization direction at one end to the magnetization direction at the other end along the central axis of the magnet ring, and The included angles between the tangents of the changing magnetization directions and the central axis of the magnet ring are continuously distributed. The magnetization trend can change clockwise from top to bottom along the central axis of the magnet ring, or can change counterclockwise from top to bottom along the central axis of the magnet ring. Figure 2a and Figure 2b only show one of the changes. When as shown in Figure 2a and Figure 2b along the axis of the magnet ring from top to bottom clockwise 360 ° step change, the main magnetic field direction of the resulting magnet ring 21 is upward; otherwise, along the axis of the magnet ring from When the counterclockwise 360° step changes from top to bottom, the main magnetic field direction of the obtained magnet ring 21 is downward.

The magnetization direction of each magnetic lobe 211 in the magnet ring 21 of the present invention is determined by the magnetization condition of the main magnetic pole 213 of its component unit. One or more main magnetic poles 213 whose magnetization direction changes gradually are superimposed on the left and right in the radial direction to form a magnetic lobe 211 whose magnetization direction changes gradually. As shown in Figure 2a, the magnetization direction at the magnetic hole 212, that is, the central axis of the magnet ring, is vertically upward; on both sides of the magnetic hole 212, as the distance between the main magnetic pole 213 and the magnetic hole 212 gradually increases, the direction of the main magnetic pole 213 The magnetization direction gradually deviates from the central axis of the magnet ring outward clockwise from a direction close to the vertical upward direction. As shown in Figure 2b, the magnetization direction at the magnetic hole 212, that is, the central axis of the magnet ring, is vertically upward; on both sides of the magnetic hole 212, as the distance between the main magnetic pole 213 and the magnetic hole 212 gradually increases, the main magnetic pole 213 The magnetization direction gradually deviates from the central axis of the magnet ring outwardly and counterclockwise from a direction close to the vertical upward direction.

Fig. 3a and Fig. 3b are radial cross-sectional schematic diagrams of the magnet ring 21 shown in the present invention corresponding to Fig. 2a and Fig. 2b, and the figures show the magnetization direction of each magnetic lobe changing stepwise around the central axis of the magnet ring. The magnetization direction of each magnetic lobe 211 in the radial direction is unique, and the magnetization direction of all magnetic lobes 211 on the same cross section completes a 360° stepwise change around the central axis of the magnet ring, wherein the arrows of each magnetization direction can be determined by the magnet ring. The axis points to the circumference, as shown in Figure 3a, or it can point from the circumference to the axis of the magnet ring, as shown in Figure 3b.

The magnet ring 21 of the present invention is composed of a plurality of magnetic lobes 211 in the radial direction, as shown in FIG. sex distribution. Figure 1 only shows the fully symmetric distribution.

The present invention has no special requirements on the size, quantity and material of the magnetic lobes 211 . The size of the magnetic lobe 211 can be determined according to the required size of the magnet ring 21 and the size of the imaging area 4 . The higher the field strength requirement of the magnet ring 21 is, the larger the imaging area 4 required for imaging the object will be, and the size of the magnetic lobe 211 will be larger accordingly. The number of magnetic lobes 211 is based on the principle that a closed magnet 21 can be formed in the radial direction. Theoretically it can be 2 or more. The material of the magnetic flap 211 can be any existing permanent magnet material, such as: sintered NdFeB, bonded rare earth permanent magnet, rare earth cobalt, permanent magnet ferrite, etc.

FIG. 1 also schematically illustrates that the magnetic lobes 211 of the magnet ring are formed by stacking a plurality of main magnetic poles 213 radially left and right. The magnetic lobes 211 have no special requirement on the number of main magnetic poles 213 , and theoretically there may be one or more. Each magnetic lobe 211 has no special requirements on the size, thickness and material of the main magnetic pole 213 . The size of each main magnetic pole 213 can be the same or different. The thickness of each main magnetic pole 213 can be the same or different. The main magnetic poles 213 of all magnetic lobes 211 may have the same thickness or different thicknesses. Each main magnetic pole 213 can use the same permanent magnet material or different permanent magnet materials according to the distribution of the local magnetic field. The permanent magnet material can be any existing permanent magnet material, such as: sintered NdFeB, bonded rare earth permanent magnet, rare earth cobalt, permanent magnet ferrite, etc.

