JP3897958B2 - Magnetic resonance imaging system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a magnetic resonance imaging (hereinafter referred to as MRI) apparatus, and in particular, in an MRI apparatus that employs an open magnet that does not give a sense of pressure to a subject, noise generated with an examination is reduced. The present invention relates to an MRI apparatus that provides a comfortable examination environment for a subject.
[0002]
[Prior art]
MRI apparatuses that obtain a tomographic image of the human body using the nuclear magnetic resonance (NMR) phenomenon are widely used in medical institutions. Conventionally, this MRI apparatus has used a long and slender cylindrical solenoid coil that can efficiently generate a uniform magnetic field space. However, this MRI apparatus eliminates the feeling of pressure on the subject, and also helps claustrophobic patients and children. In recent years, an open MRI system that employs a magnet with a closed front surface and a flat-plate gradient magnetic field coil to provide a closed part on the side surface so as not to give a sense of fear, or to widen the carry-in part of the subject It is popular.
[0003]
Since an open MRI device using such an open-structured magnet has a gap that allows interventional procedures to perform treatment during the examination, interventional MRI examination (hereinafter referred to as MR interventional) Is being promoted by advanced medical institutions. In this MR interventional, the operator needs to be able to confirm the MRI image being treated in real time.
In order to realize image quality improvement and enhancement of functions in real-time imaging (high-speed imaging), the MRI apparatus has a gradient magnetic field coil that operates at high speed and its driving power supply, a high-frequency coil that detects NMR signals with high sensitivity, and A magnet with a higher static magnetic field strength is needed.
With regard to the demand for high static magnetic field strength, development of magnets incorporating superconducting coils is being promoted from conventional permanent magnets and magnets using normal conducting coils (for example, JP-A-10-179546, JP 11-155831, JP-A-11-197132, etc.).
[0004]
On the other hand, with regard to the gradient magnetic field coil, development of a large-capacity switching power supply and the like has progressed, and high speed driving is possible. However, there is a problem that an electromagnetic force acts when a current for driving the gradient magnetic field is applied in a pulse shape, and noise is generated due to mechanical strain and vibration.
This problem became even more serious with the opening of the MRI system. That is, an open MRI device is more susceptible to vibration than an MRI device with a long and narrow examination space using a solenoid coil, and the vibration of the magnet reduces the stability of the generated static magnetic field and causes image deterioration. become. Further, as described above, in real-time high-speed imaging, the gradient magnetic field coil is switched at a high speed, so that noise and vibration increase. For example, in an examination using an imaging technique that switches a gradient magnetic field at high speed, such as EPI imaging, the gradient magnetic field operation sound sometimes exceeded 100 dBA.
[0005]
Conventionally, in order to solve the problems of vibration and noise of the gradient coil, a large number of lead balls in the structure of the gradient coil, which increases the rigidity of the gradient coil bobbin, increases the weight of the gradient coil itself, and limits the vibration amplitude. , Which generates vibrations with opposite phase to the noise of the gradient magnetic field, which converts the vibration energy of the gradient magnetic field coil into frictional heat between lead balls,
Each of these methods had an effect of reducing noise by about 10 dB. However, there were problems in that the weight of the gradient coil itself and the structure and control method were complicated for the amount of noise reduction.
[0006]
[Problems to be solved by the invention]
On the other hand, an MRI apparatus described in Japanese Patent Laid-Open No. 10-118043 has been proposed as a technique for further enhancing the noise reduction effect in an MRI apparatus using a solenoid coil. In this MRI apparatus, the gradient magnetic field coil is covered with an equipment capable of being hermetically sealed, and the gradient magnetic field coil is not attached to the magnet but is installed independently on the magnet installation floor. This method can reduce both the vibration of air propagation caused by the gradient coil drive and the vibration that is fixedly propagated to the magnet, and as a result, the noise of the entire device due to the vibration of the gradient coil is reduced by 20 to 30 dB. It is supposed to be possible.
