JP2015027530A - Magnetic field adjustment device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic field adjustment device which shortens a work time in shimming work.SOLUTION: The magnetic field adjustment device comprises a plurality of sheet-like magnetic substance pieces 5, and a shim tray including a plurality of pockets for storing the magnetic substance pieces 5 therein. The magnetic substance pieces 5 are stacked and stored in the pockets, and magnetic field uniformity of a magnetostatic field generated in a predetermined space by a superconducting coil is adjusted. A storage container 7 including an opening 7c in a direction different from the stacking direction of magnetic substance pieces used for shimming is disposed on the shim tray, thereby directly accessing the magnetic substance piece 5 desired to change, and the time required for shimming is shortened.

Description

  The present invention relates to a magnetic field adjustment device for adjusting the magnetic field uniformity of a magnetic field generated by a superconducting coil.

Magnetic Resonance Imaging: In (MRI M agnetic R esonance I maging ) device, an imaging area in order to obtain a sharp image: higher the precision of magnetic field homogeneity (for example, several ppm or less) in (FOV F ield O f V iew ) Achieving with magnetic field strength is required. Therefore, a superconducting coil that can generate a high magnetic field is used in the MRI apparatus. Then, in the pre-operation stage of the MRI apparatus, the magnetic field uniformity of the FOV is adjusted by adjusting the magnetic field distribution of the FOV by installing an appropriate amount of a magnetic piece for correction (hereinafter referred to as shim iron) at an appropriate position. A work called shimming to improve is being carried out. In this shimming, a method for calculating the position and amount at which shim iron is to be arranged, and a device used for shimming are disclosed (for example, see Patent Documents 1 and 2).

JP 2009-268791 A JP 2008-289703 A

  In the design stage of the MRI apparatus, the coil arrangement of the superconducting coil is optimized to satisfy the required magnetic field uniformity specifications. However, the magnetic field uniformity of the FOV is deteriorated from the design value at the design stage due to the installation error of the superconducting coil in the processing of the MRI apparatus, the assembly stage, and the magnetic field created by the magnetic material installed around the MRI apparatus.

  In order to obtain a clear image, an MRI apparatus is increasing in magnetic field. As an MRI apparatus having a high magnetic field of 1.5 T or more, a cylindrical tunnel type MRI apparatus has been developed. In the tunnel-type MRI apparatus, elongated shim trays having magnetic material storage portions called shim pockets arranged discretely in the direction of the central axis of a cylindrical shape are discretely installed in the circumferential direction of the inner surface of the tunnel. A shimming operation is performed by laminating thin shim irons in the shim pocket in the radial direction of the cylindrical shape and adjusting the amount thereof.

  The standard shimming procedure is shown below. First, the superconducting coil is excited with no shim iron (the shim tray is empty), and the magnetic field distribution of the FOV is measured. After the measurement, the shim tray is pulled out from the main body of the MRI apparatus, and the amount of shim iron in each shim pocket is adjusted. The adjusted shim tray is inserted into the MRI apparatus main body, the superconducting coil is excited again, and the magnetic field distribution of the FOV is measured. Because of the error between the amount of shim iron at the time of calculation and the amount of shim iron actually placed, it is difficult to achieve a magnetic field uniformity of several ppm with a single adjustment of the amount of shim iron. The procedure consisting of measuring the magnetic field distribution and adjusting the amount of shim iron is repeated until a predetermined magnetic field uniformity is obtained.

  In the shimming operation, the magnetic field distribution measurement and the shim iron amount adjustment are repeated. Therefore, in order to shorten the work time of the entire shimming operation and improve the operating rate of the MRI apparatus, the time required for one adjustment of the shim iron amount It is an important issue to shorten the time. Further, since the heater of the permanent current switch is energized every time the superconducting coil is excited or demagnetized, a large amount of refrigerant is consumed. For this reason, in order to reduce running costs, it is important to reduce the number of times of excitation and demagnetization of the superconducting coil.

  That is, the problem to be solved by the present invention is to provide a magnetic field adjusting device capable of adjusting the amount of shim iron in a short time in shimming work.

Reference examples relating to the present invention are:
A superconducting coil that generates a static magnetic field in a predetermined space (FOV);
In the magnetic field adjustment method of a magnetic field generator (MRI apparatus) having a shim tray in which a magnetic piece (shim iron) for adjusting the magnetic field uniformity of the static magnetic field in the predetermined space is arranged.
Before generating the static magnetic field in the predetermined space and measuring the static magnetic field, the magnetic piece is arranged on the shim tray.

In order to solve the above problems, the present invention provides:
A plurality of thin plate-like magnetic pieces and a shim tray provided with a plurality of pockets for storing the magnetic pieces, the magnetic pieces are stacked and placed in the pocket, and the superconducting coil is placed in a predetermined space. In the magnetic field adjustment device for adjusting the magnetic field uniformity of the generated static magnetic field,
It has an opening for taking in and out the magnetic material pieces in a direction different from the stacking direction of the magnetic material pieces, and includes a storage container for storing a plurality of the magnetic material pieces and being stored in the pockets. .

  ADVANTAGE OF THE INVENTION According to this invention, the magnetic field adjustment apparatus which can adjust shim iron amount in a short time can be provided.

