JPH0219430B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to magnet devices, particularly magnet devices suitable for use in nuclear magnetic resonance (NMR) imaging. In recent years, NMR imaging technology has been developed, making it possible to image various parts of the human body. NMR takes advantage of the fact that certain atoms in the human body resonate at specific frequencies (due to precession) in strong magnetic fields. Spatial discrimination can be achieved by applying orthogonal magnetic field gradients across the human body. An example of an NMR device is described in US Pat. No. 4,021,726. To implement these NMR imaging techniques,
It is necessary to supply a strong and uniform magnetic field within the working space formed by the hole of the magnet. The typical magnetic field strength of the magnetic field inside the hole (hereinafter referred to as the inside-hole magnetic field) is 2.0T. When such a magnetic field is generated using a conventional magnet device, there is a problem in that a large external magnetic field is also generated at the same time as the in-hole magnetic field, which has an adverse effect. It is therefore desirable to shield the magnet arrangement to limit such external magnetic fields. Typically, an iron shield that is part of the magnet system is used to suppress the external magnetic field, or iron plates are placed on the walls of the room that houses the magnet system to confine the external magnetic field to the room. Iron shields are expensive and get in the way. For example, if we try to reduce the external magnetic field outside the 5m radius to 5.0G or less, a magnet device that generates a 1.5KG in-hole magnetic field will require 3 tons of iron, and a 20KG in-hole magnetic field will be generated. To shield the magnet device for 40
Tons of iron are required. Not only is the use of such a large amount of iron expensive, but the closer the iron shield is to the magnet system, the more the in-hole magnetic field is distorted. This is particularly undesirable in NMR imaging magnet systems that are based on a precisely controlled, uniform in-hole magnetic field. Another method of shielding a magnet is to juxtapose a coil around the magnet, as described, for example, in US Pat. No. 3,671,902. However, in this case, the coil used to shield the magnet generates a small local magnetic field, and complex calculations are required to determine the magnetic field at a particular radius and then design a coil that provides the appropriate shielding field. becomes. Furthermore, this prior art device is designed to shield a magnet that generates an in-hole magnetic field of relatively low strength, unlike the magnetic field strength to which the present invention is concerned. In addition, especially for NMR imaging, it is necessary to change the strength of the in-hole magnetic field, but conventional devices do not solve this problem. Changing the in-hole magnetic field also changes the external magnetic field, making it necessary to make complex modifications to the various shield coils. For example, in addition to changing the excitation current flowing through the shield coil, it is also necessary to change the number of turns in the coil and the position of the coil relative to the magnet. U.S. Pat. No. 3,333,162 also discloses a small-scale shielding device. This device also does not help solve the problems mentioned above. The reason for this is, first of all,
Since the main magnetic field vector inside the hole rotates in an NMR device, it is absolutely impossible to use this coil device in an NMR device, and therefore it is useless to use it as is, or even if it is scaled up. The second magnetic field handled is too small for NMR. Even in the hole magnetic field of the largest coil, its Z-axis component is approximately 460 Gauss, and the peripheral magnetic field at a point 20 to 30 cm apart is estimated to be less than the earth's magnetic field. It is an object of the present invention to provide a magnet device in which the peripheral magnetic field very close to the surface of the magnet device is reduced to the strength of the earth's magnetic field, and in which both sides are open for patient access. The present invention provides a magnet device 1 comprising first superconducting coil assemblies A, A' to C, C' that generate a first magnetic field and second superconducting coil assemblies DF that generate a second magnetic field. 1. First superconducting coil assembly A, A′ to C, which generates a magnetic field.