[0007]
However, in this method, since the gradient coil needs to be installed independently on the magnet installation floor, this installation occupies the floor surface around the magnet and the gap, which is inconvenient for the MR interventional procedure. There is a fear. In addition, when this method is applied to an open MRI apparatus using a high magnetic field open type magnet, it is not easy to install the upper gradient magnetic field coil directly on the magnet installation floor among the upper and lower gradient magnetic field coils. Absent. For example, a plurality of support columns are required to stably support the upper gradient coil, but such support columns will be located on the side of the apparatus, which may hinder MR interventional procedures.
[0008]
The present invention has been made in view of the above viewpoint, and an object of the present invention is to secure a gap around the magnet and reduce vibrations accompanying the gradient magnetic field drive and noises associated therewith in an open MRI apparatus. Another object of the present invention is to provide an open MRI apparatus that can provide a comfortable examination environment for a subject and that can perform a high-speed imaging technique and MR interventional technique using the same with low noise.
[0009]
[Means for Solving the Problems]
The MRI apparatus of the present invention that achieves the above object comprises a pair of static magnetic field generating means, a pair of gradient magnetic field generating means for giving a magnetic field strength gradient to the magnetic field generated by the static magnetic field generating means, and the pair of static magnetic field generating means In the MRI apparatus provided with a static magnetic field generating means supporting portion that supports the magnetic field generating means, each of the pair of gradient magnetic field generating means is not in contact with the static magnetic field generating means and is fixed to the static magnetic field generating means support portion. It is characterized by being.
[0010]
In the MRI apparatus of the present invention, the static magnetic field generation means further includes a gap penetrating in the same direction as the magnetic field direction, and the gradient magnetic field generation means is fixed to the static magnetic field generation means support portion via the gap. It is characterized by. By directly fixing the gradient magnetic field coil to the static magnetic field generating means supporting portion, vibration of the coil itself can be suppressed and noise caused by the vibration can be reduced.
In particular, when the static magnetic field generating means supporting portion is an iron yoke , the yoke portion generally has a much larger mass than the static magnetic field magnet, so that the vibration of the gradient magnetic field coil is not transmitted from the yoke to the static magnetic field magnet. Therefore, it is possible to effectively suppress the fixed propagation of vibration, and to prevent the influence of vibration on the static magnetic field magnet, that is, the fluctuation of the magnetic field and the accompanying image quality deterioration. Furthermore, since the yoke is disposed on both sides of the static magnetic field magnet, the gradient magnetic field coil can be fixed without providing a special space on the magnet installation floor or the side of the apparatus. This allows MR interventional procedures to be performed without being hindered by the structure of the device.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
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 includes a pair of static magnetic field generating magnets 2 arranged so as to sandwich a space in which the subject 1 is placed, a gradient magnetic field coil 3 arranged inside each of the static magnetic field generating magnets 2, and further inside A high-frequency coil 5 arranged and a detection coil 7 for detecting an NMR signal generated from the subject 1 are provided. The gradient coil 3 and the high-frequency coil 5 have a pair of upper and lower plate-like structures so as not to disturb the open shape.
[0012]
The MRI apparatus further includes a sequencer 9 that controls the operation timing of each coil, a computer 10 that controls the apparatus and processes and images NMR signals, and a subject 1 in the central space of the static magnetic field generating magnet 2. And a table 14 to be installed.
[0013]
In the illustrated embodiment, the static magnetic field generating magnet 2 is composed of a superconducting magnet divided into upper and lower parts. A pair of superconducting magnets arranged vertically opposite to each other generates a uniform static magnetic field around the subject 1 in a direction perpendicular to the body axis. The magnetic field strength is 1.0 Tesla, for example, and the direction of the magnetic flux is from the floor to the ceiling as shown by the arrow 15, and the magnetic field uniformity is about 5 ppm or less in the spherical space where the subject 1 is disposed. Has been adjusted. This magnetic field uniformity adjustment uses a passive shimming method in which a plurality of magnetic material pieces (not shown in the figure) are attached to the surface of a superconducting magnet.
Further, an iron yoke (yoke part) 16 is provided so as to surround the upper and lower superconducting magnets. The iron yoke 16 constitutes a magnetic circuit together with the superconducting magnet, and reduces the magnetic flux density leaking outside the magnet.
