JP2012217573A - Magnetic resonance imaging apparatus, and gradient coil - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic resonance imaging apparatus 1 for highly accurately and uniformly adjusting the magnetic field of a uniform magnetic field space 3 by arranging shims 6c even when the shape of an opening, in which a subject is carried, is not a circular shape.SOLUTION: The magnetic resonance imaging apparatus includes: a magnet device 2 having a cylindrical outer shape and generating the uniform magnetic field space 3 inside the cylindrical shape; and a magnetic field adjusting means 6 for arranging the shims 6c made of a magnetic material on a shim arrangement curved surface C1 inside the cylindrical shape to uniform the magnetic field of the uniform magnetic field space 3. The shape of a cross-section on a plane perpendicular to the axis z of the shim arrangement curved surface C1 is not a circular shape. A distance between the shim arrangement curved surface C1 and the axis z on the plane varies depending on places on the shim arrangement curved surface C1. There are places with the large distances and the places with the distances smaller than that of the places with the large distances. Intervals between the shims 6c adjacent in the peripheral direction of the axis z at the places with the large distances are wider than intervals at the places with the smaller distances.

Description

  The present invention relates to a magnetic resonance imaging apparatus and a gradient magnetic field coil in which shims are arranged to make a magnetic field in a uniform magnetic field space more uniform.

  Magnetic Resonance Imaging (MRI) equipment uses the magnetic resonance phenomenon that occurs when a high-frequency pulse is irradiated to a subject placed in a uniform magnetic field of a static magnetic field. An image representing a property can be taken, and is used especially for medical purposes. The magnetic resonance imaging apparatus includes a magnetic field generation source (magnet device) that generates a uniform magnetic field space in an imaging region into which a subject is carried, an RF coil that irradiates a high-frequency pulse toward the imaging region (uniform magnetic field space), and imaging A receiving coil that receives a response from the region (homogeneous magnetic field space) and a gradient magnetic field coil that superimposes a gradient magnetic field for giving positional information of the resonance phenomenon to the imaging region (homogeneous magnetic field space).

  One of the requirements for determining the image quality of an image captured by the magnetic resonance imaging apparatus is the uniformity of the static magnetic field in the imaging region (homogeneous magnetic field space). The uniformity of the static magnetic field is calculated, for example, as the ratio of the difference between the maximum magnetic field strength and the minimum magnetic field strength in the imaging region (homogeneous magnetic field space) to the average magnetic field strength in the imaging region (homogeneous magnetic field space). Can do. The homogeneity of the static magnetic field varies depending on the imaging target. For example, in order to obtain a general image, a uniformity of 30 ppm or less is required in a spherical imaging region (uniform magnetic field space) having a diameter of 30 cm. In order to remove the received signal of the fat portion, a uniformity of 3 ppm or less is required in a spherical imaging region (uniform magnetic field space) having a diameter of 10 cm.

  In the magnetic resonance imaging apparatus, in order to achieve such a high degree of uniformity in the imaging region (homogeneous magnetic field space), a magnetic field adjustment operation called shimming is performed in the manufacturing stage or installation stage of the magnetic resonance imaging apparatus. For shimming, an active shimming method that energizes the built-in coil to improve the uniformity of the magnetic field generation source or the gradient magnetic field coil, or a shim of a magnetic material such as iron or permanent magnet is placed around the imaging region (uniform magnetic field space). There is a passive shimming method in which a uniform magnetic field is realized by arranging and controlling magnetic flux. Currently, many magnetic resonance imaging apparatuses employ a passive shimming method.

  As a method for realizing this passive shimming method, for example, a method of arranging a shim between a main coil and a secondary coil (shield coil) of a gradient magnetic field coil has been proposed (see Patent Document 1 and the like). Furthermore, in this arrangement, a method has been proposed in which shims are arranged at equal intervals in the axial direction and the circumferential direction of a cylindrical gradient magnetic field coil (see Patent Document 2, etc.). In Patent Document 2, it is also proposed to arrange shims at unequal intervals in the axial direction for the purpose of increasing the uniformity. Furthermore, a method has been proposed in which an axial interval for arranging shims can be arbitrarily set (see Patent Document 3). In addition, a method of arranging a shim on the inner peripheral side of the gradient coil has been proposed (see Patent Document 4).

JP-A-8-299304 JP 2009-273930 A Japanese Patent No. 4368909 JP-A-8-53187

  The magnet device of the magnetic resonance imaging device has a double cylindrical appearance, and the imaging region (uniform magnetic field space) is generated inside the outer cylinder of the appearance of the magnet device. For this reason, the subject is carried into the inner cylinder for imaging. Some people who are subjects may feel a pressure about being carried into the inner cylinder. Therefore, various proposals have been made to alleviate the feeling of pressure. For example, in order to make the shape of an opening (bore) into which a subject is carried in as close to a human shape as possible, it has been proposed to make it non-circular, for example, an ellipse or a race track. Accordingly, the cross-sectional shape of the inner cylinder of the gradient magnetic field coil arranged in the inner cylinder of the magnet device is made non-circular.

