JP2007014473A - Magnetic shield room and magnetic resonance imaging system including the magnetic shield room - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the shield property of a leakage magnetic field in the axial direction of annular coils of an MRI apparatus and reduce an influence on the uniformity of a magnetic field of an imaging space. <P>SOLUTION: This magnetic shield room 1 stores an vertical magnetic field type magnetic resonance imaging apparatus 2 formed by disposing the annular coils 4a and 4b forming the magnetic field in the imaging space, vertically opposing to each other on both sides of the imaging space and has a shield surface shielding the magnetic field from the inside and outside. This magnetic shield room is formed of a magnetic plate 10a disposed in a wide area of a floor shield face 10 and an annular magnetic plate 10b supersposedly disposed with a position where the axes of the annular coils cross with each other as a center, and improves the uniformity of the magnetic field of the imaging space using the antithetical influences of the magnetic plate 10a and the annular magnetic plate 10b on the magnetic field error. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a magnetic shield room for a magnetic resonance imaging apparatus and a magnetic resonance imaging system formed by installing a magnetic resonance imaging apparatus in the magnetic shield room.

  A magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus) is an apparatus that forms a strong magnetic field in an imaging space and images a tomographic image of a subject. If a strong magnetic field generated by such an MRI apparatus leaks to the surroundings, there is a risk of adverse effects on surrounding electronic devices. Further, when a magnetic field enters the MRI apparatus from the outside, the uniform magnetic field in the imaging space may be disturbed, and the image may be affected.

  Therefore, the MRI apparatus is surrounded by a shield surface made of a magnetic plate or the like to shield the magnetic field from inside and outside (Patent Document 1). In addition, the shield surface is generally formed using a high magnetic permeability magnetic plate such as a laminated steel plate called an electromagnetic steel plate.

  On the other hand, in the technical field of the MRI apparatus, the magnetic field leaked from the MRI apparatus increases as the magnetic field and the openness of the magnetic pole increase, and a magnetic shield room with higher shield performance than ever before is required. ing. In particular, an MRI apparatus referred to as a vertical magnetic field system forms a strong magnetic field by arranging a pair of annular coils or a group of annular coils (hereinafter simply referred to as annular coils) facing each other across an imaging space. It is like that. In the case of this MRI apparatus of the vertical magnetic field type, conventionally, there is known a configuration in which a magnetic flux emitted from one of a pair of annular coils is returned to the other through a magnetic path made of an iron yoke. When such an iron yoke is provided, for example, most of the magnetic flux directed to the floor surface immediately below is attracted to the iron yoke, so that the leakage magnetic field that passes through the floor surface and leaks to the outside is relatively small.

  However, from the viewpoint of weight reduction and miniaturization of the MRI apparatus, as described in Patent Document 2, a vertical magnetic field type MRI apparatus without a yoke has been adopted. According to Patent Document 2, an MRI apparatus is installed in a magnetic shield room in which a ceiling, a floor, and a wall surface are formed of a shield surface made of a magnetic plate, and a magnetic flux emitted from one of a pair of annular coils is transmitted to the floor shield surface, the wall It is configured to return to the other annular coil via the magnetic plate on the shield surface and the ceiling shield surface. In particular, according to this document, a plurality of rings concentric with the annular coil are provided on the magnetic plate of the shield surface facing the axial direction of the annular coil so as to be inclined in accordance with the flow of the magnetic flux so that the magnetic flux can easily enter. It has been proposed to reduce the weight of the shield surface.

JP 2003-135424 A JP 2002-172101 A

  By the way, since the vertical magnetic field type high magnetic field MRI apparatus without a yoke described in Patent Document 2 can be formed in a light weight, there is a demand to install it on the first floor where there is an underground space such as two or more floors and a parking lot. . Further, due to the demand for replacement of the low magnetic field MRI apparatus, there is a demand for installing a vertical magnetic field type high magnetic field MRI apparatus in an existing narrow magnetic shield room. However, in order to shield a leakage magnetic field to a room or a parking lot below the floor, it is necessary to take measures such as increasing the thickness of the magnetic plate on the floor shield surface as described in Patent Document 2.

