JPH0782930B2 - Static magnetic field magnet for magnetic resonance imaging device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Object of the Invention (Industrial field of application) The present invention utilizes a magnetic resonance (MR) phenomenon to detect a specific nuclear spin in a specific cross section of a living subject. CT image (ComputedTomography) by so-called computed tomography (CT) based on density distribution
The present invention relates to a static magnetic field magnet used in a magnetic resonance imaging apparatus for imaging as an image.

(Prior Art) For example, in a medical magnetic resonance imaging apparatus used for biological diagnosis, in order to obtain a tomographic image of a specific part of a subject as a living body, as shown in FIG.
A very uniform static magnetic field H ₀ along the direction is generated and acted by a static magnetic field magnet (not shown), and a linear magnetic field gradient G _x is applied to the static magnetic field H ₀ by a pair of gradient coils 100A and 100B. Here, the specific nucleus with respect to the static magnetic field H ₀ resonates at the angular frequency ω ₀ represented by the following equation.

ω ₀ = γH ₀ (1) In this equation (1), γ is a gyromagnetic ratio, which is unique to the type of atomic nucleus. Therefore, a rotating magnetic field H ₁ having an angular frequency ω ₀ that causes only specific nuclei to resonate is applied to the subject P via, for example, a pair of transmission coils 200A and 200B provided in the RF coil (probe head).

By doing so, a specific slice portion S for obtaining a tomographic image selectively acts only on the xy plane portion shown in the drawing which is selectively set in the Z-axis direction by the linear magnetic field gradient G _x.
The magnetic resonance phenomenon occurs only in (a portion on a plane but actually has a certain thickness). This magnetic resonance phenomenon is caused by, for example, a pair of receiving coils 300A and 300B provided in the RF coil.
Observed as a free induction decay signal (hereinafter referred to as "FID signal") via M
Used as R signal. By Fourier transforming this FID signal, a single spectrum is obtained at the rotation frequency of the specific nuclear spin.

To obtain a tomographic image as a CT image, xy of the slice portion S
Projections in multiple directions in the plane are required. Therefore, after the slice portion S is excited to cause the magnetic resonance phenomenon, a linear gradient is applied to the magnetic field H ₀ in the x′-axis direction (coordinate system rotated by an angle θ from the x-axis) as shown in FIG. When the linear magnetic field gradient G _{x Y} possessed is applied by a gradient coil (not shown), the equal magnetic field line E in the slice portion S of the subject P becomes a straight line, and the rotation frequency of the specific nuclear spin on this equal magnetic field line E is (1) ) Is represented by the formula.

For convenience of explanation, an equal magnetic field lines E and E ₁ to E _n, considered to result signals D ₁ to D _n which is a type of FID signals, respectively by the magnetic field on these such as magnetic field lines E ₁ to E _n. The amplitude of the signal D ₁ to D _n is proportional to the specific nuclear spin density on such field lines E ₁ to E _n each penetrating the slice portion S. However,
The FID signal actually observed is a combined FID signal in which all the signals D _{1 to} D _n are added. Therefore, the projection information (one-dimensional image) PD of the slice portion S on the x ′ axis is obtained by Fourier transforming the combined FID signal. Next, let this x'axis be x-
While rotating in y plane, which for example x by gradient coils of the two pairs, the magnetic field gradient G _X for the y direction, G _Y
Create a magnetic field gradient G _XY as the synthetic magnetic field of
This is done by changing the composition ratio of G _X and G _Y. By rotating this magnetic field gradient G _XY , projection information in the angular direction in the xy plane can be obtained in the same manner as above, and based on these information
The CT images will be combined.

The above is the principle of magnetic resonance imaging. Next, as a specific example, FIG. 11 shows a conventional magnetic resonance imaging apparatus. The subject, that is, the patient 1, is placed on the bed 2. An RF coil (probe head: high-frequency transmission / reception coil) 3 is provided around the patient 1, and a shim coil 4 for magnetic field correction and a gradient coil 5 for generating a gradient magnetic field are arranged around the RF coil 3. All these coil systems
The large static magnetic field magnet 6 is housed inside a room temperature bore 7 (usually, bore inner diameter is about 1 m). As the static magnetic field magnet, a superconducting magnet, a normal conducting magnet, or a permanent magnet is used.