It is sectional drawing which cut | disconnected the MRI (magnetic resonance imaging) apparatus (magnetic field generator) provided with the magnetic field adjustment apparatus which concerns on the 1st Embodiment of this invention by the plane parallel to an X-axis Y-axis. It is sectional drawing which cut | disconnected the MRI (magnetic resonance imaging) apparatus (magnetic field generator) provided with the magnetic field adjustment apparatus which concerns on the 1st Embodiment of this invention by the plane which passes along Y-axis Z-axis. 1 is a plan view of a magnetic field adjustment device according to a first embodiment of the present invention. 1 is a side view of a magnetic field adjustment device according to a first embodiment of the present invention. It is arrow sectional drawing of the AA direction of FIG. 2A. It is sectional drawing of the storage container periphery part of the magnetic field adjustment apparatus which concerns on the 1st Embodiment of this invention, and is a one part enlarged view of FIG. 2C. It is a perspective view of the storage container (the state which opened the lid | cover) of the magnetic field adjustment apparatus which concerns on the 1st Embodiment of this invention. It is a perspective view of the storage container (state which opened the lid | cover) of the magnetic field adjustment apparatus which concerns on the 2nd Embodiment of this invention. It is a flowchart of the magnetic field adjustment method of the MRI apparatus (magnetic field generator) which concerns on the 3rd Embodiment of this invention. It is a flowchart of the simulation method (the 1) of the prior shim iron distribution in the magnetic field adjustment method of an MRI apparatus (magnetic field generator). It is a top view of the examination room where an MRI apparatus (magnetic field generator) is installed. It is a flowchart of the simulation method (the 2-1) of prior shim iron distribution in the magnetic field adjustment method of the MRI apparatus (magnetic field generator) which concerns on the 4th Embodiment of this invention. It is a flowchart of the simulation method (the 2-2) of prior shim iron distribution in the magnetic field adjustment method of the MRI apparatus (magnetic field generator) which concerns on the 4th Embodiment of this invention. It is a flowchart of the simulation method (the 3) of prior shim iron distribution in the magnetic field adjustment method of the MRI apparatus (magnetic field generator) which concerns on the 5th Embodiment of this invention. It is a flowchart of the simulation method (the 4) of prior shim iron distribution in the magnetic field adjustment method of the MRI apparatus (magnetic field generator) which concerns on the 6th Embodiment of this invention.

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

(First embodiment)
FIG. 1A is a cross-sectional view of an MRI (magnetic resonance imaging) apparatus 1 (magnetic field generation apparatus 2) provided with a magnetic field adjustment apparatus 4 according to the first embodiment of the present invention, cut along a plane parallel to the X axis and the Y axis. FIG. 1B shows a cross-sectional view of the MRI apparatus 1 (magnetic field generator 2) cut along a plane passing through the Y axis and the Z axis. The MRI apparatus 1 includes a cylindrical magnetic field generator 2 capable of introducing a subject into an internal imaging region (FOV, predetermined space) 3 and a living tissue of the introduced subject although not shown in the drawings. An irradiation coil (not shown) for irradiating a high-frequency signal to cause nuclear magnetic resonance to occur in the nucleus, and a gradient magnetic field generator (not shown) for giving positional information to each magnetic resonance signal emitted from the subject. A receiving coil (not shown) for receiving a magnetic resonance signal emitted from the subject, a bed (not shown) for loading the subject, and the like.

  The magnetic field generator 2 generates a uniform magnetic field in the FOV 3 in order to orient the spins of atoms constituting the biological tissue of the subject. The magnetic field generator 2 has a cylindrical shape with a z-axis parallel to the horizontal direction as a central axis. The magnetic field generator 2 includes a plurality of main coils 2a that are superconducting coils, a plurality of shield coils 2b that are superconducting coils, a refrigerant container 2d that houses and cools the main coils 2a and the shield coils 2b together with a refrigerant, and a refrigerant container 2d. And a magnetic field adjusting device 4 provided on the FOV 3 side surface (inner peripheral surface) of the vacuum container 2c.

  The plurality of main coils 2a have a ring shape having the Z axis as a common central axis. A plurality of main coils 2a (four in the example of FIG. 1B) are arranged in the Z-axis direction. The plurality of main coils 2 a generate a static magnetic field that is a uniform magnetic field in the FOV 3. In addition to the FOV 3, the plurality of main coils 2a generate a static magnetic field. In particular, a leakage magnetic field is generated at a position farther than the main coil 2a with respect to the Z axis, and the magnetic field adjustment device 4 is also disposed. Generate a static magnetic field. The plurality of shield coils 2b can reduce the magnitude of the leakage magnetic field, and also generate a static magnetic field at the arrangement position of the magnetic field adjusting device 4. The plurality of shield coils 2b have a ring shape having the Z axis as a common central axis. A plurality of shield coils 2b are arranged in the Z-axis direction (two (a pair) in the example of FIG. 1B). The plurality of shield coils 2b are disposed in the vicinity of the pair of main coils 2a disposed at both ends of the plurality of shield coils 2b arranged in the Z-axis direction. The plurality of shield coils 2b are arranged farther from the pair of main coils 2a arranged at both ends of the Z axis in the X-axis and Y-axis directions. The main coil 2a and the shield coil 2b are energized in opposite directions. The main coil 2a and the shield coil 2b are optimized for their cross-sectional center, side length, and magnetomotive force (current x number of turns) so as to increase the magnetic field uniformity in the FOV 3 and reduce the leakage magnetic field. .

