JP5891063B2 - Magnetic resonance imaging system - Google Patents

Description

  The present invention relates to a magnetic resonance imaging apparatus.

A magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus) uses a nuclear magnetic resonance phenomenon that occurs when an object placed in a uniform static magnetic field is irradiated with a high-frequency pulse to physically and chemically apply to the object. It is an apparatus for obtaining a cross-sectional image showing properties, and is particularly used for medical purposes.
The MRI apparatus mainly generates a magnetic device that generates a uniform static magnetic field in an imaging region into which a subject is inserted, and a magnetic field having a spatially gradient intensity in order to give position information to the imaging region. A gradient coil, an RF coil that irradiates a subject with a high-frequency pulse, a receiving coil that receives a magnetic resonance signal from the subject, and a computer system that processes the received signal and displays an image.

As a means for improving the performance of the MRI apparatus, there is an improvement in the strength of the static magnetic field generated by the magnet apparatus. As the static magnetic field strength is higher, clearer images and various cross-sectional images can be obtained. Therefore, the MRI apparatus is continuously developed with a higher static magnetic field strength.
Other means for improving the performance include improving the gradient magnetic field strength and driving the gradient magnetic field pulse at high speed. These contribute to shortening of imaging time and improvement of image quality, and are frequently used in high-speed imaging methods that have been actively used in recent years. In particular, improvement in the performance of the drive power source for the gradient magnetic field coil has enabled high-speed switching and energization with a large current.

A pulsed current flows through the gradient coil. For this reason, an eddy current is generated in the metal container portion of the magnet device by the pulsed magnetic field, and the magnetic field due to the eddy current affects the image. Further, the magnet device vibrates by generating an electromagnetic force with an eddy current and a static magnetic field, and this vibration causes disturbance of the magnetic field, which adversely affects the image.
Furthermore, when a superconducting coil is used in the magnet device, when an eddy current is generated in the metal container of the cryostat, heat is generated, so that the amount of refrigerant consumed increases.
Therefore, in the gradient magnetic field coil that conducts a large current at a high speed in recent years, the main coil that generates the gradient magnetic field in the imaging region and the shield coil that prevents the pulsed magnetic field from leaking to unnecessary parts other than the imaging region In many cases, an active shield type structure is adopted.

By the way, in order to suppress the generation of eddy current by the shield coil and prevent the eddy current from being adversely affected, it is necessary to manufacture the main coil and the shield coil as intended by the design. In practice, the gradient coil is made by cutting a conductive material such as a metal plate, bending it into a coil winding shape, laminating these coil windings in multiple layers, and curing with resin as an insulating material. Has a structure. For this reason, it is slightly different from the intended position in the design. When the difference in position is very small, the magnetic field leaking to the unnecessary part is small, and the eddy current in the metal container part does not affect the image, but if the position difference is large, the influence of the image due to the eddy current cannot be ignored. .
Further, since the current flowing through the gradient magnetic field coil cannot normally be changed in the coil, the magnetic field leaking to unnecessary portions cannot be completely shielded even by the shield coil.

  For this reason, shielding with a good conductor is considered as means for reducing the magnetic field leaking from the gradient magnetic field coil to unnecessary portions. As this means, as disclosed in US Pat. No. 6,501,275 B1 (Patent Document 1), there is one in which a conductive conductor having a highly conductive cylinder shape is mechanically rigidly connected to a gradient coil. As disclosed in US Pat. No. 5,278,502 (Patent Document 2), as a means for reducing eddy current, there is one in which eddy current is not generated in a metal container of a magnet device.

US Pat. No. 6,501,275 B1 US Published Patent No. 5278502

By the way, in the MRI apparatus, there is a request to obtain a clear image at high speed. For this purpose, it is required to generate a gradient magnetic field having a magnetic flux density as large as possible in the gradient magnetic field coil at a high speed, and a large current is high speed. Energization is performed with a pulse waveform that changes to. However, when a large gradient magnetic field is generated by a large current, the leakage magnetic field increases.
In addition, when energization is performed with a pulse waveform that changes at high speed, the amount of magnetic field change per hour also increases.
For this reason, the eddy current generated in the metal container increases, and the eddy current magnetic field that affects the image also increases.

