WO2015178411A1

WO2015178411A1 - Mri device having magnet with extremely narrow leakage magnetic field

Info

Publication number: WO2015178411A1
Application number: PCT/JP2015/064435
Authority: WO
Inventors: 充志阿部; 竹内　博幸
Original assignee: 株式会社日立メディコ
Priority date: 2014-05-20
Filing date: 2015-05-20
Publication date: 2015-11-26
Also published as: JP6486344B2; CN106456047B; JPWO2015178411A1; CN106456047A

Abstract

In the present invention, an electromagnet, which is configured by a main coil group (positive current) and a shield coil group (negative current) that constitute a magnetic resonance tomography imaging device (MRI) used for medical diagnosis, contains three or more negative current shield coil blocks, and at least one or more of the shield coil blocks is positioned beyond a main coil block and has a radius that is smaller than the other shield coil blocks.

Description

Extremely narrow magnetic field magnet type MRI system

The present invention relates to a horizontal magnetic field type MRI apparatus.

In the diagnosis using nuclear magnetic resonance by the nuclear magnetic resonance tomography apparatus (MRI) used for medical diagnosis, the magnetic field intensity corresponds to the diagnosis location, so the accuracy required for the magnetic field intensity generated by the magnet system is A fluctuation of about one millionth of the magnetic field strength becomes a problem. Here, there are roughly three types of magnetic fields used in the MRI apparatus.
(1) Several ppm in a space (usually a 30-40cm sphere or ellipsoidal space with a diameter of 0.1 to several tesla), which is steady in time and constant in space Magnetic field that falls within the fluctuation range of about (2) A magnetic field that is changed with a time constant of about 1 second or less and that is spatially inclined. (3) A magnetic field by a high-frequency electromagnetic wave having a frequency (several MHz or more) corresponding to nuclear magnetic resonance. The magnetic field leaks not only to the originally required imaging region 6 (see FIG. 3) but also to the periphery of the apparatus. This leakage magnetic field is completely unnecessary for imaging, but cannot be eliminated. For this reason, the MRI apparatus is devised to reduce the leakage magnetic field to the surroundings. A device applied to the MRI apparatus in order to suppress this leakage magnetic field is called a magnetic shield, but there are roughly the following two types.
(1) A system in which a magnetic material is arranged on the wall of the room where the MRI apparatus is arranged, and the leaking magnetic field lines are captured in the magnetic material to reduce the leakage magnetic field outside the room.
(2) Apart from the main coil (MC) coil block (CB) group that generates a uniform magnetic field exclusively in the imaging region 6, a negative magnetic field is created in the imaging region 6, but a leakage magnetic field around the main coil is generated. A method in which the magnet itself has a shield coil (SC) that generates a magnetic field that cancels around the device (active magnetic shielding method).

And a general method is a method that shares these two types. Hereinafter, the magnetic shield using the shield coil mentioned in the above (2) will be discussed in detail.

Active magnetic shielding methods are detailed in Patent Document 1, Patent Document 2, Patent Document 3 and Patent Document 4.

Patent Document 1 discloses that a shield coil block (SC-CB) is used so that the magnetic field of the entire magnet is substantially zero, and then the higher-order components are adjusted so that the leakage magnetic field is quickly attenuated as the distance from the device increases. Disposing groups is disclosed.

In the method of Patent Document 1, there is a CB that passes both positive and negative currents in the SC-CB group in order to reduce the leakage magnetic field in the surroundings. For this reason, an increase in the number of CBs, an increase in the amount of conductors, an increase in the magnetomotive force of individual CBs due to the adjacent current flowing in the opposite direction, an increase in the amount of wire that realizes the magnetomotive force, and an increase in electromagnetic force Strengthening the CB support structure and the like cause an increase in electromagnet manufacturing costs.

