JP2011240164A - Gradient magnetic field coil device, nuclear magnetic resonance imaging device, and coil pattern design method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nuclear magnetic resonance imaging device, restricting generation of error magnetic field and eddy current, and improving image quality of a section image.SOLUTION: The nuclear magnetic resonance imaging device includes the gradient magnetic field coil device provided with a first coil to produce linear magnetic field distribution in an imaging range, a second coil to restrict a leakage magnetic field to a static magnetic field coil device to produce uniform magnetic field distribution in the imaging range from the first coil, and a static magnetic field coil device disposed near the gradient magnetic field coil device to produce a uniform static magnetic field in the imaging range. When a return line 13 from a spiral coil pattern is disposed on the opposite side of the static magnetic field coil device 2c interposing a transition line 14 crossing the return line 13 in at least either one of the first and the second coils, the width of the return line 13 of the transition line 14 in the width direction is equal to or more than four times and equal to or less than ten times the width of the return line 13.

Description

  The present invention relates to a nuclear magnetic resonance imaging (hereinafter referred to as MRI) apparatus, a gradient coil apparatus used in the MRI apparatus, and a method for designing a coil pattern of a coil provided in the gradient coil apparatus.

  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 coil device for generating a pulse, an RF 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 to obtain the cross-sectional image And a computer system for displaying.

  In order to improve the performance of the MRI apparatus, a gradient magnetic field coil apparatus that generates a gradient magnetic field in which the magnetic field strength is linearly gradient has been proposed (see Patent Document 1).

JP 2001-353137 A (FIG. 1)

  A conventional gradient magnetic field coil apparatus is provided with a coil having a complicated coil pattern. In the coil pattern, a plurality of loop-shaped main lines opened at one place on one surface are arranged in multiple so as to be inside the adjacent main lines, and connecting wires connecting the adjacent main lines; A return line drawn from the inner main line to the outer side of the outer main line is provided so as to partially overlap each other.

  A plurality of main lines are formed in a loop shape opened at one place and arranged in a multiple manner, and the adjacent main lines are connected by a crossover line to form a spiral coil pattern in which the multiple main lines are connected. And by providing a return line, it is possible to flow current through a plurality of main lines. However, the conventional gradient coil apparatus is designed so that a linear gradient magnetic field is formed when current flows only on a plurality of main lines. Due to this, an error magnetic field is generated. This error magnetic field generates an eddy current in the static magnetic field coil device, and this eddy current may form a magnetic field that disturbs the cross-sectional image in the imaging region.

  Accordingly, an object of the present invention is to provide a gradient magnetic field coil apparatus, an MRI apparatus, and a coil pattern design method capable of suppressing the generation of an error magnetic field and further an eddy current and improving the image quality of a cross-sectional image.

  In order to achieve the above object, the present invention provides a first coil that creates a linear magnetic field distribution in an imaging region, and a leakage magnetic field from the first coil to a static magnetic field coil device that creates a uniform magnetic field distribution in the imaging region. In a nuclear magnetic resonance imaging apparatus, comprising: a gradient magnetic field coil device including a second coil to be suppressed; and a static magnetic field coil device that is disposed in proximity to the gradient magnetic field coil device and generates a uniform static magnetic field in the imaging region In at least one of the first coil and the second coil, a return line from the spiral coil pattern is disposed on the opposite side of the static magnetic field coil device across a crossover that intersects the return line. The crossover line has a width in the width direction of the return line that is not less than 4 times and not more than 10 times the width of the return line.

  Moreover, in this invention, when the said return wire is arrange | positioned between the said crossover wire and the said static magnetic field coil apparatus in at least any one of the said 1st coil and the said 2nd coil, the said crossover wire is It is preferable that the coil pattern meanders so as to go up and down like a valley and a mountain.

  Further, in the present invention, at least one of the first coil and the second coil, a power supply line to the coil pattern and the return line are disposed between the coil pattern and the static magnetic field coil device. It is preferable that the coil pattern is detoured so that one section of the coil pattern intersecting the power supply line and the return line is bent into a convex shape.

  The present invention also provides a first coil that creates a linear magnetic field distribution in the imaging region of the MRI apparatus, and a first magnetic field that suppresses a leakage magnetic field from the first coil to the static magnetic field coil device that creates a uniform magnetic field distribution in the imaging region. A method for designing a coil pattern of at least one of two coils, wherein a return line from a spiral coil pattern is arranged on the opposite side of the static magnetic field coil device across a jumper line intersecting with the return line. In this case, based on an initial coil pattern prepared in advance, an error magnetic field that enters the static magnetic field coil device is calculated, a correction current component that flows on the jumper wire and cancels the error magnetic field is calculated, and based on the correction current component Then, the width of the return line in the width direction of the return line of the initial coil pattern is deformed so as to be not less than 4 times and not more than 10 times the width of the return line.Further, the magnetic field coil apparatus and the MRI apparatus have at least one of the first coil and the second coil designed and manufactured by using this coil pattern design method.