  A plurality of magnetic field adjusting devices 4 are provided on the surface (inner peripheral surface) of the vacuum vessel 2c on the FOV 3 side. The plurality of magnetic field adjustment devices 4 are arranged such that their longitudinal directions are parallel to the Z-axis direction. The magnetic field adjusting device 4 is arranged discretely and at equal intervals on the inner peripheral surface of the vacuum vessel 2c on the FOV 3 side in the circumferential direction around the Z axis. The operator performs a shimming operation using the magnetic field adjustment device 4, thereby obtaining an environmental magnetic field in the place where the MRI apparatus 1 is installed and an error magnetic field due to an installation error of the main coil 2 a and the shield coil 2 b of the superconducting coil. In the FOV3, the magnetic field uniformity in the FOV3 is set to a predetermined value (for example, several ppm) or less.

  FIG. 2A shows a plan view of the magnetic field adjustment device 4 according to the first embodiment of the present invention, and FIG. 2B shows a side view thereof. FIG. 2C shows a cross-sectional view in the direction of the arrow AA in FIG. 2A, and FIG. 2D shows an enlarged portion of FIG.

  As illustrated in FIG. 2A, the magnetic field adjustment device 4 includes a shim tray 6 and a storage container 7. The shim tray 6 and the storage container 7 can be made of a non-magnetic material such as resin or aluminum. In the shim tray 6, a plurality of shim pockets 6a are arranged in the longitudinal direction (L direction) discretely and in a line at equal intervals. The shim tray 6 can be fixed to the vacuum vessel 2c (see FIG. 1A) using the screw hole 6b.

  As shown in FIG. 2B, the shim pocket 6a is a depression formed in the T direction (stacking direction) in the shim tray 6, and one storage container 7 is stored in each of the plurality of shim pockets 6a. ing. The storage container 7 can be fitted into the shim pocket 6a by moving from the T direction of the shim pocket 6a to the shim pocket 6a. The storage container 7 is fitted into the shim pocket 6a so that it hardly moves in the L direction or the W direction relative to the shim pocket 6a.

  As shown in FIG. 2A, the storage container 7 includes a container body 7a and a lid 7b that covers the opening 7c of the container body 7a. The lid 7b can be removed from the opening 7c of the container body 7a by moving in the L direction relative to the container body 7a. In a state where the storage container 7 is fitted in the shim pocket 6a, the lid 7b cannot be moved in the L direction relative to the container main body 7a, and therefore the lid 7b is not removed.

  As shown in FIG. 2C and FIG. 2D, inside the storage container 7, a plurality of shim iron pieces (magnetic body pieces) 5 (51, 52, 53) and a plurality of spacers 5a (54, 55, 56, 57) are stacked in the T direction (stacking direction). As the shim iron piece (magnetic piece) 5, a thin plate-like magnetic body, for example, iron or a silicon steel plate can be used. As the spacer 5a, a thin non-magnetic material such as resin or ceramic can be used. The spacer 5a is provided in the storage container 7 so that a gap is formed so that the shim iron piece (magnetic body piece) 5 does not move. By adjusting the amount of shim iron pieces 5 (the amount of shim iron) stored in the storage container 7, the magnetic field strength distribution of the static magnetic field in the FOV 3 (see FIG. 1A) can be changed, and the magnetic field uniformity can be adjusted. Specifically, the magnetic field strength distribution of the static magnetic field in the FOV 3 (see FIG. 1A) is changed by adjusting (changing) the number of shim iron pieces 5 stored in the storage container 7 and the thickness of each shim iron piece 5. To do. In order to change the number of shim iron pieces 5, for example, the shim iron piece 51 is pulled out from the storage container 7, and instead the spacer 5a having the same thickness is inserted, or the spacer 5a (57) is pulled out from the storage container 7, It is only necessary to insert shim iron pieces 5 having the same thickness. In order to change the thickness of the shim iron piece 5, for example, the shim iron piece 51 and the spacer 57 are pulled out of the storage container 7, and instead, the shim iron piece 5 having a thickness that combines the shim iron piece 51 and the spacer 57 is inserted, The shim iron piece 53 may be pulled out of the storage container 7 and the shim iron piece 5 and the spacer 5a having the same thickness as the shim iron 53 may be inserted.