The present invention has no special requirement on the shape of the magnetic lobe 211 . The magnetic lobes 211 in each magnet 21 may have the same shape or different shapes. The inner surface of the magnetic lobe 211 may be in the shape of points, straight lines, arcs, broken lines, or curves; the outer surface of the magnetic lobe 211 may be in the shape of straight lines, arcs, broken lines, or curves. Therefore, the radial section of the magnetic lobe 211 can be fan-shaped, triangular, pointed cone, trapezoidal, arcuate, flat-bottomed cone, square, or any other shape, as shown in Fig. 4a, Fig. 4b, Fig. 4c, FIG. 4d, FIG. 4e, FIG. 4f, FIG. 4g, FIG. 4h, FIG. 4i and FIG. 4j exemplarily show radial cross-sectional schematic diagrams of several magnet ring magnetic lobes 211 of the present invention.

The magnet ring 21 of the present invention is composed of a plurality of magnetic lobes 211 in the radial direction, so the shape of the magnetic lobes 211 determines the shape of the magnet ring 21 .

The present invention has no special limitation on the shape of the magnet ring 21 . Wherein, the outer shape of the magnet 21 can be a regular shape or an irregular shape, such as any one of a cylinder, a cone, a polygonal prism, a polygonal pyramid, and a frame, or the cross section is an irregular shape Any kind of cylinder; wherein, the inner surface shape of the magnet 21 can be a regular shape or an irregular shape, such as any one of a cylinder, a cone, a polygonal prism, a polygonal pyramid, and a frame Or any cylinder with an irregular cross-section. Fig. 3 a and Fig. 3 b have shown the radial cross-sectional schematic view of the magnet ring 21 under the shape of one of the magnetic lobes 211. At this time, the shapes and sizes of the magnetic lobes 211 are the same, and the inner and outer surfaces of the magnet ring 21 are regular cylinders. type.

The present invention also provides a magnet device for magnetic resonance imaging using the above-mentioned magnet ring 21 . The magnet device includes a yoke 1 , a main magnet 2 and upper and lower pole pieces 3 .

Wherein, the main magnet 2 is a closed structure, and the magnetization direction of the main magnetic field at the central axis of the magnet ring is axial; the main magnet 2 is composed of the above-mentioned plurality of magnet rings 21 distributed in a stepped manner along the central axis of the magnet ring and stacked up and down. The magnetization direction of the superimposed main magnet 2 on any cross section is a 360° stepwise change around the central axis of the magnet ring, and the magnetization direction on any half of the longitudinal section is along the center of the magnet ring. The axis changes stepwise by 360°, and the tangents of the magnetization directions of the magnet rings 21 of each stage on the same half longitudinal section are continuously distributed; the axial length of the main magnet 2 is greater than the maximum diameter of the magnet ring 21 to length.

5 to 7 schematically show the structure and magnetization of the main magnet 2 of the present invention. The magnetization trend can be clockwise from top to bottom along the central axis of the magnet ring on any half of the longitudinal section, or counterclockwise from top to bottom along the central axis of the magnet ring. Figure 7 shows only one of these variations. When the clockwise 360° stepwise change along the central axis of the magnet ring is shown in Figure 7, the resulting magnetic field direction of the main magnet 2 is upward; otherwise, it is reversed from top to bottom along the central axis of the magnet ring. When the hour hand changes stepwise by 360°, the obtained magnetic field direction of the main magnet 2 is downward.