On the other hand, it is desirable that the MRI apparatus has a structure in which a space for entering a subject is formed as wide as possible (a large opening is formed) so that the subject, that is, a patient does not feel a closed feeling.
Further, as a request from the examination side, it is also required to image a wide range of the subject as much as possible. For this reason, the gradient magnetic field coil is required to be configured as small as possible between the space in which the subject enters and the magnet device. Therefore, the interval between the main coil of the gradient magnetic field coil and the magnet device tends to be narrow, and accordingly, an eddy current is easily generated in the magnet container due to the leakage magnetic field from the gradient magnetic field coil.
Furthermore, in an MRI apparatus having a relatively low static magnetic field strength (˜0.5 T), a gradient magnetic field coil without a shield coil may be employed in order to simplify the structure.

In general, the active shield type gradient magnetic field coil is configured by stacking a total of six coils of three sets of main coils and shield coils in order to independently generate a gradient magnetic field in three directions in the imaging region. Therefore, the active shield type gradient magnetic field coil is thicker than the gradient magnetic field coil having no shield coil. Further, in the active shield type gradient magnetic field coil, in order to reduce the leakage magnetic field and generate the gradient magnetic field with a lower current and voltage, it is better to make the distance between the main coil and the shield coil as wide as possible.
On the other hand, since a gradient magnetic field coil without a normal shield coil has a large leakage magnetic field, it is desirable to install it on the imaging region side away from the static magnetic field magnet in order to suppress the generation of eddy current, and take a large opening. Is difficult.

  As described above, due to the improvement in magnetic field performance required for the gradient coil and the decrease in volume occupied by the gradient coil, the leakage magnetic field is not sufficiently suppressed by the conventional gradient coil alone, or the main magnetic field in the gradient coil is insufficient. If a manufacturing error occurs in the combination of the coil and the shield coil, the original image acquisition performance cannot be exhibited.

  The present invention has been made in view of such circumstances, and provides an MRI apparatus having a thin gradient magnetic field coil and a large opening while reducing image degradation due to eddy currents generated by a gradient magnetic field. The task is to do.

  The present invention provides an MRI apparatus including a magnet that generates a static magnetic field in an imaging region of a subject and a gradient magnetic field coil that generates a magnetic field having a gradient intensity in the imaging region. An eddy current shielding shield is provided in between, the eddy current shielding shield is divided into two or more, and each of the divided eddy current shielding shields is supported via a position adjusting means. And

  According to the present invention, it is possible to reduce an image deterioration caused by an eddy current generated by a gradient magnetic field, to reduce the thickness of the gradient magnetic field coil, and to obtain an MRI apparatus having a large opening.

1 is a cross-sectional view showing a vertical MRI apparatus according to a first embodiment of the present invention. It is a model external appearance perspective view similarly. It is a top view which similarly shows a gradient magnetic field coil and an eddy current shielding shield. It is a fragmentary sectional view which similarly shows an eddy current shielding shield, a gradient magnetic field coil, a magnetic pole, and a position adjustment means. It is explanatory drawing for demonstrating the adjustment by a position adjustment means. It is a figure which shows the structure of the eddy current shielding shield and thin plate electroconductive member used for the MRI apparatus in 2nd Embodiment of this invention, (a) is a partial top view, (b) is a side view. It is a fragmentary sectional view which shows the MRI apparatus in 3rd Embodiment of this invention. It is a fragmentary sectional view which shows the MRI apparatus in 4th Embodiment of this invention. It is a figure which similarly shows the arrangement | positioning of an eddy current shielding shield, a magnetic pole, and a gradient magnetic field coil, (a) is a partial top view, (b) is sectional drawing. It is a model perspective view which shows the horizontal MRI apparatus of 5th Embodiment of this invention. It is a fragmentary sectional view showing the composition similarly. It is a side view similarly. It is a fragmentary sectional view showing the MRI apparatus in a 6th embodiment of the present invention. Similarly, it is a schematic front view. It is a top view which shows the gradient magnetic field coil and eddy current shielding shield in a modification.

