JP2008130707A - Superconducting magnet device and nuclear magnetic resonance imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a superconducting magnet device and a nuclear magnetic resonance imaging apparatus which can generate a static magnetic field having a high intensity while reducing a mutually attracting force between a pair of opposing static magnetic field generators in a technique for generating a static magnetic field having a high intensity. <P>SOLUTION: A pair of opposing magnetic field generators (30, 40) forming a superconducting magnet device (11) comprise a coolant container (35) containing a coolant (L) for cooling a main coil (31) and a shield coil (32), a fixing member (23) for concentrically fixing a first magnetizing member (21) and a second magnetizing member (22), and a linkage member (24) for linking an extension part (36) of the coolant container (35) to the fixing member (23). The first magnetizing member (21) is arranged so that the opposing inner end faces of the first magnetizing member are projected from an end face of the main coil (31) and outer end faces thereof opposed to the opposing inner end faces are recessed from the opposing end face of the main coil (31). <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a superconducting magnet apparatus that forms a high-intensity static magnetic field, and more particularly to a superconducting magnet apparatus suitable for application to a nuclear magnetic resonance imaging apparatus.

A nuclear magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus) measures an electromagnetic wave emitted by a hydrogen nuclear spin due to a nuclear magnetic resonance (hereinafter referred to as NMR) phenomenon, and processes the signal. Thus, the tomography of the subject is taken as an image depending on the density distribution of the hydrogen nuclei.
When performing measurement using this MRI apparatus, the superconducting magnet device, which is a component thereof, forms a static magnetic field (about 10 ppm) with high magnetic field strength (0.2 T or more) and high magnetic field density uniformity in the imaging region. There is a need to.

In this superconducting electromagnet apparatus, in order to increase the strength of the static magnetic field, it is necessary to increase the value of the permanent current circulating through the superconducting coil as a component. However, when the value of the permanent current is increased, the critical value necessary for maintaining the superconducting state of the superconducting coil may be exceeded.
Therefore, conventionally, a technique has been known in which a magnetized member made of a ferromagnetic material is arranged in a magnetic circuit formed by a superconducting electromagnet apparatus and the strength of a static magnetic field in this imaging region is increased (for example, Patent Document 1). -5). However, in these known technologies, when a magnetized member having a large heat capacity is accommodated in a refrigerant container together with a refrigerant (liquefied helium), there is a problem that the amount of refrigerant consumed increases during the initial cooling. In addition, in the case of adopting a structure in which the magnetized member is supported by a vacuum vessel, the magnetized member is also a good thermal conductor, so the amount of heat that enters the inside of the apparatus from the outside increases, and the amount of refrigerant consumed during normal operation There is a problem that increases.

Therefore, in order to avoid such a problem, a known technique is disclosed that has a structure in which the magnetizing member is connected to the refrigerant container (Patent Document 6).
US Pat. No. 6,570,475 JP 2001-224571 A Special table 2003-512872 gazette JP 11-318858 A Japanese Patent Laid-Open No. 11-283823 JP 2006-102060 A FIG.

  However, in the conventional technique (Patent Document 6) disclosed as a structure in which a magnetizing member is supported by a refrigerant container, a pair of magnetizing members facing each other across an imaging region attract each other when an attempt is made to increase the strength of a static magnetic field. Power increases. For this reason, it is necessary to increase the strength of the member connecting the magnetized member and the refrigerant container. As a result, the contact area between the member, the magnetized member, and the refrigerant container increases, and the amount of heat entering the refrigerant container from the magnetized member. Will increase. For this reason, there exists a problem that the effect which suppresses the consumption of the refrigerant | coolant made into the initial objective will reduce.

  The present invention aims to solve the above-described problems, and a superconducting magnet device capable of generating a high-strength static magnetic field while adopting a configuration in which a pair of opposing static magnetic field generators reduces the attractive force of each other. It is another object of the present invention to provide a nuclear magnetic resonance imaging apparatus.

  In order to solve the above-described problems, the present invention provides a superconducting electromagnet apparatus in which a pair of static magnetic field generators are opposed to each other, a refrigerant container that contains a refrigerant that cools a main coil and a shield coil, a first magnetizing member, and a second magnetizing member. A fixing member that concentrically fixes the magnetizing member, and an extension part of the refrigerant container and a connecting member that connects the fixing member, wherein the inner end face on the opposite side of the first magnetizing member is the main member The outer end face that protrudes from the end face of the coil and that is opposite to the opposite side is arranged so as to be recessed from the end face opposite to the main coil.

