Gradient magnetic field coil device, nuclear magnetic resonance imaging device, and coil pattern design method

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

An MRI device is a device that obtains cross-sectional images showing the physical and chemical properties of a subject by utilizing the nuclear magnetic resonance phenomenon that occurs when a subject placed in a uniform static magnetic field is irradiated with high-frequency pulses. Yes, especially for medical use. The MRI device mainly consists of a static magnetic field coil device that generates a uniform static magnetic field in the imaging region in which the subject is inserted, and a gradient magnetic field in which the magnetic field strength is spatially gradient in order to give position information to the imaging region. A gradient magnetic field coil device that generates a pulse shape, an RF coil that irradiates a subject with a high-frequency pulse, a receiving coil that receives a magnetic resonance signal from the subject, and a cross-sectional image obtained by processing the received magnetic resonance signal. Has a computer system to display.

Then, 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 is proposed (see Patent Document 1).<patcit num="1"><text>Japanese Unexamined Patent Publication No. 2001-353137 (Fig. 1)</text></patcit>

<p> The conventional gradient magnetic field coil device is provided with a coil having a complicated coil pattern. In the coil pattern, a plurality of loop-shaped main lines opened at one location are arranged in multiple positions on one surface so as to be inside the adjacent main lines, and a crossover connecting the adjacent main lines. , The return line drawn from the inner main line to the outside of the outer main line is provided so as to partially overlap with each other.</p><p> A plurality of main lines are arranged in a loop shape open at one place in a plurality of manners, and the adjacent main lines are connected by a crossover to form a spiral coil pattern in which the multiple main lines are connected. Then, by providing a return line, it is possible to pass a current through a plurality of main lines. However, the conventional gradient magnetic field coil device is designed so that a linear gradient magnetic field is formed when a current flows only through a plurality of main lines. Causes an error magnetic field. This error magnetic field generates an eddy current in the static magnetic field coil device, and this eddy current may form a magnetic field in the imaging region that disturbs the cross-sectional image.</p><p> Therefore, an object of the present invention is to provide a gradient magnetic field coil device, an MRI device, and a coil pattern design method capable of suppressing the generation of an error magnetic field or an eddy current and improving the image quality of a cross-sectional image.</p>

<p> In order to achieve the above object, the present invention provides a first coil that creates a linear magnetic field distribution in the imaging region of the MRI apparatus, and a static magnetic field coil apparatus that creates a uniform magnetic field distribution from the first coil to the imaging region. A gradient magnetic field coil including a second coil that suppresses a leakage magnetic field, and at least one of the first coil and the second coil has a meandering crossing line portion that intersects with a return line from a spiral coil pattern. It is characterized by being an apparatus and further being an MRI apparatus equipped with the apparatus.</p><p> Further, the present invention suppresses the leakage magnetic field from the first coil that creates a linear magnetic field distribution in the imaging region of the MRI apparatus and the static magnetic field coil apparatus that creates a uniform magnetic field distribution in the imaging region from the first coil. Two coils are provided, and in at least one of the first coil and the second coil, the width of the crossing line portion intersecting the return line from the spiral coil pattern is four times or more the width of the return line. It is characterized by being a gradient magnetic field coil device having a frequency of 2 times or less, and further being an MRI device equipped with this.</p><p> Further, the present invention suppresses the leakage magnetic field from the first coil that creates a linear magnetic field distribution in the imaging region of the MRI apparatus and the static magnetic field coil apparatus that creates a uniform magnetic field distribution in the imaging region from the first coil. Two coils are provided, and at least one of the first coil and the second coil bypasses one section of the coil pattern that intersects the feeding line to the spiral coil pattern and the return line from the coil pattern. It is characterized by being a gradient magnetic field coil device and an MRI device equipped with the coil device.</p><p> Further, the present invention suppresses the leakage magnetic field from the first coil that creates a linear magnetic field distribution in the imaging region of the MRI apparatus and the static magnetic field coil apparatus that creates a uniform magnetic field distribution in the imaging region from the first coil. It is a method of designing a coil pattern of at least one of two coils, and the error magnetic field entering the static magnetic field coil device is calculated based on an initial coil pattern prepared in advance, and is placed on the same surface as the initial coil pattern. It is characterized in that a correction current component that cancels the error magnetic field is calculated so as to exist, and the correction current component and the initial coil pattern are combined. Further, it is characterized in that it is a gradient magnetic field coil device and an MRI device having at least one of the first coil and the second coil, which are designed and manufactured by using this coil pattern design method.</p>

<p> According to the present invention, it is possible to provide a gradient magnetic field coil device, an MRI device, and a method for designing a coil pattern, which can suppress the generation of an error magnetic field or an eddy current and improve the image quality of a cross-sectional image.</p>

Next, an embodiment of the present invention will be described in detail with reference to the drawings as appropriate. In each figure, the same reference numerals are given to common parts, and duplicate description will be omitted.

