JPH10146326A - Magnetic field generator for mri - Google Patents

Abstract

PROBLEM TO BE SOLVED: To adjust the magnetic field distribution, and improve uniformity of a magnetic field by winding/installing a magnetic field adjusting coil round/on a peripheral edge part of magnetic pole pieces arranged on magnetic poles in a subject generator to generate a magnetic field in its void by electrically joining a pair of magnetic poles opposed by forming the void by a yoke. SOLUTION: A magnetic field generator, a cross-sectional pentagonal annular projection 12 is arranged on a void opposed surface of a magnetic material base 11, and an inside magnetic pole piece 13 is arranged inside it, but in this case, a magnetic field adjusting coil 14 is wound round a peripheral edge part of the annular projection 12. A material by molding soft magnetic powder together with an electric insulating material or the like can be adopted as a material to constitute the inside magnetic pole piece 13, but generation of an eddy current and residual magnetism generated in the magnetic material base 11 at pulse magnetic field impressing time can be reduced by adopting a laminated body such as a silicon steel plate on which coersive force is small and electric resistance is high. According to this, magnetic field intensity can be increased and decreased with a simple structure without changeing a magnetic circuit shape or without increasing magnet weight and the area.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement of a magnetic field generator used for a magnetic resonance tomography apparatus for medical use (hereinafter referred to as "MRI") and the like. The present invention relates to a magnetic field generator for MRI which generates a strong, highly accurate and uniform magnetic field in a gap by winding a magnetic field adjusting coil.

[0002]

2. Description of the Related Art In MRI, a part or the whole of a subject is inserted into a gap of a magnetic field generator for forming a strong magnetic field, and a tomographic image of an object is obtained and the properties of the tissue are described. It is a device that can do. In the magnetic field generator for MRI described above, the gap needs to be large enough to allow a part or all of the subject to be inserted, and in order to obtain a clear tomographic image, the gap is usually within the imaging field of view in the gap. , 0.0
It is required to form a stable and strong uniform magnetic field having an accuracy of 2 to 2.0 T and 1 × 10 ⁻⁴ or less.

As a magnetic field generator used for MRI, FIG.
As shown in (A) and (B), RF is used as a magnetic field generation source.
One end of each of a pair of permanent magnet structures 1 and 1 using an e-B magnet is fixedly opposed to one end of each of the pole pieces 2 and 2, and the other end is connected with a yoke 3 to connect the pole pieces 2 and 1. A configuration for generating a static magnetic field in a gap 4 between the two is known (Japanese Patent Publication No. 2-28).
No. 010). In order to improve the uniformity of the magnetic field distribution in the air gap 4, the magnetic pole pieces 2 and 2 are provided with an annular projection 5 at the peripheral edge or a convex projection (not shown) at the center.
Is adopted (actually fair 5-37446), etc.
Usually, it is composed of a plate-shaped bulk (integral) formed by cutting out a magnetic material such as electromagnetic soft iron and pure iron. In the figure, reference numeral 6
Is a gradient magnetic field coil.

However, in the above-described magnetic field generating device, the magnetic flux from the magnetic pole tends to leak into the air gap, and the magnetic field strength on the vertical line of the air gap becomes higher as it is closer to the magnetic pole surface. In order to obtain a high uniform magnetic field, a large amount of expensive permanent magnets must be used, and the gap between the magnetic poles and the magnetic pole area must be increased. However, there is a problem that the cost is high.

[0005]

Therefore, the applicant has proposed that the magnetic field generating source be 8 atomic% to 30 atomic% of R (where R is at least one of the rare earth elements including Y) and 2 atomic% to 2 atomic% of B.
By disposing a permanent magnet mainly composed of 8 atomic% and 42 atomic% to 90 atomic% of Fe and having a main phase made of tetragonal crystal and applying the permanent magnet to the magnetic field generator, the size of the magnetic field generator can be reduced. Further, from the temperature characteristics of the permanent magnet, it was found that by keeping the permanent magnet cooled, an extremely high maximum energy product was obtained, and the weight of the magnet could be further reduced (Japanese Patent Publication No. 4-22009).