The static magnetic field magnet 6 is excited and demagnetized by the exciting power source 8 via the current lead 9 (this is not necessary in the case of the permanent magnet system). In the case of a superconducting magnet, the current lead 9 is normally removed after excitation in order to operate in the permanent current mode and to reduce the consumption of liquid helium, which is a refrigerant, so that a magnetic field is always generated. ing. Normally, the direction of this static magnetic field is 10 directions shown in many magnets,
That is, the body axis direction of the patient 1. The gradient coil 5 is composed of a GX coil that gives a magnetic field gradient in the X-axis direction, a GY coil in the Y-axis direction, and a GZ coil in the Z-axis direction, and is connected to exciting power sources 11, 12, and 13, respectively. These excitation power supplies
11, 12, 13 are connected to the central control unit 14. The RF coil 3 is composed of a transmission coil and a reception coil.
The RF oscillator 15 and the RF receiver 16 are connected to each other, and these are further connected to the central controller 14. The central control unit 14 is connected to the display / operation panel 17, and is operated by this.

In the above description of the static magnetic field magnet, the superconducting magnet was mainly described. Here, the structure and operation of a conventional permanent magnet for a magnetic resonance imaging apparatus will be described with reference to FIGS. 12 and 13.
About this permanent magnet, for example, "DIAGNOSTIC IMAGI
NG "magazine, April 1984 issue.

Figure 12 shows the appearance of the permanent magnet. A permanent magnet 20 is formed by arranging eight trapezoidal blocks 19 made of a permanent magnet material in an annular shape. Room temperature bore 7 in the center of the magnet
Is open. In this case, the magnetic field applied to the patient is perpendicular to the patient's body axis. Ferrite ceramics, rare earth samarium / cobalt, neodymium / poron / iron, etc. are used as permanent magnet materials.

FIG. 13 shows the magnetization direction 21 of each trapezoidal block 19 and the magnetic field lines 22 when assembled as a permanent magnet. The magnetization direction of each trapezoidal block is individually determined as shown in order to obtain a uniform magnetic field, that is, a uniform distribution of magnetic force lines in the room temperature bore 7. Due to such a magnetization direction, the magnetic force lines penetrate through the trapezoidal blocks arranged in an annular shape with almost no leakage to the outside of the permanent magnet, and a uniform magnetic force line distribution can be obtained in the room temperature bore.

Next, the operation of the conventional magnetic resonance imaging apparatus configured as described above will be described.

In order to obtain a whole-body tomographic image of the patient 1, the magnetic field uniform space 18 is normally as wide as 40 to 50 cm, and high uniformity of 50 ppm or less is required. Therefore, in the case of the superconducting system, the static magnetic field magnet 6 is required to have a huge size of 2.4 m in length, 2 m in width, 2.4 m in height, and 5 to 6 tons in weight.

Even with such a large magnet, the homogeneity within the 40-50 cm sphere due to only the magnet is only several hundred ppm at most. The shim coil 4 for magnetic field correction is used to reduce this to 50 ppm or less. The diagnostic site of the patient is brought into this uniform magnetic field space 18. Then, a high-frequency magnetic field is applied by the RF oscillating device 15 and the RF coil 3 in a direction perpendicular to the static magnetic field 10 to excite required atomic nuclei in human cells, for example, hydrogen atomic nuclei. At the same time, GX excitation power supply 11 and GY excitation power supply 1
A gradient magnetic field is applied in the X, Y, and Z directions by the 2, GZ excitation power supply 13 and the gradient coil 5.

The RF and gradient pulse sequences are optimally selected according to the lesion site and image processing method.

This pulse sequence operation is controlled by the central controller 14. After applying the gradient and RF, a magnetic resonance signal is emitted from the body of the patient 1. This signal is the RF receiver 16
Is received and amplified by and is input to the central controller 14.
Image processing is performed here, and a required human tomographic image is displayed on the CRT of the display / operation panel 17.

(Problems to be Solved by the Invention) The conventional static magnetic field magnet for a magnetic resonance imaging apparatus described above has the following advantages and disadvantages.

That is, in the case of a superconducting magnet, a high magnetic field can be easily generated as an advantage, so that it can be applied to spectroscopy diagnosis in which a magnetic field of 15000 gauss or more is required.