  However, in Patent Document 2, no consideration is given to reducing the error magnetic field in the imaging space caused by the magnetic material plate on the floor shield surface being magnetized by the leakage magnetic field. For example, the magnetic field in the imaging space is required to have a uniformity on the order of ppm. On the other hand, when the thickness of the magnetic plate is increased in order to improve the shielding performance, a large magnetization is generated in the magnetic plate, and the magnetic field uniformity generated in the imaging space is lowered by the magnetic field generated by the magnetization. Such a problem also applies to the relationship between the wall shield surface and the horizontal magnetic field type MRI apparatus that uses the magnetic field formed in the cylinder of the annular coil as the imaging space.

  On the other hand, if the shield room is formed large, the distance from the device to the shield material can be increased, so that the leakage magnetic field strength acting on the shield material is weakened and the shield effect is improved. Further, the influence of the magnetic field generated by the magnetization of the shield material on the imaging space is also reduced according to the distance. In general, however, it is often difficult to increase the size of the magnetic shield room due to building restrictions.

  An object of the present invention is to realize a magnetic shield room that can improve the shielding performance against the leakage magnetic field in the annular coil axis direction of the MRI apparatus and reduce the influence on the magnetic field uniformity of the imaging space.

  In order to solve the above problems, the error magnetic field in the imaging space caused by the magnetic material of the shield surface facing the axial direction of the annular coil was examined, and the knowledge shown in FIG. 2 was obtained. That is, when a thick magnetic body having sufficient shielding performance is disposed on the shield surface facing the axial direction of the annular coil, as shown by the line 22 in FIG. An error magnetic field in which the magnetic field of the part (negative side) increases to the positive side appears. On the other hand, when only the annular magnetic body is disposed concentrically with the annular coil on the shield surface facing the axial direction of the annular coil, that is, the magnetic body is in a certain circle centering on the position where the axis of the annular coil intersects. 2 is formed, the leakage flux from the annular coil toward the shield surface is attracted to the annular magnetic body, and the leakage flux spreads in the radial direction, as shown by line 23 in FIG. It was found that the error magnetic field in the positive direction due to the magnetization of the magnetic material does not appear, and conversely, the error magnetic field at the south pole (negative side) of the meridian angle increases in the negative direction due to the spread of the leakage magnetic flux. That is, the error magnetic field in the south pole of the imaging space increases in the positive direction due to the magnetization of the magnetic material arranged in a certain central region centered on the position where the axes of the annular coils intersect, so as to surround the central region We found the phenomenon that if only the annular magnetic body (perforated shield) is placed, the error magnetic field in the south pole of the imaging space increases in the negative direction.

  Therefore, the present invention is based on the phenomenon obtained from the above knowledge, the first magnetic body portion disposed within a certain region from the position where the axes of the annular coils intersect, and disposed outside the certain region. By forming the shield surface with the annular second magnetic body portion provided, the influence of both magnetic body portions on the error magnetic field is offset, and the influence on the magnetic field uniformity of the imaging space is reduced. Reduction is the main point. In this case, it is preferable that the thickness of the magnetic body of the first magnetic body portion is thinner than that of the second magnetic body portion.

  Further, the third magnetic body portion disposed on the outer side in the radial direction of the second magnetic body portion may have a magnetic resistance that is at least reduced as the distance from the second magnetic body portion increases.

  Further, the magnetic body includes an annular second magnetic layer disposed outside a predetermined region from a position where the first magnetic body plate disposed on the entire shield surface and the axis of the annular coil on the shield surface intersect. The first magnetic body plate can be formed thinner than the sum of the thicknesses of the first magnetic body plate and the second magnetic body plate.

