JP2014039633A - Magnetic resonance imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low-noise magnetic resonance imaging apparatus 100.SOLUTION: The magnetic resonance apparatus comprises: a magnet 1 for generating a static magnetic field 6 on an imaging region 9; a gradient magnetic field coil 2 for generating a gradient magnetic field 10 with inclined intensity distribution on the imaging region 9; an exterior cover 13 for covering the magnet 1 and the gradient magnetic field coil 2; a first conductive member 16 provided on the exterior cover 13; and a second conductive member 15 that is provided on the magnet 1 or the exterior cover 13, electrically insulated from the first conductive member 16, and is relatively independently movable, with approximated to the first conductive member. The surface of the first conductive member 16 and the surface of the second conductive member 15 face in parallel with each other. A spacer may be partially provided or a spacer with elasticity lower than that of the exterior cover may be provided between the first conductive member 16 and the second conductive member 15 such that the first conductive member 16 and the second conductive member 15 vibrate mutually independently.

Description

  The present invention relates to a magnetic resonance imaging (hereinafter referred to as MRI; Magnetic Resonance Imaging) apparatus including a magnet for generating a static magnetic field and a gradient magnetic field coil.

  The MRI apparatus utilizes the nuclear magnetic resonance phenomenon that occurs when a high-frequency pulse is irradiated to a subject placed in a static magnetic field (imaging region) having a uniform direction and intensity, and the physical and chemical properties of the subject. Is a device for obtaining a cross-sectional image showing, particularly for medical use. The MRI apparatus mainly includes a magnet device that generates the static magnetic field in an imaging region in which a subject is inserted, and a magnetic field (gradient magnetic field) that has a spatially gradient intensity to give positional information to the imaging region. A gradient magnetic field coil to be generated in a pulse form, an RF coil that irradiates the subject with the high-frequency pulse, a receiving coil that receives a magnetic resonance signal from the subject, and an image obtained by processing the received magnetic resonance signal And a computer system for displaying.

  In order to improve the performance of the MRI apparatus, the magnetic field strength of the static magnetic field is increased. The higher the magnetic field strength of the static magnetic field, the clearer and various cross-sectional images can be obtained. For this reason, the MRI apparatus has been continuously developed with the aim of increasing the magnetic field strength of the static magnetic field. Further, in order to improve the performance of the MRI apparatus, the magnetic field strength of the gradient magnetic field is increased and the frequency of generation of the gradient magnetic field pulses is increased (high-speed driving). These contribute to shortening the imaging time and improving the image quality of the cross-sectional image, thereby realizing a high-speed imaging method that has been actively used in recent years. These were realized by improving the performance of the gradient coil drive power supply and enabling high-speed switching and energization with a large current.

  A pulsed current flows through the gradient coil in order to generate a pulsed gradient magnetic field. The gradient magnetic field coil is placed not in the imaging region but in a static magnetic field created by the magnet device. For this reason, time-varying electromagnetic force acts on the gradient magnetic field coil, and the gradient magnetic field coil vibrates. Moreover, the magnet apparatus which supports a gradient magnetic field coil also vibrates by this vibration. The vibration of the gradient coil and the magnet device is transmitted as noise to the subject and the operator of the MRI apparatus. This noise (vibration) tends to increase as the current of the gradient magnetic field coil and the magnetic field strength of the static magnetic field of the magnet device increase. For this reason, the noise increased when trying to improve the performance of the MRI apparatus.

  For this reason, techniques for reducing this noise have been proposed. For example, a technique for attaching a perforated eddy current screen to a magnet device has been proposed (see, for example, Patent Document 1). When the magnet device vibrates, the eddy current screen also vibrates, and eddy current flows through the eddy current screen. Noise is reduced by attenuating this eddy current with an eddy current screen. In addition, a technique for reducing noise by filling a space between a magnet device and an external decorative cover (exterior cover) with a solid foam layer and suppressing initial vibration of the magnet device has been proposed (for example, patents). Reference 2).

Special table 2007-529259 JP 2009-261940 A

  Since noise (vibration) propagates through the air and reaches the subject or the like, it is effective to reduce the noise by blocking the propagation by placing an exterior cover on the propagation path. Furthermore, in patent document 2, the solid foam layer divides air finely and prevents noise from propagating through the air. Thus, although the noise (vibration) is reduced by the exterior cover, the exterior cover itself vibrates due to the noise (vibration) propagated through the air or the solid foam layer. The vibration of the exterior cover becomes a new noise source. The exterior cover is designed to attenuate its own vibration, which makes it possible to reduce noise. However, if the ability of the exterior cover to attenuate this vibration (damping force) can be increased. Noise can be further reduced and is useful.

  Therefore, the problem to be solved by the present invention is to provide a low noise MRI (magnetic resonance imaging) apparatus.