(First Embodiment) FIG. 1 shows a perspective view of the MRI (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 arranged so as to sandwich the subject 5 lying on the bed 6 from above and below with respect to the imaging region 8 into which the subject 5 is inserted, and a pair of upper and lower static magnetic field coil devices 2 that generate 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 strength is spatially gradient gradient is generated in a pulse shape in order to give position information to the imaging region 8. 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 that receives the magnetic resonance signal from the subject 5 (not shown), and the received magnetism. It has a computer system (not shown) that processes a 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 (cylinder) shape with the axis of symmetry 10 as a common axis. The subject 5 is carried to the imaging region 8 by the movable bed 6, but since only the thin connecting pillar 17 connects the pair of upper and lower static magnetic field coil devices 2, the subject 5 can see the surroundings and reduce the feeling of claustrophobia. can do. In addition, to facilitate understanding of the explanation described later, the z-axis is set in the vertical direction parallel to and coincides with the axis of symmetry 10, and the x-axis and y-axis are set so as to be perpendicular to each other in the horizontal direction. There is.

FIG. 2 shows a cross-sectional view of the MRI apparatus 1 according to the first embodiment of the present invention cut along the yz plane including the axis of symmetry 10 (z axis). 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 in the upper and lower pair of static magnetic field coil devices 2. The pair of upper and lower static magnetic field main coils 2a and the upper and lower pair of static magnetic field shield coils 2b each have an annular shape with the axis of symmetry 10 as a common central axis. Further, the pair of upper and lower static magnetic field main coils 2a and the upper and lower pair of static magnetic field shield coils 2b are housed in a container having a three-layer structure. First, the pair of static magnetic field main coils 2a and the pair of static magnetic field shield coils 2b are housed in the refrigerant container 2e together with the refrigerant 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 inside of the vacuum container 2c in a vacuum while accommodating the refrigerant container 2e and the heat radiation shield 2d. Even if the vacuum vessel 2c is placed in a room at a normal room temperature, the inside of the vacuum vessel 2c is evacuated, so that the heat in the chamber is not transferred to the refrigerant container 2e by conduction or convection. Further, the heat radiation shield 2d suppresses the transfer of indoor heat from the vacuum container 2c to the refrigerant container 2e by radiation. Therefore, the pair of static magnetic field main coils 2a and the pair of static magnetic field shield coils 2b can be stably set to an extremely low temperature, which is the temperature of the refrigerant, and can function as a superconducting electromagnet. A non-magnetic member is used for the refrigerant container 2e, the thermal radiation shield 2d, and the vacuum container 2c so that a force due to a magnetic field does not act, and a non-magnetic metal is used for ease of processing. Therefore, a current, particularly an eddy current, can flow in the refrigerant container 2e, the thermal radiation shield 2d, and the vacuum container 2c.