[0006] The pole faces of the pair of permanent magnet structures are
By arranging a separate permanent magnet structure around the flat permanent magnet structure so as to form a concave curved surface with respect to the air gap, the magnetic field uniformity and the magnetic field strength in the magnetic circuit air gap are improved. It has been found that a high-precision and uniform magnetic field region can be expanded (Japanese Patent Publication No. Hei 3-20053).

However, the former requires a cooling device,
The latter requires a plurality of permanent magnet structures, and has a problem that the structure is complicated, the assembly is troublesome, and the price is high.

[0008] Further, for the purpose of improving the uniformity of the magnetic field in the air gap, the applicant assigns at least one auxiliary permanent magnet having the same magnetization direction as the permanent magnet to the inside of the permanent magnet and / or
Alternatively, a structure in which the amount of magnetic flux generated in the air gap is adjustable so as to be movable in the magnetization direction of the permanent magnet (Japanese Patent Publication No. 3-7124), and a magnetic force adjusting rod made of a magnetic material is provided at least in the air gap. A variable amount of protrusion into the yoke is provided on the yoke,
A configuration capable of adjusting the amount of magnetic flux short-circuit (actually 2-18441)
No.). However, if the amount of magnetic flux short-circuited by these auxiliary permanent magnets or magnetic force adjusting rods is too large, the magnetic field strength may be reduced.

Furthermore, in order to form a static magnetic field of high uniformity and high stability, a static magnetic field comprising a pair of permanent magnets and a magnetic yoke which magnetically couples the magnets and forms a substantially closed space. In the generating means, a plurality of auxiliary coils for correcting the uniformity of the static magnetic field are disposed in proximity to each of the pair of permanent magnets. There has been proposed a nuclear magnetic resonance apparatus in which the influence of the magnetic yoke on the magnetic field distribution and the magnetic field formed by the auxiliary coil is determined so as to be canceled out (Japanese Patent Application Laid-Open No. Sho 61-17053).

However, in the above configuration, the pair of auxiliary coils is arranged on the surface of each pole piece facing the air gap, and the air gap is narrowed because the auxiliary coils are arranged. If the gap is widened so as not to give the subject a feeling of oppression, there is a problem that the strength of the static magnetic field is weakened and a high-quality image cannot be obtained.

In addition, during imaging using the above-described MRI magnetic field generator, movement of an automobile or a train near a building, movement of an elevator in a building in which the apparatus is installed, and change in magnet temperature are caused. In addition, when the magnetic field strength suddenly changes dynamically, there is a problem that the conventional mechanical magnetic field adjustment method cannot cope.

The present invention is proposed in view of the above-mentioned current situation in a desired magnetic field strength of a magnetic field generator for MRI, and does not give a feeling of oppression to a subject. M which is capable of stably obtaining a uniform static magnetic field and obtaining a highly sensitive and clear image
The purpose is to provide a magnetic field generator for RI.

[0013]

Means for Solving the Problems The inventors of the present invention have conducted various studies to achieve the above object, and as a result, have formed a gap and magnetically coupled a pair of opposed magnetic poles with a yoke. In a magnetic field generator for MRI that generates a magnetic field, a magnetic field adjusting coil is wound around the periphery of a pole piece disposed on the magnetic pole, and the coil is conducted, so that the magnetic field intensity can be increased or decreased, and the current can be reduced. It has been found that by adjusting the direction, the magnetic field distribution can be adjusted and the magnetic field uniformity can be improved, and the present invention has been completed.

Further, the inventors have proposed an MR having the above-described structure.
In the magnetic field generator for I, the pole piece includes an annular protrusion,
A configuration in which a magnetic field adjusting coil is wound around the peripheral edge of the annular projection;
In addition, as a configuration capable of adjusting the magnetic field distribution with high accuracy, a configuration is proposed in which the annular projection is composed of a plurality of arcuate annular projection members, and a magnetic field adjustment coil is wound around each of the annular projection members.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION An MRI magnetic field generator to which the present invention is applied has a configuration in which a pair of magnetic poles facing each other with a gap are magnetically coupled by a yoke. It is applicable to any configuration without limitation.
Further, the shape and size of the yoke for forming the magnetic path may be appropriately selected according to various characteristics such as the required gap size, magnetic field strength, and magnetic field uniformity.