C′ is formed by each coil being functionally inseparable and electrically connected in series, and the second superconducting coil assembly DF that generates the second magnetic field is formed by each coil being functionally inseparable and electrically connected in series. The second superconducting coil assembly is connected in series with the first superconducting coil assembly, and the second superconducting coil assembly includes:
The first superconducting coil assembly and the magnetic poles are arranged in reverse order, and the first superconducting coil assembly and the second superconducting coil assembly are arranged so that their coil axes coincide,
The first magnetic field and second magnetic field generated during use are:
generating a composite uniform magnetic field within the hole working space 3 of the first superconducting coil assembly,
The zero-order magnetic field component of the magnetic field generated by each assembly substantially contributes to the magnitude of the uniform magnetic field, and the corresponding higher-order magnetic field components of the magnetic field generated by each assembly have approximately the same magnitude. The second magnetic field is formed outside the magnet device 1 in opposition to the first magnetic field. It is something. The present invention provides a self-contained magnet arrangement that does not require the addition of external shielding and can generate a useful (i.e., high strength) uniform magnetic field within the working space. As mentioned above, conventional shields using coils have been proposed, but these coils generate a small external magnetic field. However, in the present invention, the second superconducting coil assembly not only provides a shielding magnetic field, but also significantly contributes to the composite magnetic field in the working space. Since the higher-order magnetic field terms of both coil assemblies are balanced to zero, uniformity of the magnetic field is obtained. In principle, the first and second coil assemblies could be balanced separately so that both coil assemblies provide a uniform magnetic field and the result is uniform when the coil assemblies are superimposed. . A composite magnetic field in the working space is generated based on the difference in zero-order terms between the magnetic fields generated by both coil assemblies. This allows for considerably improved control of the uniformity of the in-hole magnetic field. A further important advantage of one aspect of the invention is that the two superconducting coil assemblies are electrically connected in series. To this end, when the strength of the in-hole magnetic field is changed by changing the current flowing through the first superconducting coil assembly, the second magnetic field is automatically changed by a sufficient amount, so that the same degree of shielding can be achieved. . In other words, the shielding system has linearity. A superconducting coil assembly is used to provide the magnetic field strength and precision magnetic field required for NMR imaging. To achieve the superconducting state of the coil, conventional cryogenic or refrigeration techniques are used to cool the coil to cryogenic temperatures of around 4.2K. The coil may then be kept in continuous operation to prevent noise magnetic fields. The overall size of the magnet device according to the present invention, even including the cooling device, is only slightly larger than a conventional non-shielded magnet device. The self-contained magnet device of the present invention has the advantage of reducing installation costs. Furthermore, by using the second superconducting coil assembly, the shielding action can be performed with high precision without reducing the uniformity of the magnetic field within the working space. The shielding field also contributes in practice to the control of the magnetic field in the working space. Furthermore, the shielding effect can be improved compared to those using conventional iron plate shields. Typically, the direction of current flowing in the second superconducting coil assembly is opposite to the direction of current flowing in the first superconducting coil assembly. In the simplest configuration of the magnet device according to the present invention, the first and second coil assemblies each consist of a single coil, and the two coils are electrically connected in series. Preferably, one or both of the first and second coil assemblies includes a plurality of coaxial coils arranged symmetrically with respect to a center plane of the magnet device perpendicular to the axis of the coils. Using two superconducting coil assemblies,
The problem is that considerable stress is generated during use. However, by arranging the second superconducting coil assembly such that the value of the magnetic field within the coil windings is that of an unshielded magnet as a whole,
The above stress can be reduced to a considerable extent. Only the increase in current required to provide a net magnetic field in the working volume increases the working force in the winding. When the first and second coil assemblies both include a plurality of coils, typically the two axes are coincident and the two central planes are also coincident. This makes it possible to uniformize the resultant magnetic field within the working space (the hole of the magnet device) in a very simple manner. Furthermore, the inner radius of the coil of the second superconducting coil assembly is
be larger than the outer radius of the coil in the superconducting coil assembly. Particularly advantageous when using a coaxial coil is that the first coil assembly (magnet) and the second
Since it is possible to maintain a balance with the coil assembly (shield), if the magnet assembly and shield assembly are considered separately, a uniform magnetic field cannot be obtained, and only when they work together. , it is possible to point out that uniformity has been achieved. In one embodiment, the first and second superconducting coil assemblies each include six superconducting coils, and the coils constituting the first superconducting coil assembly are:
The superconducting coil assembly is provided radially inside the coils constituting the second superconducting coil assembly, and the entire coil is arranged coaxially and symmetrically with respect to a common central plane perpendicular to the axis of the coils. The shielding effect, especially in the axial direction, provided by the magnet arrangement described above is sufficient for normal practical purposes. However, in some cases it may be advantageous to further include a shield of magnetic material around at least the first superconducting coil assembly. This additional shielding thus further reduces the radial fringe field. This additional shielding may be provided, for example, around the second superconducting coil assembly, preferably between the first and the second superconducting coil assembly.