FIG. 2 shows the concept of the conventional example of Patent Document 2. In FIG. 2, the axial direction is taken up and down, the radial direction is taken in the lateral direction, and the equal magnetic flux lines and CB positions are written. The main coil corresponds to MC10 and MC11, and the shield coil corresponds to SC10 and SC11. Since Patent Document 2 employs a shield coil arrangement that is as simple as possible, the leakage magnetic field region in the axial direction (vertical direction in the figure) is widened. In this figure, isomagnetic flux lines (also magnetic lines) and magnetic field strength contour lines are written on the left side. The outermost contour line 2 is a 5 gauss line. In addition, what is written in a square with a radius in the vicinity of 0.5 m to 1.0 m is a cross section of the coil block (MC10 or MC11, SC10 or SC11). On the right side, the CB name is written with a square. MC10 to MC30 are MC-CB11 corresponding to the main coil, and SC10 and SC11 are coil blocks SC-CB15 corresponding to the shield coil. Thus, in a conventional magnet for an MRI apparatus, a 5-gauss line has a radius of 2 m or more and about 3.5 m or more from the apparatus center in the axial direction due to a leakage magnetic field to the surroundings. The straight line running up and down is R = 2.25m and 2.5m in the radius R direction position.

On the other hand, in Patent Document 3, in order to improve the magnetic field uniformity in the imaging region, the magnetic field change due to the CB deformation is optimized from the response of the deformation and the magnetic field change to improve the uniformity. However, the variation of the magnetic field due to the size of the deformation was discussed, and the distribution of the magnetic field reference was not determined. As a result, it was difficult to achieve the best uniformity.

In Patent Document 4, a large number of SC-CBs are arranged. However, since the arrangement is concentrated at a relatively short position in the axial direction, it is difficult to exert a sufficient shielding effect. In addition, since the generation of a uniform magnetic field in the imaging region is discussed in terms of spherical harmonic functions, the function for calculating the current distribution becomes complicated and many functions are superimposed, so that the accuracy is likely to deteriorate.

Japanese Patent No. 4043946 JP 2009-397 A International Publication No. 2012-086644 JP-T 2009-502031

As described above, in the conventional technique, it has been difficult to realize active magnetic shielding capable of reducing the leakage magnetic field to the surroundings while maintaining the magnetic field of the imaging region generated by the magnet with good accuracy. An object of the present invention is to provide an MRI electromagnet that has a uniform magnetic field in an imaging region, while having an active magnetic shielding function that can suppress a leakage magnetic field to the surroundings.

Since the SC-CB group used for magnetic shielding is arranged at a position farther from the imaging area than the main coil group, the influence on the imaging area is relatively small. However, as described in Patent Document 1, extreme magnetic shielding tends to increase the magnetomotive force and deteriorate the uniform magnetic field. For this reason, the minimum number of SC-CBs was used, but a somewhat large leakage magnetic field was allowed. Therefore, in the present invention, improvements are made including other methods. That is, a CB arrangement having a position and a shape that maintains good uniformity of the magnetic field in the imaging region is performed. And the leakage magnetic field to the outside is also suppressed while the uniformity is kept good.

In solving the above-described problems, the present invention provides, as one form thereof, “a horizontal magnetic field type MRI apparatus having a cylindrical magnet open at both ends, wherein the magnet includes an annular main coil, A plurality of annular shield coils each having a diameter larger than that of the main coil and coaxially arranged with the main coil, and at least three of the shield coils are arranged on the shaft, The main coil is also disposed closer to the center of the magnet than the shield coils disposed at both ends in the axial direction, and the shield coil has a larger diameter in the axial direction as it is closer to the center of the magnet. '' It is characterized by.

According to the present invention, it is possible to provide a magnet capable of satisfactorily suppressing the leakage magnetic field to the outside and forming a good uniform magnetic field in the imaging region. Since the leakage magnetic field region is narrow, it can be said that the area and room size required for installing the MRI apparatus can be reduced, and the installation restrictions can be further reduced. In addition, good uniform magnetic field performance can be ensured regardless of the magnetic shielding to the outside.

