JPWO2012086644A1 - Static magnetic field coil apparatus, nuclear magnetic resonance imaging apparatus, and coil arrangement method for static magnetic field coil apparatus - Google Patents

Abstract

Provided are a static magnetic field coil device having a good static magnetic field uniformity while avoiding an increase in cost, and a nuclear magnetic resonance imaging apparatus (MRI apparatus) using the static magnetic field coil device. A static magnetic field coil device (2) comprising a plurality of coaxially arranged coils (2a) for generating a static magnetic field in a predetermined region (8), wherein the plurality of coils (2a) The coil (2a) is arranged in plane symmetry with a plane perpendicular to the central axis (10) of 2a) as the symmetry plane (11), and the coil (12a) located farther from the symmetry plane (11) Long in the axial direction and wide in the radial direction.

Description

  The present invention relates to a static magnetic field coil device that generates a static magnetic field, a nuclear magnetic resonance imaging device (hereinafter referred to as an MRI (Magnetic Resonance Imaging) device) using the static magnetic field coil device, and a coil arrangement method for the static magnetic field coil device. .

  An MRI apparatus is a device that obtains a cross-sectional image representing the physical and chemical properties of a subject by utilizing a nuclear magnetic resonance phenomenon that occurs when a subject placed in a uniform static magnetic field is irradiated with a high-frequency pulse. In particular, it is used for medical purposes. The MRI apparatus mainly includes a static magnetic field coil device that generates a uniform static magnetic field in an imaging region into which a subject is inserted, and a gradient magnetic field in which the magnetic field strength is spatially inclined to give position information to the imaging region. A gradient magnetic field coil for generating a pulse, an RF (Radio Frequency) coil for irradiating a subject with a high frequency pulse, a receiving coil for receiving a magnetic resonance signal from the subject, and processing the received magnetic resonance signal A computer system for displaying the cross-sectional image.

  The uniform static magnetic field generated by the static magnetic field coil device in the imaging region (for example, a sphere having a radius of about 15 to 30 cm or a spheroid having a size close to that) is a strong magnetic field (for example, 0.1 to several). Tesla or higher) and a very uniform magnetic field (for example, the deviation of the magnetic field is within about 3 ppm).

  In addition, the magnetic field generated by the static magnetic field coil device cannot be formed only in the imaging region, and the magnetic field leaks around the static magnetic field coil device. Since this leakage magnetic field is unnecessary for imaging by the MRI apparatus, attempts have been made to reduce the leakage magnetic field. For example, there has been proposed an active magnetic shielding type magnet device including a main coil group that generates a uniform static magnetic field in an imaging region and a shield coil group that generates a magnetic field that cancels a leakage magnetic field to the surroundings (for example, Patent Document 1).

  Moreover, as a device for generating a uniform static magnetic field, Patent Document 2 is characterized in that 7 to 9 small superconducting coils are installed so that the number of turns of the small superconducting coil (main coil) increases toward the outside. A superconducting magnet assembly for MRI is disclosed. In Patent Document 2, the position of the small superconducting coil is defined so as to ensure a uniform magnetic field by using an nth-order Legendre function for expressing the magnetic field.

  In addition, as a device for adjusting the generated static magnetic field, Patent Document 3 discloses a magnetic field adjustment device for a superconducting magnet in which a magnetic material shim mechanism is arranged in the inner peripheral axis direction of a cylindrical superconducting magnet. A combination shim tray in which a plurality of divided shim trays divided in the axial direction and shim tray spacers inserted between the shim trays are linearly coupled, and a magnetic shim for magnetic field adjustment is stored in the divided shim tray A magnetic field adjustment device for a superconducting magnet is disclosed.

JP 2009-397 Japanese Patent Laid-Open No. 05-190323 JP 2008-289703 A

  In the static magnetic field coil apparatus, the number of coils (main coil, shield coil) is not increased, the magnetomotive force of each coil is made as small as possible, and the leakage magnetic field to the surroundings is reduced while the MRI apparatus is used. There is a demand for magnetomotive force arrangement (coil arrangement) of an active magnetic block system with good imaging performance when mounted.

However, the method disclosed in Patent Document 2 does not have sufficient convergence in determining the position and shape of the small superconducting coil (main coil). Moreover, the cross-sectional shape of the small superconducting coil is not mentioned. In addition, although the uniformity of the static magnetic field is ensured using seven or more small superconducting coils, the smaller the number of small superconducting coils, the more advantageous in terms of cost and the miniaturization of the apparatus becomes possible.
In addition, in the method disclosed in Patent Document 3, sufficient discussion has not been made on the positional resolution (corresponding to the cross-sectional shape of the coil) required in the axial direction.

  Accordingly, the present invention provides a static magnetic field coil apparatus with good static magnetic field uniformity while avoiding an increase in cost, a nuclear magnetic resonance imaging apparatus (MRI apparatus) using the static magnetic field coil apparatus, and a coil arrangement method for the static magnetic field coil apparatus It is an issue to provide.

  In order to solve such a problem, the invention according to claim 1 is a static magnetic field coil device including a plurality of coils arranged coaxially to generate a static magnetic field in a predetermined region, The coil is arranged symmetrically with a plane perpendicular to the central axis of the coil as a plane of symmetry, and the coil located farther from the plane of symmetry is longer in the axial direction and wider in the radial direction. It is wide.

  According to the present invention, there are provided a static magnetic field coil device with good uniformity of a static magnetic field while avoiding an increase in cost, a nuclear magnetic resonance imaging device (MRI device) using the static magnetic field coil device, and a coil arrangement method of the static magnetic field coil device. Can be provided.

1 is an external perspective view of a nuclear magnetic resonance imaging (MRI) apparatus according to the present embodiment. It is sectional drawing which cut | disconnected the MRI apparatus which concerns on this embodiment by the yz plane containing a center object axis | shaft (z axis). It is a conceptual diagram which shows arrangement | positioning of the main coil and shield coil of a static magnetic field coil apparatus. In this example, the magnetic field distribution is obtained using seven eigenmodes. It is the graph which showed the relationship between the coil group total axial length and the uniformity in an imaging region for every number of used eigenmodes. It is a conceptual diagram explaining the arrangement | positioning of a main coil, and the determination method of cross-sectional shape. It is a conceptual diagram explaining adjustment of arrangement | positioning of the main coil. It is a figure which shows arrangement | positioning and shape of the main coil determined by this method. It is the result of having calculated | required the magnetomotive force arrangement | positioning which can generate | occur | produce a uniform magnetic field with respect to the coil group whole axial length given beforehand. It is the table | surface which showed each numerical value with respect to coil group total axial length. It is the graph which plotted the magnetomotive force relative value and the conductor amount relative value as a function of coil group total axial length / inner diameter (inner coil radius). It is a conceptual diagram of a shim tray.

  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. In each figure, common portions are denoted by the same reference numerals, and redundant description is omitted.