Arranged between coil assemblies. The additional shield may also be adjacent to the coils of the second superconducting coil assembly. Such an arrangement can be realized very easily by using the winding frame supporting the second superconducting coil assembly as the additional shield. Making the additional shield made of iron reduces the amount of expensive superconductor in the second coil assembly.
The reduction effect is remarkable because only 6000 kg of iron is required for the 1.5 T in-hole magnetic field. The shielding effect can be further improved by providing additional coil assemblies adjacent to each other at both ends of the working space, if necessary. These additional coil assemblies are arranged to oppose and cancel the combined magnetic field generated by the first and second superconducting coil assemblies at the additional coil positions. Since the required magnetic field generated by these additional coil assemblies is relatively small, resistive coils are usually used. Therefore, the diameter of the coil can be increased without using a large cryostat. However, the additional coil assembly may also be superconducting, in which case the first,
electrically connected in series with a second superconducting coil assembly; A magnet device according to the present invention and an NMR device incorporating this magnet device will be described in detail below using Examples. FIG. 1 shows a partially cutaway exploded view of the magnet device 1. FIG.
The magnet arrangement 1 comprises a cylindrical former 2 forming a magnet working space consisting of a hole 3 having an axis 4, said former 2 being made of a glass fiber epoxy composite. An aluminum cylindrical winding frame 5 coaxial with the axis 4 is disposed on the outside of the winding form 2 in the radial direction.
Three pairs of coils AA', BB' and CC' (six pairs in total) are provided on the winding frame 5, each arranged symmetrically with respect to the center plane 6 of the device perpendicular to the axis 4 (FIG. 2).
Typically, each coil A, A' to C of the coil pair,
C′ consists of thin strands of type alloy superconductor, a normal good conductor base material, and an electrically insulating surface that insulates the energizing voltage and fault voltage. Buried in an array. The position and number of turns of each coil will be described later. Each of the coils A, A' to C, and C' of the coil pair is wound separately so that one is located inside the other in the radial direction. These coils are embedded in a wax composition and then surrounded by clamp rings (not shown). Then, each coil pair is connected to each annular rib pair of the winding frame 5.
Place it between 5A, 5B, etc. The slight movement of the coil generates a small amount of heat, leading to the destruction of the superconducting state (known as quenching).
Movement of the coil windings during use is a problem, and the wax composition and ribs are provided to prevent this. Since the distance between the coil pairs AA' to CC' is small, the force acting between adjacent coils in the axial direction is approximately 200,000 kg.
The winding frame 5 must be configured to cope with the large force exerted on the winding. Furthermore, it is necessary to reduce the overall weight of the magnet device by making the winding frame 5 as light as possible, and to make it as perfectly cylindrical as possible. A second aluminum winding frame 7 is attached to the winding frame 5 on the outside in the radial direction. This winding frame 7 is provided with six shield coils D to F (FIG. 2) arranged symmetrically with respect to the central plane 6 of the magnet device, as will be described later. These coils D to F are connected to the winding frame 7.
Between each rib pair 7F, 7E, coils A, A' to C,
It is attached in the same way as C′. In this case as well, a clamp ring (not shown) and wax are used to suppress movement of the coil winding. The provision of a clamp ring is particularly important since the current flowing in opposite directions in the two sets of coils creates very large radial forces. In order to place the windings of coils A to F in a superconducting state, these windings must be cooled to about 4.2K. This temperature is the boiling point of helium, so 2
The winding frames 5, 7 are defined by an outer cylindrical wall 8 and an inner wall having a central cylindrical member 9 and a pair of cylindrical members 10 radially outward from this (only one visible in FIG. 1). Place it in a helium container.
The cylindrical members 9 and 10 are connected and integrated by an annular web member 11. Then, the helium container is closed using a pair of ring members 12. Wall 8, cylindrical member 9,1
0, 11 and ring member 12 are all made of stainless steel. Liquid helium is supplied to the helium container via an inlet 13 provided in the turret 14. Aluminum cylindrical radiation shields 15, 15' are installed radially outside and inside the helium container and coaxially therewith.