The figure which shows typically magnetic field distribution and arrangement | positioning of a coil block regarding embodiment of this invention. The figure which shows typically magnetic field distribution and arrangement | positioning of a coil block regarding a prior art example. The figure which shows the concept of the new calculation method regarding coil block arrangement | positioning utilized by this invention. The conceptual diagram of the electric current on the edge | side corresponding to the coil block position movement and deformation | transformation utilized for magnetomotive force arrangement | positioning calculation of this invention. The figure which showed the convergence condition of the repetition calculation of the 2nd step of the magnetomotive force arrangement | positioning calculation method of this invention. The figure which showed the convergence condition of the repetition calculation of the 3rd step of the magnetomotive force arrangement | positioning calculation method of this invention. The magnetomotive force arrangement | positioning figure which shows an example using the magnetic field adjustment method of this invention. The magnetomotive force arrangement and magnetic field distribution when the number of blocks of the main coil is six in the magnetomotive force arrangement of the present invention. Magnetic field distribution and leakage magnetic field reference diagram when the radii of four SC-CBs are the same. An overview of a magnet with a built-in coil block with magnetomotive force calculated by a new calculation method.

Thereafter, a method of constructing a uniform magnetic field so as to satisfy the good uniformity and good leakage magnetic field devised by the present inventors at the same time, and a coil that can obtain a good uniformity by applying the method to uniform magnetic field design. The arrangement will be described.

Also, by applying the present method and designing a uniform magnetic field, the SC-CB can be arranged and the leakage magnetic field to the surroundings can be effectively suppressed without worrying about deterioration of the uniformity of the imaging region 6. In addition, although a present Example is described hereafter using the word of a coil block (CB) suitably, as shown in FIG. 1, a coil block is arrangement | positioning and magnitude | size of a main coil and a shield coil in the horizontal direction cross section of a magnet. Reflecting the above, each coil is schematically shown. The coil block corresponding to the main coil (MC) is represented as MC-CB11, and the coil block (SC) corresponding to the shield coil is represented as SC-CB15.

In the case of the MRI magnet, when considering the generated magnetic field, separate the two directions of the axial direction and the radial direction. That is, for example, assuming a horizontal magnetic field type MRI apparatus, the electromagnet (magnet) is cylindrical. The cylindrical magnet is open at both ends, and an annular main coil and a shield coil are included therein. In the present invention, this cylindrical type electromagnet is considered by separating the leakage magnetic field into the following two regions.

(1) Radial leakage magnetic field from the cylindrical body (2) Axial leakage magnetic field from the end of the shaft Here, in particular, the radial leakage magnetic field of (1) is used for the magnetic moment of the entire electromagnet and the leakage in the radial direction. The problem is solved by paying attention to the magnetic field distribution that generates the magnetic field, that is, the number of magnetic poles.

In the magnet body of the present embodiment, positive and negative current coil blocks are not arranged in the angle and axial directions as in the conventional examples of Patent Document 1 and Patent Document 3, but the axial and radial magnetic shields are separated. Think about it. That is, in order to reduce the leakage magnetic field in the radial direction, the SC-CB 15 having a large radius is arranged with a negative current. This is the same as in Patent Document 2.

On the other hand, since it is still necessary to reduce the leakage magnetic field in the axial direction, a negative current is arranged in the axial direction at a position away from the center of the device (or the imaging region 6) from all MC-CBs. By setting the radius of these negative current SC-CB15 to be larger than that of MC-CB11, the influence of the magnetic field generated by the shield coil on the imaging region can be reduced.

Further, the influence of the arrangement of each SC-CB 15 is prevented from affecting the uniform magnetic field in the imaging region 6. Therefore, the method of making the magnetic field distribution uniform in the imaging region 6 is also improved from Patent Document 3. That is, by arranging the CB group according to the three steps as described below, the magnetic field uniformity of the imaging region 6 is kept good, and a good arrangement is also obtained for the leakage magnetic field. The CB group here refers to an MC-CB group that is a set of MC-CB11 and an SC-CB group that is a set of SC-CB15.

FIG. 3 is a diagram showing a concept of a new technique related to the coil block arrangement used in the present invention. The left diagram schematically shows three steps from the top to the bottom, and the right diagram shows the CB to be obtained. It is a schematic diagram of group arrangement.

