JP4749699B2 - Magnetic resonance imaging system - Google Patents

Description

  The present invention relates to a magnetic resonance imaging apparatus, and more particularly to a magnetic resonance imaging apparatus that reduces the influence of a leakage magnetic field.

  Currently, in a magnetic resonance imaging apparatus (MRI apparatus), a horizontal magnetic field type using a cylindrical magnet and a vertical magnetic field type using a counter magnet are used.

  The former is advantageous for generating a high magnetic field, but is disadvantageous in that the measurement space has a feeling of blockage and lacks openness for the subject.

  On the other hand, the latter is advantageous in terms of openness to the subject and easy access to the subject, but is a disadvantageous structure for suppressing the leakage magnetic field.

  As described above, the vertical magnetic field type using opposed magnets has a disadvantageous structure for suppressing the leakage magnetic field, and when the central magnetic field strength becomes high, the spread of the leakage magnetic field tends to be larger than that of the cylindrical magnet. It is in.

  In order to suppress the leakage magnetic field from the magnet, there are a passive shield method in which a magnetic circuit is formed from a ferromagnetic material, and an active shield method by a shield coil that flows current in a direction to suppress the leakage magnetic field.

  These leakage magnetic field suppression methods are employed even in the conventional vertical magnetic field method using opposed magnets. However, as the vertical magnetic field increases and the leakage magnetic field increases, the passive shield method has a problem that the amount of ferromagnetic material used increases and the magnet weight increases.

  Also in the active shield system, with the increase in the current value of the shield coil, there is a limit value of the current value due to the limitation of the electromagnetic force reinforcing means and the material of the shield coil, making magnet design difficult. For this reason, as a means for suppressing the leakage magnetic field from the magnet, means using a magnetic shield room is also used.

  In this magnetic shield room, a magnetic shield made of a ferromagnetic material such as an electromagnetic steel plate (silicon steel plate) or permalloy is disposed on the wall surface, ceiling, or floor surface of the magnetic shield room.

  Normally, the 0.5 mT line that adversely affects the pacemaker is suppressed in the shield room, and if necessary, the 0.1 mT line that adversely affects the precision equipment such as the CRT display and the X-ray apparatus of the personal computer is also suppressed.

  Since the main magnetic field generation direction is the vertical direction, the magnetic flux generated from the lower (upper) side of the magnet passes through a position away from the side of the magnet and the upper (lower) of the magnet. It features a route back to the side.

  Due to the characteristics of this leakage magnetic field, the leakage magnetic field from the magnet is large, and when suppressing the leakage magnetic field to the side of the opposed magnet, that is, the side wall surface direction of the magnetic shield room, not only the side surface of the magnetic shield room but also the floor It is effective to arrange a magnetic shield on the surface.

  Here, in order to improve the effect of suppressing the leakage magnetic field by the magnetic shield room, it is conceivable to increase the thickness of the magnetic material used for the magnetic shield. However, if the thickness of the magnetic material is increased, the weight of the magnetic material increases and the weight of the structure support material of the magnetic shield room increases, which is not desirable.

  Therefore, Patent Document 1 describes a configuration of a magnetic shield room that is light and can effectively suppress a leakage magnetic field.

  In the technique described in Patent Document 1, the wall surface or ceiling surface of the magnetic shield room has a double structure of a ferromagnetic ring layer and a flat ferromagnetic plate, and the side surface of the ferromagnetic ring is flat. The structure is inclined with respect to the ferromagnetic material.

  With this structure, the magnetic flux toward the ceiling surface or wall surface becomes substantially parallel to the flat ferromagnetic plate by the inclined side surface of the ferromagnetic ring. Therefore, the ferromagnetic material is easily magnetized and can effectively shield the leakage magnetic field with a small amount of iron used.

JP 2002-172101 A

  However, among the surfaces of the magnetic shield room, the floor surface is generally located at the position closest to the center of the magnetic field, and the ferromagnetic material on the floor surface is easily saturated, and the effect of suppressing the leakage magnetic field is reduced. For this reason, even if the technique described in Patent Document 1 is used, it is necessary to keep the magnetic shield on the floor away or to increase the amount of use of the magnetic shield on the floor.