  Since the shim is also arranged in the inner cylinder of the magnet device, like the gradient magnetic field coil, it is desirable to arrange the shim so that the shape of the bore becomes as close to a human figure as possible and becomes non-circular. However, even with this shim arrangement, the magnetic field in the imaging region (homogeneous magnetic field space) must be uniform with high accuracy.

  Accordingly, an object of the present invention is to provide a uniform magnetic field in an imaging region (homogeneous magnetic field space) by arranging a shim even if the shape of an opening (bore) into which a subject is carried is non-circular. It is an object of the present invention to provide a magnetic resonance imaging apparatus that can be adjusted to the above.

In order to achieve the above object, the present invention provides:
A magnet device having a cylindrical external shape and generating a uniform magnetic field space inside the cylindrical shape;
A magnetic material shim disposed on the inner shim arrangement curved surface of the cylindrical shape, and a magnetic field adjusting means for making the magnetic field of the uniform magnetic field space more uniform,
The shape on the cross section in a plane perpendicular to the cylindrical central axis of the shim arrangement curved surface is non-circular,
The distance between the shim placement curved surface and the central axis on the plane varies depending on the location on the shim placement curved surface, there are locations where the distance is large and locations smaller than the large location, and where the distance is large The distance between the shims adjacent to each other in the circumferential direction of the central axis is wider than the distance at the position where the distance is small.

The present invention also provides:
In a gradient magnetic field coil that is cylindrical and is provided inside a magnet device that generates a uniform magnetic field space inside the cylindrical shape, and for superimposing a gradient magnetic field on the uniform magnetic field space,
It is arranged on the shim arrangement curved surface, and has a storage hole that makes the magnetic field of the uniform magnetic field space more uniform by storing the shim of the magnetic material,
The shape on the cross section in a plane perpendicular to the cylindrical central axis of the shim arrangement curved surface is non-circular,
The distance between the shim placement curved surface and the central axis on the plane varies depending on the location on the shim placement curved surface, there are locations where the distance is large and locations smaller than the large location, and where the distance is large An interval between the storage holes adjacent to each other in the circumferential direction of the central axis is wider than the interval at the place where the distance is small.

  According to the present invention, even when the shape of the opening (bore) into which the subject is carried is non-circular, the magnetic field in the imaging region (homogeneous magnetic field space) is uniformly adjusted with high accuracy by arranging the shim. It is possible to provide a magnetic resonance imaging apparatus and a gradient coil that can be used.

1 is a perspective view of a magnetic resonance imaging apparatus according to a first embodiment of the present invention. It is sectional drawing which cut | disconnected the magnetic resonance imaging device which concerns on the 1st Embodiment of this invention by yz plane. It is sectional drawing which cut | disconnected the gradient magnetic field coil of the magnetic resonance imaging device which concerns on the 1st Embodiment of this invention by xy plane. It is a perspective view of a shim tray. It is a figure which shows an example of the analysis grid used when calculating the space | interval which should arrange | position a shim (shim tray) in the circumferential direction. It is a graph which shows an example of distribution of the magnetic moment centering on the x-axis direction (major axis direction) calculated using the calculation method at the time of calculating the interval which should arrange | position a shim (shim tray) in the circumferential direction. It is a graph which shows an example of distribution of the magnetic moment centering on the y-axis direction (minor axis direction) calculated using the calculation method at the time of calculating the space | interval which should arrange | position a shim (shim tray) in the circumferential direction. It is sectional drawing which cut | disconnected the gradient magnetic field coil of the magnetic resonance imaging device which concerns on the 2nd Embodiment of this invention by xy plane.

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

(First embodiment)
FIG. 1A shows a perspective view of a magnetic resonance imaging apparatus 1 according to the first embodiment of the present invention. The magnetic resonance imaging apparatus 1 includes a magnet device 2 that generates a uniform static magnetic field in an imaging region (homogeneous magnetic field space) 3, and a magnetic field strength that is spatial in order to give positional information to the imaging region (homogeneous magnetic field space) 3. A gradient magnetic field coil 4 for generating a gradient magnetic field gradient in a pulse shape, an RF coil 5 for irradiating a subject introduced into an imaging region (uniform magnetic field space) 3 with a high frequency pulse, and a magnetic resonance signal from the subject. It has a receiving coil (not shown) for receiving, and a computer system (not shown) for processing the received magnetic resonance signal and displaying an image. According to the magnetic resonance imaging apparatus 1, the physical and chemical properties of the subject are determined by using the nuclear magnetic resonance phenomenon that occurs when the subject placed in a uniform static magnetic field is irradiated with a high-frequency pulse. An image such as a tomographic image can be obtained, and the image is used particularly for medical purposes.