<Nuclear magnetic resonance imaging device>
FIG. 1 is an external perspective view of a nuclear magnetic resonance imaging (MRI) apparatus 1 according to this embodiment.
The MRI apparatus 1 is provided with a static magnetic field coil device 2 that generates a uniform static magnetic field in the imaging region 8 in which the subject 5 is inserted while lying on the bed 6, and for providing position information to the imaging region 8. Gradient magnetic field coil 3 that generates a gradient magnetic field with a spatial gradient of magnetic field intensity in a pulse shape, RF coil 4 that irradiates the subject 5 with a high-frequency pulse, and a reception coil that receives a magnetic resonance signal from the subject 5 (Not shown) and a computer system (not shown) for processing the received magnetic resonance signal and displaying the cross-sectional image. According to the MRI apparatus 1, the physical and chemical properties of the subject 5 are determined using the nuclear magnetic resonance phenomenon that occurs when the subject 5 placed in a uniform static magnetic field is irradiated with a high-frequency pulse. A cross-sectional image can be obtained, and the cross-sectional image is particularly used for medical purposes. Here, the static magnetic field coil device 2, the gradient magnetic field coil device 3, and the RF coil 4 have a cylindrical shape, and the cylindrical central symmetry axes 10 substantially coincide with each other.

FIG. 2 is a cross-sectional view of the MRI apparatus 1 according to the present embodiment cut along a yz plane including the central symmetry axis 10 (z axis).
The MRI apparatus 1 is a horizontal magnetic field type MRI apparatus in which the direction 7 of the static magnetic field is the horizontal direction. The subject 5 is carried to the imaging region 8 by the movable bed 6.
In order to facilitate understanding of the description to be described later, the x-axis, y-axis, and z-axis are set so as to be perpendicular to each other, and the origin is set near the center of the imaging region 8. The z-axis is provided so as to coincide with the central symmetry axis 10 of a cylindrical coil (coils 2a and 2b, gradient magnetic field coil 3 and RF coil 4 of a static magnetic field coil device 2 to be described later), and the (magnetic field line) direction of the static magnetic field 7 is also consistent. The y axis is provided in the vertical direction, and the x axis is provided in the direction perpendicular to the paper surface. In addition, about the coil of 2a, 2b, when not distinguishing both, it describes as coil 2a, 2b, and when distinguishing both, it describes as the main coil 2a and the shield coil 2b.

The static magnetic field coil device 2 includes a main coil 2a that generates a static magnetic field so as to form a pair on the left and right (parts where z <0 and z> 0) with respect to the symmetry plane 11 (z = 0 plane), A shield coil 2b that suppresses leakage of the magnetic field to the surroundings is used. Each of these coils 2a and 2b has an annular shape having a central axis of symmetry 10 as a common central axis.
Further, in order to obtain a high magnetic field and a temporally stable magnetic field as the static magnetic field generated by the static magnetic field coil device 2, superconducting coils are used as the coils 2a and 2b. Therefore, the coils 2a and 2b are housed in a three-layer container. First, the coils 2a and 2b are accommodated in the refrigerant container 2e together with the liquid helium (He) as a refrigerant. The refrigerant container 2e is contained in a heat radiation shield 2d that blocks heat radiation to the inside. The vacuum container 2c holds the refrigerant container 2e and the heat radiation shield 2d while keeping the inside in a vacuum. Even if the vacuum container 2c is disposed in a room at a normal room temperature, the inside of the vacuum container 2c is in a vacuum, so that the heat in the room is not transmitted to the refrigerant container 2e by conduction or convection. Moreover, the heat radiation shield 2d suppresses that the heat in the room is transmitted from the vacuum container 2c to the refrigerant container 2e by radiation. For this reason, the coils 2a and 2b can be stably set to the cryogenic temperature which is the temperature of the refrigerant, and can function as a superconducting electromagnet. The refrigerant container 2e, the heat radiation shield 2d, and the vacuum container 2c are made of a nonmagnetic member so as not to generate an unnecessary magnetic field, and further, a nonmagnetic metal is used because it is easy to maintain a vacuum. .

  The gradient magnetic field coil 3 has a cylindrical shape, and this cylindrical gradient magnetic field coil 3 is arranged so as to incorporate the imaging region 8. The RF coil 4 also has a cylindrical shape, and the cylindrical RF coil 4 is disposed so as to incorporate the imaging region 8. The gradient magnetic field coil 3 alternately generates a gradient magnetic field 9 in which the magnetic field intensity of the component in the same direction as the direction 7 of the static magnetic field is inclined in each of the three directions of the x direction, the y direction, and the z direction. The gradient magnetic field coil 3 has a function capable of generating independent gradient magnetic fields 9 in three directions of the x direction, the y direction, and the z direction so as to overlap the static magnetic field. FIG. 2 shows a gradient magnetic field 9 inclined in the y direction.

<Static magnetic field coil device>
As shown in FIG. 2, the coils 2 a and 2 b of the static magnetic field coil device 2 are formed by winding a wire (conductor, superconducting wire) around a winding frame (not shown), and share a central symmetry axis 10. It is coaxially arranged as the central axis. The coils 2a and 2b of the static magnetic field coil device 2 are arranged so as to be plane-symmetric with respect to the symmetry plane 11 (z = 0 plane).

FIG. 3 is a conceptual diagram showing the arrangement of the main coil 2 a and the shield coil 2 b of the static magnetic field coil device 2.
The static magnetic field coil device 2 shown in FIG. 3 has three pairs and six coils (main coil MC10, main coil MC20, main coil MC30, main coil MC31, main coil MC21) which become the main coil 2a inside the vacuum vessel 2c. , Main coil MC11). The main coil 2a is arranged so as to be symmetrical with respect to the plane of symmetry 11, the main coil MC10 and the main coil MC11 form a pair of coils 12a, and the main coil MC20 and the main coil MC21 form a pair. The coil 12b is formed, and the main coil MC30 and the main coil MC31 form a pair of coils 12c.
Moreover, the static magnetic field coil apparatus 2 shown in FIG. 3 has one pair and two coils (shield coil SC10, shield coil SC11) which become the shield coil 2b inside the vacuum vessel 2c. The shield coil 2b is arranged so as to be plane-symmetric with respect to the symmetry plane 11, and the shield coil SC10 and shield coil SC11 form a pair of coils 12d.

The main coils MC10, MC11, MC20, MC21, MC30, and MC31 can generate a uniform static magnetic field in the imaging region 8 by flowing a constant current in the same direction and forming a magnetic moment.
The shield coils SC10 and SC11 generate a magnetic field in the opposite direction by causing a constant current to flow in the opposite direction to the main coils MC10, MC11, MC20, MC21, MC30, and MC31 and forming magnetic moments, respectively. The leakage of the magnetic field from the apparatus 2 to the outside is reduced.