[0014]
The gradient magnetic field coil 3 is composed of three sets of coils wound so as to change the magnetic flux density in three axial directions of x, y, and z orthogonal to each other, each connected to the gradient magnetic field power source 4, and the gradient magnetic field generating means Constitute. The gradient magnetic field power supply 4 is driven in accordance with the control signal from the sequencer 9 to change the value of the current flowing through the gradient coil 3, so that the gradient magnetic fields Gx, Gy, Gz consisting of three axes are applied to the subject 1. Yes. This gradient magnetic field is used to grasp the spatial distribution of the NMR signal obtained from the examination site of the subject 1.
[0015]
The gradient magnetic field coil 3 is obtained by integrating the x, y, and z coils in a flat plate shape in each of the upper and lower sides, and is directly fixed to the iron yoke 16. The structure of the gradient magnetic field coil 3 and the mounting structure to the iron yoke will be described in detail later.
[0016]
The high-frequency coil 5 is connected to the high-frequency coil 5 and a high-frequency power amplifier 6 for flowing a high-frequency current, and a high-frequency magnetic field for resonantly exciting the nucleus (usually a hydrogen nucleus) of the examination site of the subject 1 Is generated. The high frequency power amplifier 6 is also controlled by the control signal of the sequencer 9.
[0017]
The detection coil 7 is connected to the receiver 8 and constitutes means for detecting the NMR signal. The receiver 8 amplifies and detects the NMR signal detected by the detection coil 7 and converts it into a digital signal that can be processed by the computer 10. The operation timing of the receiver 8 is also controlled by the sequencer 9.
[0018]
The computer 10 performs operations such as image reconstruction and spectrum calculation using the NMR signal converted into a digital quantity, and controls the operation of each unit of the MRI apparatus via the sequencer 9 at a predetermined timing. A computer 10, a storage device 11 for storing data, a display device 12 for displaying processed data, and a console 13 for performing operation input constitute an arithmetic processing system.
[0019]
FIG. 2 shows a detailed structure of the static magnetic field generating magnet 2 and the gradient magnetic field coil 3 of the open MRI apparatus shown in FIG. 1 as the first embodiment of the present invention.
[0020]
First, the structure of the static magnetic field magnet 2 will be described in detail. In the illustrated embodiment, the static magnetic field generating magnet 2 is a superconducting magnet system, and a pair of upper and lower static magnetic field generating magnets 2 are fixed to an iron yoke 26 across a magnetic field space in which the subject 1 is disposed. Both the upper and lower static magnetic field generating magnets are a cryonut 21 having a donut shape, a liquid helium tank 24 disposed in the cryo 21 via a heat shield plate 22, and a superconducting coil 23 housed in the liquid helium tank 24. And.
[0021]
The inside of the cryo 21 is a vacuum, and the low temperature of liquid helium is maintained. In addition, the liquid helium tank 24 and the heat shield plate 22 are supported by wires (not shown) having excellent heat insulation characteristics in order to reduce the evaporation amount of liquid helium and prevent heat from entering from the outside.
[0022]
Although only a single heat shield plate 22 is shown in the figure, a plurality of heat shield plates 22 are usually arranged. Also, in the figure, a single superconducting coil 23 is shown both on the top and bottom, but in order to improve the uniformity of the generated magnetic field or to reduce the leakage magnetic field to the outside, a plurality of superconducting coils of different sizes can be incorporated. There is also.
[0023]
Further, as shown in FIG. 3, the upper and lower cryos 21 are connected by two connecting pipes 31 so that the amount of liquid helium in each helium tank 24 becomes equal. Note that a cryocooler 32 is incorporated in the upper cryo 21 for the purpose of reducing the consumption of liquid helium.
[0024]
A DC current of about 400 amperes flows through the superconducting coil 23, and a magnetic field strength of 1.0 Tesla is generated in the central space. In order to prevent such high magnetic field magnetic flux from leaking to the outside and minimize the high magnetic field space, an iron yoke 26 constituting a magnetic circuit is incorporated in the outer periphery of the upper and lower cryos 21. It is.