  When the magnetic shield room of the present invention is applied to a vertical magnetic field type magnetic resonance imaging apparatus, the magnetic plate disposed in a wide area of the floor shield surface and the position where the axis of the annular coil intersects It can implement | achieve by forming from the cyclic | annular magnetic material board arrange | positioned by overlapping.

  In addition, when applied to a horizontal magnetic field type magnetic resonance imaging apparatus, at least one wall shield surface facing the axial direction of the annular coil, a magnetic plate disposed in a wide area of the wall shield surface, and the annular It can be constituted by an annular magnetic plate disposed so as to overlap with the position where the axes of the coils intersect.

  ADVANTAGE OF THE INVENTION According to this invention, while being able to improve the shielding performance with respect to the leakage magnetic field of the annular coil axial direction of an MRI apparatus, the influence which acts on the magnetic field uniformity of imaging space can be reduced.

Embodiments of the present invention will be described with reference to the drawings.
(Embodiment 1)
FIG. 1 is a longitudinal sectional view of a magnetic shield room according to an embodiment of the present invention. As shown in FIG. 1, a vertical magnetic field type magnetic resonance imaging apparatus 2 is accommodated in a magnetic shield room 1. In the magnetic resonance imaging apparatus 2, a pair of annular coils 4 a and 4 b that form a magnetic field in the imaging space 3 are arranged so as to face each other up and down across the imaging space 3. The pair of annular coils 4a and 4b is formed by storing one annular coil or a plurality of annular coil groups in a cylindrical container, with a common coil axis. The pair of upper and lower annular coils 4a, 4b are held at a predetermined interval by a plurality of pillars 5 through a container, and the periphery of the imaging space 3 is thereby opened.

  The magnetic shield room 1 includes a floor shield 10 that covers a floor surface, a wall shield 11 that covers a side wall, and a ceiling shield 12 that covers a ceiling. In general, these are joined to each other and magnetically coupled so that magnetic flux can be transferred. For example, the floor shield 10 is disposed in a peripheral region 15 surrounding a central region 14 in a certain range centering on a position where the magnetic plate 10a disposed on the entire floor surface and the coil axis of the annular coil 4b intersect. And an annular magnetic plate 10b. In other words, the magnetic plate of the floor shield 10 is formed such that the plate thickness of the central region 14 in a certain range centering on the position where the coil axes of the annular coil 4 b intersect is thin, and the annular peripheral region 15 surrounding the central region 14. The plate thickness of at least a certain range is formed thick.

  Next, the magnetic shield action of the embodiment configured as described above will be described. A magnetic field in the vertical direction is formed in a region including the imaging space 3 by the pair of annular coils 4a and 4b. In particular, the uniformity of the magnetic field formed in the imaging space 3 that is a spherical space is required to be high. A magnetic flux 16 that forms a magnetic field in a spatial region including the imaging space 3 enters the floor shield 10 having a high magnetic permeability as a leakage magnetic flux directly below the lower annular coil 4b as indicated by the solid line arrow shown in the figure. To do. The magnetic flux that has entered the floor shield 10 flows to the wall shield 11 and the ceiling shield 12 through the magnetic plates 10a and 10b, and returns from the ceiling shield 12 to the upper annular coil 4a.

  At this time, the magnetic plate 10a is magnetized by the magnetic flux 16 entering the magnetic plate 10a immediately below from the annular coil 4b, and a magnetization vector 17 is generated. The magnetic field generated by the magnetization vector 17 becomes an error magnetic field of the uniform magnetic field in the imaging space 3, and as will be described later, the magnetic field distribution on the meridian of the virtual sphere imagined in the imaging space 3 is distorted to reduce the magnetic field uniformity. There is.