5, 15' and the helium container there is an exhaust space 16.
form. Shield 15, 15' is turret 1
It is cooled in contact with helium by the mediating action of a heat exchanger (not shown) provided in the chamber 4. This heat exchanger extracts heat from the radiation shield 15, 15';
The heat is transferred to the cold helium gas vaporizing inside the helium container. Further, radiation shields 17 and 17' made of aluminum are attached coaxially with the shields 15 and 15' on the outside and inside in the radial direction to form an exhaust space 18. Shield when in use 17,1
Liquid nitrogen is supplied to a cooling pipe (not shown) wound around 7' and connected to a blind opening 19 of the turret 14. Finally, a cylindrical outer case 20 made of stainless steel is attached coaxially to the shield 17 to form a vacuum space 21. Aluminum end plate 22
~ 24 close the ends of the spaces 16, 17, 21, while a pair of end rings 25 are connected to the inner shield 17'
Attach to. In FIG. 1, only one end plate and end ring are shown. To reduce heat loads (losses), each shield is supported by a glass rod (not shown) mounted on a corresponding mounting plate. Arranging these rods in a three-dimensional array of supports can support a 4000 kg magnet with less than 0.04 watts of heat leakage. During use, the helium container is filled with 4.2K liquid helium. As the liquid helium vaporizes, the gas produced flows into a heat exchanger within the turret 14 and cools the shields 15, 15' to about 40°. Due to the vaporization of liquid helium, the helium container wall
It is kept at 42〓. Liquid nitrogen in a cooling tube (not shown) maintains the temperature of the shields 17, 17' at approximately 77K.
This envelope of liquid nitrogen and the vacuum within spaces 16, 18, and 21 serve to maintain the temperature of the helium vessel at 4.2K. Since the spaces 16, 18, and 21 are connected to the atmosphere through valves (not shown) in the turret 14,
These spaces can be evacuated. To understand the arrangement of coils A-F, consider the magnetic field at the center of a set of circular coplanar coils disposed on a cylindrical surface. For each value of current flowing through the coil, the magnetic field is proportional to the radius. If the magnetic field inside the cylindrical hole (in-hole magnetic field) is B _p and the magnetic field at an external point of the magnet device (hereinafter referred to as external point magnetic field) is B _f ,
Regarding the magnet (M) composed of inner coils A, A' to C, and C' and the counter magnet (A) composed of coils D to F, B _p ^M /B _f ^M = R ₁ B _p ^A /B _f ^A = R ₂ is obtained. In order to completely cancel the external magnetic field at a given external point, B _f ^A = -B _f ^M must be satisfied. Further, the combined effective in-hole magnetic field due to both arrays of coils is B _p ^M + (-B _p ^A ) = B _p ^M (1-R ₂ /R ₁ ). Thus, to obtain a constant borehole magnetic field, a fully shielded system requires a higher current than an unshielded magnet system. Due to the influence of this increase in current, the force acting on the conductor in the coil winding also increases, so the measures described above are required. The radial and axial positions of coils A to F are:
The magnetic field within the hole 3 is chosen to be at a selected value and approximately uniform over the entire hole. Additionally, the size and location of the coils are selected such that the magnetic field strength outside the magnet device does not exceed a certain value. The position of each coil A-F is determined by four dimensions. These dimensions are as shown in Figure 2 (focusing on coil C): inner radius a ₁ , outer radius
a ₂ , the shortest distance b ₁ that the coil is spaced from the central plane 6 parallel to the axis 4 and b ₂ the longest distance that the coil is spaced parallel to the axis 4 from the central plane 6 . As can be seen from FIG. 2, coils A to F all have rectangular cross sections. Table 1 below shows how the in-hole and fringe fields change for different coil diameters. In each case the radius of the coil is acm and it has 1432664 ampere turns. The in-hole magnetic field is represented by B _p , the peripheral magnetic field at a radius of 400 cm in the central plane of the coil is represented by B ^r , and the peripheral magnetic field at a distance of 400 cm from the center of the coil along the axis of the coil is represented by B ^Z. The fringe field is represented by its Z component, ie, the component parallel to the axis of the coil.