  FIG. 3 shows a perspective view of the storage container 7 taken out from the shim pocket 6a (see FIG. 2A) and opened with the lid 7b. To change the shim iron amount of the shim iron pieces 5 to be stored in the storage container 7, that is, the number of shim iron pieces 5 and the thickness of each shim iron piece 5, as shown in FIG. Take out from the pocket 6a (see FIG. 2A) and remove the lid 7b from the container body 7a of the storage container 7. As a result, the opening 7c of the container body 7a is opened, the end surfaces of all the shim iron pieces 5 (51, 52, 53) stored in the storage container 7 and all the spacers stored in the storage container 7. The end face of 5a (54, 55, 56, 57) is exposed. The operator of the shimming work only looks at the opening 7c, and the number and thickness of all the shim iron pieces 5 (51, 52, 53) stored in the storage container 7 and the storage container 7 It is possible to know the number of all the spacers 5a (54, 55, 56, 57) accommodated therein and their respective thicknesses. Then, the shim iron pieces 5 (51, 52, 53) and the spacers 5a (54, 55, 56, 57) having a thickness to be drawn can be pulled out without pulling out the other shim iron pieces 5 and the spacers 5a. . And the alternative shim iron piece 5 and the spacer 5a can be inserted. As described above, the shim iron pieces 5 (51, 52, 53) and the spacers 5a (54, 55, 56, 57) can be pulled in and out only. 53) and the shim iron pieces 5 (51, 52, 53) from the L direction and the W direction (L direction in the example of FIG. 3) other than the stacking direction (T direction) of the spacers 5a (54, 55, 56, 57). This is because the spacer 5a (54, 55, 56, 57) is taken in and out. If the lid 7b of the storage container 7 is opened, it can be known at a glance where the shim iron piece 5 having an appropriate thickness to be removed exists. And only the shim iron piece 5 having an appropriate thickness can be directly removed, and the time required for adjustment can be shortened. Further, since the shim iron piece 5 other than the shim iron piece 5 to be taken out is not taken out from the storage container 7, the shim iron piece 5 is scattered during the shimming operation, the misplacement of the spacer 5a and the shim iron piece 5 at the time of re-stacking, It is possible to prevent a change in magnetization due to deformation or the like and improve shimming accuracy. By improving the shimming accuracy, the number of shimming operations can be reduced.

  Thus, by installing the storage container 7 having the opening 7c in the direction different from the stacking direction (T direction) (L direction in the example of FIG. 3 but may be the W direction) on the shim tray 6, the storage container 7 The shim iron piece 5 can be removed and the shim iron piece 5 having the optimum thickness can be selected and removed from the opening 7c, and the work time can be reduced because there is no need to stack again. Moreover, since arrangement | positioning other than the shim iron piece 5 to remove is not changed, the scattering of the shim iron piece 5, the arrangement | positioning mistake at the time of lamination | stacking, etc. can be prevented.

(Second Embodiment)
FIG. 4 is a perspective view of the storage container 7 (with the lid 7b opened) of the magnetic field adjustment device according to the second embodiment of the present invention. The storage container 7 of the second embodiment is different from the storage container 7 of the first embodiment in that the storage space 7d in which the shim iron pieces 5 and the spacers 5a are stored is divided in the stacking direction (T direction). This is a point having a partition plate 7e. The partition plate 7e is fixed to the container body 7a.

  Although details will be described later, in the present invention, in the shimming operation, magnetic field distribution measurement and shim iron amount adjustment are performed. Prior to the magnetic field distribution measurement, shim iron amount adjustment is performed. By adjusting the shim iron amount prior to the magnetic field distribution measurement, the shim iron piece 5 and the spacer 5a are placed in the storage container 7, so that the adjustment of the shim iron amount after the subsequent magnetic field distribution measurement is already in the storage container 7. The placed shim iron pieces 5 can be removed. In adjusting the amount of shim iron prior to the magnetic field distribution measurement, shim iron pieces 51 are arranged on the upper side of the partition plate 7e as shown in the example of FIG. 4, and shim iron pieces 52 and 53 are arranged in the lower storage space 7d. Yes. Thus, the shim iron piece 51 is arranged at least in the upper storage space 7d. And in adjustment of the amount of shim iron after magnetic field distribution measurement, shim iron pieces 52 and 53 (5) arranged in lower storage space 7d are increased / decreased. When measuring the magnetic field distribution, the FOV 3 (see FIG. 1A) is positioned upward in the T direction (stacking direction), so the distance between the FOV 3 side shim iron piece 51 and the FOV 3 that is not increased or decreased by adjusting the amount of shim iron changes. It becomes constant. As a result, the reproducibility of the magnetic field generated by the shim iron pieces 5 (51, 52, 53) in the FOV 3 can be improved. And the frequency | count of adjustment of shim iron amount in a shimming operation | work can be reduced.

(Third embodiment)
FIG. 5 shows a flowchart of a magnetic field adjustment method (shimming operation) of the MRI apparatus 1 (magnetic field generator 2, see FIG. 1A) according to the third embodiment of the present invention. In the third embodiment, in the shimming operation, the magnetic field distribution measurement and the shim iron amount are adjusted, but the shim iron amount is adjusted prior to the magnetic field distribution measurement. And the adjustment of the amount of shim iron prior to the magnetic field distribution measurement is performed based on the result of the simulation of shim iron distribution performed in advance. For this reason, before the magnetic field distribution measurement for generating the static magnetic field in the FOV 3 by the superconducting coil and measuring the static magnetic field is performed, the shim iron amount is adjusted, and the shim iron piece 5 is disposed in the storage container 7 of the shim tray 6. It will be.

  As shown in FIG. 5, first, in step S <b> 1, the computer performs simulation of pre-shim iron distribution.

  Next, in step S2, the operator of the shimming work adjusts the amount of shim iron based on the simulation result of the pre-shim iron distribution. Specifically, the shim iron piece 5 and the spacer 5a are disposed (inserted) in the storage container 7, the storage container 7 is disposed in the shim tray 6, and the shim tray 6 is disposed in the vacuum container 2c.

  In step S3, the MRI apparatus 1 (magnetic field generator 2) excites the main coil 2a and the shield coil 2b to generate a static magnetic field in the FOV 3. The operator of the shimming work measures the magnetic field distribution B1 in the FOV 3.