  On the other hand, since the annular magnetic plate 10b disposed in the peripheral region 15 is formed thick and has a small magnetic resistance, the leakage magnetic flux that flows out directly from the annular coil 4b is easily attracted. Therefore, the leakage magnetic flux that flows out directly from the annular coil 4b tends to spread in the radial direction, and the magnetic field distribution on the meridian is distorted in a direction that cancels the error magnetic field due to the magnetization vector 17. As a result, according to the floor shield 10 of the present embodiment, the shielding performance against the leakage magnetic field in the annular coil axis direction of the vertical magnetic field type MRI apparatus can be improved, and the magnetic field distribution by the magnetic plate 10a and the magnetic plate 10b can be improved. The distortion is canceled out, and the magnetic field uniformity in the imaging space 3 can be maintained or improved.

  The effect of this embodiment will be described in detail with reference to FIGS. FIG. 2 shows the virtual spherical meridian angle virtually set in the imaging space 3 on the horizontal axis, and the vertical axis shows the normalized error magnetic field strength. That is, it shows the relative magnitude of the error magnetic field, and if it is on the horizontal axis passing through 0, the error magnetic field is zero. In the figure, the line 21 is the error magnetic field intensity distribution of the MRI main body, the line 22 is the error magnetic field intensity distribution by only the magnetic plate 10a, and the line 23 is the error magnetic field strength distribution by only the annular magnetic plate 10b. Further, as shown in FIG. 4, the virtual spherical meridian angle is an arbitrary angle on the sphere, with the equator 25 of the virtual sphere 24 set to 0 °, the north pole N and the south pole S set to 90 ° and −90 °, respectively. The position of the point 26 is represented by an angle on the meridian.

  As can be seen from FIG. 2, the error magnetic field intensity distribution of the MRI apparatus body indicated by line 21 has a positive error magnetic field at the lower part (near the south pole) and the upper part (near the north pole) of the virtual sphere, and on the horizontal plane (near the equator). It is shown that there is a negative error magnetic field, and the distribution is symmetric with respect to the meridian angle. When this MRI apparatus is installed in a magnetic shield room, the magnetic field decreases as a whole, resulting in a negative error magnetic field. However, the magnetic field that decreases as a whole can be dealt with by lowering the target magnetic field or increasing the excitation current. Can do. Further, the symmetrical error magnetic field of the MRI apparatus main body of FIG. 2 can be corrected by a known magnetic field correction such as shimming.

  On the other hand, the intensity distribution of the error magnetic field only by the magnetic plate 10a shown by the line 22 is an asymmetric error magnetic field that increases toward the positive side on the south pole side of the meridian angle. Therefore, this error magnetic field cannot be reduced even if the target magnetic field is lowered or the excitation current is increased. On the other hand, the shield using only the annular magnetic plate 10b corresponds to a so-called perforated shield in which the magnetic plate in the central region 14 in FIG. When the floor shield 10 having a hole directly under the annular coil 4b is used, an asymmetrical error magnetic field that increases toward the negative side on the south pole side of the meridian angle is obtained as shown by a line 23 in FIG.

  That is, as shown in FIG. 2, when the floor shield 10 having only the magnetic plate 10a is installed, the error magnetic field in the south pole is greatly strengthened in the positive direction as compared with the north pole. On the other hand, when the floor shield 10 including only the perforated annular magnetic plate 10b is installed, the error magnetic field in the south pole is strengthened in the negative direction compared to the north pole. As described above, the magnetic field plate 10a and the annular magnetic body plate 10b arranged in a wide area have opposite behaviors of the error magnetic field at the lower part of the phantom spherical surface (south pole side) of the imaging space.

  That is, in the present invention, in consideration of the conflicting effects of the magnetic plate 10a and the annular magnetic plate 10b on the magnetic field error, the magnetic field error is canceled by combining them as shown in FIG. is there.

  Therefore, according to the floor shield 10 of the present embodiment, as shown by the line 28 in FIG. 3, the magnetic field errors due to the magnetic plates 10a and 10b are canceled out, the error magnetic field can be minimized, and on the meridian. The distribution is symmetrical with respect to the angle. Therefore, it can be corrected by overall adjustment of lowering the target magnetic field or increasing the excitation current, and asymmetry can be easily corrected by magnetic field correction such as shimming.

