JP2005237501A - Magnetic circuit and magnetic field adjustment method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To further improve a uniformity in the magnetic field in a magnetic circuit with counter permanent magnets. <P>SOLUTION: The magnetic circuit 1 with counter permanent magnets comprises a pair of permanent magnets 2 which are made to face each other isolated with a clearance and magnetized across the thickness thereof, a yoke 3 which is disposed outside the permanent magnets and the clearance and magnetically linked to the permanent magnets and a pair of magnetic pole pieces 4 which are disposed on the surface sides facing each other of the permanent magnets and each have a peripheral protrusion in the direction of facing each other. It also comprises an added magnetic body 6 provided at the upper part of the magnetic circuit. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a permanent magnet type magnetic circuit and a magnetic field adjustment method.

  Magnetic field generating apparatuses using permanent magnets for magnetic resonance imaging apparatuses (hereinafter also referred to as MRI) are mainly magnet facing magnetic circuits using rare earth magnets. FIG. 4 shows a schematic front view (a), plan view (b), and side view (c) of a conventional permanent magnet facing magnetic circuit. As illustrated in FIG. 4, in the magnet facing magnetic circuit 101, the yoke 103 and the magnet 102 for passing the magnetic flux from the magnet 102, and generally soft iron or the like on the magnet surface to generate a uniform magnetic field on the air gap side. A pole piece 104 made of a soft magnetic material is provided.

In this permanent magnet facing type magnetic circuit, a gap is sandwiched between upper and lower permanent magnets, and a magnetic field is generated in the gap, so that the subject's feeling of blockage that the subject has a wider opening is reduced compared to a superconducting solenoid coil type. It has the feature. However, since the magnets are divided into upper and lower parts, there has been a problem that non-uniformity in the vertical direction is likely to occur in the magnetic field in the gap. As a method for correcting the non-uniformity of the magnetic field in the upper and lower directions, a method of adjusting the upper and lower magnets by providing a position adjusting mechanism using a magnetic material as described in Patent Document 1 is used. However, since there is a limit to the amount of correction by such a mechanism, it is preferable that there is as little magnetic field non-uniformity as possible in the vertical direction. Therefore, the magnet-facing magnetic circuit is generally vertically symmetric.
Japanese Patent No. 3131515

  On the other hand, the base of the magnetic circuit generally has sufficient strength and inexpensive iron is used. However, since the pedestal is made of iron, the lower magnet has a higher magnetic efficiency, which is also a factor in causing non-uniformity in the vertical direction of the magnetic field in the gap. Furthermore, the room temperature is generally lower in the lower part of the magnetic circuit and higher in the upper part. In general, as the temperature of a permanent magnet increases, the magnetic flux density decreases and the generated magnetic field decreases. This temperature problem has also contributed to increasing the magnetic field from the magnet below the magnetic circuit.

  An object of the present invention is to further improve magnetic field uniformity in a permanent magnet facing magnetic circuit.

  The above problem occurs due to the difference in magnetic efficiency of the magnet due to the base made of iron etc. under the magnetic circuit and the difference in temperature between the upper and lower sides of the magnetic circuit. The lower magnetic field was stronger. Therefore, the present invention proposes a magnetic circuit that can cope with an increase in the lower magnetic field by increasing the magnetic efficiency of the upper magnet.

  That is, according to one aspect of the present invention, a pair of permanent magnets opposed to each other with a gap and magnetized in the thickness direction, the permanent magnet and the outside of the gap are provided, and the permanent magnet is magnetically A permanent magnet facing magnetic circuit comprising a yoke to be coupled and a pair of magnetic pole pieces provided on opposite surfaces of the permanent magnet and having peripheral protrusions in the facing direction, There is provided a permanent magnet-facing magnetic circuit further including an additional magnetic body provided on the top of the permanent magnet.

  According to another aspect of the present invention, a pair of permanent magnets opposed to each other with a gap therebetween and magnetized in the thickness direction, the permanent magnets and the outer side of the gap, and magnetically coupled to the permanent magnets An additional magnetic body is provided on the upper part of the permanent magnet facing magnetic circuit including a yoke to be coupled and a pair of magnetic pole pieces provided on each of the facing surfaces of the permanent magnet and having peripheral protrusions in the facing direction. Provided is a magnetic circuit magnetic field adjustment method for providing and adjusting a magnetic field.

  As will be described in detail below, according to the present invention, there is provided a permanent magnet facing magnetic circuit in which the magnetic efficiency of the upper and lower portions of the magnetic circuit is balanced and the magnetic field uniformity is further improved.