  The magnet device 2 has a double-cylindrical appearance, and the center axes of the inner cylinder 2a and the outer cylinder 2b substantially coincide with each other, and furthermore, they coincide with the z-axis. The shape on the cross section in the plane perpendicular to the central axis (z axis) of the inner cylinder 2a and the outer cylinder 2b is substantially circular. A gradient magnetic field coil 4 is provided on the inner side of the inner cylinder 2 a of the magnet device 2.

  The gradient coil 4 also has a double cylindrical appearance, and the central axes of the inner cylinder and the outer cylinder substantially coincide with each other, and furthermore, they coincide with the z axis. The shape on the cross section in the plane perpendicular to the central axis (z-axis) of the outer cylinder of the gradient coil 4 is substantially circular. The shape on the cross section in a plane perpendicular to the central axis (z-axis) of the inner cylinder of the gradient coil 4 is a non-circular ellipse. In the first embodiment, the non-circular shape is an ellipse that resembles a human shape (cross-sectional shape to be inspected) rather than a circular shape. A race track may be used.

  An RF coil 5 is provided inside the non-circular inner cylinder of the gradient magnetic field coil 4 along the inner cylinder. The RF coil 5 has a cylindrical shape, and the central axis of the cylindrical shape coincides with the z axis. The shape on the cross section in the plane perpendicular to the central axis (z-axis) of the RF coil 5 is a non-circular ellipse. The space inside the cylindrical shape of the RF coil 5 becomes an opening (bore) into which the subject is carried. For this reason, the shape of the opening (bore) into which the subject is carried is also a non-circular ellipse. An imaging region (uniform magnetic field space) 3 is generated in an opening (bore) into which the subject is carried. In the present embodiment, the y-axis direction is set upward in the vertical direction. The x-axis direction is set to the horizontal direction, and is further set to the direction in which the screw advances when the screw is turned from the y-axis direction to the z-axis direction.

  The gradient magnetic field coil 4 is provided with magnetic field adjusting means 6. The magnetic field adjusting means 6 arranges a plurality of shim trays 6 a containing magnetic material shims 6 c in a ring shape in the gradient magnetic field coil 4 to make the magnetic field in the imaging region (uniform magnetic field space) 3 more uniform. The shim 6c is stored in the pocket 6d of the shim tray 6a. The shim tray 6 a is stored in a tray storage hole (storage hole) 6 b provided in the gradient magnetic field coil 4. The depth direction of the tray storage hole 6b is parallel to the direction of the central axis (z axis). A plurality of tray storage holes 6b are arranged in the circumferential direction of the central axis (z axis). The tray storage hole 6b is arranged on the shim arrangement curved surface C1. The shim arrangement curved surface C1 has a cylindrical shape, and the central axis of the cylindrical shape coincides with the z-axis. The shape on the cross section in a plane perpendicular to the central axis (z-axis) of the shim arrangement curved surface C1 is a non-circular ellipse. The larger the distance between the shim arrangement curved surface C1 and the central axis (z axis) on the cross section in the plane perpendicular to the central axis (z axis) of the shim arrangement curved surface C1, the greater the circumferential direction of the central axis (z axis). The interval between adjacent shim trays 6a (shim 6c) is widened. The distance between the shim arrangement curved surface C1 and the central axis (z axis) on the plane differs depending on the location on the shim arrangement curved surface C1, and is smaller than the location where the distance is large (for example, the location on the x axis) and the large location. There are locations (for example, locations on the y-axis), and the distance between the tray storage holes (storage holes) 6b (shim 6c) adjacent in the circumferential direction of the central axis (z-axis) at the location where the distance is large is The distance is wider than the interval between the tray storage holes (storage holes) 6b (shim 6c) at the small portion.

  FIG. 1B shows a cross-sectional view of the magnetic resonance imaging apparatus 1 according to the first embodiment of the present invention cut along a yz plane. The magnet device 2 is provided with a superconducting coil group 2 f that generates a uniform static magnetic field in the imaging region (uniform magnetic field space) 3. The superconducting coil group 2f has an annular shape with the z axis as a common central axis. Superconducting coil group 2f is housed in a three-layer container. The superconducting coil group 2f is accommodated in the helium vessel 2e together with the refrigerant helium (He). The helium container 2e is contained in a radiation shield 2d that blocks heat radiation to the inside. The double cylindrical vacuum container 2c holds the helium container 2e and the radiation shield 2d while keeping the inside in a vacuum. Even if the vacuum vessel 2c is disposed in a normal room temperature room, since the vacuum vessel 2c is evacuated, the heat in the chamber is not transmitted to the helium vessel 2e by conduction or convection. Moreover, the radiation shield 2d suppresses that the heat in the room is transmitted from the vacuum vessel 2c to the helium vessel 2e by radiation. For this reason, the superconducting coil group 2f can be stably set to an extremely low temperature that is the temperature of liquid helium, and can function as a superconducting electromagnet. As a result, the superconducting coil group 2 f can maintain a large current with zero electrical resistance, and can generate a strong and uniform magnetic field in the imaging region (uniform magnetic field space) 3.