The gradient magnetic field coil device 3 also has a pair of upper and lower tilt magnetic field coils 3, and the pair of upper and lower gradient magnetic field coil devices 3 are arranged vertically with the imaging region 8 interposed therebetween. The RF coils 4 also have a pair of upper and lower ones, and the pair of upper and lower RF coils 4 are arranged one above the other with the imaging region 8 in between. The pair of upper and lower inclined magnetic field coil devices 3 are arranged close to each other between the pair of upper and lower static magnetic field coil devices 2 and the pair of upper and lower RF coils 4. Similarly, the upper and lower pair of lower inclined magnetic field coil devices 3 are arranged close to each other between the upper and lower pair of lower static magnetic field coil devices 2 and the upper and lower pair of lower RF coils 4. .. The pair of upper and lower gradient magnetic field coil devices 3 generate 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 in a pulsed manner. The gradient magnetic field coil device 3 has a function of being able to generate an independent gradient magnetic field 9 in three directions of the x direction, the y direction, and the z direction by superimposing it on 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 magnetic field coil devices 3. The gradient magnetic field coil device 3 has a pair of upper and lower gradient magnetic field coils GC arranged one above the other with the imaging region 8 in between. The pair of upper and lower tilted magnetic field coils GC consists of a pair of upper and lower tilted magnetic field main coils (first coils) GMC arranged vertically with the imaging region 8 in between, and a pair of upper and lower tilted magnetic fields arranged above and below the imaging region 8. It has a magnetic field shield coil (second coil) GSC. A pair of upper and lower gradient magnetic field shield coils (second coil) GMCs are arranged between the upper and lower pairs of gradient magnetic field main coils GMC with the imaging region 8 sandwiched between them. The pair of upper and lower inclined magnetic field main coils GMC and the pair of upper and lower inclined magnetic field shield coils GSC support each other via the support member 3a. Similarly, the pair of upper and lower tilted magnetic field main coils GMC and the pair of upper and lower tilted magnetic field shield coils GSC support each other via the support member 3a. The pair of upper and lower inclined magnetic field coil devices 3, particularly the pair of upper and lower inclined magnetic field shield coils GSC, are close to the pair of upper and lower static magnetic field coil devices 2, especially the pair of upper and lower vacuum containers 2c. It is arranged. Similarly, the pair of upper and lower pairs of lower tilted magnetic field coil devices 3, in particular the pair of upper and lower pairs of lower tilted magnetic field shield coils GSC, are the pair of upper and lower pairs of lower static magnetic field coil devices 2, in particular the pair of upper and lower pairs of lower sides. It is located close to the vacuum vessel 2c.

A pair of upper and lower gradient magnetic field main coils GMC creates a gradient magnetic field that changes linearly in the x direction, and a pair of upper and lower x-direction gradient magnetic field main coils xGMC that are arranged vertically across the imaging region 8 and a pair of upper and lower gradient magnetic fields that change linearly in the y direction A pair of upper and lower y-direction gradient magnetic field main coils yGMC that are arranged vertically across the imaging region 8 and a gradient magnetic field that changes linearly in the z direction are created and arranged vertically across the imaging region 8. It has a pair of upper and lower z-direction gradient magnetic field main coils zGMC. 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 each form a layer (3 layers) for each pair of gradient magnetic field coil devices 3. Three layers of gradient magnetic field main coils xGMC, yGMC, and zGMC are paired, and these three layers are laminated in the z direction with the insulating layer of the support member 3a in between.

The pair of upper and lower gradient magnetic field shield coils GSC suppresses the leakage of the magnetic field formed by the x-direction gradient magnetic field main coil xGMC to the surroundings, and the upper and lower pair of tilted magnetic field shield coils xGSC are arranged vertically with the imaging region 8 in between. A pair of upper and lower y-direction gradient magnetic field shield coils yGSC and a z-direction gradient magnetic field main coil that suppress the leakage of the magnetic field formed by the y-direction gradient magnetic field main coil yGMC to the surroundings and are arranged vertically across the imaging region 8. It has a pair of upper and lower z-direction gradient magnetic field shield coils zGSC that suppress the magnetic field formed by zGMC from leaking to the surroundings and are arranged vertically with the imaging region 8 in between. 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 (3 layers) for each pair of gradient magnetic field coil devices 3. Three layers of gradient magnetic field shield coils xGSC, yGSC, and zGSC are paired, and these three layers are laminated in the z direction with the insulating layer of the support member 3a in between.

Figure 4 (a) shows the layout of the y-direction gradient magnetic field main coil yGMC and the y-direction gradient magnetic field shield coil yGSC. The y-direction gradient magnetic field main coil yGMC is arranged in two disc-shaped layers (not shown) centered on the z-axis, for a total of four. Each of the four y-direction gradient magnetic field main coils yGMC is a fan-shaped coil that is approximately semi-circular and spiral, but the shape of the spiral is omitted and only the rough current direction is shown. The four y-direction gradient magnetic field main coils yGMC are divided into two by the x-axis-y-axis plane and have a plane-symmetrical structure in that plane. In addition, the four y-direction gradient magnetic field main coils yGMC are divided into two by the z-axis-x-axis plane and have a plane-symmetrical structure in that plane. The arrow indicates the direction of the current flowing through the coil, and the same applies to the following.

