CN101847486B - Permanent magnetic system of medical nuclear magnetic resonance imager - Google Patents

Abstract

The invention relates to a permanent magnetic system of a medical nuclear magnetic resonance imager, which belongs to the field of magnetic circuit design. In the permanent magnetic system, magnets are arranged on the inner surface of a cylindrical yoke, and nonmagnetic steel is arranged on the inner surfaces of the magnets. The magnetic system is arranged in an axial layering way; each layer consists of the magnets with different magnetic properties; and the magnetic properties of the magnets symmetrically arranged from a middle layer to the layers on the two sides are increased layer by layer. Due to the layering design, the uniformity of a magnetic field in the magnetic system can be ensured on the premise that the high magnetic field is obtained, which solves the problem of magnet aging caused by the application of a compensating coil. Due to the cylindrical design, the intensity of the magnetic field can exceed 1 T, which cannot be realized by other methods for designing the permanent magnetic system of the medical nuclear magnetic resonance imager at present. The magnetic system is relatively shorter and is only 1.6 meters long so as to greatly reduce the fear, of confined spaces, of patients and meet functional check requirements. The intensity of the magnetic field of the invention can be within a range of 0.2 to 1.5 T, while the uniformity is less than 100 ppm.

Description

Permanent magnetic system of medical nuclear magnetic resonance imager

Technical field

The present invention relates to a kind of permanent magnetic system of medical nuclear magnetic resonance imager, belong to the magnetic Circuit Design field.

Background technology

Medical magnetic resonance imaging (MRI) system is the advanced medical image equipment in the modern clinical diagnosis, because its harmless, high outstanding advantage of soft tissue resolution is used widely clinically.The field intensity of the performance of MRI system magnetic used with it system has direct relation, and field intensity is high more, and performance is also high more.The size of magnetic field intensity to the influence of MRI equipment drawing picture is: aspect signal to noise ratio, magnetic field intensity is high more, and signal strength signal intensity is big more, signal to noise ratio high more (but not being linear relationship).Magnetic field intensity is high in addition, and sweep time is short.In general, people call low field device to the MRI appearance of magnet field intensity below 0.5T, 0.5T field device in claim between the 1.0T, the high field device of the title between 1.0 to 2.0T.The High-Field that magnetic system of (MRI) system of medical magnetic resonance imaging is at present reached is all realized through superconducting magnet, but superconducting magnet MRI needs liquid helium that the operational environment of coil is provided, so the selling price of superconducting magnet MRI and operating cost are all higher relatively.Sale and the operating cost of permanent magnetism low field intensity M R I are relatively low, and development in recent years is rapid, become the mainstream model of domestic routine clinical diagnosis at present, but the development of permanent magnetism MRI are limited by its field intensity, is thought low-grade MRI by the people for a long time always.China Xinao Bowei Technology Co., Ltd. successfully develops 0.7T permanent magnet type magnetic resonance imaging system technology xSTAR 7000.This is the permanent magnet type magnetic resonance imaging system art that field intensity is the highest in the world at present.MRI system requirements permanent magnetic system working area field has very high uniformity.The magnet of permanent magnetism MRI adopts open flat design at present; This kind magnetic circuit can not reach very high magnetic field but be easy to and improve the uniformity in magnetic field at the outside bucking coil that adds of magnet; Can make permanent magnet generation irreversible loss but the shortcoming of the method is the bucking coil heating, and influence the magnetic property of permanent magnet material.If being installed, the magnet temperature stabilizing arrangement then make volume, technical merit, the cost of magnetic system all receive influence.If magnetic system is designed to cylindrical shape then magnetic field can reach 1.5T; The magnetic of being narrated like Chinese patent CN85106663A is; Its magnetic system comprises the cylindrical shape yoke, and the yoke inner surface forms annular magnetic system by the polylith fan-shaped magnet, and this kind magnetic system is owing to be difficult for installing bucking coil; Can't solve the problem of magnetic field homogeneity, so it still can not be applied to medical magnetic resonance imaging (MRI) system.