  FIG. 3 also shows magnetic field contour lines (magnetic lines) 16 and magnetic field intensity contour lines generated by the static magnetic field coil device 2 depending on the arrangement of the main coils MC10, MC11, MC20, MC21, MC30, MC31 and the shield coils SC10, SC11. 17 is shown. These magnetic flux contour lines 16 and magnetic field strength contour lines 17 are calculated by simulation. The magnetic field strength contour line 17 draws the magnetic field strength 3T + 3 ppm and the magnetic field strength 3T-3 ppm in a region surrounded by the inner peripheral side surface of the vacuum vessel 2c. In addition, although the magnetic field intensity | strength which the MRI apparatus 1 requires changes for every apparatus, it demonstrates as 3T here. In the region surrounded by the magnetic field strength contour line 17, a plurality of dots are described. Each dot indicates a position where the magnetic field strength is calculated by simulation. Only the dots at positions where the magnetic field intensity at that position has a magnetic field intensity of 3T or more are shown. That is, the magnetic field strength is calculated by simulation even at a position where no dot is described, but is not described because the magnetic field strength is outside the above range. The boundary between the area where the dots are described and the area where the dots are not described is the reference magnetic field strength 3T. Further, a magnetic field strength contour line 17 of 3T ± 3 ppm drawn along this boundary surrounds the imaging region 8. From this, it can be seen that in the imaging region 8, a 3T strong magnetic field can be realized with high uniformity such that the range of the maximum and minimum values is about 3 ppm. The magnetic field formed in the imaging region 8 is a temporally steady and spatially constant magnetic field, and can be set to a magnetic field strength of 0.1T to 7T or more. The imaging region 8 having a uniformity of about several ppm can be set in a range of a sphere (or ellipsoid) having a diameter (or major axis) of 30 cm to 40 cm.

≪Optimization of magnetomotive force arrangement≫
Next, optimization of magnetomotive force arrangement of the static magnetic field coil device 2 that generates an electrostatic field will be described. The magnetomotive force arrangement is determined by the arrangement and shape (cross-sectional shape) of the main coil 2a and the shield coil 2b. Also, optimizing the magnetomotive force arrangement refers to arranging the coils so that a good uniform magnetic field can be generated with a small amount of coil conductors.
In addition, about the optimization with respect to a leakage magnetic field is shown by patent document 1 (Unexamined-Japanese-Patent No. 2009-397), In this embodiment, the magnetomotive force arrangement | positioning which produces | generates a favorable static magnetic field, ie, the main coil 2a, is shown. An arrangement and shape determination method will be described.

<Singular value decomposition>
Here, singular value decomposition will be described before the description of the arrangement and shape determination method of the main coil 2a.
In the reference (Mr. Abe, Ph.D., "Research on Magnetic Field Reconstruction and Control Using Singular Value Decomposition and Its Application to Fusion Research", The Graduate University for Advanced Studies 2009), the response matrix from magnetomotive force distribution to magnetic field distribution is singular. A distribution function obtained by value decomposition is used. This embodiment also uses this singular value decomposition. In general, the equation to be solved is
B = AI (1)
B is a vector representing the magnetic field distribution (unit: Tesla (T)), I is a parameter (unit: ampere (A)) representing the current of each circuit, and A is its response matrix (unit: T / A).
The vectors b _i and i _i representing the eigendistribution obtained by the singular value decomposition are respectively normalized basis vectors of the magnetic field and the current distribution, and the subscript _i can be increased up to the rank number of the response matrix A. That means
^{_{A = ΣC i b i t λ}} i i i ··· (2)
It is. The superscript t represents transposition. The sum Σ is performed for the subscript i, and is added so that the required magnetic field accuracy is obtained by the combination of the sizes of C _i . Here, C _i is a ratio of adding the eigen distribution, but it can also be obtained by, for example, the inner product of the magnetic field distribution vector B and the eigen distribution vector b _i . The eigen distribution for executing the sum is selected so that the target magnetic field accuracy is obtained. λ _i is mathematically a singular value, but here corresponds to the magnetic field strength per unit current. That is, if a large eigen distribution of λ _i is used, a target magnetic field distribution, here a magnetic field distribution with good uniformity can be realized with a small magnetomotive force.
This is the basic idea, but in the following, an actual calculation example will be shown. Also, it will be referred to as eigenmodes with the b _i ^t lambda _i i _i collectively in the following description, also is referred to as i-th eigenmode using subscript i.

<Determination of the number of main coils>
Hereinafter, a magnetomotive force arrangement for generating a good static magnetic field according to the present embodiment, that is, a method for determining the arrangement and shape of the main coil 2a will be described.
First, determination of the number of coils of the main coil 2a of the static magnetic field coil device 2 will be described as optimization of the magnetomotive force arrangement of the static magnetic field coil device 2.
FIG. 4 is an example in which the magnetic field distribution is obtained using seven eigenmodes.
Here, the current flowing through the main coil 2a is considered as a collection of a large number of wire ring currents. The ring wheel current is a circular current having a predetermined radius in which a line current having a zero cross-sectional area flows about the central symmetry axis 10 at a certain axial position, and each element of the current potential vector I is The current value of the ring wheel is 18.

The response from the current potential vector I having each ring wheel current value 18 as an element to the magnetic field vector B can be expressed as B = A ₁ I using the response matrix A ₁ .
Here, assuming that a current of a unit current amount flows in the ring current, the magnetic field vector B for the ring current of the unit current amount can be calculated using the Biosavall equation. For each line wheels current, it is possible to determine the response matrix A ₁ is calculated sequentially magnetic field vector B.
Then, the current potential vector I (a collection of the ring current values 18 corresponding to the axial position) is calculated from the required magnetic field vector B (the static magnetic field in the imaging region 8 is uniform) by the singular value decomposition method. Can do. Then, based on the current potential vector I calculated by the method of singular value decomposition, it is possible to calculate the magnetic field vector B (magnetic field distribution) using a response function A _1.

  In the singular value decomposition, the eigenmode may be an eigenmode that is antisymmetric with respect to the symmetry plane 11, but the coils 2 a and 2 b (see FIG. 2) of the static magnetic field coil device 2 are with respect to the symmetry plane 11. Therefore, only eigenmodes that are symmetric with respect to the symmetry plane 11 are handled, that is, eigenmodes having a current distribution that is an even function are handled with respect to the axial position (z-axis).

  Each arrow extending upward in FIG. 4 indicates a wire ring current arranged at the root position, and the arrow length indicates a wire ring current value 18 that is the magnitude of the wire ring current. When the direction is upward, it indicates that a positive current is flowing, and when the direction of the arrow is downward, it indicates that a negative current is flowing. As shown in FIG. 4, there are the same number of peaks as the number of eigenmodes (seven in FIG. 4) in the current distribution along the axial direction.

  As will be described later, the main coil 2a (see FIG. 2) is disposed in the vicinity of this peak. In the example of FIG. 4, there are seven peaks in the ring current value 18, and the main coil 2a Seven coils are arranged. The shield coil 2b (see FIG. 2) is arranged in advance with reference to Patent Document 1 (Japanese Patent Laid-Open No. 2009-397), and the current value is the optimum described in Patent Document 3 (Japanese Patent Laid-Open No. 2008-289703). Current value.