  According to the present invention, it is possible to provide a gradient magnetic field coil apparatus, an MRI apparatus, and a coil pattern design method capable of suppressing the generation of an error magnetic field and further an eddy current and improving the image quality of a cross-sectional image.

1 is a perspective view of an MRI (nuclear magnetic resonance imaging apparatus) apparatus according to a first embodiment of the present invention. It is sectional drawing which cut | disconnected the MRI apparatus which concerns on the 1st Embodiment of this invention by the yz plane containing a symmetry axis (z axis). It is sectional drawing of the gradient magnetic field coil apparatus which concerns on the 1st Embodiment of this invention. (A) is a layout view of the y-direction gradient magnetic field main coil and the y-direction gradient magnetic field shield coil of the gradient coil apparatus according to the first embodiment of the present invention, and (b) is the first embodiment of the present invention. FIG. 2 is a layout diagram of an x-direction gradient magnetic field main coil and an x-direction gradient magnetic field shield coil of the gradient magnetic field coil apparatus according to FIG. It is an arrangement view of a coil and a z-direction gradient magnetic field shield coil. (A) is a pattern figure of the y direction gradient magnetic field shield coil of the gradient magnetic field coil apparatus which concerns on the 1st Embodiment of this invention, (b) is sectional drawing of the AA direction of (a), c) is a sectional view in the BB direction of (a). It is a flowchart of the design method of coil patterns, such as a y direction gradient magnetic field shield coil, of the gradient magnetic field coil apparatus which concerns on the 1st Embodiment of this invention. (A) is an example of the initial GC coil pattern prepared in step S1 of the coil pattern design method, and (b) is a cross-sectional view in the AA direction of (a). It is an example of the coil surface divided | segmented with the triangular mesh finite element in step S2 of the design method of a coil pattern. (A) is an enlarged view around the return line and the first crossover of the initial GC coil pattern prepared in step S1 of the coil pattern design method, and (b) is a correction current (component) calculated in step S4. (C) is an enlarged view of the vicinity of the return line and the first crossover line of the modified GC coil pattern to which the correction current component is added in step S5. It is a distribution map of the error magnetic field component on the vacuum vessel (conductor) generated by the return line and the first crossover. It is a figure which shows the change of the flow of the current gravity center of GC coil pattern before and behind addition of the correction | amendment electric current component of step S5. (A) is an enlarged view around the feeding line, return line, and main line of the initial GC coil pattern prepared in step S1 of the coil pattern design method, and (b) is a correction current (component) calculated in step S4. (C) is an enlarged view of the periphery of the feeding line, the return line, and the second connecting line of the modified GC coil pattern to which the correction current component is added in step S5. (A) is a pattern figure of the periphery of the return line of the y direction gradient magnetic field shield coil of the gradient magnetic field coil apparatus which concerns on the 2nd Embodiment of this invention, and a 1st crossover, (b) is a return line and 1st It is sectional drawing of the periphery of a crossover. It is a distribution map of the error magnetic field component on the vacuum vessel (conductor) generated by the return line and the first crossover line, and (a) is the case where the ratio of the first crossover width W3 to the return line width W4 is 4 times. Yes, (b) is when the ratio of the first transition width W3 to the return line width W4 is 6 times, and (c) is when the ratio of the first transition width W3 to the return line width W4 is 8 times. Yes, (d) is the case where the ratio of the first transition width W3 to the return line width W4 is 10 times.

  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. 1 shows a perspective view of an MRI (nuclear magnetic resonance imaging) apparatus 1 according to the first embodiment of the present invention. The MRI apparatus 1 is a vertical magnetic field type MRI apparatus in which the direction of the static magnetic field 7 is the vertical direction. The MRI apparatus 1 is disposed so as to be sandwiched from above and below an imaging region 8 into which a subject 5 lying on a bed 6 is inserted, and generates a uniform static magnetic field 7 in the imaging region 8. And a connecting column 17 that supports the pair of upper and lower static magnetic field coil devices 2 apart from each other, and a gradient magnetic field in which the magnetic field intensity is spatially inclined in order to give position information to the imaging region 8 is generated in pulses. The gradient magnetic field coil device 3, the RF coil 4 that irradiates the subject 5 inserted in the imaging region 8 with a high-frequency pulse, the receiving coil (not shown) that receives the magnetic resonance signal from the subject 5, and the received magnetism And a computer system (not shown) that processes the resonance signal and displays a cross-sectional image of the subject 5. The pair of upper and lower static magnetic field coil devices 2, the gradient magnetic field coil device 3, and the RF coil 4 have a disk (cylindrical) shape having the symmetry axis 10 as a common axis. Although the subject 5 is carried to the imaging region 8 by the movable bed 6, only the thin connecting column 17 connects the pair of upper and lower static magnetic field coil devices 2, so the subject 5 can look around and reduce the feeling of closedness. can do. In order to facilitate understanding of the explanation to be described later, the z axis is set in the vertical direction parallel to or coincident with the symmetry axis 10, and the x axis and the y axis are set so as to be perpendicular to each other in the horizontal direction. Yes.