In order to solve the above problems, the present invention provides:
A magnet that generates a static magnetic field in the imaging region;
A gradient coil for generating a gradient magnetic field having a gradient intensity distribution in the imaging region;
In a magnetic resonance imaging apparatus comprising an exterior cover that covers the magnet and the gradient coil,
A first conductive member provided on the exterior cover;
A second conductive member provided on the magnet or the exterior cover, electrically insulated from the first conductive member, and capable of moving relatively independently while staying close to the first conductive member; It is characterized by having.

  According to the present invention, a low noise MRI (magnetic resonance imaging) apparatus can be provided.

1 is a longitudinal sectional view of a magnetic resonance imaging (MRI) apparatus (horizontal magnetic field type) according to a first embodiment of the present invention. 1 is a schematic perspective view of an MRI apparatus according to a first embodiment of the present invention. It is a model diagram which shows the principle of the vibration reduction effect of a proximity | contact conductive member, (a) shows a mode that the 1st conductive member vibrates with a static magnetic field, (b) shows a 1st conductive member and 1st conductive member. 2 shows a state in which eddy current is generated in the two conductive members, and FIG. 3C shows a state in which electromagnetic force is acting on the first conductive member and the second conductive member. (A) is the adjacent electroconductive member of 1st Embodiment, and the partial expanded sectional view of the periphery of it, (b) is the modification 1, (c) is the modification 2. (A) is an adjacent electroconductive member in the MRI apparatus which concerns on the 2nd Embodiment of this invention, and its partial expanded sectional view of the periphery, (b) is the modification. (A) is an adjacent electroconductive member in the MRI apparatus which concerns on the 3rd Embodiment of this invention, and its partial expanded sectional view of the periphery, (b) is the modification. It is a partial expanded sectional view of a proximity conductive member and its circumference in an MRI apparatus concerning a 4th embodiment of the present invention. It is a longitudinal cross-sectional view of the MRI apparatus (vertical magnetic field type) which concerns on the 5th Embodiment of this invention. It is a typical perspective view of the MRI apparatus which concerns on the 5th Embodiment of this invention.

  Next, embodiments of the present invention will be described in detail with reference to the drawings as appropriate. In each figure, common portions are denoted by the same reference numerals, and redundant description is omitted.

(First embodiment)
FIG. 1 shows a longitudinal sectional view of a magnetic resonance imaging (MRI) apparatus (horizontal magnetic field type) 100 according to the first embodiment of the present invention, and FIG. 2 shows a schematic perspective view thereof. The MRI apparatus 100 has a static magnetic field magnet device (magnet) 1 that generates a static magnetic field 6 in an imaging region 9 in which a subject 7 is inserted, and a spatially gradient intensity in order to give positional information to the imaging region 9. Gradient magnetic field coil 2 for generating a pulsed magnetic field (gradient magnetic field) 10, an RF coil 12 for irradiating a subject 7 with a high frequency pulse, and a receiving coil for receiving a magnetic resonance signal from the subject 7 (illustrated) (Not shown), a computer system (not shown) that processes the received magnetic resonance signal and displays an image, a movable bed 25 that moves the subject 7 to the imaging region 9 while lying down, and the static magnetic field magnet apparatus 1 And an exterior cover 13 that covers the gradient magnetic field coil 2, and a proximity conductive member 11 provided in the exterior cover 13 and the static magnetic field magnet device 1. As a result, the MRI apparatus 100 uses the nuclear magnetic resonance phenomenon that occurs when the subject 7 placed in the static magnetic field 6 having a uniform direction and intensity in the imaging region 9 is irradiated with a high-frequency pulse. A cross-sectional image showing the physical and chemical properties of the examiner 7 can be obtained under low noise. Although not shown, the MRI apparatus 100 is provided with a power supply device for driving the static magnetic field magnet device 1, the gradient magnetic field coil 2, and the RF coil 12. These power supply devices are controlled by the computer system.

  The static magnetic field magnet device 1 forms a static magnetic field 6 having a uniform direction and intensity in the imaging region 9 in order to orient the spins of atoms constituting the living tissue of the subject 7. The static magnetic field magnet apparatus 1 has a cylindrical shape with a z-axis parallel to the horizontal direction as a central axis (see FIG. 2). The gradient coil 2 is provided on the imaging region 9 side of the static magnetic field magnet device 1. The gradient magnetic field coil 2 has a cylindrical shape having the same central axis as the static magnetic field magnet apparatus 1 (with the z axis as the central axis). The RF coil 12 is provided on the imaging region 9 side of the gradient magnetic field coil 2. The RF coil 12 has a cylindrical shape having the same central axis as the static magnetic field magnet apparatus 1 (with the z axis as the central axis).

  The static magnetic field magnet apparatus 1 includes a plurality of superconducting coils 3, a liquid helium container 8 that houses and cools the superconducting coils 3 together with a refrigerant, and a radiation shield 5 that covers the liquid helium container 8 and shields radiant heat radiated from the vacuum container 4. And the vacuum container 4 that houses the liquid helium container 8 and the radiation shield 5 in a vacuum environment and insulates them. The gradient coil 2 is attached to the vacuum vessel 4 via a support member 14. The superconducting coil 3 can be cooled to a cryogenic temperature by liquid helium and a refrigerator (not shown), and the cooling state can be maintained. In addition, it replaces with the superconducting coil 3, and the coil used at normal temperature may be used. In this case, the liquid helium container 8, the radiation shield 5, and the vacuum container 4 can be omitted.