FIG. 5 is a graph showing the relationship between the total axial length of the coil group and the uniformity of the magnetic field in the imaging region 8 (see FIG. 4) for each number of eigenmodes used. Each point in FIG. 5 (points indicated by □ ○ △ in FIG. 5) is calculated by simulation by calculating the magnetic field distribution in the same manner as in FIG. 4 by setting the number of eigenmodes and the total axial length of the coil group. The uniformity of the static magnetic field in the imaging region 8 is obtained from the calculated magnetic field distribution, and the relationship between the total axial length of the coil group in each eigenmode and the uniformity of the static magnetic field in the imaging region 8 is plotted.
Here, the “coil group total axial length” means the center of the main coil 2a that is symmetrically arranged from the end in the direction of the central symmetry axis 10 of the main coil 2a that is the coil that is located farthest from the symmetry plane 11. This is the distance to the end in the direction of the symmetry axis 10, that is, the total length of the main coil 2a in the direction of the center symmetry axis 10 (see FIG. 6), and the center object in which the wire ring current value shown in FIG. It is the space | interval of the wire ring current located in the both ends of the axis | shaft 10 direction. The “uniformity” is a deviation of the magnetic field in the imaging region 8, and the smaller the uniformity, the more suitable the static magnetic field.
FIG. 5 shows the relationship between the total axial length of the coil group and the uniformity of the static magnetic field in the imaging region 8 when the number of coils of the main coil 2a (that is, the number of eigenmodes) is 5 to 7. It was. The imaging region 8 was in a sphere having a diameter of 40 cm, the inner diameter of the main coil 2a was 985 mm, and the inner diameter of the shield coil 2b was 2046 mm.

As shown in FIG. 5, if the total axial length of the coil group is increased, the uniformity of the static magnetic field in the imaging region 8 is improved (decreased).
Here, the total axial length of the coil group is the axial length of only the coil portion, and further, a coil winding frame (not shown), a refrigerant container 2e, a heat radiation shield 2d, a vacuum container 2c, etc. are added to the static magnetic field coil device. 2 in size. Therefore, if the total axial length of the coil group is increased, the static magnetic field coil device 2 is also increased, and the MRI apparatus 1 is also increased.
For this reason, the maximum value of the total axial length of the coil group is set in consideration of carrying into the room where the MRI apparatus 1 is installed. In the following description, the maximum value of the total axial length of the coil group is assumed to be 1.6 m.

Under the condition that the total axial length of the coil group is within 1.6 m, the uniformity of the static magnetic field in the imaging region 8 when the number of main coils is 5 is about 5 ppm. Since the uniformity of the static magnetic field in the imaging region 8 required for the high-performance MRI apparatus 1 is 3 ppm or less, when the number of main coils is 5, the static magnetic field in the imaging region 8 necessary for imaging is reduced. Uniformity is not achieved.
Therefore, the number of coils of the main coil 2a of the static magnetic field coil device 2 in the present embodiment is 6 or more.

On the other hand, when the number of main coils is 7, the uniformity of the static magnetic field in the imaging region 8 is improved, but the manufacturing cost of the static magnetic field coil device 2 increases because the number of coils increases.
As described above, in the static magnetic field coil device 2 according to the present embodiment, the number of coils of the main coil 2a is preferably six in order to satisfy the uniformity required for imaging (3 ppm or less) and prevent an increase in manufacturing cost. It becomes.

By the way, shortening the total axial length of the coil group not only deteriorates (increases) the uniformity of the static magnetic field in the imaging region 8, but also causes problems in terms of coil current.
As shown in FIG. 4, a negative current (wire ring current value 18 indicated by a downward arrow) is distributed in the vicinity of the axial position ± 0.5 m. The region where such a negative current is generated is a line drawn from the upper left to the lower right as shown in FIG. 5 in the relationship between the number of coils of the main coil 2a (number of eigenmodes) and the total axial length of the coil group. This is the left area (the shaded area 52 in FIG. 5) of the two areas 51.
As described above, in order to shorten the total axial length of the coil group and ensure the uniformity of the static magnetic field in the imaging region 8, a negative current (current opposite to the current flowing through the main coil 2a) is aligned with the main coil 2a. ) Is required to be placed.

When a coil for passing a negative current is arranged, in order to secure the magnetic field strength of the static magnetic field in the imaging region 8, the current passed through the main coil 2a further needs a current to compensate for the negative current. The magnetomotive force and the amount of the conductor (superconducting wire) of the main coil 2a increase. For this reason, the manufacturing cost of the static magnetic field coil apparatus 2 increases. Further, since the positive and negative currents are adjacent to each other, the empirical magnetic field of the conductor (superconducting wire) of the coil (main coil 2a, coil through which the negative current flows) also increases.
For this reason, in order to ensure the stable superconducting electricity supply, it is necessary to increase the quantity of a conductor (superconducting wire). Thus, in the region 52 where the negative current is generated, the manufacturing cost of the static magnetic field coil device 2 increases.

From the above examination, the number of coils of the main coil 2a is 6 (that is, the natural mode is 6), and the static magnetic field is as short as possible in the total length of the coil group where no negative current is generated. It can be said that the coil device 2 is advantageous in terms of cost and performance.
That is, it is compact to design the static magnetic field coil device 2 near the intersection (line 53 in FIG. 5) of the line 51 in FIG. 5 and the line indicating the relationship between the coil group total axis length and the uniformity in the eigenmode 6. -It is advantageous in terms of performance and cost.

<Discrete coil arrangement>
Next, as an optimization of the magnetomotive force arrangement of the static magnetic field coil device 2, the arrangement of the main coil 2a and the cross-sectional shape of the static magnetic field coil device 2 will be described.
The eigenmode is 6 and the total axial length of the coil group that does not generate a negative current is set (see FIG. 5), and the current potential vector I (the ring wheel current value 18 (see FIG. 4)) is calculated by simulation. And the main coil 2a (MC10, MC11, MC20, MC21, MC30, MC31) (refer FIG. 3) is arrange | positioned in the position where the wire ring electric current value 18 becomes a peak.

The ring current distribution (current potential vector I) obtained by the simulation has a number of peaks (peaks) equal to the number of eigenmodes as shown in FIG. 4 (however, the eigenmode is 7 in FIG. 4). When the number of eigenmodes is six, six equal peaks are formed. The axial length of each crested portion is defined as the axial length of each main coil 2a. Then, the magnetomotive force (approximate value) of each main coil 2a is calculated by adding the wire ring current value 18 at each mountain. For each main coil 2a, the radial width of each main coil 2a is calculated based on the calculated magnetomotive force (approximate value), the current density described later, and the axial length of each main coil 2a.
Here, the current density is determined as a fixed value assuming a conductor (superconducting wire) and a winding method to be used. When the main coil 2a is a superconducting coil, for example, the current density value is about 50 to 250 A / mm ² .
The position and cross-sectional shape (the axial length and the radial width of the main coil 2a) determined in this way are used as initial values, and optimization is performed so that a uniform static magnetic field can be generated in the imaging region 8.