  According to the present invention, there are provided a superconducting magnet apparatus and a nuclear magnetic resonance imaging apparatus capable of generating a high-intensity static magnetic field while adopting a configuration that reduces the force with which a pair of opposing static magnetic field generation units attract each other. Is done.

(First embodiment)
Hereinafter, a nuclear magnetic resonance imaging apparatus (MRI apparatus) according to a first embodiment of the present invention will be described with reference to the drawings.
As shown in the overall side view of FIG. 1, the MRI apparatus 10 includes a first static magnetic field generation unit 30 and a second static magnetic field generation unit 40 such that the central axis Z facing the vertical direction is a rotationally symmetric axis. Are placed on a superconducting magnet device (see FIG. 2) which is fixed to the support column 12 and the gradient magnetic field generators 13 and 13 arranged so as to sandwich the imaging region R and the subject P are placed on the imaging region. And a bed table D positioned at R.
Further, as components not shown, the MRI apparatus 10 includes an RF (Radio Frequency) coil that irradiates an electromagnetic wave having a resonance frequency that causes an NMR phenomenon toward the imaging region R, and a receiving coil that receives a response signal from the imaging region R. A control device that controls these components, and an analysis device that processes and analyzes the received signal.
With this configuration, the MRI apparatus 10 causes the NMR phenomenon to appear only in the region of interest (usually a slice surface having a thickness of 1 mm) in the imaging region R, and based on the electromagnetic waves emitted from the hydrogen nuclear spins. The tomogram is imaged.

  The MRI apparatus 10 is arranged so that the first static magnetic field generation unit 30 and the second static magnetic field generation unit 40 are paired in the vertical direction, and the magnetic field density of the static magnetic field is uniform and faces the vertical direction at the center of the gap. A region (imaging region R) is formed. Then, the subject P is inserted into the imaging region R, and a tomographic image of the subject in this region is captured using a nuclear magnetic resonance phenomenon (NMR phenomenon).

  The gradient magnetic field generators 13 and 13 are provided on the surfaces where the first magnetic field generator 30 and the second magnetic field generator 40 face each other, as shown in the XZ and YZ partial cross-sectional views of FIG. It is arrange | positioned in a pair of hollows. The gradient magnetic field generators 13 and 13 apply a gradient magnetic field to the static magnetic field formed by the superconducting magnet device 11 in the imaging region R to give position information of the NMR phenomenon.

As shown in FIG. 2, the first static magnetic field generation unit 30 includes a main coil 31, a shield coil 32, a refrigerant container 35, a vacuum container 37, a first magnetization member 21, and a second magnetization member 22. The fixing member 23 and the connecting member 24 are provided as at least constituent elements. Although not shown, a radiation shield that shields heat radiation from the vacuum container 37 toward the refrigerant container 35 is provided between the refrigerant container 35 and the vacuum container 37.
The inside of the second static magnetic field generation unit 40 is configured so as to share the central axis Z with the first static magnetic field generation unit 30 and be mirror-symmetric.
The support column 12 supports the pair of first static magnetic field generation units 30 and second static magnetic field generation units 40 so as to face each other in the vertical direction. Although not shown, the inside of the support column 12 is configured such that the refrigerant container 35 communicates the first static magnetic field generation unit 30 and the second static magnetic field generation unit 40. Similarly, the vacuum vessel 37 communicates the first static magnetic field generation unit 30 and the second static magnetic field generation unit 40 inside the support column 12.

The main coil 31 is a superconducting coil in which a permanent current circulates in a predetermined direction (forward direction) to generate a static magnetic field for measurement in the imaging region R, and is a coil bobbin (not shown) arranged around the central axis Z. ) Is formed by winding a superconducting wire.
Here, the superconducting coil is changed from the normal conducting state to the superconducting state when cooled to a temperature lower than the critical temperature by the refrigerant L (for example, liquid helium) filled in the refrigerant container 35, and the electric resistance becomes zero. The annular current circulates forever without being attenuated.

  The shield coil 32 is configured to share the central axis Z with the main coil 31 and to have a large diameter. An annular permanent current flows through the shield coil 32 in the direction opposite to the forward direction flowing through the main coil 31. In this way, the shield coil 32 cancels the magnetic field for measurement leaking outside the superconducting magnet device 11.