  When the magnetic shield on the floor is kept away, for example, when installing an MRI apparatus on the first floor of a building, it is necessary to dig up the floor or raise the magnet, and the installation of shield rooms and magnets deteriorates. End up.

  On the other hand, when the usage amount of the magnetic shield on the floor is increased, when a magnet is installed, a magnetic field is attracted to the magnetic shield on the floor, and an asymmetric magnetic field component appears in the vertical direction.

  Generally, it is conceivable to adjust the magnetic field of this asymmetric magnetic field component by a shimming mechanism provided on the opposite surface of the static magnetic field generating means. However, the magnetic field adjustment by this shimming mechanism is not sufficient and may exceed the magnetic field adjustment capability.

  Furthermore, there are various demands for suppressing the leakage magnetic field around the magnetic shield room, and the amount of magnetic shield material used is also different. In this case, the shimming mechanism in the magnet is required to have a wide range of magnetic field adjustment capability, so that the shimming mechanism becomes large-scale and the apparatus becomes large.

  Further, since the shimming mechanism in the vertical magnetic field method using the opposed magnet is usually disposed on the opposing surface of the static magnetic field generating means, the large-scale shimming mechanism impairs the openness of the vertical magnetic field method MRI apparatus, and the magnet structure Makes the magnet design difficult.

  An object of the present invention is to realize a magnetic resonance imaging apparatus capable of suppressing a leakage magnetic field and making a magnetic field distribution symmetric without increasing the size of the apparatus in a vertical magnetic field MRI apparatus.

In order to achieve the above object, the present invention is configured as follows.
(1) The magnetic resonance imaging apparatus of the present invention includes a static magnetic field generating means 101 that generates a uniform static magnetic field in the measurement space 9, a gradient magnetic field generating means 102, high-frequency transmitting / receiving means 103 and 104, and an image processing means 105. And storage containers 2a and 2b for storing the static magnetic field generating means 101.

In the magnetic resonance imaging apparatus, an auxiliary static magnetic field generation is provided that includes a bottom structure 32 having a plurality of concentric circular ferromagnets at the bottoms of the storage containers 2a and 2b, and compensates for a vertical asymmetric component of the static magnetic field. Means 21, 22, and 23 were provided.

( 2 ) Preferably, in the above ( 1 ), the static magnetic field correction means 21 and 22 are accommodated in the containers 2a and 2b.

( 3 ) Preferably, in the above ( 1 ), the static magnetic field correcting means 23 is disposed on the measurement space 9 side of the containers 2a and 2b.

( 4 ) Preferably, in the above ( 1 ), the static magnetic field correction means 21, 22, and 23 are a superconducting coil or a ferromagnetic material.

( 5 ) Preferably, in the above ( 3 ), the static magnetic field correction means 23 is a conductor coil or a ferromagnetic material.

( 6 ) Preferably, in the above ( 1 ), the bottom structure 32 is formed of a composite member made of a ferromagnetic material and a non-magnetic material.

( 7 ) Preferably, in the above ( 1 ), the plurality of concentric members of the bottom structure 32 are arranged at intervals, and the plurality of nonmagnetic steel materials 34 are arranged at the intervals.

( 8 ) Preferably, in the above (1), a temperature control means for making the temperature of the bottom structure constant is provided.

  Without increasing the size of the apparatus, it is possible to realize a magnetic resonance imaging apparatus capable of suppressing a leakage magnetic field and making the magnetic field distribution symmetrical.

  In the magnetic resonance imaging apparatus of the present invention described above, the magnetic field design of the magnet is performed in advance by computer simulation so that the auxiliary static magnetic field generating means compensates for the shortage of suppression of the asymmetric component of the measurement space by the bottom structure. Keep it.

  As a result, the leakage magnetic field from the lower bottom side of the superconducting magnet is reduced, and the amount of magnetic shield used on the floor surface arranged to suppress the leakage magnetic field in the side wall surface direction of the shield room can be reduced.

  As a result, the difference in the magnetic field distribution in the measurement space between the installation in the magnetic shield room and the initial magnetic field design becomes small, and the shimming mechanism used at the time of installation in the magnetic shield room is sufficient with the magnetic field adjustment capability in a small range.