  The plurality of superconducting coils 3 have a ring shape having the z axis as a common central axis. The plurality of superconducting coils 3 generate a static magnetic field 6 having a uniform direction and strength in the imaging region 9. The plurality of superconducting coils 3 generate the static magnetic field 6 in addition to the imaging region 9, particularly also in the exterior cover 13 and its periphery. The imaging region 9 is surrounded by a cylindrical static magnetic field magnet device 1. The static magnetic field magnet device 1 may have a magnetic material, and may have a magnetic material instead of the superconducting coil 3.

  In the static magnetic field magnet apparatus 1, the gradient magnetic field coil 2 is supported via a support member 14. The gradient magnetic field coil 2 generates a magnetic field (gradient magnetic field) 10 in which the magnetic flux density (magnetic field strength distribution) is inclined in an arbitrary direction spatially and fluctuates in a pulse shape in the imaging region 9 in time. Normally, the direction of the static magnetic field 6 in the imaging region 9 is taken as the z-axis, and the x-axis (horizontal direction) and the y-axis (vertical direction) are taken in two directions orthogonal to the z-axis. Gradient magnetic fields 10 that are independent from each other are generated in the three directions of the x-axis, y-axis, and z-axis. In FIG. 1, the left-right direction of the paper surface is the z axis, the vertical direction of the paper surface is the y axis, and the direction perpendicular to the paper surface is the x axis (not shown in FIG. 1). For example, the gradient magnetic field 10 shown in FIG. 1 is a gradient magnetic field in which the magnetic field strength is inclined in the y-axis direction and the magnetic field direction is in the z-axis direction. The gradient coil 2 is provided with a plurality of small pieces of magnetic material called shims (not shown). This shim is a mechanism for adjusting the magnetic field strength of the static magnetic field 6 to be uniform in the imaging region 9 including the influence of the magnetic field generated by the external device of the MRI apparatus 100.

  The static magnetic field magnet device 1, the gradient magnetic field coil 2, and the RF coil 12 are covered with an exterior cover 13 made of a nonconductive and nonmagnetic member such as FRP. The RF coil 12 may be installed on the exterior cover 13 or may also serve as a part of the exterior cover 13.

  A proximity conductive member (conductive member) 11 is installed in the vacuum container 4 which is an outer container constituting the static magnetic field magnet device 1 and the exterior cover 13. The proximity conductive member 11 is electrically insulated from the conductive member (first conductive member) 16 on the exterior cover 13 side provided on the exterior cover 13 and the conductive member 16, and is close to the conductive member 16. It has the electroconductive member (2nd electroconductive member) 15 by the side of the static magnetic field magnet apparatus 1 which can move relatively independently. The proximity conductive members 11 (15, 16) are provided on both end surfaces in the z-axis direction of the static magnetic field magnet device 1 and the vicinity thereof. The proximity conductive member 11 (15, 16) is provided on the cylindrical outer cylinder wall of the static magnetic field magnet apparatus 1 and its vicinity. If the static magnetic field magnet device 1 is installed on the floor and the floor side of the static magnetic field magnet device 1 does not vibrate, the proximity conductive member 11 (15, 16) is arranged on the floor side of the static magnetic field magnet device 1. It can be omitted. The surface of the conductive member (first conductive member) 16 and the surface of the conductive member (second conductive member) 15 are separated from each other and face each other in parallel. The conductive member (first conductive member) 16 and the conductive member (second conductive member) 15 are close to each other, but are not mechanically connected and are electrically insulated. It is in the state. That is, the conductive member 15 and the conductive member 16 are formed of, for example, an aluminum or copper foil, a plate, a net, or a coating film coated with a conductive paint, and mechanically do not transmit vibration to each other. The structure is such that electrical continuity does not occur. Such a structure is realized by holding the vacuum vessel 4 of the static magnetic field magnet device 1 and the outer cover 13 so as not to contact each other and independently of each other. Or it is also realizable by mutually connecting the vacuum vessel 4 and the exterior cover 13 of the static magnetic field magnet apparatus 1 via flexible structures, such as elastic members, such as rubber | gum. When the vacuum vessel 4 is made of a metal material, the vacuum vessel 4 also serves as the conductive member 15 and the conductive member 15 can be omitted. When the static magnetic field magnet device 1 does not have the conductive vacuum container 4 or when the vacuum container 4 is made of a material having a large specific resistance such as a stainless steel material, the specific resistance such as a copper material or an aluminum material is used. A foil, a plate, a net, or a coating film of a conductive paint is installed on the vacuum vessel 4 as the conductive member 15.