The main magnet 2 in the present invention is composed of the above-mentioned plurality of magnet rings 21 distributed up and down in a stepped manner along the central axis of the magnet ring, wherein the magnetization direction of each magnet ring 21 on any half of the longitudinal section is along The magnetization direction of the magnet ring axis changes stepwise from one end to the other end, and the angle between the magnetization direction of each magnet ring 21 on the same half longitudinal section and the magnet ring axis is in the wave band distribution, and share the band-type change from 0° to 360°. By stacking each magnet ring 21 up and down in the order of gradual change in the magnetization direction, the magnetization direction of the main magnet 2 from one end to the other end can be easily changed along the 360° stepwise change of the central axis of the magnet ring. .

Fig. 5 schematically illustrates the stepped distribution among the magnet rings 21 of each step in the main magnet 2, and this stepped distribution may be fully symmetrical or axially symmetrical or asymmetrical.

Figure 6 not only shows the stepped distribution between the magnet rings 21 of each step in the main magnet 2, but also shows that the magnet rings 21 of each step include a plurality of magnetic lobes 211 in the radial direction, and each magnetic lobe 211 may contain one or A plurality of main magnetic poles 213 .

The main magnet 2 has no special requirements on the size, thickness and material of the magnet rings 21 of each stage. Each magnet ring 21 may have the same size or different sizes. The thickness of each magnet ring 21 can be the same or different. Each magnet ring 21 can use the same permanent magnet material or different permanent magnet materials. The permanent magnet material can be any existing permanent magnet material, such as: sintered NdFeB, bonded rare earth permanent magnet, rare earth cobalt, permanent magnet ferrite, etc.

The present invention has no special limitation on the shape of the main magnet 2 . The shapes of the magnet rings 21 of each stage jointly determine the shape of the main magnet 2 . The outer shape of the main magnet 2 can be a regular shape or an irregular shape, and Fig. 7 shows a schematic longitudinal cross-sectional view of the main magnet 2 in the shape of one of the magnetic lobes 211. At this time, the shapes of the magnetic lobes 211 are the same , the same size, the inner and outer surfaces of the magnet rings 21 of each step are regular cylindrical shapes, and the outer shape of the main magnet 2 is obtained by a plurality of cylindrical bodies distributed in a stepped manner.

The magnet device for magnetic resonance imaging shown in FIGS. 8a to 8b is an embodiment of the magnet device of the present invention.

As shown in Figure 8a and Figure 8b, in the magnet device for magnetic resonance imaging of the present invention, the function of the yoke iron 1 is to form a magnetic circuit, enhance the magnetic field at the imaging area 4, shield magnetic flux leakage, and can be used to fix the The magnet ring 21 strengthens the structural stability of the main magnet 2 and provides considerable convenience for the installation of the entire magnet device.

The yoke 1 of the present invention is made of soft magnetic material with high saturation, and the material can be carbon steel or engineering pure iron. The structure of the yoke 1 is governed by the shape of the main magnet 2 . In the magnet device of the present invention, the inner surface of the yoke 1 is determined by the outer surface of the main magnet 2 and is distributed in steps, and the number of steps matches the number of magnetic layers of the magnet ring 21 . The yoke 1 can be an integral structure or an assembled structure with respect to each magnet ring 21 , and can also have some other auxiliary components. The yoke 1 is wrapped around the entire periphery of the main magnet 2 .

It should be noted that, in the magnet device of the present invention, the main magnet 2 is parallel to the direction of the main magnetic field, both ends of the magnet are closed structures, and the imaging area 4 is located at the center of the main magnet 2, as shown in Figures 8a and 8b. In Fig. 8b, in the direction perpendicular to the main magnetic field, the main magnet 2 is a transparent structure, and there is a channel 5 for taking and placing the imaging object. This channel 5 can also exist in another direction perpendicular to the main magnetic field.