The y-direction gradient magnetic field shield coil yGSC is arranged in two disc-shaped layers (not shown) centered on the z-axis, for a total of four. Each of the four y-direction gradient magnetic field shield coils yGSC is a substantially semicircular, spiral fan-shaped coil, which is arranged so as to cover the corresponding y-direction gradient magnetic field main coil yGMC. The shape of the spiral is not shown, and only the rough current direction is shown. The four y-direction gradient magnetic field shield coils yGSC are divided into two by the x-axis-y-axis plane and have a plane-symmetrical structure in that plane. In addition, the four y-direction gradient magnetic field shield coils yGSC are divided into two by the z-axis-x-axis plane and have a plane-symmetrical structure in that plane.

Figure 4 (b) shows the layout of the x-direction gradient magnetic field main coil xGMC and the x-direction gradient magnetic field shield coil xGSC. The x-direction gradient magnetic field main coil xGMC is arranged in two disc-shaped layers (not shown) centered on the z-axis, for a total of four. Each of the four x-direction gradient magnetic field main coils xGMC is a fan-shaped coil that is approximately semi-circular and spiral, but the shape of the spiral is omitted and only the rough current direction is shown. The four x-direction gradient magnetic field main coils xGMC are divided into two by the x-axis-y-axis plane and have a plane-symmetrical structure in that plane. In addition, the four x-direction gradient magnetic field main coils xGMC are divided into two by the y-axis-z-axis plane and have a plane-symmetrical structure in that plane.

The x-direction gradient magnetic field shield coil xGSC is arranged in two disc-shaped layers (not shown) centered on the z-axis, for a total of four. Each of the four x-direction gradient magnetic field shield coils xGSC is a substantially semicircular, spiral fan-shaped coil arranged to cover the corresponding x-direction gradient magnetic field main coil xGMC. The shape of the spiral is not shown, and only the rough current direction is shown. The four x-direction gradient magnetic field shield coils xGSC are divided into two by the x-axis-y-axis plane and have a plane-symmetrical structure in that plane. In addition, the four x-direction gradient magnetic field shield coils xGSC are divided into two by the y-axis-z-axis plane and have a plane-symmetrical structure in that plane.

Figure 4 (c) shows the layout of the z-direction gradient magnetic field main coil zGMC and the z-direction gradient magnetic field shield coil zGSC. The z-direction gradient magnetic field main coil zGMC is arranged in two disc-shaped layers (not shown) with the z-axis as the central axis, one for each. The two z-direction gradient magnetic field main coils zGMC are circular and spiral coils, but the illustration of the spiral is omitted and only the rough current direction is shown. The two z-direction gradient magnetic field main coils zGMC are each divided by the x-axis-y-axis plane and have a plane-symmetrical structure in that plane.

Two z-direction gradient magnetic field shield coils, zGSC, are arranged in two disk-shaped layers (not shown) centered on the z-axis. The two z-direction gradient magnetic field shield coils zGSC are circular and spiral circular coils arranged so as to cover the corresponding z-direction gradient magnetic field main coil zGMC. It should be noted that the illustration of the spiral is omitted and only the rough current direction is shown. The two z-direction gradient magnetic field shield coils zGSC are each divided by the x-axis-y-axis plane and have a plane-symmetrical structure in that plane.

Figure 5 (a) shows a pattern diagram of the y-direction gradient magnetic field shield coil yGSC. Further, FIG. 5 (b) is a cross-sectional view of FIG. 5 (a) in the AA direction, and FIG. 5 (c) is a cross-sectional view of FIG. 5 (a) in the BB direction. In contrast to the coil pattern of the y-direction gradient magnetic field shield coil yGSC, the y-direction gradient magnetic field main coil yGMC has a similar coil pattern with a slightly smaller size. 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 coil pattern of a slightly smaller size and similar shape.

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 areas (three in FIG. 5A). In the first region, the main line 12 is arranged in a quadruple (multiplex) so as to be inside the adjacent main line 12. In the second region, the main line 12 is doubly arranged 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 feeder line 11 for supplying power to the main line 12 in each region and a return line 13 for returning current from the main line 12 arranged along the feeder line 11 and fed by the feeder line 11 are provided. The point where the feeder line 11 and the return line 13 straddle the main line 12 is a correction section 15 in the main line 12, and in this 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 feeding line 11 and the return line 13, the magnetic field generated by the main line 12 bypassed by being bent into a convex shape in the correction section 15 near the feeding line 11 and the return line 13. Since the error magnetic field can be canceled, the eddy current generated in the vacuum vessel 2c of the static magnetic field coil device 2 can be suppressed, and the image quality of the cross-sectional image can be improved. The width W2 of the correction section 15 is set to be larger than the distance between the feeder line 11 and the return line 13.

