JP3624255B1 - Superconducting magnet device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make a magnetic field in an imaging region axially symmetric among regions where magnetic lines of force forming a static magnetic field are distributed.
A disk-shaped magnetic pole 12 is arranged on the inner peripheral side of a main coil 10 that forms a magnetic field space 16 of a static magnetic field, and a central portion of the magnetic pole 12 is connected to a small-diameter portion 18 of a yoke 14 so that the small-diameter portion 18 is connected. Is formed with a diameter smaller than that of the magnetic pole 12 using a ferromagnetic material that is easily magnetically saturated. For this reason, the lines of magnetic force generated from the main coil 10 are concentrated on the small diameter portion 18 in the process of moving from the magnetic pole 12 to the yoke 14 using the yoke 14 as a return circuit, and even if the axis of the main coil 10 is displaced. The magnetic field in the magnetic pole 12 can be axisymmetric without being affected by the magnetic field accompanying the axial deviation of the main coil 10, and as a result, the magnetic field in the imaging region can be axisymmetric.
[Selection] Figure 1

Description

  The present invention relates to a superconducting magnet device, and more particularly to a superconducting magnet device suitable for use as a magnetic field generation source of a magnetic resonance imaging apparatus.

  Magnetic Resonance Imaging (Physical Resonance Imaging) uses physical and chemical properties of a subject by utilizing the nuclear magnetic resonance phenomenon that occurs when the subject (test object) placed in a uniform static magnetic field space is irradiated with electromagnetic waves. An image representing the property can be obtained, and it is particularly used for medical purposes. The magnetic resonance imaging apparatus mainly includes a superconducting magnet device as a static magnetic field generating means for applying a uniform static magnetic field in an imaging region (measurement space) into which a subject is carried, and an electromagnetic wave toward the imaging region. And an RF coil that receives electromagnetic waves from the imaging region, and a gradient magnetic field coil that applies a gradient magnetic field (dynamic magnetic field) for giving positional information of the resonance phenomenon in the imaging region. (See Patent Document 1).

  In magnetic resonance imaging devices, the resonance signal strength increases due to an increase in the static magnetic field strength, shortening the imaging time, and enabling high-functional imaging. In an imaging apparatus, a superconducting coil or a ferromagnetic material (magnetic pole) is often used as a magnetic field generation source.

In this type of magnetic resonance imaging apparatus, as a superconducting magnet device, in a device using a pole-facing magnet, a pair of superconducting coils (main coils) arranged in opposition to each other on the inner peripheral side. Attempts have been made to improve the openness by minimizing the number of pillars that connect the magnetic poles to each other to a minimum, for example, one, and widening the space between the magnetic poles. This support | pillar is comprised with the magnetic body and the nonmagnetic body.
JP 2002-102205 A (pages 4 to 6, FIG. 1)

  However, when the support is made of a magnetic material, the flow of the magnetic lines of force generated from each superconducting coil becomes non-axisymmetric depending on the position of the support, and accordingly, the magnetic field in the observation region is also non-axisymmetric. Further, when the support column is made of a non-magnetic material, it is inevitably made up of a plurality of superconducting coils, so that the axial misalignment of the coils directly causes non-axis symmetry of the magnetic field in the observation region. The superconducting coil needs to be firmly fixed to support its electromagnetic force, but on the other hand, it must be supported by a long material with a small cross-sectional area to reduce the amount of heat penetration because it needs to be kept at a very low temperature. However, the structure becomes complicated and it is difficult to improve the installation accuracy. When the number of yokes is one, when the main coil has an axis deviation, the effect is generated as it is in the magnetic field distribution of each magnetic pole, and the magnetic field inside each magnetic pole becomes asymmetric with respect to the axis. To correct that the magnetic field inside each magnetic pole is asymmetric with respect to the axis, in addition to the main coil, a plurality of correction coils are arranged on the inner peripheral side of the main coil, or the amount of iron for correction is set on each magnetic pole. However, even if these configurations are employed, it is not sufficient for correcting the magnetic field inside each magnetic pole to be asymmetric with respect to the axis. In addition, when the magnetic field inside each magnetic pole is asymmetric with respect to the axis, if the dynamic magnetic field (pulse magnetic field) generated from the gradient magnetic field coil enters the ferromagnetic material such as each magnetic pole, it is generated in the ferromagnetic material. Eddy currents and the non-linear characteristics or hysteresis characteristics of the BH curve of the ferromagnetic material adversely affect the magnetic field uniformity of the static magnetic field in the imaging region (measurement space) in the magnetic field space, resulting in distortion in the image. It will be. For this reason, in the apparatus described in Patent Document 1, a gradient magnetic field leakage allowable region that allows leakage of a gradient magnetic field is provided in the outer peripheral portion of the gradient magnetic field coil, and the outer peripheral portion of the magnetic pole that faces this gradient magnetic field leakage allowable region Adopts a structure that easily saturates the magnetic field, reducing the adverse effects of the magnetic poles on the magnetic field in the measurement space.