The upper and lower pole pieces 3 are made of high-saturation soft magnetic material, which can be selected from carbon steel, engineering pure iron and some combination thereof with silicon steel sheets and iron-based nanocrystals. The upper and lower pole pieces 3 are located at the two end faces of the imaging area, providing the imaging area 4 with the best possible uniform magnetic field, so as to achieve the purpose of directly performing magnetic resonance imaging.

According to the second content of the present invention, the present invention provides a method for manufacturing a magnet ring, the method comprising: forming main magnetic poles 213 of different shapes and different magnetization directions; A magnetic lobe 211 that changes stepwise along the central axis of the magnet ring and whose tangent to the magnetization direction is continuously distributed; according to the uniqueness of the magnetization direction of the magnetic lobe 211 in the cross section, a plurality of magnetic lobes 211 are assembled in the radial direction to form a The magnetization direction of the main magnetic field at the central axis of the ring is axial magnet ring 21 .

The main magnetic pole 213 can be made into the main magnetic pole 213 with the required shape and required magnetization direction by adopting the selected permanent magnet material, performing wire cutting on it, and then magnetizing, splicing, gluing and so on.

The magnetic lobes 211 have different magnetization directions depending on their positions in the magnet ring 21 . The previously obtained main magnetic poles 213 are sequentially spliced and glued according to their magnetization directions to obtain required magnetic lobes 211 of various specifications.

Then, according to the uniqueness of the magnetization direction of the magnetic lobes 211 in the radial direction, each magnetic lobes 211 are arranged in the order of 360° stepwise change around the central axis of the magnet ring (as shown in Fig. 3a and Fig. 3b). placed, fixed by rivets, and finally the magnet ring 21 is formed.

The present invention also provides a manufacturing method of a magnet device for magnetic resonance imaging using the above-mentioned magnet ring 21 . The magnet device includes a yoke 1 , a main magnet 2 , and upper and lower pole pieces 3 .

The forming method of the main magnet 2 includes stacking the magnet rings 21 of various stages up and down according to the order of the magnetization direction gradually changing along the central axis of the magnet ring from 0° to 360° in sub-bands on any half of the longitudinal section, A closed annular main magnet 2 can be conveniently formed. The magnetization tendency of the obtained main magnet 2 can change clockwise from top to bottom along the central axis of the magnet ring, or can change counterclockwise from top to bottom along the central axis of the magnet ring. The magnet rings 21 of each step of the main magnet 2 are distributed in steps, and this step distribution can be axisymmetric or non-axisymmetric.

The manufacturing method of the main magnet 2 can be realized according to the manufacturing method of the above-mentioned magnet ring 21 of the present invention.

In the magnet device of the present invention, the pole piece 3 can be manufactured by machining. The upper and lower pole pieces are installed on opposite end surfaces of the main magnet imaging area 4 .

In the magnet device of the present invention, the yoke 1 is composed of a plurality of units. The number of units of the yoke 1 is the same as that of the magnet ring 21 , and the inner surface of the yoke 1 corresponds to the corresponding outer surface of the magnet ring 21 one by one. Each yoke unit is directly obtained by machining, and the yoke is wrapped around the outer circumference of each magnet ring 21 . Finally, a plurality of magnet rings 21 are assembled to form an imaging magnet device.

The advantages of the present invention are further illustrated below through examples.

Example:

Step 1: Use NdFeB materials to manufacture 7 batches of 8 identical magnetic lobes, of which 6 batches of magnetic lobes have a height of 55mm, and 1 batch of magnetic lobes has a height of 20mm. Among them, the thickness of the first batch of magnetic lobes and the seventh batch of magnetic lobes is 40mm, and there is only one main magnetic pole, and the main magnetic pole is made of magnetic materials with a centered magnetic energy level; the thickness of the second batch of magnetic lobes and the sixth batch of magnetic lobes is 80mm, It is composed of two levels of main magnetic poles. The main magnetic poles close to the axis use magnetic materials with a middle magnetic energy level, and the main magnetic poles far away from the axis use magnetic materials with lower magnetic energy levels; the thickness of the third batch of magnetic lobes and the fifth batch of magnetic lobes is 120mm , consisting of three levels of main magnetic poles, the main magnetic pole close to the axis uses a magnetic material with a higher magnetic energy level, the main magnetic pole located in the middle uses a magnetic material with a lower magnetic energy level, and the main magnetic pole farthest from the axis uses a magnetic material with a middle magnetic energy level. material; the fourth batch of magnetic lobes is located in the imaging area, with a thickness of 106.5mm. It is composed of two levels of main magnetic poles. magnetic material.