As a drawback, the current superconducting coil exhibits the superconducting phenomenon only at a cryogenic temperature of 4K. Therefore, it is necessary to periodically inject liquid helium that is cooling the superconducting coil in order to maintain the cryogenic state. Also, liquid helium is expensive, so the operating cost is high, and MRI
Even if the hospital introduces, the investment funds will not be recovered. Furthermore, it is difficult to handle liquid helium, which is a cryogenic refrigerant, and the regular liquid injection work is complicated.

In the case of a permanent magnet, the advantage is that no electric power or refrigerant is used, so the operating cost is zero and no periodic maintenance is required.

The drawbacks are as follows.

High magnetic field cannot be generated. The limit is 3000 gauss. Therefore, in principle, spectroscopy diagnosis cannot be performed, and even in proton imaging, the magnetic field is weak, so the image quality deteriorates compared to the superconducting method, and because high-speed imaging that requires a high magnetic field cannot be performed, the diagnosis time Hospital profits deteriorate because the diagnosis becomes longer and the diagnostic efficiency decreases.

The magnetic properties of permanent magnet materials are easily affected by changes in ambient temperature. Therefore, when the temperature of the magnet installation room changes, the magnetic field strength also changes. MRI magnets are essentially magnetic field homogeneity and stability, and if they change, the image quality deteriorates.

Ordinary permanent magnets perform complicated temperature control in order to avoid this.

Since the permanent magnet material is almost the same as the specific gravity of iron, the magnet
It weighs 10 tons to 100 tons. The installation floor strength of general hospitals is 10 tons or less, which limits the hospitals where permanent magnets can be installed.

As described above, currently used superconducting magnets and permanent magnets have advantages and disadvantages, and there was no magnet capable of completely fulfilling the market needs, which hindered the spread of MRI.

Therefore, an object of the present invention is to eliminate the drawbacks of conventional superconducting magnets and permanent magnets, and to supply a static magnetic field magnet having the advantages of both magnets. That is, a high magnetic field can be generated.

Make the operating cost almost zero.

Eliminate maintenance work.

It aims to promote the popularization of MRI by providing a static magnetic field magnet that is lightweight and compact and has a low price.

[Structure of Invention]

(Means for Solving Problems) The static magnetic field magnet for a magnetic resonance imaging apparatus according to the present invention is configured as follows in order to solve the above problems and achieve the object. That is, as the superconducting material, a ceramic superconducting material that exhibits a superconducting phenomenon at a temperature of liquid nitrogen or higher is used, and this is sintered to form a tubular or ring-shaped superconductor. A plurality of superconducting blocks formed by exciting the cylindrical or ring-shaped superconductors and aligning the magnetic field directions to make a plurality of superconducting blocks face each other so as to form voids. Arrange the superconducting block so that a magnetic field is generated.

(Operation) When a cylindrical or ring-shaped superconductor applies a magnetic field from the outside and causes a permanent current to circulate in the circumferential direction inside the cylinder wall of the superconductor by electromagnetic induction, the external magnetic field is removed due to the basic properties of superconductivity. Since the permanent current remains after the heating, the magnetic field having the required strength is generated in the axial direction of the tubular superconductor by the permanent current. Therefore, a superconducting block obtained by assembling a plurality of these superconductors with their magnetic field directions aligned with each other functions similarly to a permanent magnet.

(Embodiment) An embodiment of the present invention will be described with reference to FIGS. 1 to 5.

(Structure of Example) A cylindrical superconductor 23 shown in FIG. 3 or a ceramic (for example, Y—Ba—Cu—O) -based superconducting material exhibiting a superconducting phenomenon at a liquid nitrogen temperature (77 K) or higher is sintered. The ring-shaped superconductor 24 shown in FIG. 4 is made.

During sintering, large voids are generated between grain boundaries when only ceramics are used, but Pb, Id, Cu, Al
By mixing a metal powder having a high electric conductivity such as, the metal powder can fill the voids, and a tubular or ring-shaped superconductor having better electric characteristics can be obtained.

As shown in FIG. 5, the cylindrical superconductor 23 or the ring-shaped superconductor 24 is placed coaxially with the direction of the magnetic field B26 generated by the external superconducting magnet 25. At this time, each superconductor is in a superconducting state by immersion in liquid nitrogen or the like. In the case of a room-temperature superconductor, leave it as it is. When this external magnetic field 26 is removed, the original magnetic field is to be preserved as a basic property of superconductivity, so that a cylindrical wall of a cylindrical or ring-shaped superconductor is generated so that a magnetic field equivalent to this external magnetic field 26 is generated. Permanent current 28 flows in the inner circumferential direction and the magnetic field B is preserved. That is, the constant magnetic field 27 is always generated in the axial direction of the cylindrical superconductor 23 or the ring-shaped superconductor 24 by the permanent current 28 flowing in the superconductor.