  As a result, the installation restriction due to the leakage magnetic field is relaxed, and the shimming work at the time of installation becomes easier.

  An opposed superconducting magnet using a superconducting coil as a static magnetic field generation source has a structure in which opposed hollow vacuum vessels are connected by a plurality of connecting tubes, and is a structure that is vulnerable to vibration.

  Furthermore, in order to improve the openability as much as possible, the thickness and number of connecting pipes are limited, and structural reinforcement is difficult. In this structure, a pulse current flows through the gradient magnetic field coil arranged on the opposing surface in a static magnetic field.

  For this reason, Lorentz force acts on the gradient magnetic field coil and vibrates. The vibration of the gradient magnetic field coil is transmitted to the static magnetic field generation source of the vacuum vessel, and vibrates the static magnetic field generation source.

  The vibration of the static magnetic field generation source fluctuates the uniform magnetic field in the measurement space, and as a result, appears as an artifact in the MR image.

  In the configuration of the present invention, the bottom structure of the vacuum container is provided with a bottom structure made of a ferromagnetic material, and the bottom structure is firmly integrated with the vacuum container to improve the rigidity of the vacuum container. It is also possible to reduce the vibration transmitted from the magnetic field coil to the vacuum vessel.

Embodiments of the present invention will be described below with reference to the accompanying drawings.
FIG. 1 is an overall schematic configuration diagram of an MRI apparatus to which the present invention is applied.

  In FIG. 1, an MRI apparatus is for obtaining a tomographic image of a subject using a magnetic resonance phenomenon, and includes a static magnetic field generation means 101, a gradient magnetic field generation means 102, a transmission system 103, and a reception system 104. A signal processing system 105, a control unit 106 that controls the operation of the static magnetic field generation unit 101, a central processing unit 107, and an operation unit 108.

  The static magnetic field generating means 101 generates a uniform static magnetic field in a direction perpendicular to or parallel to the body axis around the subject 108 from a magnet arranged in a wide space around the subject 108. .

  The gradient magnetic field generation means 102 includes a gradient magnetic field power supply 110 and a gradient magnetic field coil 109, and generates gradient magnetic fields in the three axial directions of the X axis, the Y axis, and the Z axis in an imaging space where the subject 108 is arranged. To do. The imaging cross section of the subject 108 is set by applying this gradient magnetic field.

  The transmission system 103 includes a high-frequency oscillator 111, a modulator 112, a high-frequency amplifier 113, and a high-frequency irradiation coil 114. The transmission system 103 is output from the high-frequency oscillator 111 in order to excite the atomic nuclei constituting the biological tissue of the imaging cross section of the subject 108 set by the gradient magnetic field generating means 102 to cause nuclear magnetic resonance. The high frequency pulse is supplied to the high frequency amplifier 113 via the modulator 112. Then, after being amplified by the high-frequency amplifier 113, the high-frequency pulse is applied to the subject 108 by being supplied to the high-frequency irradiation coil 114 installed in the vicinity of the subject 108.

  The receiving system 104 includes a high-frequency receiving coil 115, a receiving circuit 116, and an analog / digital (hereinafter referred to as “A / D”) converter 117. Then, an NMR signal, which is an echo signal due to magnetic resonance of the nuclei of the biological tissue of the subject 108 due to the electromagnetic waves irradiated from the high-frequency irradiation coil 114 of the transmission system 103, is placed near the subject 108. Detect with. The NMR signal detected by the high frequency receiving coil 115 is input to the A / D converter 117 via the receiving circuit 116 and converted into a digital signal.

  The A / D converter 117 sends the signal to the signal processing system 105 as the collected data sampled at the timing according to the command from the control unit 106.

  The control unit 106 operates under the control of the CPU 107, and repeatedly generates slice encode, phase encode, and frequency encode gradient magnetic fields and high frequency magnetic field pulses in a predetermined pulse sequence. Then, the control unit 106 sends various commands necessary for acquiring tomographic image data of the subject 108 to the gradient magnetic field generating means 102, the transmission system 103, and the reception system 104.

  The signal processing system 105 includes a CPU 107, a signal processing device 118, a memory 119, a magnetic disk 120, an optical disk 121, and a display (display unit) 122.