In the first region and the second region, the plurality of main lines 12 arranged in double or quadruple multiplex each have a loop shape that is open at one place such as a U-shaped shape. The open portions in this loop shape (U-shaped shape) are arranged in a row, and at this portion, the crossover line 14 connects between the adjacent main lines 12. By this connection, a spiral coil pattern in which a plurality of main lines 12 are connected is formed. The width of this portion, that is, the width (crossover width) W1 in which the crossover line 14 described later is installed is set wider than the line width of the wiring of the return line 13. The crossover line 14 meanders so that the angle of 90 degrees or less formed with the return line 13 is 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. The return line 13 is connected not only to connect the regions but also to pull out the wiring from the inner main line 12 arranged in multiple layers to the outside 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 in the vicinity of the return line 13, so that the static magnetic field coil device 2 can be used. The eddy current generated in the vacuum vessel 2c or the like can be suppressed, and the image quality of the cross-sectional image can be improved.

As shown in FIG. 5 (b), the return line 13 is arranged between the crossover line 14 and the vacuum vessel 2c of the static magnetic field coil device 2. That is, the return line 13 is arranged closer to the crossover line 14 with respect to 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 be large, and in order to cancel this large error magnetic field, a large magnetic field is generated in the vacuum vessel 2c by the crossover line 14 far from the vacuum vessel 2c. The angle between the crossing line 14 and the return line 13 and 90 degrees or less is smaller than the angle between the return line 13 and 90 degrees or less when the adjacent main lines 12 are connected by a straight line. I'm letting you.

As shown in FIG. 5 (c), the feeder line 11 and the return line 13 are arranged between the correction section 15 of the main line 12 and the vacuum vessel 2c of the static magnetic field coil device 2. That is, the feeder line 11 and the return line 13 are arranged closer to the vacuum container 2c than the correction section 15 of the main line 12. The strength of the error magnetic field generated in the vacuum vessel 2c by the feeding 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, the degree of bending into a convex shape in the correction section 15 is adjusted accordingly.

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

First, in step S1, the shape (including the arrangement position) of the main line 12 of the y-direction gradient magnetic field shield coil yGSC, etc. is calculated, and as shown in FIG. 7 (a), the crossover line 14 is set as a straight line. Wire (connect) to 12 and determine the initial GC coil pattern.

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

In step S3, the 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. Since the error magnetic field generates a magnetic field line 16 that penetrates the conductor surface of the vacuum vessel 2c of the static magnetic field coil device 2 as shown in FIG. 7 (b), the magnetic field line 16 is generated in the vacuum vessel 2c or the like as shown in FIG. The error magnetic field can be calculated by calculating the direction and distribution of the magnitude (strength) 19 of the magnetic field entering the conductor surface.

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

More specifically, first, a current potential is given to the contacts of the finite surface element, and the current represented by the vector T showing the current potential distribution having the current potential as the element cancels the error magnetic field B on the conductor surface. Determine the current potential distribution T. The current density vector is represented by the vector product of the gradient of the current potential and the normal of the current (coil) plane. Then, by using a method that applies singular value decomposition to this approximate solution, the current potential T of the canceling current component, which is not complicated and can suppress the generation of eddy currents and improve the accuracy of the magnetic field, is obtained. be able to.

Once the current potential distribution T corresponding to the canceling current component is determined by this method, the displacement of the conductor (coil) position is calculated next. From the distance d between the conductors and the current Ic of the conductor, 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 calculated current potential value T0, the displacement of the conductor (coil) position can be calculated by the formula T / T0.

The method reported in the following literature can be used for the calculation of the correction current component. The literature is M. ABE, T. NAKAYAMA, S. OKAMURA, K. MATSUOKA, A new technique to optimize coil winding path for the particularly 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 GC coil pattern is completed by adding the corrected current component to the current component along the initial GC coil pattern and using the deformed current component as the coil pattern of the y-direction gradient magnetic field shield coil yGSC as shown in Fig. 5 (a). Can be made to. Specifically, the correction current components 18a and 18b shown in FIG. 9 (b) are added / combined with respect to the current component along the crossover 14 shown in FIG. 9 (a), and as shown in FIG. 9 (c). The angle of 90 degrees or less with the return line 13 is changed to a meandering crossover line 14 so that the angle of 90 degrees or less with 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. ..