  FIG. 2 shows a cross-sectional view of the MRI apparatus 1 according to the first embodiment of the present invention cut along a yz plane including the symmetry axis 10 (z axis). In the pair of upper and lower static magnetic field coil devices 2, a pair of upper and lower static magnetic field main coils 2a and a pair of upper and lower static magnetic field shield coils 2b are used. Each of the pair of upper and lower static magnetic field main coils 2a and the pair of upper and lower static magnetic field shield coils 2b has an annular shape with the symmetry axis 10 as a common central axis. The pair of upper and lower static magnetic field main coils 2a and the pair of upper and lower static magnetic field shield coils 2b are housed in a three-layer container. First, the pair of static magnetic field main coils 2a and the pair of static magnetic field shield coils 2b are accommodated in the refrigerant container 2e together with the liquid helium (He). 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 pair of static magnetic field main coils 2a and the pair of static magnetic field shield coils 2b can be stably set to a cryogenic temperature that is the temperature of the refrigerant, and can function as superconducting electromagnets. A nonmagnetic member is used for the refrigerant container 2e, the heat radiation shield 2d, and the vacuum container 2c so that a force due to a magnetic field does not act, and a nonmagnetic metal is used for ease of processing. For this reason, a current, in particular, an eddy current can flow through the refrigerant container 2e, the heat radiation shield 2d, and the vacuum container 2c.

  The gradient magnetic field coil device 3 also has a pair of upper and lower gradient coil devices, and the pair of upper and lower gradient magnetic field coil devices 3 are arranged vertically with the imaging region 8 in between. The RF coil 4 also has a pair of upper and lower sides, and the pair of upper and lower RF coils 4 are arranged above and below with the imaging region 8 interposed therebetween. The pair of upper and lower upper gradient magnetic field coil devices 3 are disposed between the pair of upper and lower upper static magnetic field coil devices 2 and the pair of upper and lower upper RF coils 4 in proximity to each other. Similarly, a pair of upper and lower lower gradient magnetic field coil devices 3 are disposed between a pair of upper and lower lower static magnetic field coil devices 2 and a pair of upper and lower lower RF coils 4 in proximity to each other. . The pair of upper and lower gradient coil devices 3 generate a pulse of a gradient magnetic field 9 in which the magnetic field strength in the same direction as the static magnetic field 7 is inclined in an arbitrary direction. The gradient magnetic field coil device 3 has a function capable of generating independent gradient magnetic fields 9 in the x direction, the y direction, and the z direction so as to overlap the static magnetic field 7. FIG. 2 shows a gradient magnetic field 9 inclined in the y direction.

  FIG. 3 shows a cross-sectional view of a pair of upper and lower gradient coil devices 3. The gradient magnetic field coil device 3 has a pair of upper and lower gradient magnetic field coils GC arranged above and below the imaging region 8. The pair of upper and lower gradient magnetic field coils GC are a pair of upper and lower gradient magnetic field main coils (first coils) GMC disposed above and below the imaging region 8 and a pair of upper and lower gradients disposed above and below the imaging region 8. A magnetic field shield coil (second coil) GSC. Between the pair of upper and lower gradient magnetic field shield coils (second coils) GSC, a pair of upper and lower gradient magnetic field main coils GMC sandwiching the imaging region 8 are arranged. The pair of upper and lower gradient magnetic field main coils GMC and the pair of upper and lower gradient magnetic field shield coils GSC support each other via the support member 3a. Similarly, the upper and lower pair of lower gradient magnetic field main coils GMC and the upper and lower pair of lower gradient magnetic field shield coils GSC support each other via the support member 3a. The pair of upper and lower upper gradient magnetic field coil devices 3, in particular, the upper and lower pair of upper gradient magnetic field shield coils GSC are adjacent to the pair of upper and lower upper static magnetic field coil devices 2, particularly the upper and lower pair of upper vacuum chambers 2c. Are arranged. Similarly, the upper and lower pair of lower gradient magnetic field coil devices 3, particularly the upper and lower pair of lower gradient magnetic field shield coils GSC are the upper and lower pair of lower static magnetic field coil devices 2 and particularly the upper and lower pair of lower magnetic field coil devices GSC. It is arranged close to the vacuum vessel 2c.