  As described above, the conductive members 16 and 15 are installed close to each other without being mechanically and electrically connected (proximity conductive member 11), so that the static magnetic field generated by the static magnetic field magnet apparatus 1 is obtained. 6, a great vibration suppressing effect is obtained. For this reason, the vibration of the gradient magnetic field coil 2 propagates through the support member 14 to the vacuum vessel 4 of the static magnetic field magnet apparatus 1 and further propagates from the vacuum vessel 4 through the air and the floor, and the exterior cover 13 vibrates. However, the exterior cover 13 to which the conductive member 16 is attached suppresses the generation of noise due to a large vibration suppressing effect. The radiated sound generated by the vibration of the gradient magnetic field coil 2 or the vacuum vessel 4 passes through the outer cover 13 and reaches the subject 7 or the operator of the MRI apparatus 100. 2 and the air around the vacuum vessel 4 vibrate, and when the outer cover 13 vibrates, noise transmitted through the outer cover 13 increases. Even in this case, the proximity conductive member 11 reduces the vibration of the exterior cover 13, so that the transmitted noise can be reduced.

  FIG. 3 is a schematic diagram for explaining the principle of the vibration reducing effect of the proximity conductive member 11. As shown in FIG. 3A, it can be considered that the conductive members 16 and 15 of the proximity conductive member 11 are placed in a static magnetic field 6 in which the direction of the magnetic field coincides with the direction of the arrow. The direction of the magnetic field of the static magnetic field 6 is substantially parallel to each of the opposing surfaces of the conductive members 16 and 15. When the conductive member 16 moves (moves) like the vibration 20 in the static magnetic field 6 along with the vibration of the exterior cover 13, an eddy current 21 as shown in FIG. 3B is applied to the conductive member 16. Occur. Note that the direction of the eddy current 21 is reversed according to the direction of vibration displacement instead of one direction. As shown in FIG. 3C, a Lorentz force (electromagnetic force) 23 is generated between the eddy current 21 and the static magnetic field 6, and the direction of the electromagnetic force 23 is opposite to the direction of displacement of the vibration. Become. Thereby, a vibration damping effect is obtained. Further, the eddy current 21 flows, so that the conductive member 16 generates Joule heat due to electric resistance. The kinetic energy of the vibration 20 of the conductive member 16 is converted into thermal energy via electric energy and consumed by radiating heat, thereby obtaining a vibration damping effect.

  As described above, the vibration of the conductive member 16 in the static magnetic field 6 can be reduced by the conductive member 16 alone. However, the vibration is reduced by the proximity of the conductive member 15. Can enhance the effect. Due to the induced electromotive force due to the eddy current 21, an eddy current 22 is generated in the conductive member 15. The direction of the eddy current 22 is opposite to that of the eddy current 21. Further, when the conductive members 16 and 15 are close to each other and the resistance value of the conductive member 15 is small, the eddy current 22 becomes almost the same size as the eddy current 21. At this time, since the eddy current 21 and the eddy current 22 are in the direction of canceling each other, the conductive member 16 has only a single component to cancel the magnetic field (magnetic flux) change received from the static magnetic field 6 by the vibration. An eddy current 21 larger than that in the case is generated. That is, since the apparent inductance of the conductive member 16 is reduced by the mutual inductance between the conductive members 16 and 15, the eddy current 21 increases. As a result, the electromagnetic force 23 and the Joule heat generation in the direction opposite to the vibration displacement generated by the eddy current 21 are increased, so that the effect of reducing the vibration of the conductive member 16 is also increased. In addition, the eddy current 22 flows in the opposite direction of the eddy current 21, whereby electromagnetic forces 23 and 24 in the opposite direction to the displacement of the distance between the conductive members 16 and 15 due to vibration are applied to the conductive members 16 and 15. The effect of reducing the vibration of the conductive member 16 is increased.

  In the MRI apparatus 100, the preferred position of the proximity conductive member 11 is in the static magnetic field 6 generated by the static magnetic field magnet apparatus 1, and a portion where the influence of the magnetic field (leakage magnetic field) from the gradient magnetic field coil 2 and the RF coil 12 is small. It is. As a specific example, as shown in FIG. 1 and FIG. 2, the front and rear surfaces in the z-axis direction of the static magnetic field magnet device 1 and the exterior cover 13 close to the front and rear surfaces. Other preferable installation positions are the outer peripheral side surface of the cylindrical shape of the static magnetic field magnet device 1 and the exterior cover 13 adjacent thereto.