  Hereinafter, modes for carrying out the present invention (hereinafter referred to as “embodiments”) will be described in detail with reference to the drawings as appropriate.

(First embodiment)
As shown in FIGS. 1 and 2, the vertical MRI apparatus has a structure in which the upper and lower sides of the imaging region 9 are sandwiched by disk-shaped magnet apparatuses.
The magnet device that generates a static magnetic field is composed of a plurality of annular coils 3 and a disk-shaped magnetic body 2 and includes a pair of upper and lower magnetic poles 1 and 1 supported by a support column 26 (see FIG. 2).
The magnetic pole 1 has both a case where it is constituted by both the coil 3 and the magnetic body 2 as in this embodiment, and a case where it is constituted by either one. Further, when a superconducting coil is used for the coil 3, it is necessary to maintain a superconducting state. Therefore, as shown in FIG. 1, the coil 3 is attached to the vacuum container 4, the radiation shield 5, and the liquid helium container 8 from the outside. It is enclosed and cooled to cryogenic temperature by liquid helium and a refrigerator (not shown) in the liquid helium container, and the cooling state is maintained.

The upper and lower magnetic poles 1 and 1 generate a strong and uniform static magnetic field 6 in the vertical direction in the imaging region 9. The subject (patient, see FIG. 2) 7 is carried to the imaging region 9 by the movable bed 25.
Here, since the magnetic poles 1 and 1 have substantially the same structure, one magnetic pole 1 will be described below.
On the imaging region 9 side of the magnetic pole 1, a gradient magnetic field coil 11 is supported from the magnetic pole 1 via a support member 14. The gradient coil 11 generates a magnetic field 10 (hereinafter referred to as a gradient magnetic field) in the imaging region 9 in the same direction as the static magnetic field 6 and having a magnetic flux density gradient strength in an arbitrary direction in a pulse shape. It is a coil. The gradient coil 11 normally takes the direction of the static magnetic field 6 as the z-axis and takes the x-axis and y-axis in two directions orthogonal to the z-axis, as shown in FIG. 2, and in the three directions x, y, and z. It has a function capable of generating an independent gradient magnetic field.

  Here, the description will be made assuming that the vertical direction in FIG. 1 is the z axis, the horizontal direction in the paper is the y axis, and the direction perpendicular to the paper is the x axis (not shown in FIG. 1). For example, the gradient magnetic field 10 shown in FIG. Between the magnetic pole 1 and the gradient coil 11, a plurality of small pieces of magnetic material called shims (not shown) are arranged. These shims are a mechanism for adjusting the static magnetic field generated by the coil 3 or the magnetic body so that the magnetic field strength becomes uniform in the imaging region 9 including the influence of the magnetic field other than the MRI apparatus. Furthermore, an RF coil 12 that irradiates a subject 7 (see FIG. 2; the same applies hereinafter) with a high-frequency pulsed magnetic field is provided on the imaging region side surface of the gradient magnetic field coil 11.

Usually, the magnetic pole 1, the gradient coil 11, and the RF coil 12 are covered with a cover 13 made of a nonconductive member such as FRP. The RF coil 12 may be installed on the cover 13.
In addition, although not shown, the MRI apparatus includes a power supply device for driving the gradient magnetic field coil 11 and the RF coil 12, and a computer for controlling the power supply and for imaging the signal obtained by the RF coil 12. System included.

  As shown in FIG. 1, an eddy current shielding shield 15 made of a conductive material is installed between the gradient coil 11 and the magnetic pole 1. The eddy current shielding shield 15 may be formed of a nonmagnetic material such as aluminum or copper, or may be formed of a magnetic material such as iron or silicon steel plate. When formed of a nonmagnetic material, an unnecessary magnetic field leaking from the gradient coil 11 to the magnetic pole 1 side, that is, the leakage magnetic field can be shielded by generating an eddy current in the eddy current shielding shield 15. Further, when formed of a magnetic material, it can act not only as an eddy current but also as a magnetic flux path due to a large magnetic permeability, and a leakage magnetic field toward the magnetic pole 1 can be reduced.

