JP2002263080A - Magnetic resonance imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an MRI apparatus employing an open type magnet structure suitable from an aspect of safety control by restricting a magnetic field intensity range exerting effect on peripheral machinery in a magnet arranged chamber and to use machinery high in operability employing a color CRT of high resolving power in the periphery of the magnet arranged chamber. SOLUTION: A superconductive coil 13 and a shield 14 are incorporated in a static magnetic field generating magnet 2. The magnetic flux at the center of a magnetic field has, for example, high magnetic field intensity of 0.5 millitesla and the shape and weight of the shield 14 is set so as to minimize a distribution range wherein a leak magnetic field is 0.5 millitesla. Further, the leak magnetic field to the outside of a shield room 15 is set to 0.1 millitesla or less by combining a silicon steel plate 27 with a part of the wall surface of the shield room 15. By this constitution, a color CRT display 12 displaying the operation data or examination result of the MRI apparatus can be arranged in close vicinity to the shield room 15.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic resonance imaging (hereinafter, referred to as "MRI") apparatus, and more particularly, to an open-type magnet which does not give a feeling of oppression to a subject and minimizes the leakage magnetic field. The present invention relates to an MRI apparatus that can be stored in a limited space.

[0002]

2. Description of the Related Art In recent years, interventional techniques for performing treatment during an examination have been used as an MRI apparatus, and a static magnetic field generating magnet has been conventionally used as a magnet for reducing the feeling of pressure on a subject. An open-type MRI apparatus in which a pair of permanent magnets, normal conducting magnets, and the like are arranged above and below a subject instead of an elongated cylindrical solenoid coil is developed and widely used.

On the other hand, in order to obtain MRI images in real time, high-speed imaging methods such as echo planar imaging (EPI) and fluoroscopy have been performed, and some magnets capable of realizing high magnetic field strength can be used as a static magnetic field generating magnet. It has become required.

As a technology for improving the strength of the generated magnetic field, a magnet having a superconducting coil has been developed (for example, Japanese Patent Application Laid-Open Nos. 10-179546 and 11-1555831). Gazette, JP-A-11-197132, U.S. Pat. No. 5,883,558).

[0005] An open magnet incorporating the above-described superconducting coil can achieve a magnetic field strength of 0.7 Tesla to 1.0 Tesla, which is 2.5 to 5 times larger than that of a conventional permanent magnet or a magnet using a normal conducting coil. A double value is obtained, and a sufficient image can be secured by real-time measurement.

[0006]

However, the achievement of a high static magnetic field strength in the examination space in which the subject is disposed has a problem that the magnetic flux density existing around the magnet increases. This problem is particularly remarkable in an MRI apparatus using an open-structured magnet. The magnetic flux density existing outside this magnet is called the stray magnetic field strength and is defined as the distance from the center of the magnet to a position 0.5 mT, and this distance is usually
It is desired that the size be equal to or smaller than the size of the room where the MRI apparatus (magnet apparatus) is installed. It is pointed out that a space of 0.5 millitesla or more has an adverse effect on life support devices such as a cardiac pacemaker and electronic devices, and must be managed for safety. In the above prior art, a passive shield system that reduces the leakage magnetic field by configuring a magnetic circuit that combines a yoke made of a ferromagnetic material, and a cancel coil that generates a magnetic field in the opposite direction to the coil that generates the magnetic field (main coil) An active shield method that reduces the leakage magnetic field by combining the above has been proposed.

If it is attempted to limit the leakage magnetic field to the size of the installation room only by the passive shield method, the magnetic field intensity is reduced to 0.
The weight of the shield required for a magnet from 7 Tesla to 1.0 Tesla is
Over 40 tons, the height of the magnet device combined with the shield is
Beyond 2.9 meters, it was difficult to carry them into general buildings.

On the other hand, in the active shield system, a magnetic flux is generated by the main coil and a reverse magnetic flux is generated by the cancel coil in a space where the subject is placed. Therefore, a magnetic flux that is the difference between the main coil and the cancel coil is generated in the space where the subject is placed, and this magnetic flux density has an intensity of 0.7 Tesla to 1.0 Tesla, and the leakage magnetic field outside the magnet is reduced by the magnet. When trying to fit in the installation room, a large gap between the main coil and the cancel coil must be secured, and the size of the magnet including the cancel coil exceeds 3 meters, so it can also be installed in a general building It is impossible to realize a magnet of a large size.