As a magnet constituting a magnetic field generating source, a normal conducting magnet, a superconducting magnet or a permanent magnet can be used.
When a permanent magnet is used, known magnet materials such as a ferrite magnet and a rare earth cobalt-based magnet can be used. In particular, R
The use of Fe-BR-based permanent magnets, which use Nd and Pr as resource-rich light rare earths and mainly B and Fe as main components and exhibit an extremely high energy product of 30 MGOe or more, results in extremely small size. Can be
In addition, by arranging these known permanent magnets in combination, without significantly impairing the miniaturization of the device,
An economically excellent magnetic field generator can be provided.

FIGS. 1A and 1B are a longitudinal sectional view and a perspective view showing an embodiment of a magnetic field generator according to the present invention. For example, an annular protrusion 12 made of a laminated silicon steel plate having a pentagonal cross section is disposed on a gap-facing surface of a magnetic material base 11 made of pure iron or the like, and an internal magnetic pole piece 13 is disposed inside the annular protrusion 12. Is a magnetic field adjusting coil 1
4 is wound. The object of the present invention can also be achieved by a configuration in which the annular projection 12 is directly disposed on the magnetic material base 11 without the internal magnetic pole piece 13 and the magnetic field adjusting coil 14 is wound around the periphery thereof. As shown in FIG. 2, a magnetic field adjusting coil 14 is wound around the magnetic material base 11 and the periphery of the annular projection 12 constituting the pole piece, or around the magnetic material base 11, the annular projection 12 and the periphery of the permanent magnet structure. With the configuration, the same operation and effect as those of the above configuration can be obtained.

In the present invention, the magnetic material base constituting the pole piece can be made of a material conventionally known as a pole piece such as soft magnetic iron or pure iron. The magnetic material base ensures the mechanical strength of the entire pole piece together with the equalization of the magnetic field strength, and improves the workability of assembling the magnetic field generator.

In the present invention, the material constituting the pole piece is not limited to the material of the embodiment, and pure iron or soft magnetic powder molded from an electrically insulating material can be used. Mn-Zn system with small and high electric resistance,
By employing a laminate of various soft ferrites such as a Ni-Zn type or a silicon steel plate, it is possible to reduce the generation of eddy current and residual magnetism generated in the magnetic material base when a pulse magnetic field is applied. In particular, by using a laminated silicon steel sheet and soft ferrite together, the advantages of these materials can be effectively utilized. Laminated silicon steel sheets are economically advantageous because they are less expensive than soft ferrites.

When the above-mentioned material is arranged on the magnetic material base 21 like the magnetic pole piece 20 shown in FIG. 8, if the material is a plurality of block-shaped magnetic pole piece members 23 and 24, the eddy current Also, the effect of reducing the residual magnetism phenomenon is great, and the work can be mounted with good workability. By optimizing the thickness of the pole piece members 23 and 24 and the thickness ratio of the magnetic material base, the mechanical strength of the pole piece 20 is secured, and the magnetic field strength required for the pole piece 20 is equalized and the eddy current is reduced. And the effect of preventing the remanence phenomenon is obtained.

Further, according to the present invention, in order to improve the uniformity of the magnetic field in the air gap, a projection made of a magnetic material ring 22 made of electromagnetic soft iron, pure iron or the like is formed on the periphery of the disk-shaped magnetic material base 21. Is desirable. It is desirable that these magnetic material rings be electrically insulated from the magnetic material base in order to reduce the influence of eddy current. In particular, 1 in the circumferential direction
If at least one slit is provided and divided, the effect of reducing eddy current can be further obtained.

The cross section of the annular projection is appropriately selected such as a substantially triangular or trapezoidal shape.
A good uniform magnetic field can be stably obtained. The same effect can be obtained by forming a concave curved surface having a gentle curved surface with a single or compound radius of curvature over the entire opposing surface of the pole piece.

In the present invention, the magnetic field adjusting coil is wound around the magnetic pole piece including the annular projection and energized to reinforce the magnetic field of the magnetic field generating source, thereby increasing or decreasing the magnetic field strength. If the configuration has an annular projection,
By winding the magnetic field adjusting coil around the periphery of the pole piece, a good uniform magnetic field can be obtained in combination with the magnetic field equalizing effect of the annular projection.