FIG. 6 is a conceptual diagram for explaining the arrangement of the main coil 2a and the method for determining the cross-sectional shape.
Next, based on the initial value of the position and cross-sectional shape of the main coil 2a (the axial length and radial width of the main coil 2a), the size of the cross-sectional area of the coil is changed while the current density is fixed, Optimization is performed so that a uniform static magnetic field can be generated in the imaging region 8.

Here, as shown in FIG. 6, the main coil 2a is arranged so as to be plane-symmetric with respect to the symmetry plane 11. Therefore, if the cross-sectional shape of the main coil 2a on the left side of FIG. The cross-sectional shape of the right main coil 2a is also determined.
Further, the position of the side of the coil 12a far from the symmetry plane 11 is fixed so that the total length of the coil group of the main coil 2a does not change. In addition, the inner diameter of the main coil 2a cannot be freely changed due to the geometric constraints of the static magnetic field coil apparatus 2 (MRI apparatus 1), so the inner diameter is fixed. In FIG. 6, the fixed sides of the cross-sectional shape are marked with “●”.

  From the above, as shown in FIG. 6, when the number of coils of the main coil 2a is 6 (eigenmode: 6), when the size of the coil cross-sectional area is changed, the sides of the cross-sectional shape that can be changed are 8 locations. Become. In FIG. 6, the symbols “← →” are attached to the sides to be changed.

For example, in the coil 12a, and the cross-sectional area of the coil and the [Delta] X ₁ varies in the radial direction of the coil (i.e., assuming that the radial width [Delta] X ₁ varies), the amount of change [Delta] I ₁ of the current flowing through the coil 12a is, [Delta] I ₁ = ΔX ₁ × (axial length of the coil 12a) × (current density).
Similarly, in the coil 12a, and the cross-sectional area of the coil and the [Delta] X ₂ changes in the axial direction of the coil (i.e., assuming that the axial length [Delta] X ₂ changes), the amount of change [Delta] I ₂ of the current flowing through the coil 12a is ΔI ₂ = ΔX ₂ × (radial width of the coil 12a) × (current density).

Here, the axial length and radial width of the coil 12a are obtained as initial values, and the current density is fixed. Therefore, the current change amount ΔI _i when the cross-sectional shape of the coil is changed by ΔX _i can be obtained.
That is, the response from the variation potential vector ΔX to the magnetic field vector B after changing the cross-sectional shape of the coil can be expressed as B = A ₂ ΔX using the response matrix A ₂ . Each element of the variation potential vector ΔX is a variation ΔX _i for each side.
Here, since the current change amount ΔI _i when changing one side (ΔX _i ) of the cross-sectional shape of the coil by a unit amount can be calculated, the cross-sectional shape of the cross-sectional shape can be calculated from the current change amount ΔI _i using the Biosavall equation. The magnetic field vector B with respect to the unit change amount can be calculated. The sides of the modifiable eight cross-sectional shape of a coil, by changing the sequential unit amount, it is possible to determine the response matrix A ₂ calculates the magnetic field vector B.
Then, by the technique of singular value decomposition, from the required magnetic field vector B (the static magnetic field in which the imaging region 8 is uniform), the change potential vector ΔX (the change amount of the side marked with “← →” in FIG. 6) A set of ΔX _i ) can be calculated.

  Here, as shown in FIG. 6, when the number of coils of the main coil 2 a is 6 (eigenmode: 6), as described above, there are 8 sides of the cross-sectional shape that can be changed, and the symmetry plane 11 is symmetric. This shows that eight eigen distributions of various components can be generated. This number is larger than the required number of eigendistributions described in FIG. 5 and is optimized so that a uniform static magnetic field can be generated in the imaging region 8 with the same accuracy as when approximated by the ring current. Can do.

As described above, when the total axial length of the coil group, the inner diameter limit of the main coil 2a, the current density, the uniform magnetic field distribution region and the uniformity condition are approximated by the wire ring current, Similarly, the change amount ΔX _i of the cross-sectional shape of the main coil 2a can be determined by a combination of eigenmodes obtained by singular value decomposition. That is, the position and cross-sectional shape of the main coil 2a can be determined from the position of the main coil 2a, the initial value of the cross-sectional shape, and the amount of change ΔX _i of the cross-sectional shape of the main coil 2a.

However, determining the amount of change delta _i of the cross-sectional shape of this main coil 2a (determining the position and cross sectional shape of the main coil 2a) is an optimum value by repeated calculation. This is because the change in the magnetic field with respect to the change in the cross-sectional shape is not linear, and the non-linear calculation is converged to an optimum value (position / shape) by repeated calculation.

  In the present embodiment, the coil whose cross-sectional shape is changed is described as the main coil 2a. However, the cross-sectional shape of the shield coil 2b may be changed. However, since the shield coil 2b has little influence on the uniformity of the static magnetic field, it is desirable to optimize the magnetomotive force mainly for magnetic shielding as in Patent Document 3 (Japanese Patent Laid-Open No. 2008-289703).

In the present embodiment, four sides (sides indicated with “●” in FIG. 6) are fixed and eight sides are changed. However, the present invention is not limited to this.
For example, weights W _i are introduced for all 12 sides, variable transformation is performed as “ΔX _i → ΔX _i / W _i ” and “A ₂ → A ₂ W _i ”, and a matrix composed of A ₂ W _i is singular Value decomposition may be performed. Here, when a small value is entered in W _i , only a small change in ΔX _i appears in the eigenmode obtained by singular value decomposition. Therefore, the end position of the coil cross section remains at substantially the same position. That is, sides "●" is attached is shown in FIG. 6, it is also practical way to set a smaller W _i.

<Discrete coil magnetomotive force and fine adjustment of coil position>
As described above, the position and the cross-sectional shape of the main coil 2a (the axial length and the radial width of the main coil 2a) are determined. At this stage, the coil magnetomotive force can take a continuous value. However, in the actual magnetostatic coil device 2, the coil magnetomotive force is the product of the wire current and the number of turns, and is proportional to the number of turns and takes a discrete magnetomotive force.

The winding method is determined so as to obtain a coil magnetomotive force and a shape close to the determined position and cross-sectional shape of the main coil 2a. When the winding method is determined in this way, the cross section (the number of turns in the axial direction and the number of layers in the radial direction) of the main coil 2a is determined, and only the position (radial direction and axial direction) of the main coil 2a becomes a variable.
However, the coil magnetomotive force is a discrete value (the product of the wire current and the number of turns) and cannot be matched exactly. As a countermeasure, fine adjustment of the coil position ensures magnetic field uniformity.
At this time, in order to secure a predetermined value of the in-coil radius, the coil radius position can be adjusted by changing the coil wire current value. That is, at this stage, the magnetic field distribution can be adjusted by adjusting the coil position (two-way fine adjustment for each coil) and the wire current value.