【table】

[Table] For the inner coil, choose a coil with a radius of, for example, 60 cm, and for the outer coil, choose a coil that is as close as possible to the inner coil in the radial direction, but has a slightly weaker strength to create a corresponding peripheral magnetic field. For example, if the radius of the main magnet is 60 cm, it will generate an in-hole magnetic field of 15000 Gauss and an average peripheral magnetic field of 40.1 Gauss (Table 1). The in-hole magnetic field generated by the shield coil itself with a radius of 80 cm is 11250 Gauss, and the average peripheral magnetic field is
It is 73.1 Gauss. The shield coil has opposite polarity to the main coil. To balance the fringing fields so that they cancel, the shield coil's ampere turns must be reduced by 40.1/73.1 to 786,560 ampere turns. Thus, the in-hole magnetic field generated by the shield coil is 6176 Gauss, and the average fringe magnetic field is 40.1 Gauss. Therefore, the net hole magnetic field is 15000−6176
= 8823 Gauss, and the net peripheral magnetic field is zero. An example of a series of coil dimensions suitable for use with the magnet arrangement of FIG. 1 is shown in Table 2 below.

【table】

[Table] In this table, except for winding length, other distance units are
cm. Further, the minus sign placed before the number of turns of the coils D to F indicates that the current flowing through these coils is in the opposite direction to the current flowing through the other coils. According to this specific configuration, at 30cmDSV
It has an extremely high uniformity of 74.7ppm, and
It can generate an in-hole magnetic field of 2.0T with a magnetic field stability of less than 0.1ppm/hour. 2.0T with 429 Amps current
A magnetic field of The electrical connections between the coils are shown in FIG. Through switch 27 and 0.5 ohm protection resistor 28,
A power supply 26 is connected in parallel with the coils D to F. Each coil group D to F is connected in series with coils A, A' to C, and C'. Furthermore, a 0.5 ohm protective resistor 29 is connected in parallel with all coils A, A' to C, and C', and a 0.5 ohm protective resistor 30 is connected in parallel to coils B, B' to C, and C'. . FIG. 4 shows the shielding effect that the outer coil groups D to F have on the magnet device. In the case of an unshielded magnet device generating an in-hole magnetic field of 15KG, the central plane 6 of the magnet device
An external magnetic field with a strength of 10 G is generated at a distance of 10 m from the magnet and 7.5 m from the axis 4 of the magnet arrangement. This is shown by curve 31 in FIG. A dotted curve 32 indicates the position where the external magnetic field strength has decreased to 5G. On the other hand, according to the magnet devices shown in FIGS. 1 and 2, external magnetic field strength positions are obtained where the external magnetic field strength is reduced to 10G and 5G, as shown by curves 33 and 34. FIG. 5 shows another example of coil configuration, in which two pairs of coils 35, 36 are used instead of the inner coils A, A' to C, C'. Coil 35 has 4098 turns, and coil 36 has 2000 turns. Also, one solenoid 37 with 4400 turns is used instead of the coils D to F. This solenoid is spaced 100 cm from the axis 4 and 64 cm on either side of the central plane 6.
extend. Generally, the shielding of the magnet arrangement shown in the figures above is sufficient, but in some cases special shielding is required. Such shielding is for example 6A, 6B
Do as shown. In this example, the same magnet device 1 as shown in FIG.
Surround with 8. A pair of resistive coils 39 perpendicular to the axis 4 are provided at positions spaced from both ends of the hole 3. Typically, the coil 39 is 3 m in diameter, has 50 turns, and receives a current supply of 100-150 A from a separate power source (not shown). According to this configuration, the shielding effect can be further improved without significantly affecting the in-hole magnetic field.
Furthermore, even when the magnet device is used in an NMR imaging device, the coil 39 does not obstruct the patient's access to and from the hole 3. By making the winding frame 7 made of iron, an improved device as shown in FIGS. 6A and 6B can be obtained. In this way, the iron plate 38 in FIG. 6A is attached to the magnet device 1.
When incorporated into the coils, the shielding effect of the coils D to F can be considerably improved with an extremely compact structure. One of the most important applications of the above-mentioned magnet device is its application to NMR imaging devices. Figure 7 is
A block diagram of an NMR imaging device is shown, which is similar to the conventional device except for the magnet device. The device comprises a magnet arrangement 1 incorporating a power supply (not shown). The helium vessel and the inlet to space 18 are connected to a suitable source, the operation of which is controlled by a conventional cryocontroller 40. Since the winding form 2 is provided with a large number of gradient magnetic field coils, different magnetic field gradients are formed through the holes 3.