Step 2: Make the magnetization direction of the first-level main magnetic pole in the first batch of magnetic lobes tilt outward by 30 degrees relative to the axial direction; make the magnetization direction of the second-level main magnetic poles in the second batch of magnetic lobes outward from the center axis of the magnet ring 30 degrees and 60 degrees outward relative to the axial direction in turn; the magnetization direction of the third-level main magnetic poles in the third batch of magnetic lobes is inclined 60 degrees, 120 degrees and 150 degrees; the magnetization direction of the first-level main magnetic pole in the fourth batch of magnetic lobes is inclined by 180 degrees relative to the axial direction; the magnetization direction of the third-level main magnetic poles in the fifth batch of magnetic lobes is opposite from the center axis of the magnet ring to the outside Inwardly inclined by 60 degrees, 120 degrees and 150 degrees in the axial direction; the magnetization direction of the second-level main magnetic poles in the sixth batch of magnetic lobes is inclined from the center axis of the magnet ring outward by 30 degrees and 60 degrees inwards relative to the axial direction. ; Make the magnetization direction of the primary magnetic pole in the seventh batch of magnetic lobes inwardly inclined by 30 degrees relative to the axial direction.

Step 3: Use engineering pure iron to make 3 groups (2 in each group, and the size and shape between the two are exactly the same) magnetic yoke iron rings, the upper end of the first group of yokes is 120mm in diameter and 50mm in height Circular plate, the lower end is a cylindrical ring with an inner diameter of 80mm, an outer diameter of 120mm, and a height of 25mm; the upper end of the second group of yokes is a cylindrical ring with an inner diameter of 80mm, an outer diameter of 200mm, and a height of 30mm, and the lower end is a cylindrical ring with an inner diameter of 160mm , a cylindrical ring with an outer diameter of 200 mm and a height of 25 mm; the upper end of the third group of yokes is a cylindrical ring with an inner diameter of 160 mm, an outer diameter of 360 mm, and a height of 30 mm; the lower end is a cylindrical ring with an inner diameter of 240 mm, an outer diameter of 360 mm, and a height of 65mm cylindrical ring.

Step 4: Fix the yoke rings symmetrically with screws from top to bottom according to the order of their outer diameters from small to large and then from large to small, and keep a bottom circular plate at this time.

Step 5: Feed the first batch of 8 magnetic lobes into the uppermost yoke ring along the radial direction according to the order in which their magnetization direction changes in a 360-degree step around the axis of the magnet ring. At this time, the yoke The disc of the ring is on top. In the same way, the second, third, fourth, fifth, sixth, and seventh batches of magnetic lobes are fed in sequence, and the last reserved bottom yoke circular plate is packaged. A closed ring magnet system with a 360-degree stepwise change in the lower magnetization direction along the axial direction.

Step 6: Detect the magnetic field strength in the air gap, and the obtained magnetic field strength is 3.4 Tesla.

The longitudinal length of the permanent magnet obtained in this embodiment is 450 mm, the radial length is 360 mm, and the imaging area is a cylinder with a height of 27 mm and a bottom diameter of 20 mm.