The eddy current shielding shield 15 is supported from the gradient magnetic field coil 11 via a plurality of position adjusting means 16. As shown in FIG. 3, the gradient coil 11 and the eddy current shielding shield 15 are substantially concentric with respect to the vertical center axis (z axis), and the eddy current shielding shield 15 is rotationally symmetric about the vertical center axis. The shape is divided into four (shapes that are four-phase symmetric). The eddy current shielding shield 15 is usually a metal having a thickness of several millimeters and lacks flexibility. Therefore, the eddy current shielding shield 15 is divided into a plurality of pieces so that the position can be adjusted to obtain a symmetrical eddy current.
In the present embodiment, they are arranged so as to be symmetric with respect to the x, y, and z axes.

Each eddy current shielding shield 15 has a central portion supported by position adjusting means 16 disposed on the vertical center axis, and an outer edge vicinity portion has position adjusting means 16 along the x axis and position adjusting means 16 along the y axis. Is supported by. That is, each eddy current shielding shield 15 is supported by the three position adjusting means 16.
Thereby, each eddy current shielding shield 15 can be independently adjusted by each position adjusting means 16, and the distance between the gradient coil 11 and the eddy current shielding shield 15 can be adjusted for each location. It has become.

The gradient magnetic field generated by the gradient coil 11 is designed to be symmetric with respect to each of the x, y, and z axes. This causes an asymmetric component between the leakage magnetic field and the eddy current. The asymmetry component of the eddy current has an adverse effect on the image because it generates an asymmetric magnetic field in the imaging region 9 and is difficult to cancel by the control by energizing the gradient magnetic field coil 11.
In such a case, the position adjusting means 16 can make the eddy current generated in the eddy current shielding shield 15 symmetrical by adjusting the position of the eddy current shielding shield 15.

Here, as shown in FIG. 4, the gradient coil 11 is supported with respect to the magnetic pole 1 via a support member 14 (a stud bolt or the like), but the eddy current shielding shield 15 includes a support member 14 and An insertion hole 15a is formed so as not to interfere. The support member 14 has an end portion 14 a fixed to the magnetic pole 1, a male screw 14 b formed at the other end 14 b is inserted into the bolt hole 11 a of the gradient coil 11, and a nut 11 c in the recess 11 b of the gradient coil 11. Fastened and fixed.
That is, the eddy current shielding shield 15 is supported from the gradient coil 11 only by the position adjusting means 16. Thereby, it can be set as a symmetrical eddy current irrespective of the attachment position to the magnetic pole 1 of the gradient magnetic field coil 11. FIG.
Furthermore, the eddy current shielding shield 15 can shield eddy currents generated in the magnetic pole 1 by a high-frequency magnetic field created by the RF coil 12. As a result, heat generation and eddy current magnetic field generated by generation of eddy current in the magnetic pole 1 can be reduced.

  As shown in FIG. 4, the position adjusting means 16 has a bolt 17 and a nut 18 screwed into the bolt 17. The bolt 17 has a base end portion 17 a fixed to the fixing portion 11 d of the gradient magnetic field coil 11 and standing on the gradient magnetic field coil 11, and supports the eddy current shielding shield 15 via a nut 18. A groove portion 18 a that engages and supports the eddy current shielding shield 15 is formed on the outer peripheral portion of the nut 18. In such a position adjusting means 16, by rotating the nut 18 with respect to the bolt 17, the nut 18 can be moved up and down with respect to the bolt 17 in accordance with the rotating direction. The vertical position of the shielding shield 15 (position along the axial direction of the bolt 17) can be adjusted.

  1 and 2 show the position adjusting means 16 in the vertical (z-axis) direction. However, the position adjusting means 16 is not limited to this. You may comprise so that the position of the horizontal direction of the current shielding shield 15 may be adjusted.