In order to further improve the homogeneity, strength accuracy, and magnetic field direction accuracy of the constant magnetic field 27, thin ring-shaped superconductors 24 may be axially laminated to form a tubular superconductor, as shown in FIG. The reason for this is that in the case of a thin ring, the component of the permanent current flowing in the axial direction can be almost ignored and the current becomes a complete circumferential current, so that a highly accurate constant magnetic field 27 can be obtained.

A plurality of cylindrical superconductors 23 (single-formed or ring-laminated) configured as described above may be used.
A liquid nitrogen container filled with liquid nitrogen 29 as shown in FIG.
Store in 30. At this time, the magnetic fields of the cylindrical superconductors 23 are made to coincide with each other and assembled. The outer circumference of the liquid nitrogen container 30 is surrounded by a vacuum layer 31, and the vacuum layer 31 is used to enhance heat insulation.
31 is filled with a multi-layer heat insulating material 32. All of these are contained in a vacuum container 33. The cold insulation container (cryostat) thus configured is called a superconducting block 34. The liquid nitrogen container 30 may be directly cooled by a small refrigerator such as a GM (Gifford McMahon) system.

The static magnetic field magnet 6 shown in FIG. 1 is constructed by using a plurality of the superconducting blocks 34. That is, 6 superconducting blocks
Three pieces of 34a, 34b, 34c, 34d, 34e, 34f are arranged so as to face each other so that a space for forming the room temperature bore 7 is formed and the side surface of the space forms a concave curved surface with respect to the space. Both ends of the superconducting block 34a (34b) in the middle
The inclination angles of c, 34e (34d, 34f) are selected so that the required magnetic field homogeneity can be achieved in the diagnostic space inside the room temperature bore 7.
Along the side of the superconducting block opposite the void side, the yoke 35 is 6
The superconducting blocks 34a, 34b, 34c, 34d, 34e, 34f are surrounded.

The ring-shaped or cylindrical superconductor may be magnetized after being assembled into a superconducting block.

(Operation of Embodiment) The constant magnetic field 27 generated by the tubular superconductor 23 is gathered in the superconducting block 34 and becomes a uniformly distributed magnetic field, which is emitted from the superconducting block. This situation is as if the superconducting block 34 is a magnetic field source such as one permanent magnet. By making the cross-sectional area of the cylindrical superconductor sufficiently small with respect to the area of the magnetic field generation surface of the superconducting block 34, the homogeneity of the magnetic field emitted from the superconducting block becomes a value sufficiently corresponding to the level required by the magnetic resonance imaging apparatus. can do.

The superconducting blocks 34a and 34b, which are placed parallel to each other so that the generated magnetic fields are in the same direction, form the main magnetic field 10 used for diagnosis. At the end portion of the main magnetic field 10, the magnetic force lines try to spread outward due to the end effect of the magnetic pole surface. In such a situation, the turbulence of the lines of magnetic force affects the center, degrading the magnetic field homogeneity in the diagnostic space. In order to avoid this, superconducting blocks 34c, 34d and 34 that generate magnetic field lines that suppress the spread of magnetic field lines
e and 34f are attached to the ends of the superconducting blocks 34a and 34b with an appropriate inclination. The magnetic field lines that are curved toward the center and are generated by 34c, 34d and 34e, 34f hold down the magnetic field lines at the end of the main magnetic field 10, and as a result, the magnetic field homogeneity in the diagnostic space is secured.

The return path 36 of the magnetic path is formed by the yoke 35 attached to the outer periphery of the superconducting blocks 34a, 34b, 34c, 34d, 34e, 34f, and the magnetic flux does not leak to the outside of the static magnetic field magnet 6.

In addition, the above embodiment is a cylindrical or ring-shaped superconductor 2
The case where 3 and 24 are made of a material exhibiting a superconducting phenomenon at a liquid nitrogen temperature or higher has been described, but when the room temperature superconducting material is used, the refrigerant and the cryostat are naturally unnecessary.

(Effects of Embodiments) The above embodiments have the following effects.