  The pair of upper and lower gradient magnetic field main coils GMC creates a gradient magnetic field that linearly changes in the x direction, and linearly changes in the y direction with a pair of upper and lower x direction gradient magnetic field main coils xGMC that are arranged above and below the imaging region 8. A pair of upper and lower y-direction gradient magnetic field main coils yGMC that are arranged above and below the imaging region 8 and a gradient magnetic field that changes linearly in the z direction are arranged above and below the imaging region 8. It has a pair of upper and lower z-direction gradient magnetic field main coils zGMC. Each of the x-direction gradient magnetic field main coil xGMC, the y-direction gradient magnetic field main coil yGMC, and the z-direction gradient magnetic field main coil zGMC forms a layer (three layers) for each pair of gradient magnetic field coil devices 3. Three layers of gradient magnetic field main coils xGMC, yGMC, and zGMC form a pair, and the three layers are laminated in each z direction with the insulating layer of the support member 3a interposed therebetween.

  The pair of upper and lower gradient magnetic field shield coils GSC suppresses the magnetic field formed by the x-direction gradient magnetic field main coil xGMC from leaking to the surroundings, and a pair of upper and lower x-direction gradient magnetic field shield coils xGSC are arranged above and below the imaging region 8 A pair of upper and lower y-direction gradient magnetic field shield coils yGSC which are arranged above and below the imaging region 8 while suppressing the magnetic field formed by the y-direction gradient magnetic field main coil yGMC from leaking to the surroundings, and the z-direction gradient magnetic field main coil The zGMC has a pair of upper and lower z-direction gradient magnetic field shield coils zGSC that are arranged above and below the imaging region 8 while preventing the magnetic field formed by zGMC from leaking to the surroundings. The x-direction gradient magnetic field shield coil xGSC, the y-direction gradient magnetic field shield coil yGSC, and the z-direction gradient magnetic field shield coil zGSC each form a layer (three layers) for each pair of gradient magnetic field coil devices 3, Three layers of the gradient magnetic field shield coils xGSC, yGSC, and zGSC form a pair, and the three layers are laminated in each z direction with the insulating layer of the support member 3a interposed therebetween.

  FIG. 4A shows a layout of the y-direction gradient magnetic field main coil yGMC and the y-direction gradient magnetic field shield coil yGSC. A total of four y-direction gradient magnetic field main coils yGMC are arranged in two disk-shaped layers (not shown) each having a z-axis as a central axis. Each of the four y-direction gradient magnetic field main coils yGMC is a substantially semicircular and spiral fan-shaped coil. However, the spiral shape is not shown, and only a rough current direction is shown. The four y-direction gradient magnetic field main coils yGMC are divided into two in the x-axis-y-axis plane and have a plane-symmetric structure on the plane. The four y-direction gradient magnetic field main coils yGMC are divided into z-axis and x-axis planes two by two and have a plane-symmetric structure on the planes. The arrow indicates the direction of the current flowing through the coil, and the same applies to the following.

  A total of four y-direction gradient magnetic field shield coils yGSC are arranged in two disk-shaped layers (not shown) having the z-axis as the central axis. Each of the four y-direction gradient magnetic field shield coils yGSC is a substantially semicircular and spiral fan coil, and is disposed so as to cover the corresponding y-direction gradient magnetic field main coil yGMC. In addition, illustration of the shape of a spiral is abbreviate | omitted and only the direction of the rough electric current is shown. The four y-direction gradient magnetic field shield coils yGSC are divided by two on the x-axis-y-axis plane and have a plane-symmetric structure on the plane. The four y-direction gradient magnetic field shield coils yGSC are divided into two z-axis-x-axis planes and have a plane-symmetric structure on each plane.

  FIG. 4B shows a layout of the x-direction gradient magnetic field main coil xGMC and the x-direction gradient magnetic field shield coil xGSC. A total of four x-direction gradient magnetic field main coils xGMC are arranged in two disk-shaped layers (not shown) each having a z-axis as a central axis. Each of the four x-direction gradient magnetic field main coils xGMC is a substantially semicircular and spiral fan-shaped coil, but the spiral shape is not shown, and only a rough current direction is shown. Each of the four x-direction gradient magnetic field main coils xGMC is divided into two planes on the x-axis-y-axis plane and has a plane-symmetric structure on the plane. In addition, the four x-direction gradient magnetic field main coils xGMC are divided by two on the y-axis-z-axis plane and have a plane-symmetric structure on the plane.

  A total of four x-direction gradient magnetic field shield coils xGSC are arranged in two disk-shaped layers (not shown) each having a z-axis as a central axis. Each of the four x-direction gradient magnetic field shield coils xGSC is a substantially semicircular and spiral fan-shaped coil, and is disposed so as to cover the corresponding x-direction gradient magnetic field main coil xGMC. In addition, illustration of the shape of a spiral is abbreviate | omitted and only the direction of the rough electric current is shown. The four x-direction gradient magnetic field shield coils xGSC are divided by two on the x-axis-y-axis plane and have a plane-symmetric structure on the plane. The four x-direction gradient magnetic field shield coils xGSC are divided by two on the y-axis-z axis plane and have a plane-symmetric structure on the plane.