In the above prior art, a superconducting coil is disposed above and / or below a space in which a subject is placed, and a practical size of an open structure with a static magnetic field strength of 0.7 Tesla or more that generates a magnetic flux in a vertical direction. For a magnet with a stray magnetic field of 0.5 millitesla, the range is 3.8 meters vertically and 4.8 horizontally.
Meters, which is virtually impossible to keep within the space of the installation room. That is, 3.8 meters from the center of the magnet in the vertical direction corresponds to a position above the floor on the floor. Furthermore, there is a demand for not only reducing the leaked magnetic field of 0.5 millitesla space for safety management, but also avoiding the influence on devices such as a color CRT around the installation room of the magnet device or on the floor. A color CRT is easily affected by an external magnetic field, for example, causing a screen distortion or a color shift even at about 0.1 mT. That is, in order to satisfy this demand, it is necessary to reduce the leakage magnetic field to 0.1 millitesla or less at the position just beside the installation room of the magnet device or on the desk on the floor (about 3 meters from the center of the magnet).

SUMMARY OF THE INVENTION The present invention has been made in view of the above-described circumstances, and has as its object to suppress the leakage magnetic field of an open-type high-field MRI apparatus in the space of an installation room and improve safety. Also MRI
MRI equipment that uses color CRT to improve operability of equipment
Open MR that can be used near the equipment installed
The purpose is to provide I equipment.

[0011]

To achieve the above object, an MRI apparatus according to the present invention comprises a static magnetic field generating means for generating a constant magnetic field strength in a space where a subject is placed, and a gradient magnetic field generating means for generating a magnetic field strength gradient. Means, means for generating a high-frequency magnetic field,
Means for detecting a nuclear magnetic resonance signal generated from the subject;
In an MRI apparatus comprising: means for shielding these means from an external electric field; and means for processing the nuclear magnetic resonance signal and displaying the result, the static magnetic field generating means may be located above and / or below the space. A superconducting coil arranged to generate a magnetic field in the vertical direction, and a cancel coil that generates a ferromagnetic material such as iron or a magnetic flux in the opposite direction that forms a magnetic circuit with respect to the magnetic flux generated by the superconducting coil, The electric field shielding means has a magnetic shielding effect by a ferromagnetic material such as a silicon steel plate or a cancel coil.

[0012] By combining a superconducting coil and a ferromagnetic material or a cancel coil with the static magnetic field generating means, the leakage magnetic field is greatly reduced, and at least a part of the wall surface of the magnet device installation room is formed by the ferromagnetic material or the cancel coil. By incorporating the magnetic shielding effect, the leakage magnetic field space required for safety management can be limited to the installation room of the MRI apparatus (magnet apparatus), and the safety of the MRI apparatus can be improved. For example, the size of the magnet installation room
It can suppress the leakage magnetic field of 0.5 millitesla within 1.4 to 4 meters, which is a half value of 2.8 meters (2.8 meters high). This makes it possible to use a color CRT display as a peripheral device of the MRI apparatus, thereby improving the operability and visibility of the MRI apparatus and the peripheral apparatuses.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is an overall configuration diagram of an MRI apparatus to which the present invention is applied. This MRI apparatus has a static magnetic field generating magnet 2 arranged so as to sandwich a space where a subject 1 is placed, and a gradient magnetic field coil 3 arranged inside the static magnetic field generating magnet 2, respectively.
Further, the high-frequency coil 5 arranged inside the object 1 and the subject 1
And a detection coil 7 for detecting an NMR signal generated from.

The magnetic field generation system and the signal detection system are installed in a shield room 15 having an electromagnetic wave shielding means in order to prevent external electromagnetic waves from entering the detection coil 7 and becoming a noise component of the inspection data. . The shield chamber 15 has two functions of preventing electromagnetic waves from entering from outside as noise and partitioning a safety management area having a leakage magnetic field of 0.5 mT or more.

The MRI apparatus further includes a power supply system for each coil.
(Gradient magnetic field power supply 4, high-frequency power amplifier 6), high-frequency amplifier / detector 8, sequencer 9 for controlling the operation timing of each coil, and computer 10 for controlling the apparatus and processing and imaging NMR signals, and these Is the shield room 15
It is installed outside.

A silicon steel plate is incorporated in a part of the ceiling and the wall of the shield room 15 to minimize the leakage magnetic field to the outside of the shield room 15.