  In step S4, the computer calculates the magnetic field uniformity of the magnetic field distribution B1 in the FOV 3 based on the measurement result of the magnetic field distribution B1 in the FOV 3. The operator of the shimming work determines whether or not the magnetic field uniformity of the calculation result has achieved a predetermined highly accurate magnetic field uniformity (for example, several ppm or less). If it has been achieved (step S4, Yes), the flow (chart) of this magnetic field adjustment method (shimming operation) is stopped, and the shimming operation is terminated. If not achieved (step S4, No), the process proceeds to step S5.

  In step S5, the shimming operator adjusts the amount of shim iron based on the magnetic field distribution B1 measured in step S3. Specifically, after degaussing, the shim tray 6 is pulled out from the vacuum vessel 2c. The storage container 7 is removed from each shim pocket 6a, the lid 7b is opened, the shim iron piece 5 and the spacer 5a are inserted and removed, and the shim iron amount is adjusted. In addition, since the shim iron piece 5 is inserted in the storage container 7 in step S2, it can be removed as well as insertion in step S5. The storage container 7 is placed on the shim tray 6 and the shim tray 6 is placed on the vacuum container 2c. Then, the process returns to step S3. This loop of steps S3, S4, and S5 is repeated until the magnetic field uniformity reaches a predetermined value.

FIG. 6 shows a flowchart of the simulation method (part 1) of the pre-shim iron distribution in step S1 of FIG.
First, in step S11, the computer acquires the magnetomotive force arrangement of the main coil 2a and the shield coil 2b of the superconducting coil by input from an operator or the like. Examples of the magnetomotive force arrangement to be acquired include the number of turns of the main coil 2a and the shield coil 2b, the current passed through each turn, the cross-sectional center and side length of the cross section of each turn, and the like. And a magnetomotive force can be calculated | required by the product of the electric current which flows into the coil of the main coil 2a and the shield coil 2b, and the number of turns of those coils.

  In step S12, the computer calculates a magnetic field distribution B1 (second magnetic field distribution) created in the FOV 3 (predetermined space) by the main coil 2a and the shield coil 2b of the superconducting coil based on the acquired magnetomotive force arrangement.

  Next, at least one of steps S13 to S16 constituting the A part and steps S22 to S27 constituting the B part is performed. First, A part is demonstrated.

  In step S <b> 13, the computer obtains an arrangement location of the magnetic body installed around the MRI apparatus 1 (magnetic field generation apparatus 2) by input from an operator or the like.

  FIG. 7 shows a plan view of an examination room in which the MRI apparatus 1 (magnetic field generator 2) is installed. In the MRI apparatus 1, a refrigerator is installed in order to reduce the amount of refrigerant consumed for cooling the main coil 2a and the shield coil 2b of the superconducting coil. In order to protect the motor of the refrigerator from the strong magnetic field generated by the main coil 2a and the shield coil 2b, the refrigerator is provided with a magnetic shield 8a using a magnetic material, the magnetic field generator 2 (vacuum container 2c (see FIG. 1A). )) Is installed on a part of the outside. As described above, the magnetic shield 8a is not evenly and locally arranged with respect to the magnetic field generator 2.

  In order to reduce the leakage magnetic field from the examination room, a magnetic shield 8b using a magnetic material is installed as a wall (including a ceiling and a floor) of the examination room or along the wall. In the MRI apparatus 1, a console 11 for an operator to operate is placed outside the magnetic shield 8b. For this reason, unlike the other magnetic shield surface 8b2, the magnetic shield surface 8b1 facing the console 11 in the magnetic shield 8b has a window. Further, the MRI apparatus 1 has a bed 9 for laying a subject. The subject is put in and out of the magnetic field generator 2 (FOV3 (see FIG. 1B)) while lying in the bed 9. For this reason, as shown in FIG. 7, in order to ensure the space of the bed 9 in the state pulled out from the magnetic field generator 2, the magnetic field generator 2 is disposed at a position away from the center of the examination room. One magnetic shield surface 8b2 is farther from the magnetic field generator 2 than the other magnetic shield surfaces 8b1 and 8b2. In addition, when there are no rooms on the upper floor or the lower floor of the examination room (when there are no upper and lower floors), it is not necessary to provide the magnetic shield 8b on the ceiling or floor of the examination room like the wall. Accordingly, the magnetic shield 8b is unevenly arranged with respect to the magnetic field generator 2.

  In addition, a magnetic shield (magnetic body) 8c is provided in the examination room so as to cover the precision machine in order to protect the precision machine from the strong magnetic field generated by the main coil 2a and the shield coil 2b. The magnetic shield 8c is arranged not locally but locally according to the location of the precision machine.

  These magnetic shields (magnetic bodies) 8a, 8b, and 8c are magnetized by the magnetic field generated by the main coil 2a and the shield coil 2b of the superconducting coil, and generate an irregular magnetic field in the FOV 3.

  In step S14, the computer calculates the magnetic field distribution B2 created by the main coil 2a and the shield coil 2b of the superconducting coil at the arrangement positions of the magnetic shields (magnetic bodies) 8a, 8b, and 8c based on the magnetomotive force arrangement acquired in step S11. Calculate

  In step S15, the computer calculates a magnetization distribution M1 in the magnetic shields (magnetic bodies) 8a, 8b, and 8c magnetized by the magnetic field distribution B2.