The adjustment of the eddy current shielding shield 15 by the position adjusting means 16 is, for example, a magnetic field based on eddy currents at the points Y1 and -Y1, which are equidistant across the center of the imaging region 9, as shown in FIG. And the nut 18 of each position adjusting means 16 is rotated so that the value of the magnetic field based on the eddy current at the points Y1 and -Y1 is symmetric with respect to the center of the imaging region 9. To do.
Thereby, the position of each eddy current shielding shield 15 is suitably adjusted so that the value of the magnetic field based on the eddy current at the points Y1 and -Y1, that is, the magnetic field component is symmetric with respect to the center of the imaging region 9. can do.
By performing such an operation over a plurality of points, the distribution of the eddy current magnetic field in the imaging region 9 is sandwiched between the centers of the imaging regions 9 similarly to the gradient magnetic field that is symmetric with respect to the center of the imaging region 9. Created to have a nearly symmetrical distribution.

According to the MRI apparatus of the present embodiment described above, the position adjusting means 16 adjusts the position of each divided eddy current shielding shield 15, so that the eddy current distribution of the eddy current shielding shield 15 and the imaging region 9 are adjusted. The eddy current magnetic field can be adjusted. Specifically, the position of each eddy current shielding shield 15 is adjusted so that the magnetic field component of the magnetic field created in the imaging region 9 by the eddy current is as proportional as possible to the magnetic field component of the gradient magnetic field. When the position of the eddy current shielding shield 15 is set in this way and the gradient magnetic field is generated by adding or subtracting the eddy current magnetic field component, the magnetic field due to the eddy current can be apparently canceled and the influence on the image quality is affected. It can suppress suitably.
Further, since the gradient coil 11 does not have a shield coil, it can be installed at a position close to the magnet that generates a static magnetic field. By installing the gradient magnetic field coil 11 in such a manner, the gradient magnetic field coil 11 can be thinned, so that a large-aperture MRI apparatus in which an opening into which the subject 7 is inserted is formed is obtained.

(Second Embodiment)
An MRI apparatus according to the second embodiment will be described with reference to FIG. This embodiment is different from the first embodiment in that adjacent eddy current shielding shields 15 and 15 are connected by a conductive member 19 having flexibility (expandable / bendable).

The conductive member 19 has a sheet shape, and a plurality of conductive members 19 are connected with a predetermined interval between the eddy current shielding shields 15 and 15 as shown in FIGS.
By connecting the eddy current shielding shields 15, 15 with such a conductive member 19, an eddy current flow path is formed so as to extend between the eddy current shielding shields 15, 15. The eddy current flow path is not interrupted on the shielding shield 15.

Therefore, it is possible to reduce the limitation of the eddy current flow path due to the division, and it is possible to sufficiently reduce the leakage magnetic field to the magnetic pole 1 side. Further, since the predetermined interval can be maintained without reducing the interval between the adjacent eddy current shielding shields 15 and 15, the eddy current shielding shields 15 and 15 are partially in contact with each other after adjustment by the position adjusting means 16. It can prevent reliably that it overlaps.
Even when the interval between the eddy current shielding shields 15 and 15 is set wide, the leakage magnetic field toward the magnetic pole 1 can be suitably suppressed.

The conductive member 19 is made of, for example, a thin metal foil having a thickness of 1 mm or less, and can be divided into several locations along the adjacent portions, thereby achieving both the formation of the eddy current flow path and flexibility. it can.
Further, the conductive member 19 is a member (synthetic resin material, non-woven fabric, cloth) knitted with the conductive member 19, so that the degree of freedom of the position between the eddy current shielding shields 15 and 15 is improved. Become. In this case, the entire adjacent parts may be connected by the conductive member 19, and it is possible to suitably avoid the restriction of the eddy current flow path. Thereby, the leakage magnetic field to the magnetic pole 1 side can be suppressed more suitably.

(Third embodiment)
An MRI apparatus according to the third embodiment will be described with reference to FIG. This embodiment is different from the above embodiment in that an eddy current shielding shield 15 is extended around each metal container of the vacuum container 4, the radiation shield 5, and the liquid helium container 8.

  The peripheral portion of the eddy current shielding shield 15 extends around each metal container such as the vacuum container 4 containing the coil 3, the radiation shield 5, and the liquid helium container 8, and the leakage magnetic field from the gradient magnetic field coil 11. Is shielded in a wider range.