Therefore, as can be seen from the above data, using the structure and manufacturing method of the present invention, the effectiveness of the permanent magnet material is fully brought into play, and the magnetic flux leakage is fully suppressed. When the opening degree remains unchanged and the total weight of the magnet remains substantially unchanged, the field strength of the magnet is greatly increased and the cost of magnetic materials is saved.

Claims (5)

1. a permanent-magnet that is used for magnetic resonance imaging comprises yoke (1), main magnet (2) and upper and lower pole shoe (3); It is characterized in that main magnet (2) is made up of a plurality of magnet rings (21); Magnet ring (21) is a loop configuration, radially is divided into a plurality of magnetic lobes (211), and the DOM of magnetic lobe (211) on any xsect is for to be 360 ° of stepwise change around the magnet ring axis; The DOM of each magnetic lobe (211) on any 1/2nd longitudinal sections is for to be stepwise change along the magnet ring axis, and the angle of the tangent line of each magnetic lobe (211) DOM and magnet ring axis is the continuity distribution; The main field DOM of magnet ring (21) at magnet ring axis place is axial; Main magnet (2) is a closed ring structure, and the main field DOM at magnet ring axis place is axial; Main magnet (2) is formed by stacking up and down a plurality of magnet rings along the stepped distribution of magnet ring axis (21); The DOM of the main magnet (2) of stack gained on any xsect is for to be 360 ° of stepwise change around the magnet ring axis; DOM on any 1/2nd longitudinal sections is for to be 360 ° of stepwise change along the magnet ring axis, and the angle of tangent line and the magnet ring axis of each magnetic lobe DOM of each rank magnet ring (21) is continuity and distributes on same 1/2nd longitudinal sections; The axial length of main magnet (2) is greater than the maximum radial length of magnet ring (21); Yoke (1) is wrapped in the periphery of whole main magnet (2); Last pole shoe is positioned at an end of main magnet imaging region (4), and following pole shoe is positioned at another end of main magnet imaging region (4), and upper and lower pole shoe is that imaging region (4) provides uniform magnetic field.

2. permanent-magnet as claimed in claim 1 is characterized in that, described magnet ring (21) is made up of a plurality of magnetic lobes (211) diametrically, magnetic lobe (211) be distributed in imaging region (4) around.

3. permanent-magnet as claimed in claim 1 is characterized in that, the appearance of said magnet ring (21) is a kind of in cylindrical, conical, polygon prismatic, polygon pyramid, the shaped as frame; The interior table of magnet ring (21) is shaped as a kind of in cylindrical, conical, polygon prismatic, polygon pyramid, the shaped as frame.

4. make the described manufacturing approach that is used for the permanent-magnet of magnetic resonance imaging of claim 1 for one kind; It is characterized in that, at first permanent magnetic material is carried out line and be cut into magnetic patch, magnetize then; Splice in order, glue together according to the magnetic patch DOM, process main pole (213); The main pole of a plurality of DOM gradual change (213) is stack to the left and right in the footpath; Be formed on that DOM is stepwise change along the magnet ring axis on any 1/2nd longitudinal sections, and the angle of the tangent line of DOM and magnet ring axis is the magnetic lobe (211) that continuity distributes; According to magnetic lobe (211) uniqueness of DOM diametrically, by the order that is 360 ° of stepwise change around the magnet ring axis, the main field DOM that is formed on magnet ring axis place is axial magnet ring (21) again.

5. like the said manufacturing approach that is used for the permanent-magnet of magnetic resonance imaging of claim 4; It is characterized in that; The order that is 360 ° of stepwise change according to DOM on any 1/2nd longitudinal sections along the magnet ring axis; Along magnet ring axis each rank magnet ring (21) that superposes up and down, form enclosed main magnet (2); Last pole shoe is fixed on the imaging end face of main magnet, and following pole shoe is fixed on another imaging end face of main magnet; Again with the stepped distribution of inside surface, and the soft magnetism yoke (1) that the exponent number of the progression of ladder and magnet ring (21) adapts is fixed on main magnet (2) periphery.

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