  In step S16, the computer calculates a magnetic field distribution B3 (first magnetic field distribution) created by the magnetization distribution M1 in the FOV3. Thus, the A part is completed. Next, steps S17 to S21 performed after the A part will be described. In addition, when A part is not implemented, step S17-S21 are implemented after step S12.

  In step S17, the computer uses both the magnetic field distribution B3 (first magnetic field distribution) and the magnetic field distribution B1 (second magnetic field distribution) for the magnetic field distribution B3 (first magnetic field distribution) and the magnetic field distribution B1 (second magnetic field distribution) in the FOV 3. In addition to (overlapping), the composite magnetic field distribution B4 is calculated (B4 = B1 + B3).

  In step S18, the computer calculates a differential magnetic field distribution ΔB1 (first differential magnetic field distribution) between the target magnetic field distribution B0 targeted when adjusting the magnetic field uniformity in the FOV 3 and the synthesized magnetic field distribution B4 (ΔB1 = B4-B0).

  In step S19, the computer calculates a pre-magnetization distribution ΔM1 (first additional pre-magnetization distribution) at the place where the shim tray 6 (storage container 7) is arranged so as to create a differential magnetic field distribution ΔB1 (first differential magnetic field distribution) in the FOV 3. To do. For this calculation, the singular value decomposition method described in Patent Documents 1 and 2 shown as the prior art document or the expansion method using a spherical harmonic function can be used.

  In step S20, the computer calculates a magnetic field distribution B5 (third magnetic field) created by the main coil 2a and the shield coil 2b of the superconducting coil at the place where the shim tray 6 (storage container 7) is arranged based on the magnetomotive force arrangement obtained in step S11. Distribution).

  In step S21, the computer calculates the pre-magnetization distribution ΔM1 (first pre-magnetization distribution) calculated in step S19 based on the magnetic field distribution B5 (third magnetic field distribution) calculated in step S20 at the place where the shim tray 6 (storage container 7) is arranged. ) To generate a prior shim iron distribution ΔS1 (first shim iron distribution). With the prior shim iron distribution ΔS1 (first shim iron distribution), the amount of shim iron to be inserted (removed) for each storage container 7 (the amount of shim iron pieces 5) can be acquired, and the shim iron pieces 5 can be arranged. . Magnetic shields (magnetic bodies) 8a, 8b, and 8c installed around the MRI apparatus 1 are created by arranging the amount of shim iron in the storage container 7 in accordance with the prior shim iron distribution ΔS1 (first shim iron distribution). A decrease in the magnetic field uniformity of the FOV 3 due to the magnetic field can be suppressed.

  Next, the B part comprised by step S22-S27 is demonstrated. According to the B part, it is possible to suppress the deterioration of the magnetic field uniformity of the FOV 3 due to the installation error of the main coil 2a and the shield coil 2b of the superconducting coil at the stage of processing and assembling of the MRI apparatus 1.

  In step S22, the computer obtains the maximum allowable installation error of the main coil 2a and the shield coil 2b of the superconducting coil by input from an operator or the like.

  In step S23, the computer calculates a magnetic field distribution B11 (fourth magnetic field distribution) created in the FOV 3 by the main coil 2a and the shield coil 2b of the superconducting coil shifted by the maximum allowable value of the installation error.

  In step S24, the computer calculates a differential magnetic field distribution ΔB2 (second differential magnetic field distribution) between the target magnetic field distribution B0 and the magnetic field distribution B11 (fourth magnetic field distribution) that are targeted when adjusting the magnetic field uniformity in the FOV3. (ΔB2 = B11−B0).

  In step S25, the computer calculates a differential magnetic field distribution ΔB3 (third differential magnetic field distribution) in which the magnetic field strength of the differential magnetic field distribution ΔB2 (second differential magnetic field distribution) is approximately halved (1/2) (ΔB3 = ΔB2 /). 2). The reason why the magnetic field intensity is substantially halved (1/2) is that there are a plus component and a minus component in the amount of change in the magnetic field due to the installation error, and the shim iron piece 5 corresponding to the minus component is pre-loaded in step S2 in FIG. It is for distributing to 6 (storage container 7).

  In step S <b> 26, the computer calculates a pre-magnetization distribution ΔM <b> 2 (second pre-magnetization distribution) at the place where the shim tray 6 (storage container 7) is arranged so as to create a differential magnetic field distribution ΔB <b> 3 (third differential magnetic field distribution) in the FOV 3. . For this calculation, the singular value decomposition method described in Patent Documents 1 and 2 shown as the prior art document or the expansion method using a spherical harmonic function can be used.

  In step S27, the computer calculates the pre-magnetization distribution ΔM2 (second pre-magnetization distribution) calculated in step S26 by the magnetic field distribution B5 (third magnetic field distribution) calculated in step S20 at the place where the shim tray 6 (storage container 7) is arranged. ) To generate a prior shim iron distribution ΔS2 (second shim iron distribution). The pre-shim iron distribution ΔS2 (second shim iron distribution) makes it possible to obtain the amount of shim iron to be inserted (removed) for each storage container 7 (the amount of shim iron pieces 5), and the shim iron pieces 5 can be arranged. . By placing the amount of shim iron in accordance with the prior shim iron distribution ΔS2 (second shim iron distribution) in the storage container 7, the installation error of the main coil 2a and the shield coil 2b of the superconducting coil in the processing and assembly stage of the MRI apparatus 1 It is possible to suppress the deterioration of the magnetic field uniformity of the FOV 3 due to. This completes the B part.