FIG. 7 is a conceptual diagram illustrating adjustment of the arrangement of the main coil 2a.
As shown in FIG. 7, the main coil 2a is composed of windings, and the winding state is shown in a partial region. FIG. 7 shows a round cross-section conductor 15a and a square stepped conductor 15b as examples. Further, if the conductors 15a and 15b are superconducting conductors, these conductors include superconducting conductors.
Further, in FIG. 7, directions in which the positions of the coils 12a, 12b, and 12c can be changed are indicated by arrows.

  The singular value decomposition is also applied at this stage. In other words, the new coil position is obtained by combining the eigenmodes obtained by the singular value decomposition by obtaining the response matrix to the magnetic field in the uniform magnetic field region using only the coil position and wire current change or only the coil position change as the vector on the input side. Get. This calculation is repeated and converged to obtain a coil position that secures a uniform magnetic field.

At this time, there are five variables for the position of the main coil 2a, as indicated by arrows in FIG. Although this number is less than the number of eigenmodes 6, it is possible to obtain a uniform accuracy with no practical problem. This is because the ring current is already calculated using singular value decomposition, and the position and cross-sectional shape of the main coil 2a are determined using singular value decomposition, so that the coil position and current are substantially optimized. Because it is.
At this stage, if the wire current value is treated as a variable, six variables can be obtained, and a magnetomotive force arrangement that generates a uniform static magnetic field in the imaging region 8 with high accuracy, that is, the main coil. The arrangement and shape of 2a can be determined. FIG. 8 shows the arrangement and shape of the main coil 2a determined by this method.

Here, in FIG. 7, the coil 12a (MC10, MC11 (see FIG. 3)) having the largest magnetomotive force is allowed only to change its position in the axial direction, and its radial position is fixed. This is because, by securing the size of the inner diameter of the main coil 2a, a space (bore) for carrying the subject 5 to the imaging region 8 by the movable bed 6 is secured in a necessary size.
Further, although the coil 12b (MC20, MC21 (see FIG. 3)) and the coil 12c (MC30, MC31 (see FIG. 3)) are allowed to change their position in the radial direction, the coil inner diameter is practically set to the coil 12a (see FIG. 3). MC10, MC11) cannot be made smaller. For this reason, it adjusts by moving to the direction where a coil internal diameter becomes large.
Therefore, as shown in FIG. 8, assuming that the inner diameter of the coil 12a (MC10, MC11) is Ra, the inner diameter of the coil 12b (MC20, MC21) is Rb, and the inner diameter of the coil 12c (MC30, MC31) is Rc, “Ra < Adjustment is made so that the relationship of Rb <Rc ”is established.
Here, “Rb <Rc” is used to move the coil position close to the symmetry plane 11 slightly away from the imaging region 8 in order to be robust against the influence on the coil shape error. However, this change in radius is within about 10% of Ra.

By the above procedure, it is possible to reproduce the magnetic field distribution targeting the arrangement and cross-sectional shape of the main coil 2a, reflecting the conductor wire current and the number of windings. That is, the static magnetic field coil apparatus 2 according to the present embodiment can generate a uniform static magnetic field required by the MRI apparatus 1 in the imaging region 8.
Thus, after determining the magnetomotive force of the shield coil 2b to an appropriate value, the uniformity and the uniformity of the static magnetic field in the imaging region 8 can be ensured by applying the singular value decomposition to the arrangement and shape of the main coil 2a. Can be optimized.
Further, if the eigenmode having a large singular value is selected by utilizing singular value decomposition and the magnetomotive force arrangement is determined, the magnetomotive force arrangement having a small magnetomotive force and ensuring a uniform magnetic field can be determined. In particular, since singular value decomposition is applied, the magnetomotive force, that is, the product of the wire current and the number of turns, can be kept small, and the magnetic field uniformity can be ensured. This makes it possible to ensure performance (uniformity) while reducing costs.

<Magnetomotive force arrangement determined by this method>
The magnetomotive force arrangement (coil arrangement) of the static magnetic field coil device 2 according to the present embodiment will be described with reference to FIG.
By utilizing the singular value decomposition, the magnetomotive force and the amount of wire can be reduced with respect to the given inner diameter of the main coil 2a (12a, 12b, 12c), the total axial length of the coil group, the number of coils and the required uniformity of the static magnetic field. Minimized. In this respect, the magnetomotive force is arranged with reduced cost.

  Further, the magnetomotive force arrangement (coil arrangement) of the static magnetic field coil device 2 according to the present embodiment is farthest from the symmetry plane 11 perpendicular to the central symmetry axis 10, and the coils 12 a (MC 10, MC 11) are more symmetrical to the symmetry plane 11. Compared to the close coils 12c (MC30, MC31) and the coils 12b (MC20, MC21), the coil length in the axial direction is long and the width in the radial direction is wide. The reason for this is the principle described next.

The interval between the magnetic flux contour lines (magnetic field lines) 16 in FIG. 3 is proportional to the product of the magnetic field strength and the radius, but in the figure, the magnetic field strength is also greater than the average magnetic field strength of the imaging area of about 3T with the magnetic field strength contour line 17. Regions are indicated by dots.
The magnetic field strength contour line 17 shows a contour line of ± 3 ppm accurately with respect to the average magnetic field strength. Here, the magnetic field strength is a magnetic field parallel to the arrow in the direction 7 (see FIG. 2) of the static magnetic field, and is a component B _z parallel to the z-axis direction (central symmetry axis 10).
In the imaging region 8 at the center of the static magnetic field coil device 2 (in the figure, a spherical region indicated by a solid semicircle, also referred to as a uniform magnetic field region), the magnetic field strength is within ± 3 ppm, and good uniformity is ensured. It can be seen that this is a magnetomotive force arrangement.

  When generating this uniform magnetic field region, the magnetic field distribution around the uniform magnetic field region repeats the strength of the magnetic field around the origin around the uniform magnetic field region. This is a result of obtaining the target uniform magnetic field distribution by superimposing the eigenmodes obtained by the singular value decomposition already described. In FIG. 3, the region of the strength of the magnetic field extends radially from the center of the static magnetic field coil device 2. The farther away from the uniform magnetic field, the stronger the magnetic field. Similarly, the region where the magnetic field is weak spreads away from the uniform magnetic field region. Since the strong magnetic field is naturally generated by the magnetomotive force of the coil, the coil is also present in the strong magnetic field. Therefore, the coil located farther from the symmetry plane 11 is present in a wider field with a stronger magnetic field, and the axial length of the coil becomes longer corresponding to the width of the strong magnetic field region. Further, since the magnetic field is further away from the uniform magnetic field region, a stronger magnetomotive force is required, and the coil in the radial direction becomes wider as the coil is farther from the symmetry plane 11 to increase the magnetomotive force.