The vacuum container 37 holds the refrigerant container 35 via the fastening member 33 inside the vacuum state, and prevents heat from conduction and convection from entering the refrigerant container 35. is there.
The refrigerant container 35 contains the refrigerant L that keeps the main coil 31 and the shield coil 32 at a temperature equal to or lower than the critical temperature at which the superconducting phenomenon occurs. An extending portion 36 that extends in the direction intersecting the central axis Z is provided on the inner peripheral surface of the refrigerant container 35.

Hereinafter, the description will be continued with reference to FIG. 3 in which an XZ (YZ) cross section indicated by a broken line in FIG. 2 is enlarged.
The first magnetizing member 21 is formed in an annular shape having an outer diameter smaller than the inner diameter of the main coil 31, and is arranged inside the vacuum vessel 37 so as to share the central axis Z with the main coil 31. ing. The first magnetizing member 21 has a relative positional relationship with the main coil 31 in the axial direction of the central axis Z as follows.
That is, the inner end surface 21a on the opposite side of the imaging region R of the first magnetization member 21 protrudes from the end surface 31a of the main coil 31 and is opposite to the opposite side of the first magnetization member 21 across the imaging region R. The first magnetization member 21 and the main coil 31 have a positional relationship such that the outer end surface 21b on the side is recessed from the end surface 31b on the opposite side of the main coil 31.
The effects produced by the first magnetizing member 21 and the main coil 31 having such a positional relationship will be described later.

The second magnetizing member 22 is configured to have an outer diameter smaller than the annular inner diameter of the first magnetizing member 21, and the central axis Z is shared with the main coil 31 inside the vacuum vessel 37. Are arranged. The second magnetizing member 22 is fixed concentrically by the fixing member 23 and sharing the central axis Z with the first magnetizing member 21.
The fixing member 23 is a hollow circular flat plate. The outer end surface 21b of the first magnetizing member 21 is fixed to the first surface by a fastening member or the like, and the inner end surface of the second magnetizing member 22 is fixed to the second surface. It is fixed by a fastening member or the like.
The second magnetizing member 22 may be a flat plate, a hollow ring, or a structure in which a plurality of such ring members are concentrically stacked.

The fixing member 23 is connected to the refrigerant container 35 at the extended portion 36 via the connecting member 24.
As shown in FIG. 2, the connecting member 24 is formed by arranging a plurality of rod-like bodies at intervals around the axis of the central axis Z and aligning the longitudinal direction thereof in the axial direction of the central axis Z. It is. One end of the connecting member 24 is fixed to the horizontal surface of the fixing member 23, and the other end is fixed to an extension portion 36 that projects from the inner peripheral surface of the refrigerant container 35.
By configuring the connecting member 24 as described above, the magnetizing members 21 and 22 are connected to the refrigerant container 35. Further, the connecting member 24 has a shape in which the cross section of the path of heat transmitted between the magnetizing members 21 and 22 and the refrigerant container 35 is narrowed. For this reason, the amount of heat entering the refrigerant container 35 from the magnetized members 21 and 22 can be suppressed, and the amount of refrigerant consumed during normal operation can be reduced.

  The connecting member 24 is preferably made of a material having high rigidity and a small heat transfer coefficient, and specifically, made of fiber reinforced plastics (FRP). The extending portion 36 is made of a nonmagnetic material welded to the inner peripheral surface of the refrigerant container 35 on the central axis Z side. As described above, the reason why the one end of the connecting member 24 is fixed to the extension part 36 of the refrigerant container 35 is that if the fixing member 24 is fixed to the vacuum container 37, the magnetizing member 21, This is to avoid such an undesirable phenomenon because the position 22 may change with respect to the imaging region R.

Next, with reference to FIG. 3, the effect of the 1st magnetization member 21 and the 2nd magnetization member 22 is demonstrated.
When an annular current flows through the main coil 31, a magnetic field line B is induced and a closed loop magnetic circuit is formed. When magnetized members 21 and 22 (for example, a ferromagnetic material such as pure iron) are disposed in the magnetic circuit, It is known that since the magnetic resistance in the magnetic circuit is lowered, a larger number of magnetic field lines B are induced, and the strength of the static magnetic field in the imaging region R is increased.

  Magnetic field lines B (B1, B2) shown in FIG. 3 are derived by calculation by numerical analysis. In general, it is known that the magnetic force lines B tend to contract in the length direction (tangential direction) and repel each other between adjacent magnetic force lines B.