Such an eddy current shielding shield 15 can not only reduce the asymmetric eddy current magnetic field in the imaging region 9 due to the eddy current of the metal container, but also generate heat in the radiation shield 5 and the liquid helium container 8. The amount can also be reduced. Thereby, evaporation of liquid helium which is not illustrated and operation load in a refrigerator which is not illustrated can be suppressed.
Furthermore, since the leakage magnetic field does not reach the coil 3, the oscillating electromagnetic force generated in the coil 3 can be reduced, and a more stable static magnetic field can be obtained.

  Note that the eddy current shielding shields 15 may be configured to be connected by the flexible conductive member 19 described in the second embodiment.

(Fourth embodiment)
An MRI apparatus according to the fourth embodiment will be described with reference to FIGS. This embodiment is different from the above-described embodiment in that the eddy current shielding shield 15 includes a member 20 made of a nonmagnetic conductive material and a member 21 made of a magnetic material. In addition, although the member 21 illustrates what consists of an electroconductive material, it should just be a magnetic body and may consist of a nonelectroconductive material.

The eddy current shielding shield 15 is supported from the magnetic pole 1 by the position adjusting means 16. The member 21 made of a magnetic material of the eddy current shielding shield 15 is attached to the magnetic pole 1 via the position adjusting means 22, and the member 20 made of a nonmagnetic conductive material of the eddy current shielding shield 15 is further positioned. It is supported via the adjusting means 23.
Here, the position adjusting means 23 and 22 are provided independently for the member 20 and the member 21 because the member 21 made of a magnetic material is not only an eddy current caused by a leakage magnetic field from the gradient magnetic field coil 11 but also a static magnetic field. This is because the spatial distribution of 6 (see FIG. 8) is also affected, so that the distribution of the static magnetic field 6 is not disturbed by adjusting the position of the eddy current distribution symmetrically.

In the present embodiment, the position adjusting means 24 for adjusting the position of the member 21 in the horizontal direction is arranged.
Such position adjusting means 22 to 24 can adopt the same structure as the position adjusting means 16 described in the first embodiment. Further, the members 20 and 21 constituting the eddy current shielding shield 15 are each divided, and in particular, the member 20 is connected by the flexible conductive member 19 shown in the second embodiment. The eddy current flow path of the eddy current generated in the member 20 can extend over the adjacent eddy current shielding shield 15. With such a configuration, the leakage magnetic field toward the magnetic pole 1 can be effectively reduced.

The conductive member 19 may be connected to the member 21, but the effect of connecting with the conductive member 19 is small in a member having a low conductivity (for example, ferritic steel).
In addition, when the member 21 is comprised with a nonelectroconductive material, it is necessary to use a material with a large saturation magnetization and a relative magnetic permeability.

According to the present embodiment, the member 21 made of a magnetic material constitutes a magnetic circuit with respect to the leakage magnetic field from the gradient magnetic field coil 11, and in particular in the magnetically unsaturated state, the member 21 toward the magnetic pole 1 side. The leakage magnetic field can be effectively reduced. This is the same when the member 21 is made of a non-conductive material.
On the other hand, the member 20 made of a nonmagnetic conductive material shields the leakage magnetic field from the end of the gradient magnetic field coil 11 by eddy current, and the liquid helium container 8 in which the coil 3 is housed, the radiation shield 5, and the vacuum container. 4 reduces the eddy current generated in In addition, the vibration electromagnetic force generated in the coil 3 can be reduced.
Here, when the member 21 is made of a non-conductive material, since no eddy current is generated in the member 21, only the position of the member 20 needs to be adjusted at the time of position adjustment. can get.
The member 21 may be made of a nonmagnetic conductive material. In this case, since each eddy current can be finely adjusted in combination, the degree of freedom of adjustment is increased.

(Fifth embodiment)
FIG. 10 is a perspective view of a horizontal MRI apparatus which is the fifth embodiment of the present invention.
In the horizontal MRI apparatus, as shown in FIG. 11, a coaxial cylindrical gradient magnetic field coil 11 and an RF coil (not shown) are arranged in a coaxial cylindrical magnetic pole 1 having a central axis in the horizontal direction. The gradient magnetic field coil 11 is supported by the support member 14 from the vacuum vessel 4 constituting the magnetic pole 1.