  FIG. 4C shows a layout of the z-direction gradient magnetic field main coil zGMC and the z-direction gradient magnetic field shield coil zGSC. A total of two z-direction gradient magnetic field main coils zGMC are arranged one by one on two disk-shaped layers (not shown) having the z-axis as a central axis. The two z-direction gradient magnetic field main coils zGMC are circular and spiral circular coils, but the spiral is not shown and only the direction of a rough current is shown. The two z-direction gradient magnetic field main coils zGMC are divided one by one on the x-axis-y-axis plane and have a plane-symmetric structure on the plane.

  A total of two z-direction gradient magnetic field shield coils zGSC are arranged on two disk-shaped layers (not shown) each having a z-axis as a central axis. The two z-direction gradient magnetic field shield coils zGSC are circular and spiral circular coils, and are arranged so as to cover the corresponding z-direction gradient magnetic field main coil zGMC. In addition, illustration of a spiral is abbreviate | omitted and only the direction of the rough electric current is shown. The two z-direction gradient magnetic field shield coils zGSC are divided one by one on the x-axis-y-axis plane and have a plane-symmetric structure on the plane.

  FIG. 5A shows a pattern diagram of the y-direction gradient magnetic field shield coil yGSC. 5B is a cross-sectional view in the AA direction in FIG. 5A, and FIG. 5C is a cross-sectional view in the BB direction in FIG. 5A. The y-direction gradient magnetic field main coil yGMC has a slightly smaller size than the coil pattern of the y-direction gradient magnetic field shield coil yGSC. The x-direction gradient magnetic field shield coil xGSC is a congruent coil pattern obtained by rotating the coil pattern of the y-direction gradient magnetic field shield coil yGSC by 90 degrees. The y-direction gradient magnetic field main coil yGMC rotates the coil pattern of the y-direction gradient magnetic field shield coil yGSC by 90 degrees to become a similar coil pattern with a small size.

  As shown in FIG. 5A, the y-direction gradient magnetic field shield coil yGSC has a plurality of main lines 12 on one plane (coil surface). The plurality of main lines 12 are divided into a plurality of regions (three in FIG. 5A). In the first region, the main lines 12 are arranged in a quadruple (multiplex) so as to be inside the adjacent main lines 12. In the second region, the main line 12 is doubled (multiplexed) so as to be inside the adjacent main line 12. In the third region, the main line 12 is arranged in a single layer. A power supply line 11 that supplies power to the main line 12 in each region and a return line 13 that is disposed along the power supply line 11 and that returns current from the main line 12 that is supplied with power from the power supply line 11 are provided. A portion where the feeder line 11 and the return line 13 straddle the main line 12 is set as a correction section 15 in the main line 12. In the correction section 15, the main line 12 is detoured by being bent into a convex shape on the coil surface. According to this, even if an error magnetic field is generated by the feed line 11 and the return line 13, the magnetic field generated by the main line 12 detoured by being bent into a convex shape in the correction section 15 near the feed line 11 and the return line 13. Therefore, the error magnetic field can be canceled, so that the eddy current generated in the vacuum container 2c of the static magnetic field coil device 2 can be suppressed, and the image quality of the cross-sectional image can be improved. Note that the width W2 of the correction section 15 is set to be larger than the interval between the feed line 11 and the return line 13.

  A plurality of main lines 12 arranged in a double or quadruple multiple in the first region or the second region each have a loop shape opened at one place like a U character shape. The open portions in the loop shape (U character shape) are arranged in a line, and the crossover line 14 connects the adjacent main lines 12 at this portion. This connection constitutes a spiral coil pattern in which multiple main lines 12 are connected. Note that the width of this portion, that is, the width (crossover width) W <b> 1 in which the later-described connecting wire 14 is installed is set wider than the line width of the return line 13. The connecting wire 14 meanders so that an angle of 90 degrees or less formed with the return line 13 is smaller than an angle of 90 degrees or less formed with the return line 13 when the adjacent main lines 12 are connected by a straight line. The return line 13 is connected not only to connect the regions but also to lead out the wiring from the inner main line 12 arranged in multiple to the outer side of the outer main line 12, and is arranged so as to overlap the crossover line 13. ing. According to this, even if an error magnetic field is generated by the return line 13, the error magnetic field can be canceled by the magnetic field generated by the crossover line 14 having a large inclination near the return line 13. Eddy currents generated in the vacuum vessel 2c and the like can be suppressed, and the image quality of the cross-sectional image can be improved.