  The conductive members 15 and 16 do not have to be the same size as long as the conductive members 15 and 16 have a sufficient area for generating an eddy current against vibration accompanied by noise. The vibration frequency particularly required to be reduced as noise is 500 [Hz] or more and 5 k [Hz] or less. The wavelength of sound when this noise propagates through air is 5 to 50 [cm] (the sound wavelength is 5 [cm] or more and 50 [cm] or less). In general, a plate-like material depends on the incident angle θ of sound to the plate surface, but when it vibrates at a wavelength λB that is 1 to 3 times the wavelength λ of the sound, the sound insulation effect decreases (coincidence effect, more precisely λB = for λ / sinθ). On the plate surface, the vibration amplitude includes the maximum range at a distance of 1 to 3 times the wavelength of the sound. Therefore, if both the widths of the conductive members 15 and 16 are larger than this, the eddy current associated with the vibration is sufficient. It can generate | occur | produce and can reduce the vibration of a board | plate material and can avoid the fall of the sound-insulation effect. That is, by configuring so that the minimum dimension in the in-plane direction of either one of the conductive members 15 and 16 is 50 [cm] or more, vibration having a vibration frequency of 500 [Hz] or more can be reduced. By configuring so that the minimum dimension in the in-plane direction of either one of the members 15 and 16 is 25 [cm] or more, vibrations with a vibration frequency of 1 k [Hz] or more can be reduced, and a decrease in the sound insulation effect can be prevented. Effective for noise reduction. Further, if the maximum dimension in the in-plane direction of the conductive member 15 or 16 is set to 300 [cm] so as to cover the entire outer periphery of the MRI apparatus 100 or the outer cover 13, vibration of the outer cover 13 can be suppressed and noise can be reduced. It is effective in reducing.

Further, the frequency of the conductive members 15 and 16 having a noise reduction effect varies depending on the plate thickness and conductivity. When the conductivity of the conductive member 15 or 16 is small or the plate thickness is thin, the noise reduction effect at a high frequency is large, and when both the conductivity of the conductive members 15 and 16 is large or the plate thickness is large, the low frequency. The noise reduction effect from can be expected. The minimum value at which the noise reduction effect of the plate thickness d [m] and the conductivity σ [Siemens / m] of the conductive member can be expected is also limited by the product σd, and noise of 500 [Hz] to 1 k [Hz] or more. When the reduction effect is expected, the product σd only needs to be 1000 [Siemens] or more after satisfying the minimum dimension. For example, when a copper material having a conductivity σ = 5 × 10 ⁷ [Siemens / m] is used for the conductive members 15 and 16, the plate thickness d is 0.02 [mm] or more, and the conductivity σ = 2 × When a stainless steel material of 10 ⁶ [Siemens] is used, if the plate thickness d is 0.5 [mm] or more, the vibration of the exterior cover 13 is suppressed, which is effective in reducing noise. On the other hand, since the maximum value of the plate thickness d of the conductive member 15 or 16 is about 2 [mm] in the case of a copper material, if the product σd is 100000 [Siemens] or less, vibration of the exterior cover 13 can be reduced. Effective for noise reduction.

  FIG. 4A shows a partially enlarged cross-sectional view of the proximity conductive member 11 of the first embodiment and its periphery. The proximity conductive member 11 is installed on the opposing surfaces of the vacuum vessel 4 and the outer cover 13. The conductive member 16 of the proximity conductive member 11 is provided on the exterior cover 13. The conductive member 15 of the proximity conductive member 11 is provided in the vacuum container 4. In order to avoid mechanically and electrically connecting (contacting) the conductive member 15 and the conductive member 16, a space 18 is provided between the conductive member 15 and the conductive member 16. . Alternatively, a thin insulating film may be provided on at least one surface of the conductive members 15 and 16 by coating or the like to ensure insulation between the conductive members 15 and 16. Moreover, although the space | interval between the electroconductive members 15 and 16 becomes wide, you may install the electroconductive member 16 in the surface on the opposite side of the static magnetic field magnet apparatus 1 of the exterior cover 13. FIG. When the vacuum vessel 4 is formed of an insulating member, the conductive member 15 may be installed on the surface on the opposite side of the outer cover 13 of the static magnetic field magnet device 1. When the vacuum vessel 4 is formed of a conductive member, the conductive member 15 may be omitted.

  FIG. 4B shows a partially enlarged cross-sectional view of the proximity conductive member 11 of the first modification of the first embodiment and the periphery thereof. A spacer (support structure) 26 is partially provided between the conductive members 15 and 16 so that the conductive members 15 and 16 of the adjacent conductive member 11 can vibrate (move) independently of each other. Yes. The outer cover 13 (conductive member 16) is supported by the partial spacer 26 with respect to the vacuum vessel 4 (conductive member 15). Space 18 between the conductive members 15 and 16 is maintained by the spacer (support structure) 26. The conductive members 15 and 16 are not mechanically connected. However, if the outer cover 13 is a flexible material such as a thin FRP material, the outer cover 13 (conductive The member 16) can vibrate substantially independently of the vacuum vessel 4 (conductive member 15), and the vibration reducing effect by the proximity conductive member 11 can be obtained. In the vibration of the exterior cover 13 and the conductive member 16, the portion where the spacer 26 is provided becomes a vibration node, and an antinode of vibration occurs between the adjacent spacers 26. Moreover, although the space | interval between the electroconductive members 15 and 16 becomes wide, you may install the electroconductive member 16 in the surface on the opposite side of the static magnetic field magnet apparatus 1 of the exterior cover 13. FIG. When the vacuum vessel 4 is formed of an insulating member, the conductive member 15 may be installed on the surface on the opposite side of the outer cover 13 of the static magnetic field magnet device 1. When the vacuum vessel 4 is formed of a conductive member, the conductive member 15 may be omitted.