  Here, when the floor shield 10 shown in the embodiment of FIG. 1 is constructed, a thick annular magnetic plate 10b is previously embedded in the floor, and a thin plate 10a is uniformly formed thereon. It is preferable to install them. Moreover, it is preferable to use a laminated silicon steel plate for the magnetic plates 10a and 10b.

When the thin magnetic plate 10a is not used on the entire surface, as shown in FIG. 12, a magnetic plate 10a is arranged in the center, and a thicker magnetic plate 10b is arranged in the periphery of the plate 10a. A third magnetic plate 10c may be disposed around the periphery. At this time, the third magnetic plate 10 c is connected to the wall shield 11.
(Embodiment 2)
Here, in place of the annular magnetic plate 10b of the present embodiment in FIG. 1, a quadrangular annular magnetic plate 10b can be used as shown in FIG. According to this, it is easy to procure and process the laminated silicon steel sheet, and the installation cost can be reduced. Further, instead of the annular magnetic plate 10b of the present embodiment in FIG. 1, an octagonal annular magnetic plate 10b can be used as shown in FIG.

  That is, the magnetic field generated by the quadrangular annular magnetic plate 10b shown in FIG. 5 has fourfold symmetry, and the non-axisymmetric component becomes an error magnetic field. Further, in the case of a quadrangular shape, the difference between the shortest width of the opening of the magnetic plate 10b and the length of the diagonal that is the longest width is large. Therefore, if sufficient magnetic shielding characteristics are to be maintained, the width of the opening must be reduced. I do not get. For this reason, in the quadrilaterally symmetric quadrangular magnetic plate 10b, the side of the width of the opening approaches the magnetic pole central axis, and the error magnetic field increases. In this regard, as shown in FIG. 6, when the magnetic plate 10b is configured in an octagonal ring shape, the axial symmetry of the magnetic field is further improved, and the width of the magnetic field plate 10b can be increased even in the same opening area, so that the error magnetic field is reduced. .

  The construction of the magnetic plate 10b shown in FIGS. 5 and 6 can be easily performed by combining cut silicon steel plates as shown in FIGS. When the silicon steel plate is processed into a circular shape and the magnetic plate 10b is formed into an annular shape, the error magnetic field is further reduced.

In addition, for example, the thickness of the magnetic plate 10a is about 3 to 10 mm, and the thickness of the magnetic plate 10b is about 30 to 50 mm, so that the weight of transportation equipment is necessary for handling during construction. Therefore, as shown in FIG. 9, a bundle of laminated silicon steel plates 30 is fastened with rivets 31 to form one layer, and by superimposing the bundles, a single bundle can be transported by human power. Can be suppressed.
(Embodiment 3)
Each embodiment mentioned above demonstrated what made the thickness of the circumferential direction of the cyclic | annular magnetic body board 10b constant. However, the present invention is not limited to this, and the error magnetic field in the circumferential direction can be reduced by changing the thickness of a part of the annular magnetic plate 10b in the circumferential direction.

That is, as shown in FIG. 10, generally, the MRI imaging room covered with the magnetic shield room has a rectangular shape when viewed from above, and the apparatus is not placed at the center due to the availability of space. In general, since the subject is disposed around the bed on which the subject lies, the apparatus is installed at a position eccentric from the center of the room. At this time, the magnetic field environment tends to increase the magnetic field in the region close to the wall surface. In addition, the magnetic field in a region far from the wall surface tends to become weak. This results in an error magnetic field that is not only non-axisymmetric in the circumferential direction but also has no rotational symmetry. In this case, the one-time symmetric error magnetic field can be reduced by reducing the thickness of the part 32 of the magnetic plate 10b located on the opposite side of the wall shield 11 to which the annular magnetic plate 10b is closest. . The reason is that the thickness of the magnetic plate 10b has the effect of weakening the magnetic field as shown in FIG. 2, and therefore the magnetic field can be increased by reducing the thickness of the part 32 of the magnetic plate 10b. It is. Specifically, the number of laminated silicon steel plates included in the part 32 of the magnetic plate 10b is reduced, and the rest is composed of a non-magnetic SUS plate or the like, so that the entire thickness is the same as the original thickness. It can also be made easier.
According to this embodiment, the error magnetic field in the circumferential direction of the MRI apparatus installed eccentrically on the rectangular magnetic room shield can be relaxed.
(Embodiment 4)
Each of the above embodiments is applied to a vertical magnetic field type MRI apparatus, but the magnetic shield room of the present invention is not limited to this, and is applied to the magnetic room shield of the horizontal magnetic field type MRI apparatus 35 shown in FIG. You can also