  As shown in FIG. 5B, the return line 13 is disposed between the crossover wire 14 and the vacuum vessel 2 c of the static magnetic field coil device 2. That is, the return line 13 is disposed closer to the crossover line 14 than the vacuum container 2c. The strength of the error magnetic field generated in the vacuum vessel 2c by the return line 13 near the vacuum vessel 2c tends to increase, and in order to cancel this large error magnetic field, a large magnetic field is applied to the vacuum vessel 2c by the connecting wire 14 far from the vacuum vessel 2c. Meandering so that the angle of 90 degrees or less between the connecting wire 14 and the return line 13 is smaller than the angle of 90 degrees or less with the return line 13 when the adjacent main lines 12 are connected by a straight line. I am letting.

  As shown in FIG. 5C, the power supply line 11 and the return line 13 are disposed between the correction section 15 of the main line 12 and the vacuum vessel 2 c of the static magnetic field coil device 2. That is, the feed line 11 and the return line 13 are arranged at a position closer to the correction section 15 of the main line 12 with respect to the vacuum container 2c. The strength of the error magnetic field generated in the vacuum vessel 2c due to the power supply line 11 and the return line 13 near the vacuum vessel 2c tends to increase, and in order to cancel this large error magnetic field, the correction section 15 of the main line 12 far from the vacuum vessel 2c. Then, in order to generate a large magnetic field in the vacuum vessel 2c accordingly, the degree of bending into a convex shape in the correction section 15 is adjusted.

  FIG. 6 shows a flowchart of a method for designing a coil pattern such as the y-direction gradient magnetic field shield coil yGSC of the gradient magnetic field coil apparatus 3 according to the first embodiment of the present invention.

  First, in step S1, the shape of the main line 12 (including the arrangement position), such as the y-direction gradient magnetic field shield coil yGSC, is calculated, and as shown in FIG. 12 is connected (connected) to determine an initial GC coil pattern.

  In step S2, the coil surface 20 forming the initial GC coil pattern is divided by a triangular mesh finite element and the gradient magnetic field coil correction current calculation generated by the triangular mesh finite element as shown in FIG. Create a model. FIG. 8 is a diagram shown for reference, in which a larger triangular element is drawn than that used in actual calculation.

  In step S3, an error magnetic field in the static magnetic field coil device 2 or the like is calculated based on the initial GC coil pattern prepared in step S1. The error magnetic field generates a magnetic force line 16 that penetrates the conductor surface of the static magnetic field coil apparatus 2 such as the vacuum vessel 2c as shown in FIG. 7B, so that the error magnetic field such as the vacuum vessel 2c as shown in FIG. By calculating the direction of the magnetic field entering the conductor surface and the distribution of the magnitude (strength) 19, the error magnetic field can be calculated.

  In step S4, a correction current component that cancels the error magnetic field is calculated so as to exist on the coil surface. As a result, as shown in FIG. 9B, the coil exists on the coil surface where the crossover wire 14 exists with respect to the return line 13 and the crossover wire 14 of the initial GC coil pattern as shown in FIG. 9A. Correction current components 18a and 18b can be calculated.

  More specifically, first, a current potential is applied to a contact of a finite surface element, and a current expressed by a vector T indicating a current potential distribution having the current potential as an element cancels the error magnetic field B on the conductor surface. A current potential distribution T is determined. The current density vector is expressed by the vector product of the gradient of the current potential and the normal of the current (coil) surface. Then, by using a technique that applies singular value decomposition to this approximate solution, a current potential T of a cancellation current component that is not complicated and that can suppress the generation of eddy currents and improve the accuracy of the magnetic field is obtained. be able to.

  If the current potential distribution T corresponding to the cancellation current component is determined by this method, the displacement of the conductor (coil) position is then calculated. From the distance d between conductors and the conductor current Ic, Ic / d is equivalent to the gradient of the current potential. Therefore, the current potential T of the correction current component can be converted into the displacement of the conductor position by T / (gradient). If the initial GC coil pattern is determined based on the current potential calculation value T0, the displacement of the conductor (coil) position can be calculated by the equation T / ＴT0.

  In addition, the method reported by the postscript can be used for calculation of a correction | amendment electric current component. The literature is M. ABE, T. NAKAYAMA, S. OKAMURA, K. MATSUOKA, “A new technique to optimize coil winding path for the arbitrarily distributed magnetic field and application to a helical confinement system”, Phys. Plasmas. Vol. 10 No.4 (2003) 1022.

  In step S5, the initial GC coil pattern is deformed based on the correction current component. The corrected current component is added to the current component along the initial GC coil pattern, and the modified current component is used as the coil pattern of the y-direction gradient magnetic field shield coil yGSC as shown in FIG. Can be made. Specifically, correction current components 18a and 18b shown in FIG. 9B are added to and combined with the current components along the crossover 14 shown in FIG. 9A, as shown in FIG. 9C. The angle of 90 degrees or less formed with the return line 13 is changed to a meandering connecting line 14 so as to be smaller than the angle of 90 degrees or less formed with the return line 13 when the adjacent main lines 12 are connected by a straight line. .