  Embodiments of the present invention will be described below with reference to the accompanying drawings. However, the embodiments described below do not limit the present invention.

  FIG. 1 shows a schematic front view (a), a plan view (b), and a side view (c) of a permanent magnet facing magnetic circuit according to one embodiment of the present invention. As shown in FIG. 1, a permanent magnet-facing magnetic circuit 1 according to the present invention is opposed to a pair of permanent magnets 2 which are opposed to each other with a gap therebetween and are magnetized in the thickness direction. A yoke 3 magnetically coupled to the permanent magnet, a pair of magnetic pole pieces 4 provided on opposite sides of the permanent magnet and having peripheral protrusions in the facing direction, and the magnetic And an additional magnetic body 6 provided in the upper part of the circuit. For convenience of explanation, an aspect in which the upper part and the lower part of the part excluding the base and the additional magnetic body of the permanent magnet facing magnetic circuit are symmetrical will be described. However, the present invention can be applied if the configuration is the same even if it is asymmetrical in the vertical direction.

  Since the permanent magnet 2 can be the same as the conventional one, a detailed description is omitted. A pair of permanent magnets magnetized in the thickness direction face each other with a gap therebetween, and form a main magnetic field in the gap. In particular, in the case of a magnetic circuit used for MRI, the main magnetic field generally has a magnetic field strength of 0.1 T or more. Although not particularly limited, sintered permanent magnets such as ferrite, alnico, and rare earth magnets and bonded magnets can be used as permanent magnets, and Nd—Fe—B, Sm—Co, and Sm— with high energy products. It is preferable to use an Fe—N rare earth sintered magnet because the amount of magnet used is small and the apparatus can be miniaturized. The shape of the permanent magnet is a circular shape, a quadrangular shape, or the like, and is not particularly limited, but a shape similar to that of the pole piece is preferable. The direction of magnetization is a direction (thickness direction) substantially perpendicular to the surfaces of the permanent magnets facing each other, and is generally the same direction. In particular, in the case of a magnetic circuit used for MRI, a permanent magnet that forms the main magnetic field of the magnetic circuit is generally configured using a magnet material having a residual magnetic flux density of 1T or more.

  Since the yoke 3 can be the same as the conventional one, a detailed description is omitted. A yoke is provided outside the gap between the permanent magnets to magnetically couple the permanent magnets. Thereby, a closed magnetic circuit is formed. The shape of the yoke is not particularly limited, and can be any shape, for example, a C shape, an H shape, a cylindrical shape, a round shape, or a two-pillar shape. The present invention is particularly preferably applied to a C-type magnetic circuit. The C-type magnetic circuit is relatively structurally low in strength and has a problem that vibration in the opening / closing direction of the opening is generated. This is because it can be improved. More specifically, a pair of substantially parallel plate yokes can be supported by columnar yokes, and permanent magnets can be provided on opposing surfaces of the plate yokes.

  Since the pole piece 4 can be the same as the conventional one, a detailed description is omitted. A pair of magnetic pole pieces having peripheral protrusions in opposing directions are provided on each of the opposing surface sides of the permanent magnet. Thereby, the uniformity of the magnetic field of a magnetic circuit improves. In other words, when a spherical or elliptical space (also referred to as an evaluation space) is virtually provided at the center of the gap, and the magnetic field uniformity in the evaluation space is evaluated, the magnetic pole piece is simple. In the case of a disk shape, the magnetic field intensity at the equator of the evaluation space is lower than that at the pole. On the other hand, when using a pole piece with peripheral protrusions around it, the physical distance between the equator and peripheral protrusions in the evaluation space is close and the magnetic field strength in the equator is improved. Uniformity is improved. Further, in order to further improve the magnetic field uniformity, a plurality of small circumferential projections (those having steps smaller than the peripheral projections) can be provided at the bottom of the pole piece.

  In general, a pair of gradient magnetic field coils (not shown) can be provided in the concave portion on the air gap side of the magnetic pole piece in order to generate a gradient magnetic field on the air gap side of the opposing magnetic pole piece. By this gradient magnetic field coil, the uniformity of the magnetic field can be intentionally disturbed linearly with respect to the magnetic field uniform space on the magnetic pole piece gap side. At this time, if an NMR signal including an inhomogeneous magnetic field is received, spatial information can be attached when the signal is imaged.