FIG. 9 shows a result of obtaining a magnetomotive force arrangement capable of generating a uniform magnetic field with respect to all the axial lengths of the coil groups given in advance.
In the upper part of FIG. 9, the magnetomotive force arrangement is illustrated by taking three types of coil group total axial lengths (long, medium and short) as an example. In order to generate a uniform magnetic field, the position of the strength of the magnetic field does not depend on the total axial length of the coil group and exists at a certain position. For this reason, even if the input of the total axial length of the coil group is changed, the coil position exists at substantially the same position.
Only the length (axial length) and width (radial width) of the main coils MC10 and MC11 (coil 12a) at the extreme ends are largely changed by the change in the total axial length of the input coil group. It can be said.

If the total axial length of the coil group is increased, the uniformity of the static magnetic field is improved and the empirical magnetic field of the coil conductor is lowered, and a magnet with more stable performance can be manufactured, but the amount of the conductor wire material is increased. Further, the static magnetic field coil device 2 becomes large and causes an increase in cost.
On the other hand, if the total axial length of the coil group is shortened, the outer shape of the static magnetic field coil device 2 can be reduced, but the amount of the conductor (wire length) is particularly wide in the radial width of the main coils MC10 and MC11 (coil 12a). Since the average winding radius is increased, the cost is increased. In addition, the empirical magnetic field of the conductor is increased, which also causes an increase in cost.
As the MRI apparatus 1, in the static magnetic field coil apparatus 2 having a representative coil inner radius (500 mm) and a shield coil 2b (see FIG. 2) having a function of active magnetic shielding, the coil length is about 1500 mm and the length of the conductor is the longest. Since the thickness can be reduced, it can be said that it is appropriate to design and manufacture the magnet around 1500 mm. In addition, the static magnetic field coil device 2 of the MRI apparatus 1 is more easily installed if it is made compact. Therefore, the static magnetic field coil device 2 in which the total axial length of the coil group is in the vicinity of 1500 mm and the total axial length of the coil group is the short axis length side is preferable.
However, as indicated by “◯” in the lower graph of FIG. 9, the empirical magnetic field of the coil rapidly increases as the total axial length of the coil group becomes shorter, so that the total axial length of about 1450 mm or less is not preferable.
At this time, only the length (axial direction length) and width (radial direction width) of the main coils MC10 and MC11 (coil 12a) at the end are greatly changed as the shape of the coil cross section. (See the upper part of FIG. 9).

FIG. 10 is a table showing numerical values for the total axial length of the coil group.
The “total axial length / inner diameter” standardizes the total axial length of the coil group by the inner diameter of the coil 12a (MC10, MC11). “Length ratio (MC10 / MC20)” is obtained by dividing the axial length of the coil 12a (MC10, MC11) by the axial length of the coil 12b (MC20, MC21). “Length ratio (MC10 / MC30)” is obtained by dividing the axial length of the coil 12a (MC10, MC11) by the axial length of the coil 12c (MC30, MC31). “Length / width (MC10)” is obtained by dividing the axial length of the coil 12a (MC10, MC11) by the radial width of the coil 12a (MC10, MC11). The “width ratio (MC10 / MC20)” coil 12a (MC10, MC11) radial width is divided by the coil 12b (MC20, MC21) radial width. “Relative value of magnetomotive force” refers to the magnetomotive force of the coil 12a (MC10, MC11) when the “total axial length / inner diameter” is 3.0 (the inner radius is 500 mm and the total axial length of the coil group is 1500 mm). Normalized as a value of 1.0. “Conductor amount relative value” is the total conductor amount of all coils (MC10, MC11, MC20, MC21, MC30, MC31, SC10, SC11). The conductor amount when the total axial length of the group is 1500 mm) is standardized with a reference value of 1.0.

FIG. 11 shows a typical parameter shown in FIG. 10 in which “magnetomotive force relative value” and “conductor amount relative value” are functions of “total axis length (coil group total axis length) / inner diameter (coil radius)”. This is a plotted graph.
As can be seen from the table shown in FIG. 10 and the graph shown in FIG. 11, the magnetomotive force (relative value of magnetomotive force) decreases when the “total axial length / inner diameter” is 2.92 to 3.00.
Further, when the “total axial length / inner diameter” is 3.00 to 3.10, the conductor amount (conductor amount relative value) decreases.
Considering the manufacturability of the coil, it is not preferable that the magnetomotive force of the coil 12a (MC10, MC11) increases. On the other hand, a smaller amount of conductor is advantageous in terms of cost. Considering this, it can be said that the coil should be designed so that the “total axial length / inner diameter” is around 3.0, but before and after the actual design. 2.92 to 3.04 is a preferable region in which both the magnetomotive force and the conductor amount of the coil 12a (MC10, MC11) are sufficiently small. The coil group total axial length of 1500 mm and the inner radius of 500 mm are also included in this.

When the contents discussed so far are converted into these parameters, it is appropriate that the value of the “length / width” of the coil 12a (MC10, MC11) is 4.04 to 6.47. In an actual coil, the value varies depending on the type of conductor, so it may be slightly larger or smaller than this, but it is still appropriate to set the value in the range of 4.0 to 6.5.
In this range, the “length ratio (MC10 / MC30)” of the axial length also changes, but is in the range of 2.33 to 2.82. This is a value necessary for ensuring a uniform magnetic field.

<Sim placement>
By the way, in the actual static magnetic field coil apparatus 2, there is an error in manufacturing and assembly, and the coil cannot be arranged at an accurate coil position. For this reason, when the static magnetic field coil device 2 is manufactured and the MRI apparatus 1 is installed, fine adjustment is performed so that the magnetic field is actually generated and the magnetic field is measured to obtain a uniform static magnetic field.

The coil that generates a strong electromagnetic force is firmly fixed in the static magnetic field coil device 2 and is covered with the vacuum vessel 2c, the thermal radiation shield 2d, and the refrigerant vessel 2e, so that the position of the coil is corrected. I can't do it. For this reason, methods such as arranging an iron piece (magnetic material) or adjusting a current value by adding a fine adjustment coil are usually used.
When arranging the iron pieces, as shown in Patent Document 3, prepare a shim tray that is long in the axial direction, arrange a box-shaped container (to be referred to as a divided shim tray) therein, and further There is known a method of accommodating a magnetic piece having a size adapted to a divided shim tray.
In Patent Document 3, the position of the divided shim tray is changed to a position necessary for magnetic field correction. This method is performed by changing the order of combination with the shim tray spacer.

By the way, the magnetic field adjustment has a meaning of adjusting the magnetic field distribution so as to become a strong and weak distribution as shown in FIG. Therefore, the arrangement of the magnetic shim in the shim toilet needs to have a resolution of 1/2 position with respect to the axial length of the strong and weak distribution, and further resolution is not necessary.
Therefore, if the knowledge of the present invention is utilized, it can be said that the axial position resolution is sufficient as long as the size in the axial direction is within half of the interval of the coil arrangement. The size of the shim tray in the axial direction is such that the farther away from the symmetry plane 11, the larger the length in the axial direction. Adjustment (shimming) is possible.