[0025]
FIG. 4 shows a state in which the iron yoke 26 is incorporated in the static magnetic field generating magnet 2 shown in FIG. As shown in the figure, the iron yoke 26 is composed of an upper plate 41, a lower plate 42, and left and right columns 43, 44 so as to constitute an efficient magnetic circuit for the magnetic flux generated by the superconducting coil 23. The left and right pillars 43 and 44 are attached to be shifted rearward so that the front space of the static magnetic field generating magnet 2 is widened.
[0026]
In general, it is desirable that the leakage magnetic field be within the inspection room (usually 5 × 8 meters) in which the static magnetic field generating magnet 2 is installed. Therefore, the iron yoke 26 constituting the magnetic circuit has a saturation magnetic flux that is an eigenvalue of iron. The product of the density and its cross-sectional area must be greater than or equal to the magnetic flux generated by the superconducting coil 23. When the yoke is made of iron, its weight reaches about 35 tons.
[0027]
Next, a pair of flat type gradient magnetic field coils 3 fixed to the iron yoke 26 will be described.
[0028]
FIG. 5 shows a pattern of a flat plate-type x-axis gradient magnetic field coil. A white portion in the drawing is a conductor portion, and a black line portion is an insulating portion obtained by etching the conductor portion. The points 51 and 52 of this pattern are connected by an electric wire 53. As a result, the terminal 54 to the terminal 55 exhibit a one-stroke pattern as a whole.
[0029]
When a predetermined voltage is applied between the terminals 54 and 55 of such an x-axis gradient magnetic field coil so that a current in the direction indicated by the arrow 56 flows, a magnetic flux is generated from the back of the paper to the front in the spiral pattern on the right half. To do. In the spiral pattern on the left half, a magnetic flux is generated from the front to the back of the page. As a result, a magnetic field gradient in the x direction can be applied to the static magnetic field in the direction orthogonal to the paper surface (z-axis direction).
[0030]
The configuration of the y-axis gradient magnetic field coil is exactly the same as that of the x-axis gradient magnetic field coil and is not shown in the figure, but is arranged so that its orientation is at an angle of 90 degrees with respect to the x-axis gradient magnetic field coil. . Therefore, a magnetic field gradient in the y direction can be applied to the static magnetic field by applying a predetermined voltage to the terminal of the y-axis gradient magnetic field.
[0031]
The z-axis gradient magnetic field coil has a single spiral pattern as shown in FIG. Two coils with such a pattern are placed facing each other across the inspection space, and a predetermined voltage is applied between the terminals 61 and 62 so that currents flowing in opposite directions flow in the z-axis direction (perpendicular to the paper surface). A magnetic flux having an intensity gradient is generated.
[0032]
The upper and lower gradient magnetic field coils 3 shown in FIGS. 1 and 2 are obtained by integrating xyz gradient magnetic field coils each having such a pattern with an insulating material such as an epoxy-based adhesive overlapped. The gradient magnetic field coil 3 having such a configuration is fixed to the iron yoke via the coil support rod 27 and the screw 28 via the central gap of the doughnut-shaped static magnetic field generating magnet 2. The support rod 27 is designed so that its outer diameter is smaller than the inner diameter of the air gap so that it does not contact the cryo 21 (inner wall of the air gap), and it is not so strong that it has sufficient strength against the stress applied to the gradient coil 3. It is made of a magnetic metal material (for example, stainless steel or aluminum).
[0033]
In the MRI examination, a pulsed current is applied to these gradient magnetic field coils in a static magnetic field space in which a magnetic field as described above, for example, a magnetic flux having a strength of 1.0 Tesla is generated in the z-axis direction. As a result, a complicated stress is applied to the gradient coil 3. Such stress generated in the gradient coil 3 is directly applied to the iron yoke 26 through the support rod 27 and is not directly transmitted to the upper and lower cryos 21. Further, as described above, the weight of the iron yoke 26 is about 35 tons, and the stress energy of the gradient coil 3 is absorbed, so that it is possible to prevent vibration from being transmitted to the upper and lower cryos 21 due to solid propagation.
[0034]
Furthermore, since the mechanism (support rod 27) for supporting the gradient magnetic field coil 3 can be arranged using the inside of the upper and lower cryo 21, such a mechanism occupies the space around the static magnetic field generating magnet 2. It can be used effectively by MR interventionists.
[0035]
Next, a second embodiment of the present invention will be described.