Summary of the invention

The object of the present invention is to provide a kind ofly can reach evenly, the permanent magnetic system of medical nuclear magnetic resonance imager of highfield.Technical solution:

The present invention is provided with magnet at cylindrical shape yoke inner surface, and the magnet inner surface is provided with nonmagnetic steel; Said magnetic ties up to axis direction layering setting, and each layer is made up of the different magnet of magnetic, and the remanent magnetism of the magnet 2 of symmetrical setting from the intermediate layer to both sides successively increases; Said magnet 2 is a neodymium iron boron magnetic body, and magnet 2 is made up of a series of fan-shaped magnets, and the equal angles gradual change along the circumferential direction of the direction of magnetization of these fan-shaped magnets forms two magnetic poles, and magnetic direction is perpendicular to axial direction, so arranges to obtain the highfield.

The remanent magnetism of the magnet that the intermediate layer bilateral symmetry in the magnetic system is provided with is consistent, the remanent magnetism 1～1.2T of intermediate layer magnet.

The two-layer magnet that faces mutually in the magnetic system, outer magnet improves 0.02～0.2T than the remanent magnetism of internal layer magnet.

Magnetic is intermediate layer thickness scope 30～50cm, and other layer thickness reduces by 10～15cm than the intermediate layer.

Magnetic is that radial thickness is 2 times of cylinder type inter-air space radius r.

Magnetic is that the service area is the cylinder type space of center director for 2 times of air gap radiuses at magnetic, and magnetic is working area field: 0.5～1.5T, magnetic are that the working area field uniformity is less than 100ppm.

The present invention is that method for designing relatively has the following advantages with present nulcear magnetic resonance (NMR) magnetic:

(1) magnetic is that hierarchical design can obtain guaranteeing under the prerequisite of highfield that magnetic is the uniformity of internal magnetic field, avoids the use of the aging difficult problem of magnet that the bucking coil temperature raises and brings.

(2) adopt the drum type brake design can make magnetic field surpass 1T, this is that other nuclear medical magnetic resonance appearance permanent magnetic system method for designing institute is inaccessible at present.

(3) magnetic system is shorter, and length is merely 1.6 meters, significantly reduces patient's fear of confined spaces and can satisfy the needs of functional check.

Description of drawings:

Fig. 1 is a sectional view for magnetic of the present invention;

Fig. 2 is the hierarchy sketch map for magnetic of the present invention;

Fig. 3 is a magnet direction of magnetization sketch map of the present invention;

Fig. 4 axis of the present invention magnetic field comparison diagram;

Fig. 5 cross section of the present invention space magnetic field distribution map;

Fig. 6 layering of the present invention magnetic field comparison diagram;

Fig. 7 layering magnetic field homogeneity of the present invention design sketch;

Fig. 8 magnetic of the present invention is that working area field distributes.

Embodiment

The present invention serves as that calculate on the basis with " magnetic charge " notion and magnetic Coulomb law.The main feature of the demagnetization curve of neodymium iron boron magnetic body is that its squareness is fine, promptly the working point very on a large scale in (H≤H _k), magnetization M approaches Mr, i.e. M ≈ Mr.Therefore, the working point very on a large scale in can think M ≈ Mr approx.Thus, the body magnetic charge of magnet can be ignored, and only considers surface magnet charge, and the distributed pole of surface magnet charge is similar to evenly.Because surface magnet charge density: σ=M _nSo the relation of the surface magnet charge and the magnetization is relevant with the magnet block number of each layer.If is magnetic that cross section is divided into 16 magnets, the every magnet direction of magnetization is as shown in Figure 3, and computing formula is following:

σ ₁＝M，σ ₂＝Mcos45°＝0.707M，σ ₃＝Mcos45°＝0.707M，σ ₄＝Mcos90°＝0

σ _a＝2Msin11.25°＝0.39M

σ _b＝Msin11.25°+Msin56.25°＝1.027M

σ _c＝Msin56.25°-Msin33.75°＝0.276M

σ _d＝Msin78.75°-Msin33.75°＝0.425M

The magnetic field calculation formula:

The magnetic that is divided into 16 small magnets is the magnetic field on the inner axis:

(1) 1,2,3,4 type surface magnet charges are in the magnetic field of central point:

(2) a, b, c, d type surface magnet charge is in the magnetic field of central point:

The total magnetic field does

H＝H ₁+H ₂＝4πMR

$R=\frac{1.962J+1.501K}{\mathrm{\π}}$

$K=\ln \frac{(\sqrt{D+{\mathrm{<figure-callout\; id="2"\; label="P"\; filenames="HSA00000162169600013.png"\; state="\{\{state\}\}">P</figure-callout>}}^2}+P)(\sqrt{1+M^2}+M)(\sqrt{D+{\mathrm{<figure-callout\; id="2"\; label="N"\; filenames="HSA00000162169600013.png"\; state="\{\{state\}\}">N</figure-callout>}}^2}+N)(\sqrt{1+q^2}+Q)}{(\sqrt{D+M^2}+M)(\sqrt{1+{\mathrm{<figure-callout\; id="2"\; label="P"\; filenames="HSA00000162169600013.png"\; state="\{\{state\}\}">P</figure-callout>}}^2}+P)(\sqrt{D+{\mathrm{<figure-callout\; id="2"\; label="Q"\; filenames="HSA00000162169600013.png"\; state="\{\{state\}\}">Q</figure-callout>}}^2}+Q)(\sqrt{1+{\mathrm{<figure-callout\; id="2"\; label="N"\; filenames="HSA00000162169600013.png"\; state="\{\{state\}\}">N</figure-callout>}}^2}+N)}$

$J=2({\tan }^{-1}\frac{\mathrm{tM}}{\sqrt{4{\mathrm{<figure-callout\; id="2"\; label="M"\; filenames="HSA00000162169600013.png"\; state="\{\{state\}\}">M</figure-callout>}}^2+s}}-{\tan }^{-1}\frac{\mathrm{tN}}{\sqrt{4{\mathrm{<figure-callout\; id="2"\; label="N"\; filenames="HSA00000162169600013.png"\; state="\{\{state\}\}">N</figure-callout>}}^2+s}}+{\tan }^{-1}\frac{\mathrm{tP}}{\sqrt{{4\mathrm{<figure-callout\; id="2"\; label="P"\; filenames="HSA00000162169600013.png"\; state="\{\{state\}\}">P</figure-callout>}}^2+s}}-{\tan }^{-1}\frac{\mathrm{tQ}}{\sqrt{{4\mathrm{<figure-callout\; id="2"\; label="Q"\; filenames="HSA00000162169600013.png"\; state="\{\{state\}\}">Q</figure-callout>}}^2+s}})$

Foregoing calculates magnetic field on the axis, and the magnetic field in other spaces is big or small with the magnetic field on the axis to be consistent with uniformity, and has proved through small test.

Embodiment 1

Magnetic of the present invention is parameter: magnetic is length 25cm, and internal diameter is 2cm, and the magnet radial thickness is 4cm, and the cross section is divided into 16 fan-shaped magnets, and magnet performance is 1.1T.

After utilizing bonding method with magnetic system assembling, magnetic system is measured the magnetic field on the axis and compares with result of calculation, the result is identical basically, like Fig. 4.At magnetic is each the point measurement magnetic field beyond the same cross-sectional axis, and the result is more even, like Fig. 5.Therefore this experiment can prove with the design's method and can obtain magnetic field of the goal.

Embodiment 2:

Magnetic of the present invention is to comprise cylindrical shape yoke 1, and the cylindrical shape yoke 1 inner cylinder magnet of arranging around the inter-air space 2 forms magnetic and is, magnetic is that the inner surface place is provided with nonmagnetic steel 3, and the air gap that nonmagnetic steel 3 surrounds becomes the cylinder type solid space; Magnetic ties up to axis direction and divides 5 layers of setting, and the magnetic property of the magnet 2 of intermediate layer bilateral symmetry setting is consistent, and the magnetic property from the intermediate layer to the magnet 2 of bilateral symmetry setting successively increases.Magnet 2 of the present invention is made up of a series of fan-shaped magnets, and the equal angles gradual change along the circumferential direction of the direction of magnetization of these fan-shaped magnets forms two magnetic poles, and magnetic direction is perpendicular to axial direction, and direction of magnetization arrangement like this can use less magnet to obtain higher magnetic field.

Magnetic is parameter: internal diameter is 25cm; Magnet 2 radial thicknesss are 50cm, axially divide 5 layers, 16 magnets of every layer of branch; Intermediate layer thickness 40cm, other 4 layer thicknesses are 30cm; Intermediate layer magnet magnetic (remanent magnetism) 1T, two-layer magnet magnetic (remanent magnetism) 1.1T adjacent, the two-layer magnet magnetic in edge (remanent magnetism) 1.4T with the intermediate layer.