  Finally, in step S28, the computer superimposes the prior shim iron distribution ΔS1 (first shim iron distribution) and the prior shim iron distribution ΔS2 (second shim iron distribution) at the place where the shim tray 6 (storage container 7) is arranged. The combined shim iron distribution ΔS3 is calculated (ΔS3 = ΔS1 + ΔS2). By arranging the amount of shim iron in the storage container 7 by the synthetic shim iron distribution ΔS3, the magnetic field uniformity of the FOV 3 by the magnetic field created by the magnetic shields (magnetic bodies) 8a, 8b, 8c installed around the MRI apparatus 1 can be improved. The decrease can be suppressed, and deterioration of the magnetic field uniformity of the FOV 3 due to an installation error between the main coil 2a and the shield coil 2b of the superconducting coil in the processing and assembly stage of the MRI apparatus 1 can be suppressed. With the above, the flow (chart) of the simulation method (part 1) of the advance shim iron distribution is completed.

  According to these, since the amount of shim iron can be adjusted before measuring the magnetic field distribution of the FOV, the number of times of excitation / demagnetization of the main coil 2a and the shield coil 2b of the superconducting coil can be reduced.

(Fourth embodiment)
In the fourth embodiment, the simulation method for the pre-sim iron distribution in step S1 of FIG. 5 described in the third embodiment is described as 2-1 for creating a database and 2-2 for using the database. Has been changed.

  FIG. 8A shows a flowchart of a pre-sim iron distribution simulation method (2-1) used in the fourth embodiment, and FIG. 8B shows a pre-sim iron distribution simulation method (part 2) used in the fourth embodiment. -2) is shown.

  First, in step S31 of FIG. 8A, the computer acquires the magnetomotive force arrangement of the main coil 2a and the shield coil 2b of the superconducting coil in the same manner as in step S11, and acquires the arrangement location of the magnetic material in the same manner as in step S13.

  In step S32, similarly to step S14, the computer, based on the magnetomotive force arrangement acquired in step S31, arranges the main coil 2a and the shield coil 2b of the superconducting coil into magnetic shields (magnetic bodies) 8a, 8b, 8c. The magnetic field distribution B2 created at the position is calculated.

  In step S33, similarly to step S15, the computer calculates the magnetization distribution M1 in the magnetic shields (magnetic bodies) 8a, 8b, and 8c magnetized by the magnetic field distribution B2.

  In step S34, similarly to step S16, the computer calculates the magnetic field distribution B3 (first magnetic field distribution) created by the magnetization distribution M1 in the FOV 3 for each of the magnetic shields (magnetic bodies) 8a, 8b, and 8c.

  In step S35, the computer creates a database that stores the magnetic field distribution B3 (first magnetic field distribution) for each of the magnetic shields (magnetic bodies) 8a, 8b, and 8c. This database is based on the magnetomotive force arrangement of the superconducting coils and the location of each magnetic shield (magnetic body) 8a, 8b, 8c, and the magnetic field distribution generated by the corresponding magnetic shield (magnetic body) 8a, 8b, 8c. B3 can be read. For example, in the magnetic shield 8a for protecting the motor of the refrigerator (see FIG. 7), if the shape of the magnetic shield 8a is different for each refrigerator model, the shape of the refrigerator (the shape of the magnetic shield 8a) Every time, the magnetic field distribution B3 (first magnetic field distribution) is calculated and stored in the database. Further, in the magnetic shield 8b (see FIG. 7) for reducing the leakage magnetic field from the examination room, depending on whether the magnetic shield 8b is provided on the ceiling and floor of the examination room, for each case, The magnetic field distribution B3 (first magnetic field distribution) is calculated and stored in the database. Further, in the magnetic shield 8c (see FIG. 7) for covering the precision machine, the magnetic field distribution B3 (first magnetic field distribution) is calculated for each type of precision machine (each shape of the magnetic shield 8c) and stored in the database. Let me. In the following, the synthetic shim iron distribution ΔS3 is determined (calculated) using the database created above.

  First, in step S11 of FIG. 8B, the computer acquires magnetomotive force arrangements of the main coil 2a and the shield coil 2b of the superconducting coil as in step S11 of FIG.

  In step S12 of FIG. 8B, the computer calculates the magnetic field distribution B1 (second magnetic field distribution) created in the FOV 3 (predetermined space) by the main coil 2a and the shield coil 2b of the superconducting coil, as in step S12 of FIG. .

  The A2 portion composed of steps S36 and S37 in FIG. 8B is replaced with the A portion in FIG. Therefore, similarly to the third embodiment, the process may proceed to step S22 without performing the A2 part (steps S36 and S37).

  In step S36, the worker selects a magnetic material (magnetic shield) that is in the installation environment of the MRI apparatus 1 from the database, and inputs the selection result to the computer.

  In step S37, the computer reads the magnetic field distribution B3 (first magnetic field distribution) corresponding to the selected magnetic shield from the database. Then, the process proceeds to step S17 in FIG.

  According to this, since it is possible to obtain the magnetic field distribution B3 (first magnetic field distribution) by omitting the calculation if the magnetic body arrangement has been calculated before, it is possible to increase the calculation speed and reduce the cost. Become.