FIG. 11 shows a conceptual diagram of such a shim tray. In FIG. 11, a cross section of the shim tray 24 is shown on the upper side, a cross section of the plane perpendicular to the central axis (upper right side) and a cross section on the plane including the central axis (upper left side).
The shim magnetic body 22 is arranged in a circumferential direction and an axial direction. Here, conventionally, shim magnetic bodies having the same length in the axial direction have been used. Therefore, the shim tray 24 has a structure in which magnetic bodies of the same size can be arranged.

  On the other hand, when the knowledge of the present invention is used, the magnetic bodies 22 </ b> A and 22 </ b> B inside the shim tray 20 have different sizes in the axial direction, and a longer magnetic body is disposed in the axial direction as the distance from the imaging region 8 increases. That is, the magnetic body 22B closer to the imaging area 8 is a magnetic body having the same axial length as the conventional magnetic body, but the magnetic body 22A farther from the imaging area 8 is closer to the imaging area 8. The length in the axial direction is longer than that of the magnetic body 22B. Further, the shape of the shim tray 24 has a structure in which such magnetic bodies 22A and 22B can be arranged.

  The shim tray 24 is disposed in the magnet bore portion. In FIG. 3, the radial position is around 40 cm. Since the axial length of the magnetic material is about half the wavelength in FIG. 3 (strike area and non-spot area), it is 5 to 7.5 cm near the imaging area 8 (part close to the coil 12c). Is the axial length. On the other hand, at a position close to the coil 12a (a position far from the imaging region 8), an axial length of 1.5 times is sufficient as with the coil length ratio “(MC10 / MC20)” (see FIG. 10). It can be said that magnetic field adjustment can be performed.

  As shown in FIG. 12A, the magnetic body 22 (22A, 22B) arranged on the shim tray 24 may be composed of a single magnetic body piece. As shown in FIG. By combining the thin pieces 22a, 22b, and 22c, the magnetic body 22 (22A and 22B) can be configured to be overlapped with a desired thickness. The shim tray 24 has a structure in which the shim magnetic body 22 can be disposed so as to surround the central axis. Thereby, the shim magnetic body 22 can be arrange | positioned in the arbitrary positions of an axial direction and a circumference direction.

In the above discussion, in order to optimize the leakage magnetic field, the magnetic moment of the shield coil 2b (Mres = Σ∫i _r πr drdz) is made 0.25% to 2.00% smaller than the magnetic moment of the main coil 2a. ing.
Here, i is the density of the current in the circumferential direction, and the shield coil 2b has a negative current density because a current in the direction opposite to that of the main coil 2a flows. r is the radius of the shield coil 2b, and the integral is the area integral in the cross section of all the coils. The sum is the sum of the integral values for each coil.
By making the magnetic moment of the shield coil 2b smaller than the magnetic moment of the main coil 2a, the amount of conductor of the entire coil is reduced and the area of the leakage magnetic field is also optimized.
If the ratio of reducing the magnetic moment of the shield coil 2b from the magnetic moment of the main coil 2a is greater than 2.00%, the generation of the leakage magnetic field cannot be sufficiently suppressed by the shield coil 2b. Further, if the ratio of reducing the magnetic moment of the shield coil 2b to the magnetic moment of the main coil 2a is less than 0.25%, the effect of reducing the conductor amount of the entire coil is reduced. For this reason, it is desirable to make the magnetic moment of the shield coil 2b 0.25% to 2.00% smaller than the magnetic moment of the main coil 2a.

DESCRIPTION OF SYMBOLS 1 Nuclear magnetic resonance imaging (MRI) apparatus 2 Static magnetic field coil apparatus 2a, 12a, 12b, 12c Main coil (coil)
2b, 12d Shield coil 2c Vacuum vessel 2d Thermal radiation shield 2e Refrigerant vessel 3 Gradient magnetic field coil 4 RF coil 5 Subject 6 Bed 7 Static magnetic field (lines of magnetic force) direction 8 Imaging region (predetermined region)
9 Gradient magnetic field 10 Center symmetry axis 11 Symmetry plane 24 Shim tray (Sim iron arrangement mechanism)
22, 22A, 22B Shim magnetic body (shim iron piece)

Claims (10)

A static magnetic field coil device comprising a plurality of coils arranged coaxially for generating a static magnetic field in a predetermined region,
The plurality of coils are:
The coil is arranged in plane symmetry with a plane perpendicular to the central axis of the coil as a plane of symmetry,
A static magnetic field coil device, wherein a coil located farther from the symmetry plane is longer in the axial direction and wider in the radial direction. The static ratio according to claim 1, wherein a ratio of an axial length to a radial width of a cross section of the coil existing farthest from the symmetry plane is in a range of 4.0 to 6.5. Magnetic field coil device. The total axial length of the coil group, which is the distance from the axial end of the coil located farthest from the symmetry plane to the axial end of the coil arranged symmetrically, and the inner radius of the coil The static magnetic field coil device according to claim 1, wherein the ratio is in a range of 2.92 to 3.04. The inner diameter of the coil present at a position farthest from the plane of symmetry is:
2. The static magnetic field coil device according to claim 1, wherein the coil has an inner diameter smaller than the inner diameter of the coil located closer to the symmetry plane than the coil. A shield coil for reducing a magnetic field generated by the plurality of coils outside the predetermined region;
2. The static magnetic field coil device according to claim 1, wherein a sum of magnetic moments of the shield coils is 0.25% to 2.00% smaller than a sum of magnetic moments of the plurality of coils. It further comprises an arrangement mechanism for arranging shim iron pieces for adjusting the magnetic field distribution,
The arrangement mechanism of shim iron pieces arranged at a position far from the symmetry plane is as follows:
2. The static magnetic field according to claim 1, wherein a larger shim iron piece can be arranged in a more axial direction as compared with a shim iron piece arrangement mechanism that is closer to the symmetry plane than the arrangement mechanism. Coil device. 2. The static magnetic field coil device according to claim 1, wherein each of the plurality of coils is formed by winding a wire. 3.   A nuclear magnetic resonance imaging apparatus using the static magnetic field coil apparatus according to claim 1. A coil arrangement method of a static magnetic field coil device comprising a plurality of coils arranged coaxially to generate a static magnetic field in a predetermined region,
A number determining step for determining the number and approximate position of the plurality of coils in a current distribution of a plurality of wire ring currents;
A cross-sectional shape determining step for determining a coil cross-sectional shape corresponding to the magnetomotive force based on the current distribution of the wire ring current;
And a step of optimizing the determined cross-sectional shape of the coil by singular value decomposition. The coil is formed by winding a wire,
The position optimization step of determining a winding structure of the wire based on the optimized cross-sectional shape and optimizing the position of the coil by singular value decomposition. The coil arrangement method of the static magnetic field coil apparatus.

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