  These are surrounded by an exterior 13 formed of FRP or the like having a cylindrical opening space on the central axis, thereby forming a magnet device. The subject 7 is conveyed to the center of the cylindrical opening space by the movable bed 25 and imaged.

In the imaging region 9 in the center of the opening space, a uniform and strong static magnetic field 6 is generated in the direction of the central axis of the cylinder by the coil 3 constituting the magnetic pole 1 or a magnetic material (not shown).
The coil 3 is kept at a very low temperature by the liquid helium vessel 8, the radiation shield 5, and the vacuum vessel 4 as in the vertical MRI apparatus. The gradient magnetic field coil 11 forms the gradient magnetic field 10 in two directions perpendicular to the direction of the static magnetic field 6 and the direction of the static magnetic field 6.
In FIG. 10, the direction of the static magnetic field 6 is represented as the z-axis, the vertical direction of the apparatus, that is, the vertical direction of the paper is represented as the y-axis, and the horizontal direction of the apparatus, that is, the vertical direction of the paper. An example of the gradient magnetic field is shown.

  The eddy current shielding shield 15, which is a component of the present invention, is formed of a conductive nonmagnetic material or magnetic material, and is installed between the gradient magnetic field coil 11 and the magnetic pole 1 in a cylindrical shape. In this embodiment, the eddy current shielding shield 15 is supported from the gradient coil 11 via the position adjusting means 16. By being supported from the gradient magnetic field coil 11 in this way, the eddy current generated in the eddy current shielding shield 15 and the mechanical vibration generated by the oscillating electromagnetic force due to the static magnetic field 6 are not directly transmitted to the magnetic pole 1, and the static magnetic field 6 The advantage of improved uniformity is obtained.

  If the eddy current shielding shield 15 is supported by a mechanism whose position can be adjusted as in the first embodiment, it can be supported from the magnetic pole 1 or the gradient coil 11 as in the fourth embodiment. You may comprise so that it may support from the member 14. FIG.

As shown in FIG. 12, the eddy current shielding shield 15 is divided at a position symmetrical to the x, y, and z axes passing through the center of the imaging region 9 (see FIG. 11) that generates the gradient magnetic field. , Including a portion not shown, is divided into eight.
The position adjusting means 16 is the same as that shown in FIG. 4, for example, and can adjust the distance between the gradient magnetic field coil 11 and the eddy current shielding shield 15 independently. Thereby, the eddy current generated in the eddy current shielding shield 15 is adjusted so as to create a symmetrical magnetic field in the imaging region 9 (see FIG. 11).

Further, the eddy current shielding shield 15 has a notch 15e in order to avoid interference with the support member 14 that attaches the gradient coil 11 to the vacuum vessel (not shown) of the magnetic pole 1.
Further, the divided eddy current shielding shields 15 are connected by the flexible conductive member 19 as described in the second embodiment, so that the eddy current eddy current generated in the eddy current shielding shield 15 is connected. The current flow path is not hindered, and the leakage magnetic field to the magnetic pole 1 can be reduced more effectively.

(Sixth embodiment)
A horizontal MRI apparatus according to the sixth embodiment of the present invention will be described with reference to FIGS. The present embodiment is different in that the eddy current shielding shield 15 is configured to extend not only to the cylindrical portion of the magnetic pole 1 but also to the front and rear plane portions of the cylinder so as to cover the front and rear plane portions of the cylinder.
By using such an eddy current shielding shield 15, the leakage magnetic field can be shielded in a wider range at both ends in the z direction that is the cylindrical axis, and effective shielding is possible.

In addition, the eddy current shielding shield 15 can be omitted in the central portion (the portion where the leakage magnetic field is small).
Also in this embodiment, the eddy current shielding shield 15 is divided at positions symmetrical with respect to the gradient magnetic field axes x, y, and z axes, and the position adjusting means 16 independently adjusts the position of the gradient magnetic field coil 11. can do.

  Further, the eddy current shielding shield 15 is provided with a notch 15e in order to avoid positional interference with the support member 14 of the gradient coil 11, but by making the size of the notch 15e as small as possible, A symmetrical eddy current magnetic field can be obtained in an imaging region not shown.