  FIG. 4C shows a partially enlarged cross-sectional view of the proximity conductive member 11 of the second modification of the first embodiment and its periphery. A spacer 27 having lower elasticity than the outer cover 13 is provided between the conductive members 15 and 16 so that the conductive members 15 and 16 of the close conductive member 11 vibrate (move) independently of each other. A space 18 between the conductive members 15 and 16 is filled with a flexible elastic member (low elastic spacer) 27 such as rubber. The outer cover 13 (conductive member 16) is supported by the elastic member 27 with respect to the vacuum vessel 4 (conductive member 15). A space 18 between the conductive members 15 and 16 is maintained by the elastic member 27. The conductive members 15 and 16 are not mechanically connected. However, if the outer cover 13 is a flexible material such as a thin FRP material, the outer cover 13 (conductive The conductive member 16) can vibrate substantially independently of the vacuum vessel 4 (conductive member 15), and the vibration reducing effect of the proximity conductive member 11 can be obtained. In addition, the elastic members 27 may be used for the partial spacers 26 of the first modification by combining the first and second modifications of the first embodiment.

(Second Embodiment)
FIG. 5A shows a partially enlarged cross-sectional view of the proximity conductive member 11 and its periphery in the MRI apparatus according to the second embodiment of the present invention. The second embodiment is different from Modification 2 of the first embodiment in that both the conductive members 15 and 16 of the proximity conductive member 11 are provided on the exterior cover 13. . The exterior cover 13 is divided into two layers, an outer layer 13a and an inner layer 13b, and has a structure in which a flexible elastic member 27 such as rubber is sandwiched therebetween. The conductive member 16 is provided outside the outer layer 13 a of the exterior cover 13. The conductive member 15 is provided on the vacuum container 4 side of the inner layer 13 b of the exterior cover 13. In the second embodiment as well, as in the second modification of the first embodiment, there is an exterior between the conductive members 16 and 15 so that the conductive members 16 and 15 vibrate (move) independently of each other. An elastic member (low elastic spacer) 27 having a lower elasticity than the cover 13 is provided. When the exterior cover 13 vibrates, due to the elastic member 27, the outer layer 13a (conductive member 16) and the inner layer 13b (conductive member 15) of the exterior cover 13 vibrate independently, and the conductive members 15 and 16 The distance changes while staying close to each other, a large eddy current is generated in the conductive members 15 and 16, and the vibration of the outer cover 13 is attenuated. In FIG. 5A, the conductive member 16 is installed outside the outer layer 13a of the exterior cover 13. However, the conductive member 16 is not limited to this and may be installed on the elastic member 27 side. Further, although the conductive member 15 is installed on the vacuum container 4 side of the inner layer 13b of the outer cover 13, it is not limited to this and may be installed on the elastic member 27 side. That is, the conductive members 15 and 16 may be installed so as to face each other with the elastic member 27 interposed therebetween. Further, the vibration damping effect can be further enhanced by further increasing the three layers of the elastic member 27, the exterior cover 13, and the conductive member 16 (or 15) to have two or more conductive members 16 (or 15). Can do.

  FIG. 5B shows a partially enlarged cross-sectional view of the proximity conductive member 11 and its periphery in an MRI apparatus according to a modification of the second embodiment of the present invention. The modification of the second embodiment is different from the second embodiment in that the conductive members 16 and 15 are electrically insulated and are relatively independent while being close to the conductive members 16 and 15. Thus, a conductive member (third conductive member) 17 (proximity conductive member 11) that can vibrate (move) is provided in the vacuum container 4. According to this, the vibration reduction effect of the exterior cover 13 can be further increased. An elastic member 27 may be provided between the conductive member 17 (vacuum container 4) and the conductive member 15 (exterior cover 13). Further, when the vacuum vessel 4 is a conductive member, the conductive member 17 can be omitted.

(Third embodiment)
FIG. 6A shows a partially enlarged cross-sectional view of the proximity conductive member 11 and its periphery in the MRI apparatus according to the third embodiment of the present invention. The third embodiment is different from Modification 1 of the first embodiment in that both the conductive members 15 and 16 of the proximity conductive member 11 are provided on the exterior cover 13. . The exterior cover 13 is divided into two layers, an outer layer 13a and an inner layer 13b, and a spacer (partial spacer) 26 is partially sandwiched therebetween. The conductive member 16 is provided outside the outer layer 13 a of the exterior cover 13. The conductive member 15 is provided on the vacuum container 4 side of the inner layer 13 b of the exterior cover 13. Also in the third embodiment, as in the first modification of the first embodiment, a spacer is provided between the conductive members 16 and 15 so that the conductive members 16 and 15 vibrate (move) independently of each other. 26 is partially provided. When the outer cover 13 vibrates, the outer layer 13a (conductive member 16) and the inner layer 13b (conductive member 15) of the outer cover 13 vibrate independently due to the spacer 26, and the conductive members 15 and 16 are close to each other. As the distance changes, a large eddy current is generated in the conductive members 15 and 16, and the vibration of the outer cover 13 is attenuated. In FIG. 6A, the conductive member 16 is installed outside the outer layer 13 a of the exterior cover 13. However, the conductive member 16 is not limited to this and may be installed on the spacer 26 side. Further, although the conductive member 15 is installed on the vacuum container 4 side of the inner layer 13b of the outer cover 13, it is not limited to this and may be installed on the spacer 26 side. That is, the conductive members 15 and 16 may be installed so as to face each other with the spacer 26 interposed therebetween. Further, the vibration damping effect can be further enhanced by further increasing the three layers of the spacer 26, the exterior cover 13, and the conductive member 16 (or 15) to have two or more conductive members 16 (or 15). it can.