In the illustrated embodiment, the static magnetic field generating magnet 2 has a pair of superconducting coils 13 opposed to each other and a shield 14 constituting a magnetic circuit for the magnetic flux generated by the superconducting coils 13. Therefore, the magnetic field strength is
At 0.7 Tesla, the direction of the magnetic flux is from the floor to the ceiling as shown by arrow 17, and the magnetic field uniformity is about 3 ppm or less in a 40 cm diameter spherical space where the subject 1 is placed. Has been adjusted. This magnetic field adjustment is realized by, for example, a passive shimming method in which a plurality of small magnetic pieces are attached to the surface of the superconducting coil 13.

The static magnetic field generating magnet 2 having the pair of superconducting coils 13 is supported by one or a plurality of columns so that an open space is provided therebetween, and the operator is placed in the space. It is configured so that an interventional procedure can be performed on the sample from both sides.

The gradient magnetic field coil 3 has x,
3 wound to change the magnetic flux density in the three axes of y and z
A set of gradient magnetic field coils 3 each having a gradient magnetic field power supply 4
To form gradient magnetic field generation means. Sequencer
The gradient magnetic field power supply 4 is driven according to the control signal from 9 to change the value of the current flowing through the gradient magnetic field coil 3, thereby applying gradient magnetic fields Gx, Gy, and Gz consisting of three axes to the subject 1. . This gradient magnetic field is used to grasp the spatial distribution of NMR signals obtained from the inspection site of the subject 1. The connection between the gradient magnetic field coil 3 and the gradient magnetic field power supply 4 is made via a filter circuit 16 connected to a shield conductor of a shield room 15 so that an electromagnetic wave from the outside is not mixed.

The high-frequency coil 5 is connected to a high-frequency power amplifier 6 for supplying a high-frequency current to the high-frequency coil 5 to resonately excite an atomic nucleus (generally, a hydrogen nucleus is used) at the inspection site of the subject 1. To generate a high-frequency magnetic field. The high-frequency power amplifier 6 is controlled by a control signal of the sequencer 9. The high-frequency coil 5 and the high-frequency power amplifier 6 are also connected using a coaxial cable so that electromagnetic noise from the outside is not mixed in, and the external conductor is connected to the shield conductor in the shield room 15.

The detection coil 7 is connected to a high-frequency amplification / detection circuit 8 and constitutes means for detecting an NMR signal. The high-frequency amplification / detection circuit 8 amplifies and detects the NMR signal detected by the detection coil 7, and converts the NMR signal into a digital signal that can be processed by the computer 10. The operation timing of the high-frequency amplification / detection circuit 8 is also controlled by the sequencer 9. A coaxial cable is used for the connection between the detection coil 7 and the high-frequency amplification / detection circuit 8 so that the electromagnetic waves are not mixed therein.

Computer 10 is converted to digital
Using the NMR signal, calculations such as image reconstruction and spectrum calculation are performed, and the operation of each unit of the MRI apparatus is controlled via the sequencer 9 at a predetermined timing. An arithmetic processing system is composed of the computer 10, the storage device 12 for storing data, and the display device 11 for displaying processed data.

FIGS. 2 and 3 are views for explaining the arrangement of such an MRI apparatus, and FIG. 3 is a sectional view taken along line AA ′ of FIG. The above-described magnetic field generation system and signal detection system are integrated into a gantry 21 and housed in a shield room 15 which is a magnet device installation room together with a patient table 22 on which the subject 1 is placed.

On the other hand, a gradient magnetic field power supply 4, a high-frequency power amplifier 6, and a high-frequency amplification / detection circuit 8, which are driving power supplies for the respective magnetic field generating means, are housed in two casings 23, 24, and are provided in a shield room 1.
It is installed near 5. The connection between the gradient magnetic field power supply 4, the high-frequency power amplifier 6, the high-frequency amplification / detection circuit 8 and each of the magnetic field generating coils and the signal receiving detection coil 7 in the gantry 21 via the filter circuit 16 as described above. Connected (with cable not shown).

A computer 1 for controlling each drive power supply system and signal receiving system and processing the received NMR signal.
0 and the like are incorporated in the console 25. A part of the wall of the shield room 15 is provided with a see-through window 26 on which a wire mesh for shielding electromagnetic waves is attached, and an operator (not shown in the figure) allows the inside of the gantry 21 to pass through the see-through window 26 through the console 25. You can see through. The display 12 is installed on a console 25, and the operator can move the gantry 21 and the display 12 without moving.
You can see both.