  An object of the present invention is to make the magnetic field in the imaging region axially symmetric among the regions in which the lines of magnetic force forming the static magnetic field are distributed.

In order to solve the above-mentioned problems, a superconducting magnet device according to the present invention includes a pair of superconducting coils formed in an annular shape and arranged opposite to each other, and arranged separately on the inner peripheral side of each superconducting coil. Each of the magnetic field adjusting means is formed by arranging a plurality of ferromagnetic regions where magnetic lines of force are concentrated in the radial direction of the superconducting coil, and a cross-sectional area of the plurality of ferromagnetic regions. S ₁ is the saturation magnetization of the plurality of ferromagnetic regions is 2 [T], the cross-sectional area of the superconducting coil is S ₂ , and the magnetic flux density of the observation region formed by the pair of superconducting coils is B ₀ [T]. _when, characterized by comprising set to _{S 1 <B 0 · S 2} /2.

  According to the present invention, the magnetic field in the imaging region can be axisymmetric, and the magnetic field adjustment can be facilitated.

The following description will explain the present onset Akira to the accompanying drawings. Fig.1 (a) is principal part sectional drawing of the superconducting magnet apparatus which shows the reference example of this invention, (b) is a principal part top view of (a). In FIG. 1, the superconducting magnet apparatus includes a main coil 10, a magnetic pole 12, and a yoke 14 as a magnetic field generation source of a magnetic resonance imaging apparatus. In practice, a pair of main coils and magnetic poles are arranged, a lower main coil is arranged opposite to the upper main coil 10, and a lower magnetic pole is arranged opposite to the upper magnetic pole 12. The magnetic poles are connected to the yoke 14, but the upper and lower main coils and the magnetic poles have the same configuration. Therefore, hereinafter, the configuration of the apparatus is based on the upper main coil 10 and the upper magnetic pole 12. explain.

  The main coil 10 is formed in a ring shape as a superconducting coil, and is accommodated in, for example, a coil container (not shown). The coil container includes a vacuum container formed in a substantially cylindrical shape, a radiation shield accommodated in the vacuum container, and a helium container accommodated in the radiation shield. A coil (not shown) is housed together with liquid helium as a superconducting refrigerant. A magnetic field space 16 of a static magnetic field is formed between the main coils by the magnetic field lines generated from the main coil 10, and an imaging region (not shown) serving as a magnetic field uniform region is formed in a spherical shape at the center of the magnetic field space 16. It has come to be.

  The magnetic pole 12 is formed in a disk shape using a ferromagnetic material, for example, iron, and is disposed in a region on the inner peripheral side of the main coil 10, and a central portion thereof is coupled to the small diameter portion 18 of the yoke 14. The small-diameter portion 18 is formed as a connecting portion so as to protrude from the main body of the yoke 14 and is formed in a columnar shape having a smaller diameter than the magnetic pole 12. A region connecting the magnetic pole 12 and the yoke 14 is divided into a region of a ferromagnetic material by the small diameter portion 18 and a region of a nonmagnetic material by the air, and the region of the ferromagnetic material by the small diameter portion 18 is divided into the main coil 10. It is formed as a region where the magnetic lines of force generated from are concentrated or converged.