FIG. 11 shows the crossover line 14a of the initial GC coil pattern and the crossover line 14 of the modified GC coil pattern superimposed. From this, the crossover line 14 is the return line 13 in the region where the return line 13 overlaps, when the angle of 90 degrees or less with the return line 13 connects the adjacent main lines 12 with a straight line (14a). It meanders so that it is smaller than an angle of 90 degrees or less. The flow of the current center of gravity is displayed as the crossover line 14 and the 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 becomes smaller on the lower side of the return line 13 in the center of FIG. 11 (the area surrounded by the crossover line 14 and the crossover line 14a is corrected to be smaller), and the upper side. Is corrected so that the magnetic moment becomes large.

In the above, the method of obtaining the gradient of the current potential from the conductor width and 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, but the present invention is limited to this. However, as described in relation to FIG. 11, there is also a method of correcting by considering the magnetic moment. Since the area integral value of the current potential is the magnetic moment, this integration is performed for each region representing one turn or several turns of the coil surface, and the area and current product of the region surrounded by the corresponding turn are the magnetic moments to be changed. Determine the magnitude of the displacement of the conductor (coil) position so that it becomes the magnitude of. The effect is the same for both correction methods, 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 modification example of the main line 12 when the main line 12 crosses the place where the feed line 11 and the return line 13 are passing a reciprocating current without the crossover line 14 will be described. .. Explaining along with the flowchart of the coil pattern design method, up to step S2 can be carried out in the same manner as described above.

In step S3, the 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 place where the feed line 11 and the return line 13 carry a reciprocating current.

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

In step S5, the correction current component 21 shown in FIG. 12 (b) is added / synthesized to the main line 12 shown in FIG. 12 (a), and the corrected current component 21 is formed into a convex shape in the correction section 15 as shown in FIG. 12 (c). It has been changed to the main line 12, which is detoured by being folded. The error magnetic field that generates the eddy current can also be reduced by such a modified GC coil pattern.

As described above, according to the first embodiment, in the design of the gradient magnetic field coil, the error magnetic field penetrating the proximity conductive surface of the vacuum vessel 2c or the like can be weakened, and the generation of eddy current can be suppressed, so that a clear diagnostic image can be obtained. Can be provided. Further, by suppressing the generation of eddy current, the vibration generated by the eddy current can be reduced.

(Second embodiment) FIG. 13 (a) shows a pattern diagram around the return line 13 and the crossover line 14 of the y-direction gradient magnetic field shield coil and the like of the gradient magnetic field coil device according to the second embodiment of the present invention, and FIG. 13 (b) shows. The cross-sectional view of the return line 13 and the crossover line 14 around the periphery is shown in. The difference between the second embodiment and the first embodiment is that, as shown in FIG. 13 (b), the return line 13 is arranged on the opposite side of the vacuum vessel 2c or the like with the crossover line 14 in between. It is a point. As a result, the return line 13 is located farther than 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 away from the vacuum vessel 2c is small, and in order to cancel this small error magnetic field, the small magnetic field corresponding to the vacuum vessel 2c by the crossing line 14 near the vacuum vessel 2c. Is sufficient, so that the angle of 90 degrees or less between the crossing line 14 and the return line 13 is smaller than the angle of 90 degrees or less between the crossing line 14 and the return line 13 when the adjacent main lines 12 are connected by a straight line. You don't have to meander like that. Then, in the second embodiment, in order to cancel a small error magnetic field, the inclination of the crossover line 14 is made by connecting the adjacent main lines 12 with a linear crossover line 14 and making the crossover width W3 variable. I'm adjusting.