  FIG. 6B shows a partially enlarged cross-sectional view of the proximity conductive member 11 and its periphery in an MRI apparatus according to a modification of the third embodiment of the present invention. The modification of the third embodiment is different from the third embodiment in that the conductive members 16 and 15 are electrically insulated and are relatively independent while being close to the conductive members 16 and 15. Thus, a conductive member 17 (proximity conductive member 11) that can vibrate (move) is provided in the vacuum container 4. According to this, the vibration reduction effect of the exterior cover 13 can be further increased. A spacer 26 may be provided between the conductive member 17 (vacuum container 4) and the conductive member 15 (exterior cover 13). Further, when the vacuum vessel 4 is a conductive member, the conductive member 17 can be omitted.

(Fourth embodiment)
FIG. 7 shows a partially enlarged cross-sectional view of the proximity conductive member 11 and its periphery in an MRI apparatus according to the fourth embodiment of the present invention. The fourth embodiment is different from the first to third embodiments in that at least one of the conductive members 15 and 16 facing each other (the other surface) has an uneven shape. It is a point. According to this, it becomes possible to make the opposing area of the mutually opposing surfaces of the conductive members 15 and 16 larger than the opposing area of the exterior cover 13 and the vacuum vessel 4, which occurs when the exterior cover 13 vibrates. Eddy current can be increased. And the vibration reduction effect of the exterior cover 13 can further be increased. The uneven shape is not necessarily required for both of the conductive members 15 and 16, and either one may be used. The fourth embodiment can be applied not only to the first embodiment but also to the second and third embodiments. Moreover, it is preferable that the uneven | corrugated shaped pitch of the electroconductive members 15 and 16 corresponds mutually. According to this, the uneven surface of the conductive members 15 and 16 can be brought close to each other regardless of the height of the uneven surface.

(Fifth embodiment)
FIG. 8 shows a longitudinal sectional view of an MRI apparatus (vertical magnetic field type) 100 according to the fifth embodiment of the present invention, and FIG. 9 shows a schematic perspective view thereof. The fifth embodiment is different from the first to fourth embodiments in that the direction of the static magnetic field 6 in the imaging region 9 is changed from the horizontal direction to the vertical direction. Along with this, the z-axis direction is perpendicular to the direction of the static magnetic field 6. The y-axis direction and the x-axis direction are horizontal. A static magnetic field magnet device 1 that generates a static magnetic field 6 includes a plurality of superconducting coils 3 that are paired in an upper and lower ring shape and a pair of upper and lower magnetic poles that are formed in a pair of magnetic bodies 28 that are paired in a disk shape. The pair of upper and lower magnetic poles are supported by a support column 29 (see FIG. 9) and are spaced apart from each other. As shown in FIG. 5, the static magnetic field magnet apparatus 1 may be configured by both the superconducting coil 3 and the magnetic body 28, or may be configured by either one.

  A pair of disk-shaped upper and lower gradient magnetic field coils 2 is provided on the imaging region 9 side of each of the upper and lower magnetic poles of the static magnetic field magnet device 1. The gradient coil 2 is supported from the magnetic body 28 of the static magnetic field magnet apparatus 1 via the support member 14. Similar to the first embodiment, the gradient coil 2 can generate independent gradient magnetic fields in the three directions of the x-axis, the y-axis, and the z-axis. The gradient magnetic field 10 shown in FIG. 8 shows an example of a gradient magnetic field in which the magnetic field direction is the z-axis direction and the magnetic field strength distribution is inclined in the y-axis direction. A pair of disk-shaped upper and lower RF coils 12 are provided on the imaging region 9 side of each of the upper and lower pair of gradient magnetic field coils 2. The pair of upper and lower exterior covers 13 cover the pair of upper and lower magnetic poles of the static magnetic field magnet device 1 and the pair of upper and lower gradient magnetic field coils 2. RF coils 12 are installed on the surfaces of the pair of upper and lower exterior covers 13 on the imaging area 9 side.