  Furthermore, the magnetic circuit according to the present invention may include a pedestal 5 for supporting the magnetic circuit. That is, the magnetic circuit according to the present invention may include a pedestal provided below the yoke. Although it does not specifically limit, As a material used for a base, nonmagnetic materials, such as magnetic materials, such as iron, aluminum, brass, and nonmagnetic stainless steel, can be used. Further, although not particularly limited, the shape of the pedestal can be an arbitrary leg-shaped pedestal such as a prism or cylinder. In addition, it is preferable that the pedestals be arranged so that the weight of the magnetic circuit is equally applied to each pedestal. Specifically, the magnetic circuit can be fixed by three-point support, four-point support, or the like. For example, when three points of a C-shaped magnetic circuit are supported, as shown in FIG. 1, pedestals can be installed at the open end of the plate-shaped yoke on the bottom surface and at the two corners below the columnar yoke. Further, for example, when four C-shaped magnetic circuits are supported (not shown), pedestals can be installed at the four corners of the plate-like yoke on the bottom surface. The pedestal can be directly fixed to a lower yoke in the magnetic circuit with a bolt or the like.

  Furthermore, the magnetic circuit according to the present invention includes an additional magnetic body 6 provided on the magnetic circuit. The present inventors have found that by providing an additional magnetic body, the magnetic efficiency of the upper and lower portions of the magnetic circuit can be balanced and the magnetic field uniformity can be further improved. Further, by firmly attaching the additional magnetic body of the present invention to the upper part of the magnetic circuit, an effect as a structural reinforcing material can be obtained at the same time.

  In the conventional magnetic circuit, there are magnetic factors such as a pedestal and temperature distribution at room temperature as external factors that strengthen the magnetic field on the lower side of the magnetic circuit, while there are no factors that increase the upper magnetic field. On the other hand, in the magnetic circuit according to the present invention, an additional magnetic body is provided on the upper part of the magnetic circuit, and the magnetic efficiency on the upper side of the magnetic circuit is balanced by increasing the magnetic efficiency on the upper side of the magnetic circuit in advance. Therefore, it is considered that the magnetic field uniformity can be further improved. Furthermore, in the present invention, the strength of the magnetic circuit can be increased by providing the additional magnetic body on the top of the magnetic circuit. That is, the yoke pole portion where the magnetic flux is concentrated is where the stress is concentrated due to the structure. In particular, in the case of a C-type magnetic circuit, there is a problem that vibration in the opening / closing direction of the opening occurs. However, according to the present invention, by using the additional magnetic material for reinforcing the magnetic circuit, it is possible to increase the strength of the magnetic circuit and to suppress vibrations.

  Although it is not particularly limited, the material used for the additional magnetic body is preferably soft iron, particularly steel such as structural rolled steel or carbon steel, cast steel, cast iron, pure iron or electromagnetic soft iron, and more preferably among them. An inexpensive iron material such as soft iron having a magnetic flux density of 1.6 to 2.3 T can be used. Further, it is desirable that the magnetic flux density and relative permeability of the additional magnetic material be higher. The reason for this is that the place where the additional magnetic body is attached is more effective in the portion where the magnetic flux yoke is concentrated, and the magnetic flux is leaked from the yoke and the magnetic efficiency is lowered. The higher the magnetic flux density of the additional magnetic material, the more magnetic flux can flow in the additional magnetic material, and the higher the relative permeability, the more the magnetic flux can be guided to the additional magnetic material, so the leakage magnetic flux can be reduced and magnetism can be reduced. This is because the efficiency can be increased. Although not particularly limited, the saturation magnetic flux density of the additional magnetic body is preferably 1.6 T or more, and more preferably 1.6 to 2.3 T. This is because the range of the saturation magnetic flux density of a material generally used as a yoke is about this level, and a material equivalent to or higher than the material of the yoke is desirable as the additional magnetic body. Moreover, although it does not specifically limit, The maximum relative magnetic permeability of an additional magnetic body becomes like this. Preferably it is 1000 or more, More preferably, it is 1000-12000. This is also because the range of the maximum relative permeability of a material generally used as a yoke is about this level, and the additional magnetic material is preferably equivalent to or more than the material of the yoke.

  The shape of the additional magnetic body can be selected from various shapes such as a rectangular parallelepiped, a cube, a cylinder, and a column. Among these, a cube and a rectangular parallelepiped are preferable because the amount of magnetic material to be added can be suitably adjusted by combining additional magnetic materials as necessary. The magnitude | size of a magnetic body should be suitably set according to the ratio which needs adjustment. Although not particularly limited, when the pedestal is made of a magnetic material such as iron, it is desirable to use a magnetic material having a weight equivalent to or higher than that of the pedestal as the additional magnetic body. This is because the imbalance between the upper and lower magnetic materials can be eliminated.