  The CPU 117 performs Fourier transformation and control of the sequencer 106 on the collected data. Further, the signal processing device 118 performs processing necessary for reconstructing correction calculation and acquired data into a tomographic image.

  The memory 119 stores a program of a time-lapse image analysis process and a designated measurement sequence, parameters used at the time of execution, and the like, and measurement parameters obtained in advance measurement performed on the subject. Collected data from the NMR signal detected by the receiving system 104 and an image used for setting the region of interest are temporarily stored, and parameters for setting the region of interest are recorded.

  The magnetic disk 120 and the optical disk 121 are data storage units for storing the reconstructed image data. The display 122 performs an image reconstruction calculation using the NMR signal detected by the receiving system 104 and displays the image.

  The operation unit 108 includes a trackball, a mouse, a keyboard, and the like, and is used to input control information for processing performed by the signal processing system 105.

  Images reconstructed from the NMR signals detected by the receiving system 104 are sequentially displayed on the display 122. The position and angle of the next imaging are set by the operation unit 108 on the continuously displayed images. The set information is displayed on the display 122.

  The present invention is applied to the above-described MRI apparatus. As a comparative example of the present invention, the distribution of the leakage magnetic field when only the magnetic shield room is used will be described.

  FIG. 13 is a view showing an example in which a vertical magnetic field type MRI apparatus (an example different from the present invention) using an opposed superconducting magnet is installed in a magnetic shield room.

  In FIG. 13, in order to suppress the 0.5 mT line 8 of the leakage magnetic field in the side wall surface direction, the magnetic shields 5 and 6 are arranged on the side surface and floor surface direction of the magnetic shield room.

  An auxiliary magnetic shield 61 is also arranged on the floor surface of the magnetic shield room in order to reduce the magnetic saturation of the magnetic shield 6 on the floor surface. As the material of the magnetic shields 5 and 6 and the auxiliary magnetic shield 61, a ferromagnetic material such as an electromagnetic steel plate (silica steel plate) or permalloy is used.

  In FIG. 13, the gradient magnetic field coils 4a and 4b are arranged to face each other with the measurement space 9 in between. 2a is the upper side of the cryocontainer, and 2b is the lower side of the cryocontainer. Superconducting coils 3a and 3b are disposed in the cryocontainers 2a and 2b.

  Next, FIG. 14 shows the magnetic field distribution in the measurement space 9 before the apparatus shown in FIG. 13 is installed in the magnetic shield room.

  FIG. 15 shows the magnetic field distribution in the measurement space 9 when the apparatus shown in FIG. 13 is installed in the magnetic shield room.

  FIG. 14 shows a magnetic field contour line 10 of ± several tens of ppm in a spherical cross section having a diameter of 400 cm, for example, and FIG. 15 shows a magnetic field contour line 11 of ± several 1000 ppm in a spherical cross section having a diameter of 400 cm, for example.

  In the highly uniform magnetic field distribution shown in FIG. 14, positive and negative magnetic field contour lines 10 appear alternately, and a value of ± several tens of ppm is outside the measurement space 9.

  On the other hand, in FIG. 15, the magnetic field contour lines are asymmetrical in the vertical direction due to the influence of the ferromagnetic bodies (6 and 61) on the floor surface, and the uniformity of the magnetic field is inferior to several thousand ppm inside the measurement space.

  Thus, suppression and symmetrization of the leakage magnetic field when the MRI apparatus is installed in the magnetic shield room is achieved by the following embodiment.

  FIG. 2 is a diagram showing a schematic external shape of the cryocontainers 2a and 2b of the opposed superconducting magnet for the MRI apparatus according to the first embodiment of the present invention. FIG. 3 is a schematic cross-sectional view of the cryocontainers 2a and 2b shown in FIG.

  As shown in FIGS. 2 and 3, the opposed superconducting magnet connects a pair of cryocontainers 2a and 2b disposed vertically opposite each other with the measurement space 9 interposed therebetween, and the pair of containers 2a and 2b. One or more connecting pipes 51 are provided.