  In FIG. 11, the connecting wire 14a of the initial GC coil pattern and the connecting wire 14 of the modified GC coil pattern are shown superimposed. As a result, the crossover line 14 becomes the return line 13 when the angle of 90 degrees or less formed with the return line 13 connects the adjacent main lines 12 with a straight line in the region overlapping with the return line 13 (14a). It meanders so as to be smaller than an angle of 90 degrees or less. In addition, the flow of the current center of gravity is displayed as a connecting line 14 and a main line 12 of the initial GC coil pattern and the modified GC coil pattern. Before and after the correction, the current (coil) pattern is corrected so that the magnetic moment is reduced on the lower side of the central return line 13 in FIG. 11 (the area surrounded by the connecting wire 14 and the connecting wire 14a is reduced), and the upper side is corrected. Then, the magnetic moment is corrected so as to increase.

  In the above, the method of obtaining the gradient of the current potential from the conductor width and from the original current potential calculation result and calculating the displacement amount of the conductor (coil) position corresponding to the correction current component has been described. First, as described with reference to FIG. 11, there is a method for correcting the magnetic moment. Since the integral value of the area of the current potential is the magnetic moment, this integration is performed for each region that represents one or several turns of the coil surface, and the area and current product of the region surrounded by the corresponding turn change the magnetic moment to be changed. The amount of displacement of the conductor (coil) position is determined so that the size becomes. The effect is the same in either correction method, and the modified GC coil pattern is a coil pattern that minimizes the generation of eddy currents on the adjacent conductive surface.

  Next, with reference to FIG. 12, a description will be given of a modification example of the main line 12 in the case where the main line 12 crosses the place where the crossover line 14 is not present and the feed line 11 and the return line 13 are flowing round-trip current. . If it demonstrates along the flowchart of the design method of a coil pattern, it can implement similarly to the above until step S2.

  In step S3, an error magnetic field in the static magnetic field coil device 2 or the like is calculated based on the initial GC coil pattern prepared in step S1. In the initial GC coil pattern, as shown in FIG. 12A, the main line 12 crosses the feeding line 11 and the return line 13 where a reciprocating current flows.

  In step S4, a correction current component that cancels the error magnetic field is calculated so as to exist on the coil surface. As a result, as shown in FIG. 12B, on the coil surface where the main line 12 exists, with respect to the feeding line 11, the return line 13, and the main line 12 of the initial GC coil pattern as shown in FIG. The correction current component 21 can be calculated.

  In step S5, the correction current component 21 shown in FIG. 12 (b) is added to and synthesized with the main line 12 shown in FIG. 12 (a), and a convex shape is formed in the correction section 15 as shown in FIG. 12 (c). The main line 12 is detoured by being bent. Such a modified GC coil pattern can also reduce the error magnetic field that generates eddy currents.

  Thus, according to the first embodiment, in the design of the gradient magnetic field coil, the error magnetic field penetrating through the adjacent conductive surface such as the vacuum vessel 2c can be weakened, and the generation of eddy current can be suppressed. Can be provided. Further, by suppressing the generation of eddy current, vibration generated by the eddy current can be reduced.

(Second Embodiment)
FIG. 13A shows a pattern diagram around the return line 13 and the crossover line 14 such as the y-direction gradient magnetic field shield coil of the gradient magnetic field coil apparatus according to the second embodiment of the present invention, and FIG. FIG. 3 shows a cross-sectional view of the return line 13 and the crossover line 14 in the vicinity. The second embodiment is different from the first embodiment in that the return line 13 is arranged on the opposite side of the vacuum vessel 2c and the like with the crossover 14 interposed therebetween, as shown in FIG. 13B. Is a point. Thereby, the return line 13 is arranged at a position farther from the crossover line 14 with respect to the vacuum container 2c and the like. The strength of the error magnetic field generated in the vacuum vessel 2c by the return line 13 far from the vacuum vessel 2c is small. In order to cancel this small error magnetic field, a small magnetic field corresponding to the vacuum vessel 2c by the connecting wire 14 close to the vacuum vessel 2c. Therefore, the angle of 90 degrees or less between the connecting wire 14 and the return line 13 is smaller than the angle of 90 degrees or less with the return line 13 when the adjacent main lines 12 are connected by a straight line. There is no need to meander. And in 2nd Embodiment, in order to cancel a small error magnetic field, between the adjacent main lines 12 is connected with the linear connecting line 14, and the connecting width W3 is made variable, the inclination of the connecting line 14 is made. It is adjusted.