Here, the distribution range of the leakage magnetic field of 0.5 millitesla of the static magnetic field generating magnet 2 provided with the shield 14 shown in FIG. 1 is 4 in the direction (+ y axis) of the patient table 22 on the horizontal plane (xy plane) from the center of the magnet. Meters, 3.1 meters in the opposite direction (-y axis), 3.5 meters in the horizontal direction (x axis), and 3.5 meters in the vertical ceiling direction (z axis). In this distribution shape, the angle at which the line connecting the center of the static magnetic field and the center of each of the two columns intersects is smaller than 180 degrees (or larger), so that both sides from the head of the subject placed at the center of the static magnetic field Depending on the structure of the static magnetic field generating magnet 2 having a wide open space.

On the other hand, the size of the shield room 15 in which the static magnetic field generating magnet 2 is installed has a width (x-axis) of 5.4 meters and a length (y
The axis (axis) is 6.8 meters and the height (z axis) is 2.8 meters. The distribution range of the leakage magnetic field of 0.5 mT is above the shield room 15
Will be exceeded. Shield room 15 of the present embodiment
Has a 4-mm thick silicon steel plate 27 built into the ceiling wall 151, the back wall 152, and both side walls 153, so that the leakage magnetic field is 0.
The distribution range of 5 millitesla is greatly reduced and can be confined within shielded room 15 as shown by line 28. Since the magnetic flux density cannot be visually observed, the ability to define a distribution range of the magnetic field intensity of 0.5 mT at the boundary surface of the structure as in this embodiment is an extremely effective means for safety management. Furthermore, as shown by the line 29, the range of the magnetic field distribution of 0.1 milliTesla, which has an adverse effect on devices such as a color CRT, can be significantly reduced, and leakage occurs around the shield room 15 or at the level of the desk level 30 on the floor. The device can be operated without being affected by the magnetic field.

Next, the second embodiment of the static magnetic field generating magnet according to the present invention will be described.
An example will be described. FIG. 4 is a diagram showing a side surface of the static magnetic field generating magnet, and FIG. 5 is a diagram showing a BB ′ cross section of the magnet and a shield room in which it is installed. As shown in the figure, a plurality (two in the illustrated embodiment) of superconducting coils 41 and 42 are each a cryo-tube in which a vessel containing a refrigerant such as liquid helium and a vacuum vessel for shutting off the temperature from the outside are integrated. It is incorporated in the stud 43 and is arranged vertically above and below a space 44 in which the subject is placed. The upper cryostat 43 and the lower cryostat 43 are supported by two columns 45, 45.

As in the embodiment shown in FIG. 1, the angle at which the line connecting the center of the magnetic field and the line connecting the centers of the two columns intersects is smaller than (or larger than) 180 degrees, whereby the lower cryostat 43
A widely open space is provided from the head of the subject placed thereon to both sides.

In each of the cryostats 43, a main coil disposed at a position close to the space 44 among a plurality of coils
In 41, a current flows so as to generate a magnetic flux density A in the direction of arrow 46. A current flows through the cancel coil 42 located far from the space 44 so as to generate the magnetic flux density B in the direction opposite to the arrow 34.

Therefore, the magnetic field strength at each position is the magnetic flux density
The difference between A and the magnetic flux density B is (AB) .In the space 44, the direction of the magnetic flux, which is the difference, is the direction of the arrow 46 and the target magnetic field strength, for example, 0.7 Tesla. Outside the magnetic field magnet, for example, at a position 47 that is 2.5 meters away from the center of the space 44, the magnetic field strength is adjusted to be 0.5 millitesla or less.

This adjustment depends on the number of turns of the cancel coil 43,
This is performed by adjusting the diameter and the current flowing through the coil. Generally, a coil having a smaller number of turns and a larger diameter than the main coil is used as the cancel coil, and this coil is arranged outside the main coil as shown in the figure. Can be realized by: The specific numerical value is appropriately selected according to the magnetic flux generated by the main coil and the magnetic field intensity at a target position.
There is a limit to bringing the position closer to the magnet.

Since the magnetic field strength at a position 48 (the position of the ceiling of the shield room 49 where the static magnetic field generating magnet is installed) at a distance of 1.4 meters in the vertical direction from the space 44 becomes 0.5 millitesla or more,
By incorporating a coil 50 for canceling magnetic flux only in a part of the shield room 49 and applying a constant current from the power supply 51, a magnetic field distribution of 0.5 mT or more does not exist outside the shield room 49.