  In the cryocontainers 2a and 2b, as a static magnetic field generating means, a pair of superconducting coils 3a and 3b arranged opposite to each other up and down and the superconducting coils 3a and 3b are housed and kept below the superconducting transition temperature. A helium tank (not shown) and a radiation shield (not shown) for blocking heat radiation to the helium tank are arranged.

  Further, a pair of gradient magnetic field coils 4a and 4b and a pair of high-frequency irradiation coils (not shown) facing each other are arranged on the cryocontainers 2a and 2b on the side facing the measurement space 9.

  In the opposed superconducting magnet to which the present invention is applied, a bottom structure 31 made of a ferromagnetic material such as an electromagnetic steel plate, pure iron, or general rolled steel (SS400) is provided at the bottom of the bottom cryocontainer 2b, An auxiliary static magnetic field generating means 21 that compensates for an irregular magnetic field component (vertical asymmetric component) generated in the measurement space 9 by the bottom structure 31 in the container 2a is provided. Further, the bottom structure 31 is firmly integrated with the lower cryocontainer 2b by connecting means such as bolt fastening or welding.

  With this configuration, the rigidity of the cryocontainers 2a and 2b can be improved. That is, it is possible to improve the rigidity of the lower cryocontainer 2b and reduce the influence on the vibration of the gradient magnetic field coils 4a and 4b.

  Moreover, as shown in FIG. 3, the structural reinforcement member 41 is arrange | positioned at the upper part (surface on the opposite side to the measurement space 9 side) side of the upper cryocontainer 2a. The material of the structural reinforcing member 41 is preferably a nonmagnetic member such as stainless steel. With this configuration, the structure of the cryocontainers 2a and 2b can be further strengthened.

  Further, at the time of transporting the MRI apparatus, the bottom structure 31 may be detached from the lower cryocontainer 2b, attached to the lower cryocontainer 2b on site at the time of installation, and bolted. With this configuration, it is possible to relax the weight limit during transportation.

  In the example shown in FIG. 3, the auxiliary superconducting coil 21 is used as the auxiliary static magnetic field generating means. However, as shown in FIG. 4, auxiliary ferromagnetic materials such as electromagnetic steel plates (silicon steel plates) and pure iron are used. 22 or an auxiliary conductor coil 23 such as copper may be used as auxiliary static magnetic field generating means, as shown in FIG.

  The arrangement positions of the auxiliary static magnetic field generating means 21, 22, 23 are desirably as follows.

  That is, the auxiliary superconducting coil 21 is disposed in the helium tank. The auxiliary ferromagnet 22 can be stored inside the cryocontainer 2a such as in a helium tank so as not to impair the openness of the measurement space 9 and to prevent the magnetic properties of the ferromagnet 31 from changing due to temperature. desirable. Further, the auxiliary conductor coil 23 is disposed on the opposing surface of the cryocontainer 2a so as not to impair the openness of the measurement space 9.

  The position, volume, and current value of these auxiliary static magnetic field generating means 21, 22, and 23 are obtained in advance by computer simulation, and a plurality of them may be configured in some cases.

  3 and 5, the auxiliary superconducting coil 21 and the auxiliary conductor coil 23 are arranged on the upper cryocontainer 2a side, but depending on the result of computer simulation, the direction of the current is reversed. Thus, it may be arranged on the lower cryocontainer 2b side.

  Further, the auxiliary static magnetic field generating means 21, 22, and 23 are desirably located as close to the measurement space 9 as possible in order to reduce the volume and current value. FIG. 6 is an internal cross-sectional view of the gradient magnetic field coil 4a located on the surface side facing the measurement space 9 of the upper cryocontainer 2a.

  In FIG. 6, the gradient coil 4 a includes a main gradient coil group 401 that generates X, Y, and Z components, and a shield gradient coil group 402 that cancels the leakage magnetic field of the main gradient coil group 401. Yes.

  In addition, although not shown, the MRI apparatus also includes a cooling pipe for cooling the shim coil and the gradient magnetic field coil. In addition, the generation efficiency of a gradient magnetic field becomes higher when the layer between the main gradient magnetic field coil group 401 and the shield gradient magnetic field coil group 402 is expanded. Therefore, a space is provided between the main gradient magnetic field coil group 401 and the shield gradient magnetic field coil group 402. You may arrange | position the auxiliary | assistant ferromagnetic material 22 and the auxiliary conductor coil 23 in this space.