In the present embodiment, the eddy current shielding shield 15 can be used in combination with the member 21 made of a magnetic material and the member 20 made of a nonmagnetic conductive material as described in the fourth embodiment. Even in this case, the member 21 made of a magnetic material and the member 20 made of a nonmagnetic conductive member can be independently adjusted by the position adjusting means 16.
Incidentally, as shown by a broken line in FIG. 13, a member 21 made of a magnetic conductive material can be disposed.

As mentioned above, although it demonstrated in detail based on embodiment, the content of this invention is not limited to the above description, Various modifications and changes are also included in this invention in the range which does not deviate from the meaning of this invention.
In the first embodiment, the example in which the eddy current shielding shield 15 is divided on each of the x and y axes has been shown. However, the present invention is not limited to this, and as shown in FIG. It is good also as a structure rotated 45 degrees around the z-axis.
By adopting such a configuration, the eddy current shielding shield 15 is disposed so as to overlap (strand) the x and y axes, so that it is possible to preferably avoid the restriction of the eddy current flow path. It is possible to suppress the leakage magnetic field to the magnetic pole 1 side more suitably.

  Similarly, in the fifth embodiment, the example in which the eddy current shielding shield 15 is divided on each of the x, y, and z axes has been described. However, the present invention is not limited to this, and the eddy current shielding shield 15 is not limited to this. It is good also as a structure rotated 45 degrees around the z-axis.

  In the first embodiment, etc., the eddy current shielding shield 15 is divided into four or eight. However, the present invention is not limited to this. It may be an odd division such as a division.

  Further, as the position adjusting means 16, there is a method of adjusting the position of the eddy current shielding shield 15 by appropriately interposing a space member such as a thin plate between the gradient coil 11 and the eddy current shielding shield 15. .

1 magnetic pole 2 magnetic material (magnet)
3 Coil 6 Static Magnetic Field 7 Subject 9 Imaging Area 11 Gradient Magnetic Field Coil 15 Eddy Current Shielding Shield 16 Position Adjusting Means 19 Conductive Member 20 Member 21 Member 22-24 Position Adjusting Means

Claims (8)

In a magnetic resonance imaging apparatus comprising a magnet that generates a static magnetic field in an imaging region of a subject and a gradient magnetic field coil that generates a magnetic field having a gradient intensity in the imaging region, the magnetic resonance imaging apparatus includes a magnetic field between the gradient magnetic field coil and the magnet. , Provide eddy current shielding shield,
The eddy current shielding shield is divided into two or more;
Each of the divided eddy current shielding shields is supported via a position adjusting means.   The position adjusting means is configured so that the magnetic field component of the magnetic field created in the imaging region by the eddy current flowing in each eddy current shielding shield is proportional to the magnetic field component of the magnetic field created in the imaging region by the gradient magnetic field coil. The magnetic resonance imaging apparatus according to claim 1, wherein the position of each eddy current shielding shield is adjusted.   3. The magnetic resonance imaging apparatus according to claim 1, wherein each of the eddy current shielding shields is made of a magnetic or non-magnetic conductive member.   4. The magnetic resonance imaging apparatus according to claim 1, wherein each of the eddy current shielding shields is supported by the gradient magnetic field coil via the position adjusting unit. 5.   4. The magnetic resonance imaging apparatus according to claim 1, wherein each of the eddy current shielding shields is supported by the magnet via the position adjusting unit. 5.   6. The magnetic resonance imaging apparatus according to claim 1, wherein each of the eddy current shielding shields is connected by a conductive member that can be expanded and contracted. 6.   Each of the eddy current shielding shields includes both a magnetic member and a non-magnetic conductive member. The magnetic resonance imaging apparatus according to any one of the above. The magnetic resonance imaging apparatus includes an RF coil that generates a high-frequency magnetic field,
7. Each of the eddy current shielding shields reduces an eddy current generated in the magnet by a high frequency magnetic field generated by the RF coil together with a gradient magnetic field of the gradient magnetic field coil. 2. A magnetic resonance imaging apparatus according to item 1.

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