  FIG. 2 shows a cross-sectional view of the gradient magnetic field coil 4 of the magnetic resonance imaging apparatus 1 according to the first embodiment of the present invention cut along the xy plane. The gradient magnetic field coil 4 is a coil for generating an arbitrary gradient magnetic field in the imaging region (uniform magnetic field space) 3, and has a gradient magnetic field having an arbitrary intensity on each of three orthogonal axes (x axis, y axis, and z axis). Independent main coils 7x, 7y, 7z capable of producing

  The gradient coil 4 has main coils 7x, 7y, 7z arranged on the inner side (center axis (z-axis) side) and sub-coils 8x, 8y, 8z arranged on the outer side (magnet device 2 side). ing. The main coils 7x, 7y and 7z generate a gradient magnetic field in the imaging region (uniform magnetic field space) 3, but also generate a so-called leakage magnetic field in the space where the inner cylinder 2a (see FIG. 1A) of the magnet device 2 is disposed. Let Therefore, a current in a direction opposite to that of the main coils 7x, 7y, and 7z is caused to flow through the auxiliary coils 8x, 8y, and 8z to generate a magnetic field that cancels (suppresses) the leakage magnetic field. The main coils 7x, 7y, and 7z mainly generate a gradient magnetic field in the imaging region (homogeneous magnetic field space) 3, and the sub-coils 8x, 8y, and 8z are mainly magnets generated by the main coils 7x, 7y, and 7z. Each wiring pattern is determined so as to prevent leakage to the device 2 (inner cylinder 2a).

  The main coils 7x, 7y, and 7z include a z main coil 7z that generates a gradient magnetic field that linearly changes in the z-axis direction, an x main coil 7x that generates a gradient magnetic field that linearly changes in the x-axis direction, and the y-axis direction. And y main coil 7y for generating a linearly changing gradient magnetic field. By applying a pulsed current to each of the z main coil 7z, the x main coil 7x, and the y main coil 7y, a gradient magnetic field inclined in each corresponding direction is generated, and a magnetic resonance signal is generated in the subject. Position information is given. The z main coil 7z is disposed on the z main curved surface C7z. The x main coil 7x is disposed on the x main curved surface C7x. The y main coil 7y is arranged on the y main curved surface C7y.

  The sub coils 8x, 8y, and 8z are composed of a z sub coil 8z that suppresses a leakage magnetic field generated by the z main coil 7z, an x sub coil 8x that suppresses a leakage magnetic field generated by the x main coil 7x, and a y main coil 7y. It consists of y subcoil 8y which suppresses the leakage magnetic field to generate. The z sub-coil 8z is disposed on the z sub-curved surface C8z. The x secondary coil 8x is disposed on the x secondary curved surface C8x. The y subcoil 8y is disposed on the y subcurved surface C8y.

  The x main coil 7x, the y main coil 7y, the z main coil 7z, the x subcoil 8x, the y subcoil 8y, and the z subcoil 8z are embedded and laminated in an insulating material (not shown).

  The main coils 7x, 7y, and 7z have main curved surfaces C7x, C7y, and C7z on which the main coils 7x, 7y, and 7z are arranged in order to ensure a large space in the opening (bore) into which the subject is carried. The shapes on the cross section in the plane perpendicular to the central axis (z-axis) are substantially similar to each other, and are non-circular elliptical shapes. The major axis direction of the elliptical shape is the horizontal direction (x-axis direction), and the minor axis direction is the vertical direction (y-axis direction).

  On the other hand, the auxiliary coils 8x, 8y, and 8z are spatially separated from the main coils 7x, 7y, and 7z along the inner cylinder 2a of the magnet device 2 in order to reduce the overall inductance of the gradient coil 4. Yes. Thereby, the shape on the cross section in the plane perpendicular to the central axis (z axis) of the secondary curved surfaces C8x, C8y, C8z where the secondary coils 8x, 8y, 8z are arranged is circular.