Further, in the case where the annular projection has a configuration in which a plurality of slits are provided in the circumferential direction and is divided into a plurality of annular projections, the annular projections 12 ₁ , 12 ₂ , 12 which are divided as shown in FIG.
There is also an advantage in that the magnetic field distribution can be finely adjusted by winding the magnetic field adjusting coil 14 around each of the _three and adjusting the amount of current flowing through each.

Further, the positive Y-axis magnetic field distribution of the imaging space through the X-axis as shown in FIG. 5, if it is asymmetrical in the negative direction, one of the magnetic field adjusting coil 14 _3, 14 ₄ increase the磁側In addition, by energizing the other magnetic field adjusting coils 14 ₁ and 14 ₂ to the demagnetizing side, the magnetic field distribution can be adjusted, and the magnetic field uniformity is improved.

When the magnetic field generator for MRI having the above-mentioned structure according to the present invention is used, the shape of the magnetic circuit can be largely changed,
By wrapping a magnetic field adjusting coil around the periphery of a conventionally used magnetic pole piece without increasing the weight and area of the magnet, the magnetic field strength can be increased or decreased. In addition, by adjusting the direction of the current of each of the magnetic field adjustment coils wound around the pair of magnetic pole pieces, the adjustment of the magnetic field distribution in the air gap can be easily performed, and the uniformity of the magnetic field is improved.

Further, in order to prevent eddy currents, the annular projection is provided with a plurality of slits in the circumferential direction to form an aggregate of arcuate annular projection components, and each of the annular projection components has a magnetic field adjusting coil. To adjust the amount of current to be applied to each of them, thereby making it possible to finely adjust the magnetic field distribution.

Further, when an R-Fe-B permanent magnet is used as a magnetic field generating source, the rare earth magnet has a temperature coefficient of-
0.1% / ° C, but the magnetic field adjustment coil wound around the pole piece or the pole piece and the periphery of the permanent magnet generates heat when energized continuously and keeps the permanent magnet warm, so the temperature of the magnet is constant. , And the magnetic field strength is stabilized. Therefore,
If the sensor is arranged in a magnetic field, when the magnetic field intensity changes due to a temperature change, the magnetic field adjusting coil can be energized to stably maintain the magnetic field. Further, even if the magnetic field is disturbed by the movement of the automobile or the elevator during the imaging, the change of the dynamic magnetic field can be dynamically corrected by disposing the magnetic field adjusting coil of the present invention.

[0029]

【Example】

Example 1 An Nd-Fe-B-based permanent magnet having a BHmax of 40 MGOe having an outer diameter of 1300 mm, an inner diameter of 190 mm, and a height of 150 mm was used for a magnetic field generator having the same configuration as that shown in FIGS. An annular protrusion made of low-carbon steel having an outer diameter of 1200 mm x inner diameter of 1000 mm x a height of 70 mm is circumferentially arranged on a magnetic material base made of pure iron having an outer diameter of 1200 mm x a height of 40 mm on the gap side. A pole piece made of soft ferrite having an outer diameter of 1000 mm and a height (central portion) of 25 mm was described. The soft ferrite is a Mn-Zn ferrite, Hc = 6.0.
A / m, Bs = 0.54T, ρ = 0.2Ω · m.

The distance between the pair of upper and lower annular projections is set to 500 mm, and a magnetic field adjusting coil made of an electric copper wire is wound around the periphery of the annular projections for 1000 turns.
The current A was applied to one coil on the demagnetizing side in the same direction as the static magnetic field, and to one coil on the demagnetizing side in the opposite direction to the static magnetic field. With this configuration, the central magnetic field strength is about 120 Gauss.
Could be increased.

In this case, the purpose is to reinforce the magnetic field at the center of the air gap. However, if the purpose is to correct the change in the magnetic field due to the movement of an automobile or an elevator, the magnetic field can be reduced with a smaller number of turns or current values of the conductor. Can be corrected. The winding of the conductor and the current value are appropriately selected according to the portion to be corrected and the desired magnetic field strength.

FIG. 4B shows the magnetic field distribution in each plane obtained by assuming that the imaging space is a sphere in the configuration of the first embodiment and the sphere space is divided into 15 horizontal planes. Further, a conventional magnetic circuit having a configuration similar to that of the first embodiment and having no magnetic field adjusting coil was prepared, and the result of examining the magnetic field distribution as in the first embodiment is shown in FIG. As is apparent from FIG. 4, it can be seen that the configuration according to the present invention significantly improved the uniformity of the magnetic field with a small current value.