  FIG. 14 shows the results of the coil pattern design method described in the first embodiment, which is similarly performed in the second embodiment. 14A to 14D are distribution diagrams of the direction and magnitude of the error magnetic field on the vacuum vessel (conductor) 2c generated by the return line 13 and the crossover line 14, and FIG. FIG. 14B shows the case where the ratio of the transition width W3 to the width W4 of the return line 13 is four times, FIG. 14B shows the case where the ratio of the transition width W3 to the width W4 is six times, and FIG. 14C shows the width W4. FIG. 14D shows the case where the ratio of the transition width W3 to the width W4 is 10 times. It is considered that the eddy current is less likely to occur as the magnitude of the error magnetic field in the z direction is smaller over the entire region on the vacuum vessel (conductor) 2c. The magnitude of the error magnetic field in the z direction is the smallest over the entire region when the ratio is about 8 times. The larger the ratio, the larger the magnitude of the error magnetic field in the z direction and the smaller the ratio. The magnitude of the error magnetic field in the z direction has increased. Thus, it has been found that if the ratio is 4 times or more and 10 times or less, the size of the error magnetic field in the z direction can be suppressed to be small over the entire region. According to this, since the generation of eddy current can be suppressed, the image quality of the cross-sectional image can be improved.

DESCRIPTION OF SYMBOLS 1 Magnetic resonance imaging (MRI) apparatus 2 Static magnetic field coil apparatus 2a Static magnetic field coil (static magnetic field main coil)
2b Static magnetic field coil (static magnetic field shield coil)
2c Container (vacuum container, conductor)
2d Heat radiation shield 2e Refrigerant container 3 Gradient magnetic field coil device 4 RF coil 5 Subject (patient)
6 Bed 7 Direction of static magnetic field 8 Imaging area (central area)
DESCRIPTION OF SYMBOLS 9 Gradient magnetic field 10 Symmetry axis 11 Feed line 12 Main line 13 Return line 14 Crossover line 15 Correction line 16 Magnetic field line 17 Connection pillar 18a, 18b Correction current 19 Direction and magnitude of magnetic field 20 Coil surface 21 Correction current GMC Gradient magnetic field main coil xGMC x Directional gradient magnetic field main coil yGMC y direction gradient magnetic field main coil zGMC z direction gradient magnetic field main coil GSC gradient magnetic field shield coil xGSC x direction gradient magnetic field shield coil yGSC y direction gradient magnetic field shield coil zGSC z direction gradient magnetic field shield coil W1, W3 Crossover width W2 Return line width

Claims (7)

A gradient coil device comprising: a first coil that creates a linear magnetic field distribution in an imaging region; and a second coil that suppresses a leakage magnetic field from the first coil to a static magnetic field coil device that creates a uniform magnetic field distribution in the imaging region. When,
In a nuclear magnetic resonance imaging apparatus having a static magnetic field coil apparatus that is arranged close to the gradient magnetic field coil apparatus and generates a uniform static magnetic field in the imaging region,
In at least one of the first coil and the second coil, a return line from the spiral coil pattern is disposed on the opposite side of the static magnetic field coil device across a crossover that intersects the return line. In this case, the width of the return line in the width direction of the return line is not less than 4 times and not more than 10 times the width of the return line.   When at least one of the first coil and the second coil, the return line is disposed between the crossover line and the static magnetic field coil device, the crossover line is a trough as the coil pattern. 2. The nuclear magnetic resonance imaging apparatus according to claim 1, wherein the magnetic resonance imaging apparatus is meandering so as to go down and rise down like a mountain.   When at least one of the first coil and the second coil has a power supply line to the coil pattern and a return line arranged between the coil pattern and the static magnetic field coil device, The nuclear magnetic resonance imaging apparatus according to claim 1 or 2, wherein a section of the coil pattern that intersects the electric wire and the return line is detoured so as to be bent into a convex shape. A first coil that creates a linear magnetic field distribution in the imaging region of the nuclear magnetic resonance imaging apparatus, and a second coil that suppresses the leakage magnetic field from the first coil to the static magnetic field coil device that creates a uniform magnetic field distribution in the imaging region. A method for designing at least one of the coil patterns,
When the return line from the spiral coil pattern is arranged on the opposite side of the static magnetic field coil device across the crossover that intersects the return line,
Based on the initial coil pattern prepared in advance, the error magnetic field in the static magnetic field coil device is calculated,
Calculate a correction current component that flows on the crossover and cancels the error magnetic field,
A coil, wherein the width of the return line in the width direction of the return line of the initial coil pattern is deformed based on the correction current component so as to be not less than 4 times and not more than 10 times the width of the return line. Pattern design method. In calculating the correction current component,
The coil pattern design method according to claim 4, wherein a current potential distribution on the same plane as the initial coil pattern is calculated.   A gradient magnetic field coil device comprising at least one of the first coil and the second coil designed and manufactured by using the coil pattern design method according to claim 4.   A nuclear magnetic resonance imaging apparatus comprising at least one of the first coil and the second coil designed and manufactured using the coil pattern design method according to claim 4 or 5. .