  With the above configuration, the openness of the measurement space 9 is not impaired. However, when the auxiliary ferromagnet 22 is used, it is necessary to saturate the ferromagnet 31 because of the non-linearity of the ferromagnet 31 and the influence of minor loops. In addition, it is desirable to install a cooling structure in the auxiliary conductor coil 23 in consideration of heat generation due to Joule heat.

  It is a schematic sectional drawing of the cryo container in the MRI apparatus which is the 2nd Embodiment of this invention. The second embodiment is different from the first embodiment in that the bottom structure is a concentric concentric bottom structure 32.

  FIG. 8 is a plan view of the concentric bottom structure 32 shown in FIG. As shown in FIG. 8, the concentric bottom structure 32 includes a plurality of concentric ferromagnets and a nonmagnetic resin material 33 disposed between the plurality of concentric ferromagnets. .

  In the second embodiment of the present invention, the concentric pattern of the concentric bottom structure 32 is determined from the number of turns corresponding to the magnetic field component of the measurement space 9. This concentric pattern has the function of the auxiliary static magnetic field generating means (higher-order term suppression).

  Therefore, this second embodiment can simplify the configuration of the auxiliary static magnetic field generating means as compared with the first embodiment shown in FIG. In other words, the concentric bottom structure 32 can improve the efficiency of magnetic field adjustment in the auxiliary static magnetic field generating means such as the auxiliary superconducting coil 21.

  FIG. 9 is a view showing a modified example of the concentric bottom structure 32 in the second embodiment. As shown in FIG. 9, a plurality of concentric ferromagnets are reinforced with a plurality of nonmagnetic steel materials 34 arranged radially (the bottom structure has a cross-girder structure). As the nonmagnetic steel material 34, stainless steel or the like can be used.

  Thus, the weight of the bottom structure can be further reduced by making the shape of the bottom structure a cross-girder structure.

  As another modified example of the bottom structure 32 shown in FIG. 8, materials having different permeability are arranged in a concentric manner so as to form portions having different magnetic flux passing amounts in the radial direction. You can also. Moreover, although the magnetic permeability is the same material, the circular part from which thickness differs mutually can also be arrange | positioned concentrically.

  In the above-described example, the bottom structure bodies 31 and 32 are manufactured separately from the cryocontainer 2b, and the bottom structure bodies 31 and 32 are attached to the cryocontainer 2b with bolts or the like when installed at the use location of the MRI apparatus. The structure which connects may be sufficient, and as shown in FIG. 10, the bottom part structures 31 and 32 can also be manufactured integrally with the cryocontainer 2b (bottom part 35 of cryocontainer 2a, 2b). By comprising in this way, integration with the cryocontainer 2b can be strengthened and rigidity can be improved.

  Here, when the magnetic permeability of the bottom structure (31, 32, 35) has a temperature characteristic that changes depending on the ambient temperature, the temperature of the bottom structure is made substantially constant to compensate for this temperature characteristic. It is preferable.

  For this reason, it is possible to provide a temperature control means for covering the bottom structure with a heat insulating material, detecting the temperature of the bottom structure with a temperature sensor, and controlling the heater or the cooling means so that the temperature becomes constant. . The temperature control by this temperature control means may be performed by the control unit 106 shown in FIG. 1 or by the central processing unit 107. Moreover, temperature control can also be performed by means different from these.

  FIG. 11 is a diagram showing results obtained by simulation of the leakage magnetic field suppression effect according to the present invention (second embodiment).

  Moreover, FIG. 12 is a figure which shows the result of having obtained the leakage magnetic field suppression effect by simulation, when not providing a bottom structure unlike the present invention.

  In the case of the example shown in FIG. 11, the 0.5 mT line is about 2.5 m from the bottom surface of the shield room 36 in the vertical (up and down) direction, and the horizontal dimension of the shield room 36 in the horizontal (left and right) direction (about 5.5m).

  The 0.1 mT line is about 3.0 m from the bottom of the shield room 36 in the vertical (up and down) direction, and about 7.5 m in the horizontal (left and right) direction.