  A tray storage hole 6b that stores shim trays 6a each containing a shim 6c so as to be sandwiched between the main coils 7x, 7y, and 7z and the sub-coils 8x, 8y, and 8z has a central axis (z axis). A plurality of layers are arranged in the circumferential direction. The tray storage hole 6b is disposed close to the main coils 7x, 7y, 7z and along the main coils 7x, 7y, 7z. This is because the volume of the shim 6c necessary for adjustment for obtaining a uniform magnetic field can be reduced by arranging the shim 6c (shim tray 6a) as close as possible to the imaging region (uniform magnetic field space) 3. . The tray storage hole 6b is arranged on the shim arrangement curved surface C1. The cross-sectional shape of the shim-arranged curved surface C1 on a plane perpendicular to the central axis (z-axis) is a non-circular elliptical shape that is substantially similar to that of the main curved surfaces C7x, C7y, and C7z. As in the case of the main curved surfaces C7x, C7y, and C7z, the elliptical shape, which is a shape on a cross section in a plane perpendicular to the central axis (z axis) of the shim arrangement curved surface C1, has the longest diameter (major diameter) direction. They are arranged in the horizontal direction (x-axis direction) so that the direction of the shortest diameter (minor axis) is the vertical direction (y-axis direction). In the first embodiment, the non-circular shape is an ellipse that resembles a human shape (cross-sectional shape to be inspected) rather than a circular shape. A race track may be used.

  The tray storage holes 6b are not arranged at regular intervals in the circumferential direction, but are arranged at irregular intervals. In particular, the long axis (x axis, longest diameter d5) of the main coils 7x, 7y, 7z, in which the interval (radial interval) between the main coils 7x, 7y, 7z and the sub coils 8x, 8y, 8z is the narrowest. ) In the vicinity, the circumferential interval s5 between the adjacent tray receiving holes 6b is the widest, and conversely, the intervals (radial intervals) between the main coils 7x, 7y, 7z and the sub-coils 8x, 8y, 8z. In the vicinity of the shortest axis (y-axis, shortest diameter d1) of the main coils 7x, 7y, 7z, which is the widest, the circumferential interval s1 between the adjacent tray storage holes 6b is arranged to be the narrowest. The larger the distance (diameter) between the shim arrangement curved surface C1 and the central axis (z-axis), the wider the interval between adjacent tray storage holes 6b (shim 6c). Specifically, the distance (diameter) between the shim arrangement curved surface C1 where the interval s1 between the adjacent tray storage holes 6b occurs and the central axis (z axis) is the distance d1, and similarly, the interval s2 The distance of the place where the distance s3 occurs is the distance d2, the distance of the place where the distance s3 occurs is the distance d3, the distance of the place where the distance s4 is the distance d4, and the place where the distance s5 occurs The distance d5 is the distance d5, the distance d1 is smaller than the distance d2, the distance d2 is smaller than the distance d3, the distance d3 is smaller than the distance d4, and the distance d4 is smaller than the distance d5. (D1 <d2 <d3 <d4 <d5), the interval s1 is smaller than the interval s2, the interval s2 is smaller than the interval s3, the interval s3 is smaller than the interval s4, and the interval s4 is smaller than the interval s5 ( s1 s2 <s3 <s4 <s5). That is, the distances d1 to d5 are not equal to each other, and the tray storage holes 6b are arranged at unequal intervals in the circumferential direction.

  Further, both the circumferential width of the tray storage hole 6b and the circumferential interval between the adjacent tray storage holes 6b, that is, the pitch of the tray storage holes 6b (the shims 6c) are defined from the central axis (z axis). The expected angles θ1 to θ5 are also different in size from each other so that the major axis side is larger than the minor axis side (θ1 <θ2 <θ3 <θ4 <θ5).

  FIG. 3 shows a perspective view of the shim tray 6a. The shim tray 6a is made of a nonmagnetic material such as plastic. In the shim tray 6a, a plurality of pockets 6d are formed in a line. In the pocket 6d, a shim 6c made of a magnetic material made of, for example, iron or an iron-based alloy is formed in the pocket 6d as necessary for improving the magnetic field uniformity of the imaging region (uniform magnetic field space) 3. Can be accommodated. Note that the intervals between the adjacent pockets 6d may be equal intervals or unequal intervals.

  Next, a method of determining (calculating) the circumferential intervals s1 to s5 between the adjacent tray storage holes 6b (shim 6c) shown in FIG. 2 and the angles θ1 to θ5 that allow for the pitch of the tray storage holes 6b, This will be described using mathematical expressions. In the first embodiment, an inverse problem solving method is shown. However, the present invention is not limited to this. For example, a mathematical programming method such as linear programming or other optimization methods may be used.

  FIG. 4 shows an example of an analysis grid used when calculating the intervals s1 to s5 and the like at which the tray storage holes 6b (the shims 6c) should be arranged in the circumferential direction. A calculation grid (analysis grid) is formed on the shim-arranged curved surface C1 and on the surface of the imaging region (uniform magnetic field space) 3.