  That is, the horizontal magnetic field type MRI apparatus 35 is typically a tunnel type in which the annular coil 36 for generating an imaging magnetic field is formed in a cylindrical shape and the inside of the cylinder is used as an imaging space. Such a tunnel type MRI apparatus 35 has a higher magnetic field than a vertical magnetic field type MRI apparatus, and its leakage magnetic field tends to be very large. Further, the direction of the leakage magnetic field is the direction of the side wall of the magnetic shield room. Therefore, in a medical facility in which leakage of a magnetic field into the adjacent room becomes a problem, the reduction of the leakage magnetic field becomes more important than a vertical magnetic field type MRI apparatus.

  Therefore, in the present embodiment, the wall shield 11 at which the coil axis of the tunnel-type annular coil 36 intersects is centered on the position where the magnetic body plate 11a disposed on the entire wall surface and the coil axis of the annular coil 36 intersect. For example, an annular magnetic plate 11b disposed in a peripheral region 38 surrounding a certain range of the central region 37 is formed. That is, the magnetic plate of the wall shield 11 is formed such that the plate thickness of the central region 37 in a certain range around the position where the coil axes of the annular coil 36 intersect is thin, and the annular peripheral region 38 surrounding the central region 37. The plate thickness of at least a certain range is formed thick.

  As a result, as in the vertical magnetic field method, it is possible to reduce the horizontal asymmetry of the imaging space magnetic field and improve the uniformity of the imaging space while having high shielding performance. In addition, the magnetic board of embodiment shown in FIGS. 5-10 is applicable to the magnetic board 11b.

  As described above, according to each embodiment of the present invention, the influence on the magnetic field uniformity of the imaging space can be reduced. However, the present invention is not limited to these embodiments. For example, the plate thickness of the magnetic material may be changed in a plurality of stages in the radial direction from the position of the shield surface where the axes of the annular coils intersect. Further, the effect of the present invention can be achieved not only by changing the plate thickness of the magnetic material stepwise but also by changing it continuously.

It is a longitudinal cross-sectional view of one embodiment of the magnetic shield room of the present invention. It is a diagram for explaining the principle of the present invention, and shows the distribution of the error magnetic field strength with respect to the virtual spherical meridian angle set in the imaging space. It is a diagram which shows distribution of the error magnetic field strength with respect to the angle on the meridian of the virtual sphere set in the imaging space obtained by the embodiment of FIG. It is a figure explaining the virtual spherical meridian angle. It is a top view of one embodiment of a magnetic board concerning the feature of the present invention. It is a top view of other embodiments of a magnetic board concerning the feature of the present invention. It is a top view of other embodiments of a magnetic board concerning the feature of the present invention. It is a top view of other embodiments of a magnetic board concerning the feature of the present invention. It is a figure explaining the construction method of the magnetic body plate which concerns on the characteristic of this invention. It is a top view of other embodiments of a magnetic board concerning the feature of the present invention. It is a longitudinal cross-sectional view of other embodiment of the magnetic shield room of this invention. FIG. 6 is a longitudinal sectional view of a modification of the embodiment of the magnetic shield room in FIG. 1.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Magnetic shield room 2 MRI apparatus 3 Imaging space 4a, 4b Annular coil 5 Pillar 10 Floor shield 10a, 10b Magnetic body plate 11 Wall shield 14 Central area | region 15 Peripheral area | region