  That is, since the small diameter portion 18 is formed with a smaller diameter than the magnetic pole 12 using a ferromagnetic material (iron) that is easily magnetically saturated, the magnetic field lines generated from the main coil 10 use the yoke 14 as a return circuit and the magnetic pole 12. In the process of moving to the yoke 14, it concentrates on the small diameter portion 18. As a result, the magnetic lines of force are concentrated in one place, so that the influence of the magnetic lines of force on the magnetic pole 12 is radial, that is, axially symmetric and the shape of the yoke 14 body is non-axisymmetric. That is, the magnetic field inside the magnetic pole 12 can be made axisymmetric only by managing the installation accuracy of the small-diameter portion 18 rather than the superconducting coil in which the installation accuracy is difficult to adjust. As a result, the magnetic field in the imaging region is axisymmetric. Can be.

  That is, as shown in FIG. 2, when the yoke 14 does not have the small-diameter portion 18 as in the prior art, the lines of magnetic force that have passed through the magnetic pole 12 enter the yoke 14 as they are, and therefore the non-axisymmetric effect of the yoke 14 is affected by the magnetic pole 12. The magnetic field inside the magnetic pole 12 does not become axially symmetric as it occurs in the internal magnetic field. On the other hand, according to the present invention, the lines of magnetic force generated from the main coil 10 are concentrated on the outer peripheral side of the small diameter portion 18 in the process of moving from the magnetic pole 12 to the yoke 14, so that the axis of the main coil 10 is displaced. However, the magnetic field inside the magnetic pole 12 can be made axisymmetric without being affected by the magnetic field due to the axial deviation of the main coil 10.

For this purpose, almost all the magnetic flux generated in the main coil 10 only needs to pass through the small diameter portion 18, so that the saturation magnetization of iron is 2 [T] and the magnetic field lines generated from the main coil 10 in the yoke 14 are concentrated. the cross-sectional area of the small diameter portion 18 serving as a _{S 1,} the cross-sectional area of the main coil 10 and _{S 2,} set the magnetic flux density in the imaging region when the _B 0 _[T], the _{S 1 <B 0 · S 2} /2 can do.

In the case where the small-diameter portion 18 is divided into a plurality to the area in which the outline surrounds the S _1. The superconducting coil 10 refers to a coil that generates a magnetic field in the same direction as the magnetic field in the imaging region, and when there are a plurality of, and the average value S _2.

Next, another reference example of the present invention will be described with reference to FIG. In this example , a cylindrical shaft-shaped member 22 made of a ferromagnetic material (iron) is arranged as a magnetic field adjusting means on the inner peripheral side of the plurality of main coils 10 housed in the upper coil container 20, This shaft-shaped member 22 and the shaft-shaped member disposed as magnetic field adjusting means on the inner peripheral side of the plurality of main coils housed in a lower coil container (not shown) are magnetized using air as a return circuit. The other configurations are the same as those in the example of FIG.

  That is, the shaft-shaped member 22 is configured using a ferromagnetic material in order to form a region where magnetic lines of force generated from each main coil 10 are concentrated on the inner peripheral side of each main coil 10. Arranged in the area.

In this example , the lines of magnetic force generated from each main coil 10 are concentrated on the shaft-shaped member 22 in the process of moving from the magnetic field space 16 to the shaft-shaped member 22 using air as a return circuit. As a result, the magnetic lines of force are collected radially around the shaft-shaped member 22, so that the magnetic field in the imaging region is axisymmetrical if the installation accuracy of the shaft-shaped member 22 is well managed without depending on the superconducting coil. Can be.

Next, the implementation form of the present invention according to FIG. In the present embodiment, a columnar shaft-shaped member 24 made of a ferromagnetic material (iron) is disposed as the magnetic field adjusting means on the inner peripheral side of the plurality of main coils 10 housed in the upper coil container 20. In addition, a cylindrical body 26 made of a ferromagnetic material and surrounding the shaft-shaped member 24 is arranged, and the shaft-shaped member 24 and the cylindrical body 26 and a plurality of coils housed in a lower coil container (not shown). A shaft-like member and a cylinder arranged as magnetic field adjusting means on the inner peripheral side of the main coil are magnetically coupled using air as a return circuit, and the other configuration is the same as the example of FIG.