When a shim 6c (magnetic dipole moment mi (ma, mb)) having a volume Vi and a magnetization M is arranged at a certain node i on the calculation grid on the shim arrangement curved surface C1, the shim 6c is formed in an imaging region (uniform magnetic field space). ) The magnetic field strength B (i, j) created at a certain node j on the calculation grid 3 is proportional to the volume Vi and the magnetization M as shown in the equation (1).

Here, the magnetization M is constant. Therefore, the distribution (vector m) of the magnetic moment mi of the shim 6c arranged at each node i on the calculation grid of the imaging region (homogeneous magnetic field space) 3 can be expressed as Equation (2).

Further, the distribution (vector b) of the magnetic field strength bj generated at each node j on the calculation grid of the imaging region (homogeneous magnetic field space) 3 by these magnetic moments mi can be expressed as Equation (3). .

From this, the relationship between the magnetic field distribution (vector b) and the magnetic moment distribution (vector m) can be expressed as shown in Equation (4) using the coefficient matrix A.

When the singular value decomposition method is applied to the matrix A, a generalized inverse matrix A ′ of the matrix A can be obtained. Thereby, the magnetic moment distribution (vector m) can be calculated from the generalized inverse matrix A ′ and the magnetic field distribution (vector b) as shown in the equation (5).

  The singular value decomposition method is detailed in, for example, “Projective Matrix General Matrix Singular Value Decomposition” by Haruo Yanai et al., UP Applied Mathematics Selection 10 (published in 1983). The magnetic moment distribution (vector m) obtained by Equation (5) is an approximate solution, but according to the singular value decomposition method, the accuracy of approximation can be arbitrarily set. That is, when the target (to be generated) magnetic field distribution (vector b) is determined, the necessary magnetic moment distribution (vector m) is obtained by taking a matrix product with the matrix A ′ according to Equation (5). Can be calculated with accuracy. In the first embodiment, the accuracy is the accuracy of magnetic field adjustment in the imaging region (homogeneous magnetic field space) 3 as long as sufficient accuracy is ensured for a desired uniformity.

  Specifically, as shown in FIG. 4, a case where magnetic moments ma and mb (shim 6c) are arranged on the shim arrangement curved surface C1 is considered. As shown in FIG. 2, the shim 6c having the magnetic moment ma has the long axis of the main coils 7x, 7y, 7z where the distance between the main coils 7x, 7y, 7z and the subcoils 8x, 8y, 8z is the narrowest. It is housed in a shim tray 6aa disposed in the vicinity of (x axis, longest diameter d5). On the other hand, the shim 6c having the magnetic moment mb has the shortest axis (y-axis, most) of the main coils 7x, 7y, 7z where the distance between the main coils 7x, 7y, 7z and the auxiliary coils 8x, 8y, 8z is the widest. It is housed in a shim tray 6ab disposed near the short diameter d1).

  Based on the arranged magnetic moment ma, a magnetic moment distribution (vector m) is generated as in equation (2), and the magnetic field distribution (vector b) is calculated using equation (4) for this magnetic moment distribution (vector m). calculate. The magnetic moment distribution (vector m) with a certain accuracy is calculated using the equation (5) for this magnetic field distribution (vector b).

  FIG. 5A shows a magnetic moment distribution (vector m) obtained by calculating the magnetic moment ma with a certain accuracy. The magnetic moment ma is arranged at the ma position (zero degree) in the circumferential position, but the peak of the magnetic moment distribution (vector m) has a half width ha in the circumferential direction.

  Similarly, based on the arranged magnetic moment mb, a magnetic moment distribution (vector m) is generated as in Expression (2), and the magnetic field distribution (vector) is calculated using Expression (4) for this magnetic moment distribution (vector m). b) is calculated. Using this formula (5) for this magnetic field distribution (vector b), the magnetic moment distribution (vector m) with the same accuracy as the magnetic moment ma is calculated.

  FIG. 5B shows a magnetic moment distribution (vector m) obtained by calculating the magnetic moment mb with a certain accuracy. The magnetic moment mb is arranged at the mb position (zero degree) in the circumferential position, but the peak of the magnetic moment distribution (vector m) has a half width hb in the circumferential direction.

  These are the tray storage holes for placing the shim tray 6a (shim 6c) if it is sufficient to ensure a certain accuracy in the magnetic field adjustment for improving the magnetic field uniformity of the imaging region (uniform magnetic field space) 3. 6b indicates that the half-value widths ha and hb can be arbitrarily arranged in the circumferential direction on the shim arrangement curved surface C1. That is, even if the magnetic moments ma and mb (the shims 6c) are arranged at positions shifted by about the half widths ha and hb, there is no influence on the accuracy of desired magnetic field adjustment. Therefore, it is sufficient if the tray storage holes 6b for arranging the magnetic moments ma and mb (the shims 6c) are arranged in the circumferential direction with an interval of only the half widths ha and hb.