  On the other hand, in the case of the example shown in FIG. 11, the 0.5 mT line is about 3.5 m from the bottom of the shield room 36 in the vertical (up and down) direction and about 6.5 m in the horizontal (left and right) direction. Yes.

The 0.1 mT line is about 4.0 m from the bottom surface of the shield room 36 in the vertical (up and down) direction, and is within about 10.0 m in the horizontal (left and right) direction.
As shown in FIG. 11 and FIG. 12, the bottom structure (31, 32, 35) according to the present invention improves the leakage magnetic field suppression effect.

1 is an overall schematic configuration diagram of an MRI apparatus to which the present invention is applied. 1 is a schematic external shape view of a cryocontainer of an opposing superconducting magnet for an MRI apparatus according to a first embodiment of the present invention. It is a schematic sectional drawing of the cryo container shown in FIG. It is a figure which shows the modification of the 1st Embodiment of this invention. It is a figure which shows the other modification of the 1st Embodiment of this invention. It is a figure which shows the further another modification of the 1st Embodiment of this invention. It is a cryocontainer outline appearance shape figure of a counter type superconducting magnet for MRI devices which is a 2nd embodiment of the present invention. It is a top view of the concentric bottom part structure of the example shown in FIG. It is a top view of the modification of the concentric bottom part structure of the example shown in FIG. It is a figure which shows the example at the time of integrating a bottom part structure body with a cryo container in this invention. It is a figure which shows the result of having obtained the leakage magnetic field suppression effect by this invention by simulation. It is a figure which shows the result of having obtained the leakage magnetic field suppression effect by the simulation when not providing a bottom structure unlike the present invention. It is a different example from this invention, and is a figure which shows the example which installed the MRI apparatus of the perpendicular magnetic field system in the magnetic shield room. It is a magnetic field distribution map of the measurement space before installing the apparatus shown in FIG. 13 in a magnetic shield room. It is a magnetic field distribution figure of measurement space at the time of installing the device shown in Drawing 13 in a magnetic shield room.

Explanation of symbols

2a, 2b Cryocontainer 3a, 3b Superconducting coil 4a, 4b Gradient magnetic field coil 9 Measurement space 21, 22, 23 Auxiliary static magnetic field generating means 31, 32, 35 Bottom structure 33 Nonmagnetic material 34 Structural reinforcement member of nonmagnetic steel 41 Upper Reinforcement Structure 51 Connecting Tube 101 Static Magnetic Field Generation Unit 102 Gradient Magnetic Field Generation Unit 103 Transmission System 104 Reception System 105 Signal Processing System 106 Control Unit 107 Central Processing Unit 108 Operation Unit

Claims (8)

In a magnetic resonance imaging apparatus comprising a static magnetic field generating means for generating a uniform magnetic field in a measurement space, a gradient magnetic field generating means, a high frequency transmitting / receiving means, an image processing means, and a storage container for storing the static magnetic field generating means.
The lower bottom portion of the storage container includes a bottom structure having a plurality of concentric circular ferromagnets,
A magnetic resonance imaging apparatus comprising static magnetic field correction means for compensating for the asymmetric component of the static magnetic field.   2. The magnetic resonance imaging apparatus according to claim 1, wherein the static magnetic field correction means is stored in the storage container.   3. The magnetic resonance imaging apparatus according to claim 2, wherein the static magnetic field correction means is disposed on the measurement space side of the storage container.   2. The magnetic resonance imaging apparatus according to claim 1, wherein the static magnetic field correction means is a superconducting coil or a ferromagnetic material.   4. The magnetic resonance imaging apparatus according to claim 3, wherein the static magnetic field correction means is a conductor coil or a ferromagnetic material. 2. The magnetic resonance imaging apparatus according to claim 1, wherein the bottom structure is formed of a composite member made of a ferromagnetic material and a non-magnetic material. 2. The magnetic resonance imaging apparatus according to claim 1, wherein the plurality of circular ferromagnets of the bottom structure are disposed with a space therebetween, and a plurality of nonmagnetic steel materials are disposed at the space. Magnetic resonance imaging device.   2. The magnetic resonance imaging apparatus according to claim 1, further comprising temperature control means for making the temperature of the bottom structure constant.

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