Since the superconductivity phenomenon is used, it is possible to generate a high magnetic field required for spectroscopy diagnosis and high-speed imaging.

Since the superconducting state can be maintained by liquid nitrogen, the operating cost can be drastically reduced compared to the case of using conventional liquid helium.

Since the superconducting material can be used in a sintered body without being made into a wire rod, the manufacturing is easy and the manufacturing cost is drastically reduced.

Since liquid nitrogen is dealt with, the complexity of maintenance work, which is found in conventional liquid helium, is reduced.

The magnetic field uniformity can be improved by reducing the cross section of the cylindrical or ring-shaped superconductor which is the unit of the magnetic field generation source and increasing the number of aggregates.

Since a ceramic material is used, the weight can be reduced as compared with the conventional permanent magnet.

The leakage magnetic field can be reduced by the yoke.

(Other Embodiment 1) FIG. 6 shows another embodiment 1. A pair of superconducting blocks 34 are arranged parallel to each other so that the lines of magnetic force generated are in the same direction. A yoke 35 is attached to surround the outer circumference of this superconducting block.

In the case of this embodiment, since the superconducting block for suppressing the spread of the main magnetic field is not attached to the above embodiment, the magnetic field uniformity is deteriorated by that amount, but the manufacturing cost is greatly reduced. There is.

Other Embodiment 2 Another embodiment 2 is shown in FIG. Eight trapezoidal superconducting blocks 37 are annularly arranged to form a ring-shaped magnet.
The center of the ring is the room temperature bore 7.

In order to obtain the same magnetic field distribution 22 as the conventional permanent magnet 20 shown in FIG.
Are set as shown in FIG. Each trapezoidal superconducting block 37
Since the magnetization directions of the cylindrical superconductors are as shown in the figure, the trapezoidal superconducting blocks 37 are assembled by assembling the cylindrical superconductors 23 so that the axes of the cylindrical superconductors 23 are coaxial with the required magnetization direction.
Make up. At this time, since the trapezoids are formed by accumulating the tubular superconductors, the lengths of the tubular superconductors do not match and the lengths become shorter as shown in FIG.

With the above structure, the trapezoidal superconducting block 37 shown in FIG. 7 has the same magnetic function as the trapezoidal block 19 of the conventional permanent magnet shown in FIG. Therefore, the generated magnetic force lines are uniform in the room temperature bore 7, and the trapezoidal superconducting block 37 itself serves as the magnetic path return path, so that there is almost no leakage of the magnetic force lines to the outside of the magnet. That is, the operation is exactly the same as that of the conventional example shown in FIG.

The effect is the same as that of the first embodiment shown in FIG.

(Other Embodiment 3) FIG. 8 shows another embodiment 3. The structure is such that each tubular superconductor 23 can be finely moved in its axial direction. Any mechanism may be used for this fine adjustment movement.

When the tubular superconductor 23 moves axially toward the room temperature bore 7 side, the magnetic field strength in the diagnostic space contributed by the tubular superconductor slightly increases. On the contrary, when it is moved to the side opposite to the room temperature bore 7, the magnetic field strength is slightly reduced. By this effect, the magnetic field homogeneity in the diagnostic space can be finely adjusted by moving each tubular superconductor.

As described above, the high magnetic field homogeneity required for the magnetic resonance imaging apparatus can be achieved.

〔The invention's effect〕

According to the present invention, the disadvantages of the conventional superconducting magnets and permanent magnets are eliminated, and the advantages of both magnets, that is, high magnetic field generation, operating cost reduction, maintenance reduction,
It is possible to provide a high-performance static magnetic field magnet for a magnetic resonance imaging apparatus, which is lightweight, compact, and inexpensive. Therefore, there is an effect that the spread of the magnetic resonance imaging apparatus can be promoted.