  Next, focusing on the half-value widths ha and hb, the half-value width ha shown in FIG. 5A is about 40 degrees in a range from about −20 degrees to about +20 degrees, and the half-value width shown in FIG. 5B. hb is approximately 20 degrees in a range from approximately −10 degrees to approximately +10 degrees. From this, it can be seen that the half-value width ha is approximately twice as large as the half-value width hb. In this calculation, the major dimension of the bore is about 15% longer than the minor axis. Therefore, the intervals s5 (θ5) and s1 (θ1) of the tray accommodation holes 6b for arranging the magnetic moments ma and mb (shim 6c) can be determined so as to correspond to the half widths ha and hb. The interval s5 (θ5) is set so as to be larger than the interval s1 (θ1) by about twice the interval s1 (θ1). The distance between the main coils 7x, 7y, 7z and the sub-coils 8x, 8y, 8z is the narrowest, and the distance between the main coils 7x, 7y, 7z is the widest in the vicinity of the major axis (x axis, longest diameter d5). Therefore, the shim tray 6a (shim 6c), the main coils 7x, 7y, and 7z, and the auxiliary coils 8x, 8y, and 8z can be easily arranged without reducing the accuracy of the magnetic field adjustment. be able to. In the above description, the interval s1 (θ1) and the interval s5 (θ5) are examined. However, the intervals can be similarly determined for the intervals s2 to s4 (θ2 to θ4). As shown in FIG. 2, as the distance between the shim arrangement curved surface C1 and the central axis (z-axis) is larger (d1 <d2 <d3 <d4 <d5), the adjacent tray storage holes 6b (shims 6c) become closer to each other. The intervals are continuously and smoothly widened (s1 <s2 <s3 <s4 <s5, θ1 <θ2 <θ3 <θ4 <θ5). As described above, according to the first embodiment, even if the cross-sectional shape of the shim arrangement curved surface C1 is a non-circular elliptical shape, the accuracy (ability) of magnetic field adjustment is improved as in the case of a circular shape. The magnetic field in the imaging region (uniform magnetic field space) 3 can be uniformly adjusted with high accuracy.

(Second Embodiment)
FIG. 6 is a cross-sectional view of the gradient magnetic field coil 4 of the magnetic resonance imaging apparatus according to the second embodiment of the present invention cut along the xy plane. The gradient magnetic field coil 4 is different between the magnetic resonance imaging apparatus of the second embodiment and the magnetic resonance imaging apparatus according to the first embodiment. The gradient coil 4 is different between the second embodiment and the first embodiment in that the shim arrangement curved surface C1 in which the tray storage hole (groove) 6b in which the shim tray 6a is accommodated is arranged is the main curved surface. This is a point arranged on the central axis (z-axis) side of C7x, C7y, and C7z. In the second embodiment, the shim tray 6a (shim 6c) can be arranged closer to the central axis (z-axis) (imaging region (uniform magnetic field space) 3) than in the first embodiment, so the volume of the shim 6c is reduced. be able to. Further, the tray storage hole (groove) 6b is in contact with the inner cylinder of the gradient magnetic field coil 4 and has a groove shape instead of a hole shape.

  As in the first embodiment, the larger the distance between the shim arrangement curved surface C1 and the central axis (z-axis) (d1 <d2 <d3 <d4), the adjacent tray storage holes (grooves) 6b (shim 6c). ) Are continuously and smoothly widened (s1 <s2 <s3 <s4, θ1 <θ2 <θ3 <θ4). Even in the second embodiment, even if the cross-sectional shape of the shim arrangement curved surface C1 is a non-circular elliptical shape, the accuracy (ability) of the magnetic field adjustment is maintained and the imaging region (uniformity) is maintained as in the case of the circular shape. The magnetic field of (magnetic field space) 3 can be adjusted uniformly with high accuracy.

  Further, since a wide space can be secured like the interval s4, components other than the shim tray 6a, for example, cooling pipes (not shown) for the RF coil 5 and the gradient magnetic field coil 4 are arranged in the interval s4. And a wider bore space can be realized.