  The additional magnetic body is provided on the top of the magnetic circuit. Here, the upper part of the magnetic circuit means a part above the center of the air gap in the magnetic circuit. Thereby, it is possible to balance the magnetic efficiency between the upper part and the lower part of the magnetic circuit and further improve the magnetic field uniformity. Here, the additional magnetic body can also be provided in the lower part of the magnetic circuit as long as the additional magnetic body is provided with a larger weight in the upper part of the magnetic circuit. Although not particularly limited, the additional magnetic body can be directly fixed to the yoke with a bolt or the like.

  More specifically, in the magnetic circuit in which the yoke includes an upper yoke and a lower yoke each having a pair of permanent magnets, and a columnar yoke that magnetically couples the upper and lower yokes, The additional magnetic body is preferably provided on and near the upper yoke, that is, above the side of the columnar yoke and / or on the upper yoke. Here, the upper side portion of the columnar yoke refers to a portion of the side portion of the columnar yoke that is above the surface equidistant to each of the opposing permanent magnets.

  The additional magnetic body may be provided in the upper part of the magnetic circuit. However, as shown in FIGS. 1 to 3, it is more preferable to provide the additional magnetic body in the vicinity of the joint between the upper yoke and the columnar yoke where the magnetic flux is concentrated. This is because the leakage magnetic field can be further reduced by arranging the additional magnetic body in the vicinity of the joint where the magnetic flux is concentrated. More specifically, the additional magnetic body is preferably attached so as to straddle at least a part of the joint between the upper yoke and the columnar yoke. Moreover, an additional magnetic body can also be attached so that at least one part of the part corresponded just above a columnar yoke among upper yokes may be covered. In addition, as for the position which attaches an additional magnetic body, the outer side of a yoke is desirable with respect to the space | gap between opposing permanent magnets, ie, uniform space. This is because when the additional magnetic body is provided inside the yoke, the position of the additional magnetic body is close to a uniform space, which may adversely affect the magnetic field uniformity. Further, the number of additional magnetic bodies to be used may be one or plural. This should be set as appropriate according to the necessity of magnetic field adjustment.

  More specifically, as shown in FIG. 1, plate-like upper and lower yokes each having a pair of permanent magnets, and plate-like columnar magnetically coupling the upper and lower yokes. In a C-type magnetic circuit including a yoke, a plurality of rectangular parallelepiped additional magnetic bodies 6 each reach one end of which is connected to the columnar yoke of the upper yoke. The upper yoke covers a part of the portion corresponding to the top of the columnar yoke, and the longitudinal direction of each of the upper yokes extends from the end connected to the upper yoke to the opening side. It can be arranged to face. Such an aspect is preferable in the following points. That is, it is to ensure that the magnetic flux is concentrated near the joint between the upper yoke and the columnar yoke, while the additional magnetic material does not reach the opening side where the magnetic flux concentration is reduced. Furthermore, the shape is easy to process and can be manufactured at low cost.

  FIG. 2 shows a schematic front view (a), a plan view (b), and a side view (c) of a permanent magnet facing magnetic circuit according to another embodiment of the present invention. That is, as shown in FIG. 2, plate-like upper and lower yokes each having a pair of permanent magnets, and plate-like columnar yokes that magnetically couple the upper and lower yokes. In the C-type magnetic circuit including one of the rectangular parallelepiped additional magnetic bodies 6, one end reaches the end where it is connected to the columnar yoke of the upper yoke. Among these, it can arrange | position so that the whole region of the part corresponded just above a columnar yoke and a part of other part may be covered. Such an aspect is preferable in the following points. That is, it covers almost the entire seam where the magnetic flux is concentrated, and the effect of increasing the magnetic efficiency is high. This is because a material (plate material) that can be obtained at low cost can be used.

  FIG. 3 shows a schematic front view (a), a plan view (b), and a side view (c) of a permanent magnet facing magnetic circuit according to another embodiment of the present invention. That is, as shown in FIG. 3, plate-like upper and lower yokes each having a pair of permanent magnets, and plate-like columnar yokes that magnetically couple the upper and lower yokes. In a C-type magnetic circuit including a portion of a plurality of L-shaped additional magnetic bodies 6 over a part of the joint of the upper yoke and the columnar yoke, a part of which corresponds to the portion directly above the columnar yoke It can arrange | position so that a part of other part and a part of other part may be covered. Such an aspect is preferable in the following points. That is, it is applied not only to the upper yoke but also to the columnar yoke, and further a reinforcing effect can be expected.