In addition, when a uniform magnetic field design is performed by this method, for example, an arrangement of CB groups as shown in FIG. 1 is obtained. Specifically, the horizontal magnetic field type MRI apparatus of the present embodiment is a horizontal magnetic field type MRI apparatus having a cylindrical magnet that is open at both ends, and the magnet includes an annular main coil and a main coil. And a plurality of annular shield coils each having a larger diameter and arranged coaxially with the main coil. The axis here is an axis that passes through the center of the opening at both ends of the magnet, and in many cases, it is a horizontal axis. Further, at least three shield coils are arranged on the shaft, and all the main coils are arranged closer to the center of the magnet than the shield coils with respect to the openings at both ends of the magnet.
1. First Step The arrangement of the MC-CB group is obtained as follows.

First, since the inner diameter of the magnet is obtained from the size of the imaging region 6 (DSV 6), the ring current 20 is continuously arranged in the axial direction at a predetermined position in the radial direction that is larger than the inner diameter. At this point in time, the distribution of current magnitude in the axial direction of the continuous ring current 20, in other words, the current distribution in the axial direction has not been determined.

Next, regarding the ring current 20 continuously arranged in the axial direction as described above, a current distribution in which the magnetic field generated in the imaging region 6 (DSV 6) satisfies a predetermined uniformity is changed from the current distribution to the imaging region. 6 is subjected to singular value decomposition to obtain the sum of dominant eigenmodes. The current distribution to be acquired is a combination of 10 to 14 eigen distributions of symmetrical and asymmetrical components in the axial direction.

Next, it is assumed that the MC-CB 11 is installed at a location where a large amount of the wire ring current 20 is arranged, in other words, a certain portion in the axial direction where the wire ring current 20 is concentrated. This is repeated as many times as the number of the eigen distributions described above, and the current density is given to each MC-CB 11 on the assumption that the arrangement of the MC-CB group is tentatively determined. Further, the eigen magnetic field distribution intensity when a uniform magnetic field is generated in the imaging region 6 is obtained for each eigen distribution. When the position and shape of each MC-CB 11 are changed in the MC-CB group temporarily arranged so as to correspond to the above-described eigen distribution, the MC-CB including the MC-CB 11 after the change is obtained. The target of the magnetic field generated by the group.

That is, in the first step, a current distribution of the ring current 20 is obtained in a spatially continuous state, and an eigen distribution with a dense current distribution is obtained. Then, the arrangement of the MC-CB 11 is determined in a spatially discrete state so as to correspond to the eigen distribution. In the subsequent second step, the MC has a current distribution that generates a magnetic field that substantially matches the uniform magnetic field generated in the imaging region 6 by the spatial distribution of the ring current 20 that is spatially determined previously. -Adjust the placement and shape of CB11.

The eigenmode used here includes both a symmetric component and an asymmetric component with respect to the axial position. In the case of a normal horizontal magnetic field type MRI magnet, the arrangement is symmetrical in the axial direction from the center of the apparatus. Therefore, the natural mode component to be used is half of the above and is about 5 to 7. SC is arranged as a coil block, and its magnetomotive force is determined according to the magnetomotive force of the main coil so as to shield the leakage magnetic field. The first step described above is based on the reference "M. Abe, K. Shibata," Consideration on Current and Coil Block Placements with Good Homogeneity for MRI Magnets using Truncated SVD ", IEEE Trans. Magn., Vol. 49, no. 6, pp. 2873-2880, June. 2013 ”.
2. Second Step Using the number of windings of each CB as a continuous real number, the position and shape of MC-CB11 (cross-sectional side position indicated by an arrow) are reproduced so as to reproduce the magnetic field eigendistribution intensity obtained by the ring current 20 obtained in the first step. adjust. At the same time, the magnetomotive force of the SC-CB group 15 that adjusts the leakage magnetic field is also adjusted. These adjustments are performed by repeated calculation. The details of the second step and the next third step will be described later.
3. Third Step In this step, information about the actual wire rod is given and reflected on the MC-CB 11 and SC-CB 15 whose positions and shapes have been obtained in the previous second step. Specifically, based on the wire shape to be used, an integer number of windings, coil cross-sectional shape, magnetomotive force, and coil position (adjusted in the direction indicated by the arrow) are given to the CB group for approximation. Then, the position of each MC-CB 11 is adjusted to reproduce the intrinsic magnetic field strength obtained in the first step. For SC-CB15, the leakage magnetic field is adjusted to be small. The adjustment of the leakage magnetic field here is performed by adjusting the magnetic moment of the shield coil (proportional to the square of the radius of the current x area) and the position of the SC-CB 15 in the radial direction.