  Also in the fifth embodiment, the proximity conductive member 11 is in the static magnetic field 6 by the static magnetic field magnet apparatus 1 and is less affected by the magnetic field (leakage magnetic field) generated by the gradient magnetic field coil 2 and the RF coil 12. It is desirable to place it at the place. The proximity conductive member 11 is provided on the side surface of the disk-shaped magnetic body 28 of the static magnetic field magnet apparatus 1, the surface opposite to the imaging region 9, and the surface of the exterior cover 13 facing them. Further, the proximity conductive member 11 is provided on the side surface of the annular vacuum vessel 4 of the static magnetic field magnet apparatus 1, the surface on the imaging region 9 side, and the surface of the exterior cover 13 facing them. When the vacuum vessel 4 and the magnetic body 28 are formed of conductive members, the conductive member 15 of the proximity conductive member 11 provided on the vacuum vessel 4 and the magnetic body 28 can be omitted. Also in the fifth embodiment, the spacer 26 of the second embodiment, the elastic member 27 of the third embodiment, and the uneven shape of the fourth embodiment can be applied. In the fifth embodiment, the conductive members 15 and 16 facing and adjacent to the conductive member 11 are not the same size and shape as each other, but the size, conductivity and plate thickness of the conductive members 15 and 16 are different. If the conditions described in the first embodiment are satisfied, a sufficient effect for noise reduction can be expected.

  In the first to fifth embodiments described above, the superconducting coil 3 is used in the static magnetic field magnet apparatus 1, but the present invention is not limited thereto, and a normal conducting coil or a permanent magnet (magnetic body 28) may be used.

1 Static magnetic field magnet device (magnet)
2 Gradient magnetic field coil 3 Superconducting coil 4 Vacuum container (outer container constituting the magnet)
5 Radiation shield 6 Static magnetic field (direction)
7 Subject 8 Liquid helium container 9 Imaging area 10 Gradient magnetic field (variable magnetic field)
11 (Proximity) Conductive Member 12 RF Coil 13 Exterior Cover 14 Support Member 15 Static Magnetic Field Magnet Device Side Conductive Member (Second Conductive Member)
16 Conductive member on the exterior cover side (first conductive member)
17 Third conductive member 18 Space 20 Arrow representing vibration 21, 22 Eddy current 23, 24 Electromagnetic force 25 Mobile bed 26 Support structure ((partial) spacer)
27 Elastic member ((low elasticity) spacer)
28 Magnetic material 29 Support column 100 Magnetic resonance imaging apparatus

Claims (12)

A magnet that generates a static magnetic field in the imaging region;
A gradient coil for generating a gradient magnetic field having a gradient intensity distribution in the imaging region;
In a magnetic resonance imaging apparatus comprising an exterior cover that covers the magnet and the gradient coil,
A first conductive member provided on the exterior cover;
A second conductive member provided on the magnet or the exterior cover, electrically insulated from the first conductive member, and capable of moving relatively independently while staying close to the first conductive member; A magnetic resonance imaging apparatus comprising:   2. The magnetic resonance imaging apparatus according to claim 1, wherein a surface of the first conductive member and a surface of the second conductive member are parallel and face each other.   3. The magnetic resonance imaging apparatus according to claim 1, wherein the second conductive member is provided on the magnet and is a part of an outer container constituting the magnet. 4.   The magnetic resonance imaging apparatus according to claim 1, wherein the second conductive member is provided on an outer surface constituting the magnet.   Spacers are partially provided between the first conductive member and the second conductive member so that the first conductive member and the second conductive member vibrate independently of each other. The magnetic resonance imaging apparatus according to claim 1, wherein:   A spacer having lower elasticity than the exterior cover is provided between the first conductive member and the second conductive member so that the first conductive member and the second conductive member vibrate independently of each other. The magnetic resonance imaging apparatus according to claim 1, wherein the magnetic resonance imaging apparatus is a magnetic resonance imaging apparatus. The magnetic resonance imaging apparatus according to claim 6, wherein the outer cover has a two-layer structure, and a conductive member is disposed in each of the two-layer structures.   8. The magnetic resonance according to claim 1, wherein at least one of the opposing surfaces of the first conductive member and the second conductive member has an uneven shape. Imaging device.   9. The magnetism according to claim 1, wherein each of the first conductive member and the second conductive member is one of a plate, a foil, a net, and a coating film. Resonance imaging device.   10. The minimum dimension in the in-plane direction of at least one of the first conductive member and the second conductive member is 25 cm or more and 300 cm or less. 2. A magnetic resonance imaging apparatus according to claim 1.   The product of the conductivity and the thickness of at least one of the first conductive member and the second conductive member is 1000 Siemens or more and 100,000 Siemens or less. The magnetic resonance imaging apparatus according to claim 10. A magnet that generates a static magnetic field in the imaging region;
A gradient coil that generates a varying magnetic field with a gradient intensity distribution in the imaging region;
In a magnetic resonance imaging apparatus comprising an exterior cover that covers the magnet and the gradient coil,
A first conductive member provided on the exterior cover;
A second conductive member provided on the exterior cover, electrically insulated from the first conductive member, and capable of moving relatively independently while remaining close to the first conductive member;
Third conductivity provided on the magnet, wherein the first conductive member and the second conductive member are electrically insulated, and can move relatively independently while being close to the second conductive member. And a magnetic resonance imaging apparatus.

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