FIG. 7 is a diagram showing the open MRI apparatus of the second embodiment, and the configuration other than the mounting structure of the static magnetic field generating magnet 2 and the gradient magnetic field coil 3 is the same as the MRI apparatus of FIG. Omitted.
[0036]
Also in the second embodiment, the static magnetic field generating magnet 2 is a superconducting magnet system, and its structure is substantially the same as that of the first embodiment, and a pair of upper and lower cryos 71 and a heat shield plate 72 housed in the cryo 71. And a liquid helium tank 74 in which the superconducting coil 73 is housed. Here, a plurality of heat shield plates 72 may be provided, or a plurality of superconducting coils 73 may be incorporated. In addition, an iron yoke 26 constituting a magnetic circuit is incorporated in the outer peripheral portion of the upper and lower cryos 71.
[0037]
However, in this embodiment, the cryo 71 is not a donut shape but a cylindrical shape, and the pair of gradient magnetic field coils 3 are fixed to the iron yoke 26 via a support ring 75 located outside the cylindrical upper and lower cryo 71. ing.
[0038]
In the case of this embodiment, since the outer peripheral portion of the gradient coil 3 is firmly fixed to the iron yoke 26 by the support ring 75, further rigidity can be provided. Thereby, the reduction effect of the vibration noise by solid propagation is also improved. Here, the material of the support ring 75 can be a non-magnetic metal material (for example, stainless steel or aluminum) as in the first embodiment, but the material of the support ring 75 is a ferromagnetic material. By doing so, a magnetic shielding effect can be obtained even with respect to the leakage magnetic flux from the side surface of the superconducting coil 74. Therefore, the static magnetic field generating magnet 2 can be configured more compactly than in the first embodiment. This provides a more suitable operator space for MR interventional use.
[0039]
FIG. 8 is a diagram showing an open MRI apparatus according to the third embodiment of the present invention. In this embodiment, the gradient magnetic field coil 3 is attached by combining the first embodiment and the second embodiment.
That is, the static magnetic field generating magnet 2 has a donut shape as in the first embodiment, and the gradient magnetic field coil 3 is fixed to the iron yoke 26 by the support rod 82 and the screw 84 using the gap at the center of the donut. At the same time, the periphery thereof is directly fixed to the iron yoke 26 with a support ring 83 and a screw 84.
[0040]
In the case of this embodiment, the outer peripheral portion of the gradient magnetic field coil 3 is firmly fixed to the iron yoke 26 via the support rod 82 and the support ring 83, so that further rigidity can be provided. Further, since the central portion of the gradient magnetic field coil 3 is fixed to the iron yoke 26 by the support rod 82, the vibration amplitude of the gradient magnetic field coil 3 is suppressed, and the noise accompanying the vibration of the gradient magnetic field coil 3 can be further reduced. Also here, by forming the support ring 83 of a ferromagnetic material, a magnetic shielding effect can be obtained, and the static magnetic field generating magnet 2 can be configured more compactly.
In the present embodiment, the gradient magnetic field coil 3 is covered with a sound absorbing mat 85. By using such a sound absorbing mat 85 in combination, it is possible to cope with a higher frequency vibration mode of the gradient magnetic field coil 3, and a high noise attenuation effect can be obtained.
[0041]
As described above, in the MRI apparatus of the present invention, not only the third embodiment but also other vibration noise suppression techniques known as conventional techniques can be used in combination. It can be effectively suppressed.
In the above embodiment, the case where the static magnetic field generating magnet is a superconducting magnet will be described. In that case, the high effect of the present invention can be expected, but the present invention is not limited to the superconducting magnet, and a permanent magnet or a normal conducting magnet is used. The same applies to the open MRI apparatus used.
[0042]
【The invention's effect】
As described above, by attaching the gradient magnetic field coil to the yoke, it is possible to dampen the vibration of the solid transmission accompanying the gradient magnetic field coil driving. Furthermore, noise due to vibration of air propagation can be attenuated by attaching a sound reduction cover to the gradient magnetic field coil. As a result, even when a high-speed imaging method is performed with a high magnetic field open MRI apparatus, it is possible to secure a space around the magnet and reduce vibrations caused by the gradient magnetic field drive and noise caused thereby. As a result, it is possible to provide an open MRI apparatus that enables a test environment comfortable for the subject and MR interventional procedures.