Claims (13)

  In a magnetic shield room containing a magnetic resonance imaging apparatus and having a shield surface that shields a magnetic field from inside and outside, at least one of the annular coils that form a magnetic field in the imaging space of the magnetic resonance imaging apparatus is opposed in the axial direction. The magnetic body disposed on the shield surface includes a first magnetic body portion disposed within a certain region from a position where the axes of the annular coils intersect, and a first magnetic body portion outside the certain region. And a second magnetic body portion disposed so as to surround the magnetic coil, and the thickness of the magnetic body with respect to the axial direction of the annular coil has different portions in the direction orthogonal to the axial direction of the annular coil. A magnetic shield room.   In the magnetic shield room, the magnetic body includes a first magnetic body portion disposed within a certain region from a position where the axes of the annular coils intersect, and a first magnetic body portion outside the certain region. And a second magnetic body portion disposed so as to surround the magnetic coil, and the magnetic resistance with respect to the axial direction of the annular coil has different portions in a direction perpendicular to the axial direction of the annular coil. The magnetic shield room according to claim 1.   The magnetic body is disposed so as to surround the first magnetic body portion disposed within a certain region from a position where the axes of the annular coils intersect, and outside the certain region. The magnetic body of the first magnetic body portion is formed to be thinner than the second magnetic body portion, and the second magnetic body portion is formed. The magnetic shield room described.   3. The third magnetic body portion disposed on the outer side in the radial direction of the second magnetic body portion has a magnetic resistance that decreases as the distance from the second magnetic body portion increases. Magnetic shield room as described in.   The magnetic body is disposed so as to surround a first magnetic plate disposed on the entire surface of the shield surface and a position outside a certain region from a position where the axis of the annular coil of the shield surface intersects. The first magnetic plate is formed to be thinner than the sum of the thicknesses of the first magnetic plate and the second magnetic plate. The magnetic shield room according to claim 1.   The magnetic shield room according to claim 3, wherein the second magnetic plate is formed in a circular or polygonal ring shape.   The magnetic shield room according to claim 5 or 6, wherein the second magnetic plate is provided with a portion having a high magnetic resistance in a part of the circumferential direction.   The magnetic shield according to any one of claims 5 to 7, wherein the second magnetic plate is formed by stacking a plurality of magnetic plate units formed by stacking a plurality of laminated steel plates. Room.   A magnetic resonance imaging apparatus of a vertical magnetic field system in which an annular coil that forms a magnetic field in an imaging space is arranged so as to face up and down across the imaging space is housed, and has a shield surface that shields the magnetic field from inside and outside In the magnetic shield room, the floor shield surface is an annular magnetic plate disposed so as to overlap with a magnetic plate disposed in a wide area of the floor shield surface and a position where the axis of the annular coil intersects Magnetic shield room characterized by consisting of.   In a magnetic shield room having a shield surface that shields a magnetic field from inside and outside, a horizontal magnetic field type magnetic resonance imaging apparatus having an imaging space as a magnetic field formed in a cylinder of the annular coil is housed. At least one wall shield surface facing in the axial direction is an annular magnetic layer disposed so as to overlap with a magnetic plate disposed in a wide area of the wall shield surface and a position where the axis of the annular coil intersects. A magnetic shield room comprising a body plate.   The magnetic shield room according to claim 9 or 10, wherein the annular magnetic plate is formed in a circular or polygonal annular shape.   The magnetic shield room according to claim 11, wherein the annular magnetic plate is provided with a portion having a high magnetic resistance in a part of an annular circumferential direction.   A magnetic resonance imaging system formed by installing a magnetic resonance imaging apparatus in the magnetic shield room according to claim 1.

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