Here, paying attention to the magnetic force line B1 penetrating the first magnetizing member 21, the magnetic force line B1 penetrates the first magnetizing member 21 substantially along the central axis Z in a substantially straight line. Thus, the majority of the magnetic field lines B1 penetrate the inner end face 21a and the outer end face 21b of the first magnetizing member 21. The inner end face 21a protrudes from the end face 31a of the main coil 31, and the outer end face 21b is opposite to the main coil 31. This is because it is configured to be immersed from the side end surface 31b.
By adopting such a configuration, the bundle of magnetic lines B1 penetrating the inner end face 21a side of the first magnetization member 21 is sparse, and the bundle of magnetic lines B1 penetrating the outer end face 21b side is dense.

  This is because the magnetic field lines B1 penetrating the outer end face 21b cannot be expanded by being sealed in a narrow gap by the action of the shield coil 32 arranged on the outer end face 21b. This is because the magnetic lines B1 that penetrate therethrough are repelled and spread. Due to the mutual repulsion of the magnetic lines B1 that penetrate the inner end face 21a side of the first magnetizing member 21, a force is applied to the first magnetizing member 21 in a direction opposite to the imaging region R (upward in the figure). . That is, a force is applied in a direction in which a pair of static magnetic field generation units 30 and 40 (see FIG. 1) arranged to face each other repel each other.

  Next, paying attention to the magnetic force line B2 that penetrates through the second magnetizing member 22, the magnetic force line B2 is along the central axis Z on the imaging region R side, but is bent at a substantially right angle in the second magnetizing member 22, and the second magnetizing member. It penetrates through 22 side peripheral surfaces. For this reason, a force is applied in the direction of the imaging region R (downward in the figure) due to the nature of trying to shrink the closed loop of the magnetic lines of force B2.

In this way, the pair of opposing second magnetizing members 22 attract each other, but since the repulsive forces act on the opposing pair of first magnetizing members 21, the opposing pair of static magnetic field generating units 30 attract each other. Force is reduced. Accordingly, it is possible to reduce the mechanical rigidity of the member that supports each component of the MRI apparatus 10 or the superconducting magnet apparatus 11.
Specifically, the cross-sectional area of the connecting member 24 can be reduced. As a result, the amount of heat entering the refrigerant container 35 from the first magnetizing member 21 and the second magnetizing member 22 can be reduced, and the phenomenon of vaporization of the stored refrigerant can be suppressed.

Next, the effect | action which reduces the external leakage of a static magnetic field is demonstrated using FIG. 3 similarly.
It can be said that the magnetic field line B1 induced by the main coil 31 has a larger curvature of the closed loop. This is because the first magnetizing member 21 disposed inside the main coil 31 has its inner end surface 21 a protruding from the end surface 31 a of the main coil 31, and its outer end surface 21 b is immersed from the end surface 31 b on the opposite side of the main coil 31. Depending on the arrangement.
That is, since the first magnetizing member 21 draws the magnetic force lines B1 induced in the main coil 31 toward the main coil 31, the orbit that swells outside the closed loop of the magnetic force lines B1 is shortened.

  On the other hand, the magnetic field line B2 induced by the main coil 31 tends to be bent in the direction perpendicular to the second magnetizing member 22, but a part of the magnetic force line B2 goes straight in the direction of the central axis Z and leaks to the outside of the MRI apparatus 10. . However, as described above, since the magnetic field lines B1 have a large closed loop curvature, the effect of pushing up the magnetic field lines B2 by the magnetic field lines B1 is reduced, and the number of magnetic field lines B2 leaking to the outside is reduced.

(Second Embodiment)
Next, with reference to Fig.4 (a), 2nd Embodiment of a 1st magnetization member is described.
The difference between the present embodiment and the first embodiment is that the first magnetizing member 21 is provided with perforations 25 on the outer end face 21b side.
According to this structure, the effect | action which increases the force of the direction which mutually repels added to a pair of 1st magnetization member 21 is acquired. As a result, the forces applied to the pair of second magnetizing members 22 in the attracting directions are canceled out, and the effect of reducing the force applied to the connecting member 24 is obtained.
Thereby, the cross-sectional area of the connection member 24 can be reduced, and the amount of heat penetration into the refrigerant container 35 can be further reduced. Furthermore, the effect of suppressing the spread of the leakage magnetic field in the vertical direction of the MRI apparatus 10 can be obtained.