[Brief description of drawings]

FIG. 1 is a block diagram of a static magnetic field magnet according to an embodiment of the present invention, and FIG.
FIG. 1 is a block diagram of a superconducting block used in one embodiment of the present invention, FIG. 3 is a structural diagram of a cylindrical superconductor used in one embodiment of the present invention, and FIG. 4 is a ring shape used in one embodiment of the present invention. FIG. 5A and FIG. 5B are views showing the method of applying a constant magnetic field in one embodiment of the present invention, and FIG. 6 is a static magnetic field of another first embodiment of the present invention. FIG. 7 is a configuration diagram of a magnet, FIG. 7 is a configuration diagram of a static magnetic field magnet according to another second embodiment of the present invention, and FIG. 8 is a configuration diagram of a static magnetic field magnet according to another third embodiment of the present invention. 9 is a configuration diagram of a conventional magnetic resonance imaging apparatus, FIG. 10 is a principle diagram of magnetic resonance imaging, FIG. 11 is a system diagram of the conventional magnetic resonance imaging apparatus, and FIG. 12 is a permanent diagram for the conventional magnetic resonance imaging apparatus. Fig. 13 is a structural diagram of a magnet, and Fig. 13 is a distribution diagram of magnetic force lines of a conventional permanent magnet for a magnetic resonance imaging apparatus. 6 ... Static magnetic field magnet, 7 ... Room temperature bore 10 ... Static magnetic field direction, 19 ... Trapezoidal block 20 ... Permanent magnet, 21 ... Magnetization direction 22 ... Magnetic field line, 23 ... Cylindrical superconductor 24 ... Ring superconductor 34, 34a, 34b , 34c, 34d, 34e, 34f ... Superconducting block 35 ... Yoke, 37 ... Trapezoidal superconducting block

Claims (12)

[Claims] 1. A plurality of cylindrical superconductors made of a superconducting material, in which a permanent current is circulated in a circumferential direction in a cylindrical wall and a magnetic field of a required strength is generated in a cylindrical axis direction, with the magnetic field directions aligned with each other. A static magnetic field magnet for a magnetic resonance imaging apparatus, characterized in that a plurality of superconducting blocks are arranged so as to face each other so that a void is formed so that a substantially uniform static magnetic field is generated in the void. 2. A superconducting block, in which a tubular superconductor is housed in a liquid nitrogen container filled with liquid nitrogen, the liquid nitrogen container is surrounded by a heat insulating material and a vacuum layer, and the whole is vacuumed. The static magnetic field magnet of the magnetic resonance imaging apparatus according to claim 1, having a cryostat structure housed in a container. 3. A static magnetic field magnet for a magnetic resonance imaging apparatus according to claim 2, wherein the liquid nitrogen container is directly cooled by a small refrigerator such as a GM system. 4. The static magnetic field magnet of the magnetic resonance imaging apparatus according to claim 1, wherein the tubular superconductor is formed by laminating a plurality of ring-shaped superconductors coaxially in the axial direction. 5. A static magnetic field magnet for a magnetic resonance imaging apparatus according to claim 1, wherein the void side surfaces of the plurality of superconducting blocks form concave curved surfaces with respect to the voids. 6. A static magnetic field magnet for a magnetic resonance imaging apparatus according to claim 1, wherein a plurality of superconducting blocks are surrounded by a yoke along a surface of the superconducting block opposite to the void. 7. A static magnetic resonance imaging apparatus according to claim 1, wherein a plurality of superconducting blocks having different magnetization directions are annularly arranged and the magnet is formed in a ring shape. Magnetic field magnet. 8. The magnetic resonance according to claim 1, wherein the magnetic field homogeneity in the gap is adjusted by finely moving the cylindrical superconductor in the superconducting block in the axial direction. Static field magnet for imaging equipment. 9. A superconducting material is a ceramic superconducting material that exhibits a superconducting phenomenon at a liquid nitrogen temperature or higher, and is formed by sintering a cylindrical superconductor or a ring-shaped superconductor with the ceramic material. A static magnetic field magnet of the magnetic resonance imaging apparatus according to claim 1. 10. When sintering a ceramics-based superconducting material, a ceramics powder is mixed with a metal powder having a high electric conductivity such as Pb, Id, Cu, Al to fill the voids of the ceramics grain boundaries with a metal base material. The static magnetic field magnet of the magnetic resonance imaging apparatus according to claim 9, wherein 11. A cylindrical or ring-shaped superconductor has a permanent current based on electromagnetic induction circulated in a circumferential direction by a process of applying or not applying a magnetic field using an external superconducting magnet for applying a magnetic field of strong axial direction. The static magnetic field magnet of the magnetic resonance imaging apparatus according to claim 1, wherein the static magnetic field magnet is a magnetic field magnet. 12. A static magnetic field magnet for use in a magnetic resonance imaging apparatus according to claim 11, wherein a superconductor block is used instead of the tubular or ring-shaped superconductor.