CB group arrangement with high accuracy is obtained by this method, and the CB group arrangement is determined according to the magnetomotive force arrangement of the magnet that generates a magnetic field with high accuracy regardless of the position of the SC-CB group performing magnetic shielding to the outside.

By arranging the magnetomotive force of the CB group as described above, the leakage magnetic field in the axial direction and the radial direction can be adjusted, and the leakage magnetic field in a narrow region can be obtained. At this time, since the uniform magnetic field in the imaging region 6 can be reconstructed with a uniform magnetic field distribution based on the uniform magnetic field generated by the ring current 20 having a high degree of freedom, a CB group with good uniformity can be obtained. As a result, it is possible to design a magnet with a narrow leakage magnetic field and a good uniform magnetic field at the same time. As a result, the MRI apparatus can be installed in a narrow space as compared with the conventional case. In addition, the number of magnetic shields that weaken the influence on the surroundings can be reduced, and the area required for installing the apparatus can be reduced. In addition, it is possible to realize a good uniformity that keeps the imaging performance good, and to provide an easy-to-use MRI apparatus.

An example of the first step is detailed in the above-mentioned reference, but the second and third steps are not discussed in this document. Regarding the adjustment of the coil position / cross-sectional shape in the second and third steps, it is considered that the change in the position / cross-sectional shape of the CB 13 appears as a current on each side of the CB 13. Therefore, in this method, a current is arranged on the side so as to correspond to the deformation of CB13, a response matrix between the magnetic field generated by the current and the magnetic field of the imaging region 6 is obtained, and the response matrix A is singularly decomposed. ,
A = Σu _i λ _i v _i (1)
And Here, u _i , λ _i , and v _i are the i-th eigen distribution vector of the magnetic field, the i-th singular value, and the i-th eigen vector of the current distribution on the side, respectively. The combination of these three elements is called an eigenmode, and the current to be placed on the side is obtained from the combination of these eigenmodes. Although the number of eigenmodes to be combined is determined depending on the required magnetic field accuracy, the upper limit is the number of ranks in the response example A, and the accuracy increases as the number of combinations increases.

FIG. 4 schematically shows the relationship between the current on the CB cross-sectional side and the CB movement / deformation. When the CB 13 moves as shown in the left of FIG. 4, positive and negative currents appear as shown in the center diagram due to the difference in cross-sectional position. This current is treated as the ring current 21 on the side as shown in the rightmost diagram, and the magnetic field response is summarized in the matrix A.

In the second step, the position of each side of the CB 13 is adjusted independently, but the side given the X mark in the diagram schematically showing the second step shown in FIG. 3 is not moved. This is because the inner diameter of the coil is a fixed value for the MRI apparatus and cannot be moved during magnetic design. Further, the size of movement of each side is continuous, and the change in the size of the cross section is also continuous. Therefore, the magnetomotive force (Ampere-turn) of each CB 13 is also continuous. The CB cross section shows the i-th intrinsic magnetic field strength P _i obtained by the inner product u _i B of the magnetic field eigen distribution vector u _i by the ring-wheel current 20 and the magnetic field distribution vector B,
P _i → inner product value {(i-th intrinsic magnetic field distribution of ring current 20) (uniform magnetic field distribution)} (2)
To be transformed. Adjustment is performed by continuously changing the coil shape assuming the current density.

The size of the deformation as follows considered to adjust the residual intrinsic magnetic field strength Pr _i. Assuming that a vector representing the distribution of the magnetic field generated by the CB group is Bc, these intrinsic magnetic field strengths are inner products of Bc and the intrinsic magnetic field distribution u _i due to the ring wheel current 20, and the residual intrinsic magnetic field strength is the intrinsic magnetic field distribution u _i . Is the inner product of
Pr _i = (Uniform magnetic field distribution vector−magnetic field distribution by CB group) u _i (3)
It is. This is corrected (Pr _i → 0) vector ΔI representing the current distribution on the edges using singular value lambda _i,
_{_{ΔI = Σv i Pr i / λ}} i (4)
It is. Note that ΔI is a vector having, as an element, a current that appears due to CB deformation for each CB 13. Here, the sum is executed for a number of eigenmodes that require magnetic field accuracy. When Nc MC-CBs 11 are arranged in a portion where the current due to the ring current 20 is large, the number is normally 2Nc. However, when a good magnetic field accuracy is required, 2Nc + 2 eigenmodes may be executed. Moreover, in these eigenmodes, the number of eigenmodes is half the number of magnets in which the CB 13 is arranged symmetrically with respect to the axial position. The remaining half is an eigenmode that is antisymmetric and cannot be used.