Enables NMR imaging operations. The gradient field coils are conventional rather than superconducting coils. These coils are driven by a drive source 41 controlled by a control logic circuit 42 via a waveform generator 43. The winding form 2 generates RF (radio frequency) energy,
A receiving coil (not shown) is also attached,
An RF transmitter is connected to an amplifier 44 that is connected to a spectrometer 45. An RF receiver for detecting NMR signals is also connected to the spectrometer 45. A control logic circuit 42 connected to a spectrometer 45 controls the generation of RF pulses. NMR data from spectrometer 45 is sent to data acquisition device 46 which is controlled by control logic 42 . Data from this device 46 is supplied to a logic processing circuit 47. The entire device is controlled by conventional RS (Record Separation)
This is done by a computer 48 connected to an operator input section 49 via a H.232 interface. Information for the computer is stored in the disk drive mechanism 50, while the results of the imaging operation are sent by the computer to the display device 51. The display device 51 displays the human body slice of the patient on the monitor 5.
2 Display on top. In use, the patient lies within the bore 3 along the axis 4 of the magnet device, referred to as the Z direction. Table 3 below shows a typical dimensional comparison between a conventional unshielded magnet system and the magnet system shown in FIG. The numbers are for a 1 m diameter hole and an in-hole magnetic field of 1.5 T.

【table】 [Brief explanation of drawings]

Fig. 1 is a partially cutaway exploded perspective view of an embodiment of the present invention, Fig. 2 is a diagram for explaining the position of the superconducting coil of Fig. 1, and Fig. 3 is a superconducting coil of Fig. 2. Figure 4 is a circuit diagram showing the electrical connection between the unshielded magnet device and the magnetic field strength shown in Figure 1. The graph shown in FIG. 5 is a graph for explaining the coil arrangement in the second embodiment of the present invention, and FIG.
FIG. B is a schematic diagram showing the end and side views of a third embodiment of the present invention, and FIG. 7 is a block diagram of an NMR apparatus incorporating the device shown in FIG. 1. DESCRIPTION OF SYMBOLS 1... Magnet device, 3... Working space, 4... Coil axis, 6... Center plane, 7... Winding frame, 38... Shield, 3
9... Additional coils, AA' to CC'... First superconducting coil assembly, D to F... Second superconducting coil assembly.

Claims (1)

[Claims] 1 Magnet device 1 comprising first superconducting coil assemblies A, A' to C, C' that generate a first magnetic field and second superconducting coil assemblies DF that generate a second magnetic field. , the first superconducting coil assemblies A, A' to C, and C' that generate the first magnetic field are formed by electrically connecting each coil in series in a functionally inseparable manner, and also generate the second magnetic field. The generated second superconducting coil assembly DF is formed by electrically connecting each coil in series in a functionally inseparable manner, and the second superconducting coil assembly is provided with: The magnetic poles of the first superconducting coil assembly and the second superconducting coil assembly are arranged in the opposite direction, and the first superconducting coil assembly and the second superconducting coil assembly are arranged so that their coil axes coincide with each other. The generated first magnetic field and second magnetic field generate a composite uniform magnetic field in the working space 3 in the hole of the first superconducting coil assembly, and the zero-order magnetic field component of the magnetic field generated by each assembly substantially contributes to the magnitude of the uniform magnetic field, and the corresponding higher-order magnetic field components of the magnetic field generated by each assembly have approximately the same magnitude and contribute to achieving the uniform magnetic field. A magnet device, characterized in that the second magnetic field is formed outside the magnet device 1, facing the first magnetic field. 2 The second superconducting coil assemblies D to F are connected to the first
The magnet device according to claim 1, wherein the magnet device is electrically connected in series with superconducting coil assemblies A, A' to C, and C'. 3. The first superconducting coil assembly comprises a plurality of coaxial coils A, A'...
The magnet device according to claim 1, characterized in that it comprises C and C'. 4. The second superconducting coil assembly is characterized in that it comprises a plurality of coaxial coils D to F' arranged symmetrically with respect to the central plane 6 of the magnet arrangement 1 perpendicular to the axis 4 of the coils. A magnet device according to any one of claims 1 to 3. 5. Claims 1 to 4, characterized in that an auxiliary shield 38 of magnetic material is provided around at least the first superconducting coil assembly.
The magnet device according to any one of paragraphs. 6. The magnet device according to claim 5, wherein the auxiliary shield is constituted by a winding frame 7 that supports the second superconducting coil assembly. 7. The magnet device according to claim 6, wherein the auxiliary shields 38, 7 are made of iron.