  Embodiments of the present invention will be described below with reference to the accompanying drawings. However, the embodiments described below do not limit the present invention.

  As a magnetic circuit according to the example, a C-type magnetic circuit as shown in FIG. 1 was manufactured. The magnetic circuit had a central magnetic field strength of 0.2 T and a gap of 380 mm. Further, two additional magnetic bodies having a cross section of 100 mm × 800 mm, made of soft iron, having a magnetic flux density of 1.8 T, and a relative permeability of 2100 were provided in the magnetic circuit. Each of these additional magnetic bodies 6 reaches an end where one end of the additional magnetic body 6 is coupled to the columnar yoke of the upper yoke, so that each part is directly above the columnar yoke of the upper yoke. A part of the portion corresponding to is covered, and the respective longitudinal directions are arranged so as to go from the end connected to the columnar yoke of the upper yoke to the opening side. These additional magnetic bodies were attached to the upper yoke with bolts. Further, as a magnetic circuit according to the comparative example, a magnetic circuit similar to the magnetic circuit according to the above example was manufactured except that no additional magnetic material was provided.

  The magnetic circuit according to the example and the comparative example was placed in an environment where the magnetic circuit was actually used, and the relationship between the additional magnetic material and the magnetic field uniformity was tested as follows. At this time, the temperature distribution at room temperature was 20.3 ° C. at 20 cm from the floor and 21.5 ° C. at 150 cm from the floor in the air-conditioned state. About each of the magnetic circuit concerning an Example and a comparative example, the uniformity was measured with the NMR tesla meter. The uniformity was measured as follows. That is, the magnetic field intensity at 134 points on the surface of a sphere having a diameter of 350 mm between the permanent magnets of each magnetic circuit was measured using an NMR teslameter, and the uniformity was obtained from the difference between the maximum value and the minimum value.

  In the magnetic circuit according to the comparative example having no additional magnetic material, the uniformity of the magnetic field was 347 ppm. This is considered to be due to the effect of the reinforcing bars on the floor and the difference between the upper and lower temperatures. On the other hand, in the magnetic circuit according to the example provided with the additional magnetic material, the uniformity of the magnetic field could be 93 ppm. From this result, it can be seen that according to the present invention, vertical non-uniformity can be corrected by the additional magnetic material.

FIG. 1 shows a schematic front view (a), a plan view (b), and a side view (c) of a permanent magnet facing magnetic circuit according to one embodiment of the present invention. The typical front view (a), plan view (b), and side view (c) of the permanent magnet opposing magnetic circuit according to another embodiment of the present invention are shown. The typical front view (a), plan view (b), and side view (c) of the permanent magnet opposing magnetic circuit according to another embodiment of the present invention are shown. The typical front view (a) of the conventional permanent magnet opposing type | mold magnetic circuit, the top view (b), and the side view (c) are shown.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1,101: Permanent magnet opposing magnetic circuit 2, 102: Permanent magnet 3, 103: yoke 4, 104: Magnetic pole piece 5, 105: Base 6: Additional magnetic body

Claims (3)

A pair of permanent magnets opposed across a gap and magnetized in the thickness direction;
A yoke provided outside the permanent magnet and the air gap, and magnetically coupled to the permanent magnet;
A permanent magnet-facing magnetic circuit including a pair of magnetic pole pieces provided on each of facing surfaces of the permanent magnet and having peripheral protrusions in the facing direction,
A permanent magnet facing magnetic circuit further comprising an additional magnetic body provided on the magnetic circuit.   The yoke includes an upper yoke and a lower yoke each having the pair of permanent magnets, and a columnar yoke that magnetically couples the upper yoke and the lower yoke. The magnetic circuit according to claim 1, wherein the magnetic circuit is provided above a side portion of the columnar yoke and / or on the upper yoke. A pair of permanent magnets opposed across a gap and magnetized in the thickness direction;
A yoke provided outside the permanent magnet and the air gap, and magnetically coupled to the permanent magnet;
A magnetic field is adjusted by providing an additional magnetic body on top of the permanent magnet facing magnetic circuit provided on each of the facing surfaces of the permanent magnet and including a pair of magnetic pole pieces having peripheral protrusions in the facing direction. Magnetic circuit magnetic field adjustment method.

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