FIG. 14 shows the results of implementing the coil pattern design method described in the first embodiment in the same manner in the second embodiment. 14 (a) to 14 (d) are distribution maps 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. 14 (a) is a distribution diagram. The ratio of the crossover width W3 to the width W4 of the return line 13 is 4 times, FIG. 14 (b) shows the case where the ratio of the crossover width W3 to the width W4 is 6 times, and FIG. 14 (c) shows the width W4. The ratio of the crossover width W3 to the width W3 is 8 times, and FIG. 14 (d) shows the case where the ratio of the crossover width W3 to the width W4 is 10 times. It is considered that the smaller the magnitude of the error magnetic field in the z direction over the entire region on the vacuum vessel (conductor) 2c, the less likely it is that eddy currents will be generated. The magnitude of the error magnetic field in the z direction is the smallest over the entire region when the ratio is around 8 times, and the larger the ratio, the larger the magnitude of the error magnetic field in the z direction, and the smaller the ratio. Indeed, the magnitude of the error magnetic field in the z direction has increased. Therefore, it was found that if the ratio is 4 times or more and 10 times or less, the magnitude of the error magnetic field in the z direction can be suppressed to a small value over the entire region. According to this, the generation of eddy current can be suppressed, so that the image quality of the cross-sectional image can be improved.

<figref num="1">It is a perspective view of the MRI (magnetic resonance imaging) apparatus which concerns on 1st Embodiment of this invention.</figref><figref num="2">It is sectional drawing which cut | cut the MRI apparatus which concerns on 1st Embodiment of this invention in the yz plane including the axis of symmetry (z axis).</figref><figref num="3">It is sectional drawing of the gradient magnetic field coil apparatus which concerns on 1st Embodiment of this invention.</figref><figref num="4">(a) is a layout diagram of a y-direction gradient magnetic field main coil and a y-direction gradient magnetic field shield coil of the gradient magnetic field coil device according to the first embodiment of the present invention, and (b) is a layout diagram of the first embodiment of the present invention. It is a layout diagram of the x-direction gradient magnetic field main coil and the x-direction gradient magnetic field shield coil of the gradient magnetic field coil device according to the above, and (c) is the z-direction gradient magnetic field main of the gradient magnetic field coil device according to the first embodiment of the present invention. It is a layout diagram of a coil and a magnetic field shield coil tilted in the z direction.</figref><figref num="5">(a) is a pattern diagram of the y-direction gradient magnetic field shield coil of the gradient magnetic field coil device according to the first embodiment of the present invention, (b) is a cross-sectional view of (a) in the AA direction, and (c). Is a cross-sectional view of (a) in the BB direction.</figref><figref num="6">It is a flowchart of the design method of the coil pattern such as the y-direction gradient magnetic field shield coil of the gradient magnetic field coil device which concerns on 1st Embodiment of this invention.</figref><figref num="7">(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).</figref><figref num="8">This is an example of a coil surface divided by a finite element of a triangular mesh in step S2 of the coil pattern design method.</figref><figref num="9">(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 the correction current (component) calculated in step S4. ) Is a conceptual diagram, and (c) is an enlarged view around the return line and the first crossover of the modified GC coil pattern to which the correction current component was added in step S5.</figref><figref num="10">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.</figref><figref num="11">It is a figure which shows the change of the flow of the current center of gravity of the GC coil pattern before and after the addition of the correction current component of step S5.</figref><figref num="12">(a) is an enlarged view of the feeder 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 the correction current (component) calculated in step S4. ) Is a conceptual diagram, and (c) is an enlarged view of the periphery of the feeder line, the return line, and the second crossover line of the modified GC coil pattern to which the correction current component was added in step S5.</figref><figref num="13">(a) is a pattern diagram around the return line and the first crossover of the y-direction gradient magnetic field shield coil of the gradient magnetic field coil device according to the second embodiment of the present invention, and (b) is the return line and the first crossover. It is sectional drawing around the crossover line.</figref><figref num="14">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 the case where the ratio of the first crossover width W3 to the return line width W4 is 6 times, and (c) is the case where the ratio of the first crossover width W3 to the return line width W4 is 8 times. Yes, (d) is the case where the ratio of the first crossover width W3 to the return line width W4 is 10 times.</figref>

Code description

1 Magnetic resonance imaging (MRI) device 2 Static magnetic field coil device 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 thermal radiation shield 2e Refrigerant container 3 Inclined magnetic field coil device 4 RF coil 5 Subject (patient) 6 beds 7 Direction of static magnetic field 8 Imaging area (center area) 9 Gradient magnetic field 10 axis of symmetry 11 Feed line 12 main lines 13 Return line 14 Crossover 15 Correction line 16 magnetic field lines 17 Connecting pillars 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-direction 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