In the second step of this method, the eigenmode related to the residual eigenmagnetic field strength is added according to the required magnetic field homogeneity to obtain ΔI. After obtaining ΔI, ΔI is converted into the deformation amount of CB13 based on the current density assumed previously. By reflecting this deformation amount on the original shape of the CB 13, the position of each side of the CB 13 can be adjusted independently. Then, after reflecting the deformation amount of the CB 13, the second step is executed again for the arrangement of the CB 13 after the deformation, and the deformation amount is calculated in the same manner. By repeatedly executing and adjusting this calculation until a predetermined magnetic field homogeneity is satisfied, that is, until the residual eigenmagnetic field strength is equal to or less than a desired threshold value, a magnetic field with good uniformity reflecting the deformation of the CB 13 can be generated. The arrangement of the CB group is calculated.

* Adjustment of SC-CB15 changes the magnetomotive force according to the cross-sectional size. It is a practical method to adjust the magnetic moment to the target during the repeated calculation. Normally, adjustment is performed by the magnitude of the magnetomotive force assuming the current density.

In the third step, the coil position is moved by constraining the movement of the opposite sides of the cross-sectional sides of each CB 13 to the same current value in the positive and negative directions in the movement of the cross-sectional sides of each CB 13 in the third step. To express. At this stage, the current density is determined from the assumed wire and number of windings and the CB cross-sectional area determined from them, but it is desirable that the current density be approximately the same as in the second step. If the current density and magnetomotive force value are far from each other, an inconvenient CB group arrangement greatly changed from the second step is obtained, and in this case, the second step is recalculated.

In step 3, as shown in the lower diagram of FIG. 3, the cross-sectional shape of the CB is determined by arranging the wire based on the shape of the wire in the cross section. Since the number of windings is determined, the size of the cross section remains unchanged during the adjustment iteration calculation, and the movement of the opposite side positions is the same size and direction, and is adjusted by the movement of the radial and axial positions of the CB 13. . In this adjustment, adjustment is performed by the method shown in the equations (1) to (4) except for the condition that the size of the cross section is not limited.

5 and 6 show that the convergence calculation in the second step and the third step can determine the uniform magnetic field with high accuracy, respectively. The horizontal axis shows the number of repeated calculations, the vertical axis shows the uniformity from the top, the difference between MC and SC of the magnetic moment in%, the bottom shows the residual eigenmode strength in FIG. Shows six. FIG. 5 shows the second step, and FIG. 6 shows the third step. The number Nc of MC-CB11 is six. The eigenmode intensity of the residual is sufficiently small, and the uniformity is converged to about 1 ppm. This uniformity is evaluated at a magnetic field calculation point placed on the surface of a sphere with a diameter of 40 cm. The SC magnetic moment (dipole strength) is adjusted to 99% of the MC magnetic dipole strength and converges. Generally, adjusting between 98% and 99.75% provides a reasonable magnetic shield.

From these figures, it can be understood that the repeated calculation of the adjustment of the position and the cross-sectional shape of the CB 13 performed in the second step and the third step can be converged and calculated as expected.

It can be seen from FIG. 7 that this calculation works well. In this figure, the current value of the wire ring current 20 and the coil block rCB by the continuous magnetomotive force value reproducing the magnetic field, and the resulting magnetic field lines and magnetic field strength contour lines are shown on the left. In the figure, the left and right are axial positions, and the radial positions are written from bottom to top. The striking point area is an area having a magnetic field strength of 1.5 T or more. Here, the magnetomotive force arrangement is determined so that a 1.5 T uniform magnetic field is generated in an FOV having a thickness of 20 cm and an outermost shape of 42 cm.