Embodiment 2 As shown in FIGS. 3 and 5, the annular projection is provided with a plurality of slits in its circumferential direction to form an aggregate of a plurality of arcuate annular projection members, and each of the annular projection members has a magnetic field adjustment. Coil was wound. As shown in FIG. 6B, it can be seen that the magnetic field uniformity is improved as compared with FIG. 6A when the magnetic field adjusting coil is not arranged.

[0034]

According to the present invention, a magnetic field adjusting coil is wound around the periphery of a pair of magnetic pole pieces facing each other while forming a gap, thereby changing the shape of the magnetic circuit and reducing the weight and area of the magnet. Without increasing, the magnetic field strength can be increased or decreased with a simple structure. Also, by adjusting the direction of the current conducted to the coil, when the magnetic field distribution in the imaging space is vertically asymmetric via the X-axis, by conducting one coil in the opposite direction to the other coil, The magnetic field distribution can be adjusted, and the magnetic field uniformity has been improved.

Further, in order to reduce the eddy current, the annular projection of the pole piece is provided with a plurality of slits in the circumferential direction to form an aggregate of a plurality of divided annular projection members. By winding the magnetic field adjusting coils and adjusting the current amount of each coil, fine adjustment of the magnetic field distribution became possible.

When a rare earth magnet, which is easily affected by the temperature of the atmosphere, is used as a main magnetic field generating source, the permanent magnet can be kept warm by the heat generated by conduction to the magnetic field adjusting coil, and a stable uniform magnetic field can be obtained. Can be obtained. In addition, even when the magnetic field strength dynamically changes for some reason such as movement of an automobile or an elevator during imaging, the magnetic field distribution can be adjusted immediately.

[Brief description of the drawings]

FIGS. 1A and 1B are a longitudinal sectional view and a perspective view showing an embodiment of a magnetic field generator according to the present invention.

FIG. 2 is an explanatory longitudinal sectional view showing another embodiment of the magnetic field generator according to the present invention.

FIG. 3 is an explanatory perspective view showing an embodiment of a split type annular projection of the magnetic field generator according to the present invention.

FIGS. 4A and 4B are magnetic field distribution diagrams in each plane when the sphere space is divided into 15 horizontal planes,
(A) shows the case of the conventional magnetic field generator, and (B) shows the case of the magnetic field generator according to the present invention.

FIG. 5 is an explanatory top view showing an embodiment of a split type annular projection of the magnetic field generator according to the present invention.

6A and 6B are magnetic field distribution diagrams in the X-axis direction. FIG. 6A shows a case of a conventional magnetic field generator, and FIG. 6B shows a case of a magnetic field generator according to the present invention.

FIG. 7A is an explanatory front view showing the configuration of a conventional magnetic field generator for MRI, and FIG. 7B is an explanatory transverse view thereof.

FIG. 8A is a top view showing a configuration of a pole piece used in a conventional MRI magnetic field generator, and FIG. 8B is a longitudinal sectional view.

[Explanation of symbols]

1 permanent magnet arrangement 2,20 pole pieces 3 yoke 4 voids 5,12,12 _1, 12 _2, 12 _3, 12 ₄ annular projection 6 gradient magnetic field coils 11 and 21 magnetic material base 13 inside the pole pieces 14, 14 ₁ , 14 ₂ , 14 ₃ , 14 ₄ Magnetic field adjustment coil 22 Magnetic material ring 23, 24 Magnetic pole piece

Claims (3)

[Claims] An MRI that forms a gap and magnetically couples a pair of opposed magnetic poles with a yoke to generate a magnetic field in the gap.
A magnetic field generator for MRI, wherein a magnetic field adjusting coil is wound around a periphery of a pole piece arranged on the magnetic pole. 2. The MRI magnetic field generator according to claim 1, wherein the magnetic pole piece includes an annular protrusion, and a magnetic field adjusting coil is wound around a periphery of the annular protrusion. 3. An MRI magnetic field generating apparatus according to claim 2, wherein the annular projection comprises a plurality of arcuate annular projection members, and a magnetic field adjusting coil is wound around each of the annular projection members. .