By incorporating the coil in a part of the shield room as described above, the leakage magnetic field generated outside the static magnetic field generating magnet can be substantially reduced. Further, in this embodiment, a nonmagnetic material such as aluminum can be selected as the support structure of the cryostat 43 instead of the iron yoke structure constituting the magnetic circuit, so that the weight of the static magnetic field generating magnet 2 can be reduced. Can be planned.

In the first embodiment, the superconducting coil 13
Shield 14 is placed above and below
In the second embodiment, the configuration in which the cancel coil 42 is disposed above and below the superconducting coil 41 and the configuration in which the cancel coil 50 is disposed above the shield room 49 has been described. However, the present invention is not limited to this combination, and the same effect can be obtained with the combination of the shield 14 and the cancel coil 50 or the cancel coil 42 and the silicon steel plate 27. Further, although a silicon steel plate is used for the magnetic field shielding means incorporated in the shield room, an amorphous magnetic sheet or the like may be used. Also,
The position and number of the cancel coils 42 and 50 may be appropriately determined according to the strength of the leakage magnetic field and the like.

Although the preferred embodiment of the present invention has been described above, the present invention is not limited to the above-described method and apparatus, and various modifications are possible. Is also possible.

[0038]

According to the present invention, an open type MRI apparatus using a magnet that generates a high magnetic field is provided with a leakage magnetic field reducing means for reducing the leakage magnetic field and a leakage magnetic field shielding means, so that the safety by the leakage magnetic field is provided. Since the control area can be accommodated in the magnet installation room, a device suitable for safety management can be provided. Further, since a stray magnetic field can be effectively reduced, devices such as a color CRT can be installed in the periphery.

Further, in order to achieve the reduction of the leakage magnetic field by both the leakage magnetic field reducing means and the leakage magnetic field shielding means, a ferromagnetic material and a cancel coil can be selectively used depending on the purpose. For example, when the weight is reduced as much as possible, a cancel coil can be used for the leakage magnetic field reduction means.

[Brief description of the drawings]

FIG. 1 is a diagram showing an overall configuration of an MRI apparatus to which the present invention is applied.

FIG. 2 is a diagram showing an arrangement of an MRI apparatus to which the present invention is applied.

FIG. 3 is a sectional view of FIG. 2;

FIG. 4 is a diagram showing a static magnetic field generating magnet and a shield room according to another embodiment of the present invention.

FIG. 5 is a sectional view of FIG. 4;

[Explanation of symbols]

 1 Subject 2 Static magnetic field generating magnet 3 Gradient coil 4 Gradient magnetic field power supply 5 High frequency coil 6 High frequency power amplifier 7 Detection coil 8 … High-frequency amplification / detection circuit 9 …… Sequencer 10 …… Computer 11 …… Storage device 12 ……… Display device 15 ……… Shield room

Claims (5)

[Claims] 1. A static magnetic field generating means for generating a constant magnetic field strength in a space where a subject is placed, a gradient magnetic field generating means for generating a magnetic field strength gradient, a high frequency magnetic field generating means, and a nuclear magnetic field generated from the subject A means for detecting a resonance signal; an electric field shielding means for enclosing the static magnetic field generating means, the gradient magnetic field generating means, the high frequency generating means, and the means for detecting a nuclear magnetic resonance signal to shield an external electric field; Means for processing a magnetic resonance signal and displaying the result thereof, wherein the static magnetic field generating means is disposed above and / or below the space and generates a vertical magnetic field. And a leakage magnetic field reduction unit that reduces a magnetic field leaking out of the space, wherein the electric field shielding unit shields the magnetic field by substantially reducing the leakage magnetic field. Magnetic resonance imaging apparatus comprising the stage. 2. The magnetic resonance imaging apparatus according to claim 1, wherein said leakage magnetic field reducing means is constituted by said superconducting coil and a first ferromagnetic material constituting a magnetic circuit. 3. The leakage magnetic field reducing means comprises a superconducting coil and a first cancel coil which generates a magnetic flux having a polarity opposite to that of the magnetic flux generated by the superconducting coil. 2. The magnetic resonance imaging apparatus according to 1. 4. The magnetic resonance imaging apparatus according to claim 2, wherein said magnetic field shielding means is made of a second ferromagnetic material. 5. The magnetic field shielding means according to claim 2, wherein the second cancel coil generates a magnetic flux having a polarity opposite to the magnetic flux of the leakage magnetic field.
The magnetic resonance imaging apparatus according to claim 1.