On the right side, as a result of adjusting the coil block (iCB) position in the third step so as to reproduce the magnetic field due to the wire ring current 20 as a discrete magnetomotive force value assuming a winding structure of 1 kA for the wire current It is. MC-CB11 of negative current is arranged for both rCB and iCB, and MC22 and MC23 correspond to this. This is a negative current required to shorten the axis in the axial direction, and is required separately from SC10 and SC11. This negative current also appears in the vicinity of the MC 22 in the study using the ring current 20.

It can be seen that both the magnetic field by rCB and the magnetic field by iCB can be adjusted within ± 1.5 ppm in the FOV. That is, it was shown that a good magnetic field distribution is possible with this calculation method even in such an arrangement with a very short axis and a negative current.

FIG. 1 shows an example of magnetomotive force arrangement applying the above-described new design technique and the arrangement of SC-SB groups calculated by this technique. On the left side of this figure are drawn isomagnetic flux lines (also magnetic lines of force) and magnetic field strength contour lines. The outermost contour line is a 5 gauss line 9. In addition, what is represented by a square with a radius in the vicinity of 0.5 m to 1.0 m is the cross-sectional shape of the CB group 13. On the right side, the CB name is appended to the square of the CB cross section. MC10-30 is the main coil, and SC10, SC11, SC12, and SC13 are shield coils. Compared to a normal magnet for an MRI apparatus, the magnetomotive force arrangement of this embodiment is 1.8 m in the radial direction (2 m or more in the conventional example). For R = 2.00, 2.25m, and 2.50m, an axial line is written every 0.25m as a mark, but the 5 gauss line 9 is located at a position of a small radius sufficiently. It can also be seen that it is narrower than the conventional example of FIG.

In the one-axis direction, the 5 gauss line 9 exists at a position 2.5 m or less from the center. Compared to the conventional apparatus, the area surrounded by the 5 Gaussian line 9 is narrow.

On the other hand, the figure on the right side is useful for the uniformity of the imaging region 6. Writing magnetic field lines running vertically and magnetic field contours (1.5T ± 1.5ppm) written radially. The striking area is a strong magnetic field exceeding 1.5T. As described above, the number of SC-CB 15 is increased, and the necessary uniform magnetic field can be generated even in the magnetomotive force arrangement in which the MC is smaller than the normal six.

In this magnetomotive force arrangement, there are four SC-CB15, two of which (SC10, SC11) are arranged in the axial direction away from the main coil (referred to as end SC-CB). In other words, any main coil is disposed on the center side of the magnet with respect to both ends of the magnet with respect to the shield coil. With this arrangement, the axial leakage magnetic field is limited to a narrow region. The other two are MC-CB groups and positions larger in the radial direction than SC-CB15 (end SC-CB) arranged farther in the axial direction, and the MC-CB group is arranged in the axial direction. It is placed in the position range. Further, the radius is made smaller by the farther SC-CB15. In other words, the diameter of the shield coil increases as it approaches the center of the magnet in the horizontal axis direction. With this arrangement, the axial leakage magnetic field is limited to a narrow region.

FIG. 8 shows the case where there are six MC-CB11. The SC-CB15 is the same as described above. This CB arrangement also has a narrow leakage magnetic field region (evaluated in the region surrounded by the 5 Gaussian line 9) as in the previous example (FIG. 1). That is, the same magnetic shielding performance is obtained regardless of the MC-CB arrangement.

FIG. 9 shows the case where there are four SC-CBs 15 at the end including two that are far from all the MC-CBs 11 in the axial direction, and all the SC-CBs 15 have the same radius. The uniformity is good, but it can be seen that the area surrounded by the 5 Gaussian line 9 is wide. That is, it can be understood that the radial position of the SC-CB 15 at the end must be smaller than that of the other SC-CB 15.

FIG. 10 shows a magnet (magnet) incorporating a main coil and a shield coil reflecting the arrangement of the coil block of the present embodiment described above, and shows an overview thereof. In the axial direction, the magnet is about 1.2m to 1.8m and the diameter of the cylindrical body is about 1.8m to 2.4m. With such a magnet device, approximately 1m from the surface of the magnet container 3 (cryostat). When separated, it is a magnet with a magnetic field of 5 gauss or less. An imaging space 7 having a uniform magnetic field exists at the center of the magnet device.