[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 a main part of the MRI apparatus according to the first embodiment of the present invention.
3 is a perspective view showing a cryo part of a superconducting magnet of the MRI apparatus of FIG. 2. FIG.
4 is a perspective view showing the entire superconducting magnet of the MRI apparatus of FIG. 2;
FIG. 5 is a pattern diagram of an x-axis gradient magnetic field coil.
FIG. 6 is a pattern diagram of a z-axis gradient magnetic field coil.
FIG. 7 is a diagram showing a main part of an MRI apparatus according to a second embodiment of the present invention.
FIG. 8 is a diagram showing a main part of an MRI apparatus according to a third embodiment of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Subject 2 ... Static magnetic field generating magnet 3 ... Gradient magnetic field coil 4 ... Gradient magnetic field power supply 5 ... High frequency coil 6 ... High frequency power amplifier 7 ... Detection coil 8 ... Receiver 9 ... Sequencer
10 …… Computer
21 …… Cryo
26 …… iron yoke
27 …… Supporting rod
75 …… Support ring
85 …… Sound absorption mat

Claims (8)

Each of a pair of static magnetic field generating means arranged opposite to each other with a gap space in between has a static magnetic field generation source, forms a static magnetic field in the gap space, and supports the pair of static magnetic field generation means A generating means supporting means; a pair of gradient magnetic field generating means that are disposed opposite to the inside of the static magnetic field generating means with the gap space interposed therebetween to give a magnetic field strength gradient to the static magnetic field; and the pair of gradient magnetic field generating means A magnetic resonance imaging apparatus comprising: support means for supporting each of
Each of the pair of gradient magnetic field generating means is formed in a flat plate shape having a surface perpendicular to the direction of the static magnetic field,
The magnetic resonance imaging apparatus characterized in that the supporting means supports the gradient magnetic field generating means substantially perpendicularly to the surface thereof, and is fixed to the static magnetic field generating means supporting means in a non-contact manner with respect to the static magnetic field generating means. The magnetic resonance imaging apparatus according to claim 1.
The support means secures a gap between the support means and the gradient magnetic field generation means and the static magnetic field generation means, so that the vibration of the gradient magnetic field generation means is not directly propagated to the static magnetic field generation means. A magnetic resonance imaging apparatus, wherein the gradient magnetic field generating means is fixed to the static magnetic field generating means supporting means . The magnetic resonance imaging apparatus according to claim 1.
The static magnetic field generating means includes a gap penetrating in the same direction as the magnetic field direction,
The support means has a support structure including a support member disposed in the gap,
It said gradient magnetic field generating means, at its central portion, magnetic resonance imaging apparatus characterized by being fixed to the static magnetic field generating means support means via said supporting lifting structure. The magnetic resonance imaging apparatus according to claim 1.
The support means includes a support member that surrounds the static magnetic field generation means in a non-contact manner,
Said gradient magnetic field generating means, a magnetic resonance imaging apparatus, characterized in that the at periphery is fixed to the static magnetic field generating means support means through said supporting support member. The magnetic resonance imaging apparatus according to claim 3.
The support means includes a support member that surrounds the static magnetic field generation means in a non-contact manner,
The magnetic resonance imaging apparatus according to claim 1, wherein the gradient magnetic field generating means is further fixed to the static magnetic field generating means supporting means via the support member at a peripheral portion thereof . The magnetic resonance imaging apparatus according to claim 3, the magnetic resonance imaging apparatus characterized by vibration damping member is arranged between the peripheral portion and the static magnetic field generating means of said gradient magnetic field generating means. The magnetic resonance imaging apparatus according to claim 3, wherein the gradient magnetic field generating means is a magnetic resonance imaging apparatus characterized by being covered by sound-absorbing mat. The magnetic resonance imaging apparatus according to any one of claims 1 to 7 ,
The magnetic resonance imaging apparatus according to claim 1, wherein the static magnetic field generating means supporting means includes an iron yoke that surrounds the pair of static magnetic field generating means and forms a magnetic circuit.

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