Through to magnetic field calculation, the accessible scope in magnetic field is 1.4 teslas, and magnetic field homogeneity equals 10ppm.The result show as with magnetic be layering will improve greatly magnetic field all and property, and can reach the requirement of NMR imaging instrument, magnetic is that the magnetic field homogeneity of service area (is the cylinder type space of center director for 2 times of air gap radiuses at magnetic) is seen Fig. 6, Fig. 7.

Embodiment 3:

Magnetic of the present invention is to comprise cylindrical shape yoke 1; The cylindrical shape yoke 1 inner cylinder magnet of arranging around the inter-air space 2 forms magnetic system, and magnetic is that the inner surface place is provided with nonmagnetic steel 3, and the air gap that nonmagnetic steel 3 surrounds becomes the cylinder type solid space; Magnetic ties up to axis direction and divides 5 layers of setting; With the intermediate layer is symmetry, and the magnetic property of the magnet 2 of the symmetrical setting in both sides, intermediate layer is consistent, and the magnetic property of the magnet 2 of symmetrical setting from the intermediate layer to both sides successively increases.Magnet 2 of the present invention is made up of a series of fan-shaped magnets, and the equal angles gradual change along the circumferential direction of the direction of magnetization of these fan-shaped magnets forms two magnetic poles, and magnetic direction is perpendicular to axial direction, and direction of magnetization arrangement like this can use less magnet to obtain higher magnetic field.

Magnetic is parameter: internal diameter is 15cm; The magnet radial thickness is 30cm, axially divides 5 layers, 16 magnets of every layer of branch; Intermediate layer thickness 20cm, other 4 layer thicknesses are 15cm; Intermediate layer magnet magnetic (remanent magnetism) 1.2T, two-layer magnet magnetic (remanent magnetism) 1.29T adjacent, the two-layer magnet magnetic in edge (remanent magnetism) 1.46T with the intermediate layer.

Through to magnetic field calculation, magnetic field can reach 1.4 teslas.The result shows: magnetic is that the requirement that reaches NMR imaging instrument is regulated through layering in the magnetic field of service area (is the cylinder type space of center director for 2 times of air gap radiuses at magnetic), and is as shown in Figure 8.

Claims (6)

1. permanent magnetic system of medical nuclear magnetic resonance imager, said magnetic system comprises the cylindrical shape yoke, it is characterized in that be provided with magnet at cylindrical shape yoke inner surface, the magnet inner surface is provided with nonmagnetic steel; Said magnetic ties up to axis direction layering setting, and each layer is made up of the different magnet of magnetic, and the remanent magnetism of the magnet of symmetrical setting successively increases from the intermediate layer to both sides; Said magnet is a neodymium iron boron magnetic body, and magnet is made up of a series of fan-shaped magnets, and the equal angles gradual change along the circumferential direction of the direction of magnetization of these fan-shaped magnets forms two magnetic poles, and magnetic direction is perpendicular to axial direction, so arranges to obtain the highfield.

2. permanent magnetic system of medical nuclear magnetic resonance imager according to claim 1 is characterized in that, the remanent magnetism of the magnet that the intermediate layer bilateral symmetry in the magnetic system is provided with is consistent, and the remanent magnetism of intermediate layer magnet is 1～1.2T.

3. permanent magnetic system of medical nuclear magnetic resonance imager according to claim 1 and 2 is characterized in that, the two-layer magnet that faces mutually in the magnetic system, and outer magnet improves 0.02～0.2T than the remanent magnetism of internal layer magnet.

4. permanent magnetic system of medical nuclear magnetic resonance imager according to claim 3 is characterized in that, magnetic is intermediate layer thickness scope 30～50cm, and other layer thickness reduces by 10～15cm than the intermediate layer.

5. permanent magnetic system of medical nuclear magnetic resonance imager according to claim 4 is characterized in that, magnetic is that radial thickness is 2 times of cylinder type inter-air space radius r.

6. permanent magnetic system of medical nuclear magnetic resonance imager according to claim 1; It is characterized in that; Magnetic is that the service area is that to be the center director at magnetic be the cylinder type space of 2 times of air gap radiuses, and magnetic is working area field: 0.5～1.5T, magnetic are that the working area field uniformity is less than 100ppm.

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