As described above, the horizontal magnetic field type MRI apparatus has been described with reference to a new design method devised by the present inventor and examples of magnetomotive force arrangement obtained by the method. According to this method, it is possible to design a magnet with good uniformity, which suppresses the leakage magnetic field with respect to the outside. Further, since the leakage magnetic field region is narrow, an effect of reducing the area and room size required for installing the MRI apparatus can be obtained. In other words, it can be said that the installation restrictions can be reduced for the MRI apparatus having a strong magnetic field (for example, 3T). In addition, good uniform magnetic field performance can be ensured regardless of the magnetic shielding to the outside.

In the above-described embodiment, the size of the imaging region 6, the 5 gauss region, and the uniformity of the magnetic field have been described with specific numerical values. However, this is for ease of explanation, and as necessary. Needless to say, it can be changed.

DESCRIPTION OF SYMBOLS 1 ... Magnetic flux contour 2 ... Magnetic field strength contour 3 ... Electromagnetic container outer wall 4 ... Magnetic field direction arrow 5 ... Magnetic field strength contour (± 1.5ppm)
6 ... Imaging region 7 ... Uniform magnetic field region 8 ... Support leg 9 ... Magnetic field contour (5 gauss)
11 ... MC-CB (coil block corresponding to the main coil)
12 ... Magnetic field lines (same as equal magnetic field lines)
13 ... Coil block 15 ... SC-CB (coil block corresponding to shield coil)
20 ... Ring current 21 ... Ring current on the coil block cross section

Claims

A horizontal magnetic field type MRI apparatus having a cylindrical magnet open at both ends,
The magnet
An annular main coil;
A plurality of annular shield coils each having a larger diameter than the main coil and arranged coaxially with the main coil,
At least three shield coils are arranged on the shaft,
Any of the main coils is disposed closer to the center of the magnet than the shield coils disposed at both ends in the axial direction,
A horizontal magnetic field type MRI apparatus characterized in that the shield coil has a larger diameter in the axial direction as it is closer to the center of the magnet.
The horizontal magnetic field type MRI apparatus according to claim 1,
A horizontal magnetic field type MRI apparatus characterized in that the magnetic moment of the shield coil is 99.75% to 98% of the magnetic moment of the main coil.
The ring current is continuously arranged in the axial direction at a predetermined radial position where the main coil is arranged,
Singular value decomposition of a response matrix to the target magnetic field generated in the imaging region from the arranged ring current, and calculating current distribution by obtaining 9 to 14 eigen distributions of the arranged ring current,
Determine the magnetomotive force of the shield coil according to the magnetomotive force of the main coil so as to shield the leakage magnetic field,
Replacing the eigendistribution with coil blocks, assuming current density for each coil block;
A first step of obtaining an eigen magnetic field distribution intensity for each eigen distribution;
The coil block position shape is obtained by repeatedly performing the adjustment of the main coil shape to reproduce the intrinsic magnetic field distribution strength of the magnetic field and the adjustment of the shield coil magnetomotive force by repeated calculation, with the number of windings of the coil block as a continuous real number. A second step for determining
For the coil block whose position shape was determined in the second step, a restriction is added so that the number of windings of the wire becomes an integer value,
Approximate the shape, magnetomotive force and arrangement position of the coil block obtained in the second step to the coil block to which the restriction is added,
Calculating the approximate position of the coil block that reproduces the natural magnetic field distribution obtained in the first step,
A third step of adjusting the radius of the shield coil to reduce the leakage magnetic field of the external magnetic field;
A method for designing a magnetomotive force arrangement, comprising:
In the design method of magnetomotive force arrangement according to claim 3,
Adjustment of the main coil shape that reproduces the natural magnetic field distribution intensity of the magnetic field in the second step is as follows:
The deformation of the main coil is a relocation of the ring current, and the relocation of the ring current that reproduces the natural magnetic field strength distribution is calculated,
A design method of magnetomotive force arrangement, wherein the rearrangement of the calculated wire ring current is replaced with the deformation of the main coil shape and adjusted.