  That is, the shaft-like members 24 and 26 use a ferromagnetic material to form a plurality of, for example, two regions where magnetic lines of force generated from each main coil 10 are concentrated on the inner peripheral side of each main coil 10. And is arranged in a non-magnetic material (air) region.

  In the present embodiment, the lines of magnetic force generated from each main coil 10 are concentrated on the shaft-shaped member 24 and the cylinder 26 in the process of moving from the magnetic field space 16 to the shaft-shaped member 24 and the cylinder 26 using air as a return circuit. Will do. As a result, the magnetic lines of force are collected radially toward the axial center of the shaft-like member 24 and the cylindrical body 26, so that the installation accuracy of the shaft-like member 24 and the cylindrical body 26 can be firmly managed without depending on the superconducting coil. For example, the magnetic field in the imaging region can be axisymmetric.

  It should be noted that the effect of the present invention can be obtained as long as the cylindrical body 26 is not cylindrical but is arranged symmetrically so as to surround the shaft-shaped member 24.

  Similarly, the shaft-like member 24 may be divided as long as it is arranged symmetrically.

1, when configuring the MRI apparatus using a superconducting magnet apparatus of the example and the implementation form of FIG. 2, when conveyed to the magnetic field space 14 with the patient as the subject was placed on the bed In addition, an analysis device is provided as analysis means for analyzing the nuclear magnetic resonance signal from the subject.

  In this case, in the upper and lower coil containers, on the side facing the magnetic field space 16, gradient magnetic field coils that form a dynamic magnetic field serving as positional information of the resonance phenomenon are stored in the imaging region of the magnetic field space 16, respectively. On the magnetic field space 16 side of the gradient magnetic field coil, an RF coil is disposed as a transmission / reception coil that radiates electromagnetic waves toward the imaging region and receives electromagnetic waves from the imaging region.

  In addition, although the said embodiment demonstrated what used the superconducting coil, it is possible to apply this invention also to a normal conduction type apparatus.

(A) is principal part sectional drawing of the superconducting magnet apparatus which shows the reference example of this invention, (b) is a principal part top view of (a). (A) is principal part sectional drawing for demonstrating the magnetization characteristic of the superconducting magnet apparatus which concerns on this invention, (b) is the magnetization characteristic of the superconducting magnet apparatus which concerns on this invention, and the magnetization characteristic of a prior art It is a characteristic view for demonstrating a relationship. (A) is principal part sectional drawing of the superconducting magnet apparatus which shows the other reference example of this invention, (b) is a principal part top view of (a). (A) is a fragmentary cross-sectional view of a superconducting magnet apparatus according to the implementation embodiments of the present invention, (b) are a plan view of (a).

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Main coil 12 Magnetic pole 14 Yoke 16 Magnetic field space 18 Small diameter part 20 Coil container 22, 24 Axial member 26 Cylindrical body

Claims (3)

A pair of superconducting coils formed in an annular shape and arranged opposite to each other , and a pair of magnetic field adjusting means arranged separately on the inner peripheral side of each superconducting coil, each magnetic field adjusting means comprising: A plurality of ferromagnetic regions in which the magnetic field lines are concentrated are arranged in the radial direction of the superconducting coil, and the cross-sectional area S ₁ of the plurality of ferromagnetic regions has a saturation magnetization of 2 [T ], wherein the cross-sectional area of the superconducting coil _{S 2,} when the magnetic flux density of the observation area formed by the pair of superconducting coils was _B 0 _[T], is set to _{S 1 <B 0 · S 2} /2 consisting of a superconducting magnet apparatus.   2. The superconducting magnet device according to claim 1, wherein the magnetic field adjusting means is divided into a ferromagnetic region and a non-magnetic region, respectively.   2. The superconducting magnet device according to claim 1, wherein the magnetic field adjusting means includes: a shaft-like member formed of a ferromagnetic material; and a cylindrical member disposed around the shaft-like member and formed of a ferromagnetic material. A superconducting magnet device comprising:

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