DESCRIPTION OF SYMBOLS 1 Magnetic resonance imaging device 2 Magnet apparatus 2a Inner cylinder 2b Outer cylinder 3 Uniform magnetic field space (imaging area)
4 Gradient magnetic field coil 6 Magnetic field adjusting means 6a Shim tray 6b Tray storage hole (groove)
6c shim 6d pocket 7x x main coil 7y y main coil 7z z main coil 8x x subcoil 8y y subcoil 8z z subcoil C1 shim arrangement curved surface C7x x main surface C7y y main surface C7z z main surface C8x x subsurface C8y y Sub-surface C8z z sub-surface

Claims (11)

A magnet device having a cylindrical external shape and generating a uniform magnetic field space inside the cylindrical shape;
A magnetic material shim disposed on the inner shim arrangement curved surface of the cylindrical shape, and a magnetic field adjusting means for making the magnetic field of the uniform magnetic field space more uniform,
The shape on the cross section in a plane perpendicular to the cylindrical central axis of the shim arrangement curved surface is non-circular,
The distance between the shim placement curved surface and the central axis on the plane varies depending on the location on the shim placement curved surface, there are locations where the distance is large and locations smaller than the large location, and where the distance is large A magnetic resonance imaging apparatus, wherein an interval between the shims adjacent to each other in the circumferential direction of the central axis is wider than the interval at the portion where the distance is small.   2. The magnetic resonance imaging apparatus according to claim 1, wherein a shape of a cross section of the shim-arranged curved surface on a plane perpendicular to the cylindrical central axis is an ellipse or a racetrack.   3. The magnetic resonance imaging apparatus according to claim 1, wherein the maximum value of the interval between the shims adjacent in the circumferential direction is approximately twice the minimum value of the interval.   The angle at which the pitch of the shims arranged in the circumferential direction from the central axis on the plane is estimated to be approximately 40 degrees at the maximum and approximately 20 degrees as the minimum. The magnetic resonance imaging apparatus according to any one of the above. The cylindrical central axis is arranged in a horizontal direction,
The shim-arranged curved surface is arranged so that the longest diameter direction of the cross-sectional shape in a plane perpendicular to the cylindrical central axis is the horizontal direction, and the shortest diameter direction is the vertical direction. The magnetic resonance imaging apparatus according to claim 1, wherein the magnetic resonance imaging apparatus is a magnetic resonance imaging apparatus. A gradient magnetic field coil for superimposing a gradient magnetic field on the uniform magnetic field space is provided inside the cylindrical shape,
The gradient coil is
A main coil disposed on the main curved surface and generating the gradient magnetic field;
A secondary coil that is arranged on the secondary curved surface along the cylindrical shape of the magnet device, and that generates a magnetic field that cancels a leakage magnetic field that the primary coil generates in the cylindrical shape;
The shape on the cross section in a plane perpendicular to the cylindrical central axis of the main curved surface is non-circular,
The shape on the cross section in a plane perpendicular to the cylindrical central axis of the sub-curved surface is a circle,
The shape on the cross section of the shim-arranged curved surface in a plane perpendicular to the cylindrical central axis is the shape on the cross section of the main curved surface in the plane perpendicular to the cylindrical central axis. The magnetic resonance imaging apparatus according to any one of claims 1 to 4.   The magnetic resonance imaging apparatus according to claim 6, wherein the shim-arranged curved surface is sandwiched between the main curved surface and the sub-curved surface.   The magnetic resonance imaging apparatus according to claim 6, wherein the shim arrangement curved surface is arranged on the central axis side of the main curved surface.   The shape of the shim-arranged curved surface on the plane perpendicular to the cylindrical central axis and the shape of the main curved surface on the plane perpendicular to the cylindrical central axis are elliptical or racetrack. The magnetic resonance imaging apparatus according to claim 6, wherein the magnetic resonance imaging apparatus has a shape. The cylindrical central axis is arranged in a horizontal direction,
The longest diameter direction of the shape on the cross section in the plane perpendicular to the cylindrical central axis of the shim placement curved surface is the horizontal direction, and the shortest diameter direction is the vertical direction,
The longest diameter direction of the cross-sectional shape in a plane perpendicular to the cylindrical central axis of the main curved surface is a horizontal direction, and the shortest diameter direction is a vertical direction. The magnetic resonance imaging apparatus according to claim 6, wherein the magnetic resonance imaging apparatus is a magnetic resonance imaging apparatus. In a gradient magnetic field coil that is cylindrical and is provided inside a magnet device that generates a uniform magnetic field space inside the cylindrical shape, and for superimposing a gradient magnetic field on the uniform magnetic field space,
It is arranged on the shim arrangement curved surface, and has a storage hole that makes the magnetic field of the uniform magnetic field space more uniform by storing the shim of the magnetic material,
The shape on the cross section in a plane perpendicular to the cylindrical central axis of the shim arrangement curved surface is non-circular,
The distance between the shim placement curved surface and the central axis on the plane varies depending on the location on the shim placement curved surface, there are locations where the distance is large and locations smaller than the large location, and where the distance is large The gradient magnetic field coil, wherein an interval between the storage holes adjacent in the circumferential direction of the central axis is wider than the interval at the place where the distance is small.

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