Note that “the perforation 25 is provided on the outer end surface 21b side” is not limited to the perforation 25 being open on the outer end surface 21b side, and the first magnetization member 21 is formed on the outer end surface 21b. All are applicable as long as the material volume is relatively reduced on the 21b side. This is because by adopting such an aspect, an effect of substantially immersing the outer end face 21 b of the first magnetizing member 21 with respect to the end face 31 b of the main coil 31 is obtained.
Therefore, in order to obtain a mode in which the material volume is relatively reduced on the outer end face 21b side, a discontinuous or continuous notch may be provided in the circumferential direction of the first magnetization member 21.

(Third embodiment)
Next, with reference to FIG.4 (b), 3rd Embodiment of a 1st magnetization member is described.
The difference between this embodiment and the first embodiment is that the first magnetizing member 21 is provided with a ferromagnetic protrusion 26 on the inner end face 21a side.
According to this structure, the same operations and effects as those described in the second embodiment can be obtained.

The “ferromagnetic protrusion 26 provided on the inner end face 21 a side” is a bolt that is screwed in along the axial direction of the central axis Z, or is fastened or joined by a non-magnetic bolt or welding. Or a ferromagnetic iron plate.
This is because the effect of causing the inner end face 21a of the first magnetizing member 21 to substantially protrude from the end face 31a of the main coil 31 is obtained.

  As described above, according to the present invention, it is not necessary to increase the mechanical rigidity of the constituent elements, increase the consumption of refrigerant, increase the external leakage, and generate a strong static magnetic field. Possible superconducting magnet devices and nuclear magnetic resonance imaging devices are provided.

1 is a side view showing an overview of a nuclear magnetic resonance imaging apparatus according to an embodiment of the present invention. It is a fragmentary sectional perspective view of the superconducting magnet device concerning a 1st embodiment of the present invention. It is an enlarged view of the partial cross section (dashed line part of FIG. 2) of the superconducting magnet apparatus which concerns on 1st Embodiment. (A) is a partial cross-sectional enlarged view of the superconducting magnet device according to the second embodiment of the present invention, and (b) is a partial cross-sectional enlarged view of the superconducting magnet device according to the third embodiment of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 MRI apparatus 11 Superconducting magnet apparatus 13 Gradient magnetic field generation part 21 1st magnetization member (magnetization member)
21a Inner end surface of the first magnetizing member 21b Outer end surface of the first magnetizing member 22 Second magnetizing member (magnetizing member)
23 fixing member 24 connecting member 25 perforation 26 protrusion 30 first static magnetic field generation unit (static magnetic field generation unit)
31 Main coil 31a End face of main coil 31b End face opposite to main coil 32 Shield coil 35 Refrigerant container 36 Extension part of refrigerant container 37 Vacuum container 40 Second static magnetic field generator B (B1, B2) Magnetic field line L Refrigerant P Covered Examiner R Imaging area Z Central axis

Claims (4)

In the superconducting electromagnet apparatus formed by facing a pair of static magnetic field generation units,
The static magnetic field generator is
A main coil in which a permanent current circulates in the forward direction, a shield coil in which a permanent current circulates in the reverse direction, and a refrigerant container that contains a refrigerant that cools them,
A first magnetizing member having an outer diameter smaller than the inner diameter of the main coil and configured in an annular shape;
A second magnetizing member configured to have an outer diameter smaller than an annular inner diameter of the first magnetizing member;
A fixing member for concentrically fixing the first magnetizing member and the second magnetizing member;
A connecting member that connects the extension part of the refrigerant container and the fixing member,
The first magnetizing member is
The inner end face on the opposite side protrudes from the end face of the main coil,
A superconducting electromagnet apparatus, wherein an outer end surface opposite to the opposite side is disposed so as to be immersed in an end surface opposite to the main coil. The superconducting magnet device according to claim 1,
The superconducting magnet device according to claim 1, wherein the first magnetizing member is provided with a perforation or a notch on the outer end face side. In the superconducting magnet device according to claim 1 or 2,
A superconducting magnet device, wherein a ferromagnetic protrusion is provided on the inner end face of the first magnetizing member.   A nuclear magnetic resonance imaging apparatus using the superconducting magnet apparatus according to any one of claims 1 to 3.

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