(Fifth embodiment)
The fifth embodiment relates to a method of using the database created in the fourth embodiment. In the database used in the fourth embodiment, the magnetic field distribution B3 (first magnetic field distribution) is stored for various magnetic shields. However, in the database used in the fifth embodiment, various magnetic shields are used. The magnetic field distribution B3 (first magnetic field distribution) is stored for various sets of magnetomotive force arrangements of the superconducting coils, that is, various sets (combinations) of the magnetic shield and the superconducting coil. As various superconducting coils, the center and side length of the cross section of the main coil 2a and the shield coil 2b, the number of turns, the current flowing through each turn, and the like are changed.

  FIG. 9 shows a flowchart of a simulation method of the pre-sim iron distribution used in the fifth embodiment (part 3, method of using the database).

  First, in step S41 of FIG. 9, the computer sequentially reads a set of a superconducting coil (magnetomotive force arrangement) and a magnetic shield (magnetic body) (placement position) from the database.

  In step S42, similarly to step S12, the computer, based on the magnetomotive force arrangement of the superconducting coils in the set, generates a magnetic field distribution B1 (first space) created by the main coil 2a and the shield coil 2b of the superconducting coil in the FOV 3 (predetermined space). 2 magnetic field distribution).

  In step S43, the computer reads a magnetic field distribution B3 (first magnetic field distribution) corresponding to the set from the database. Further, the magnetic field distribution B3 (first magnetic field distribution) is added (superposed) to the magnetic field distribution B1 (second magnetic field distribution) to calculate a combined magnetic field distribution B4 (B4 = B1 + B3).

  In step S44, the computer acquires the maximum value and the minimum value in the combined magnetic field distribution B4 calculated in step S43 for each set.

  In step S45, the computer calculates a difference value Bp that is a difference between the maximum value and the minimum value for each set.

  In step S46, the computer extracts a set having the minimum difference value Bp. In the group having the smallest difference value Bp, it is considered that the magnetic field distribution B3 (first magnetic field distribution) and the magnetic field distribution B1 (second magnetic field distribution) cancel each other, and the superconducting coil and the magnetic shield are The best combination. Therefore, in step S47, the operator installs the MRI apparatus 1 in this set, and the computer implements the pre-sim iron distribution simulation method described in FIG. 6 and FIG. 8B for the MRI apparatus 1. . According to this, the amount of shim iron in the whole MRI apparatus 1 can be reduced.

(Sixth embodiment)
In the sixth embodiment, a magnetic field distribution B1 (second magnetic field distribution) that cancels the magnetic field distribution B3 (first magnetic field distribution) by a typical magnetic shield arranged around the MRI apparatus 1 is generated. The amount of shim iron is reduced by designing a superconducting coil with magnetic arrangement.

  FIG. 10 shows a flowchart of a pre-sim iron distribution simulation method (part 4) used in the sixth embodiment.

  First, in step S51 of FIG. 10, the computer acquires magnetomotive force arrangement e of the main coil 2a and the shield coil 2b of the superconducting coil as a base, as in step S11.

  In step S52, the computer calculates the magnetic field distribution B1 (second magnetic field distribution) created by the main coil 2a and the shield coil 2b of the superconducting coil in the FOV 3 (predetermined space) based on the acquired magnetomotive force arrangement e, similarly to step S12. ).

  In step S <b> 53, the computer obtains a representative (specific) magnetic shield (magnetic body) placement location in the same manner as in step S <b> 13.

  In step S54, the computer calculates the magnetic field distribution B2 formed by the main coil 2a and the shield coil 2b of the superconducting coil at a representative (specific) magnetic shield arrangement position based on the magnetomotive force arrangement e, similarly to step S14. Calculate

  In step S55, the computer calculates the magnetization distribution M1 in the representative (specific) magnetic shield magnetized by the magnetic field distribution B2, as in step S15.

  In step S56, the computer calculates a magnetic field distribution B3 (first magnetic field distribution) created by the magnetization distribution M1 in the FOV 3 as in step S16.

  In step S57, similarly to step S17, the computer adds (superimposes) the magnetic field distribution B3 (first magnetic field distribution) to the magnetic field distribution B1 (second magnetic field distribution) to calculate the combined magnetic field distribution B4 (B4). = B1 + B3).

  In step S58, the computer calculates a differential magnetic field distribution ΔB1 (first differential magnetic field distribution) between the target magnetic field distribution B0 and the combined magnetic field distribution B4 (ΔB1 = B4-B0), as in step S18.

  In step S59, the computer calculates the differential magnetomotive force arrangement Δe at the location where the main coil 2a and the shield coil 2b of the superconducting coil create the differential magnetic field distribution ΔB1 (first differential magnetic field distribution) in the FOV 3. For this calculation, the singular value decomposition method described in Patent Documents 1 and 2 shown as the prior art document or the expansion method using a spherical harmonic function can be used.

  In step S60, the calculator adds the differential magnetomotive force arrangement Δe to the magnetomotive force arrangement e to calculate the net magnetomotive force arrangement e1. An operator produces the main coil 2a and the shield coil 2b of the superconducting coil based on the net magnetomotive force arrangement e1, and produces the MRI apparatus 1 including these. And this MRI apparatus 1 is installed in the place where the typical (specific) magnetic shield (magnetic body) is arrange | positioned.

  In step S61, the calculator describes the location of the representative (specific) magnetic shield (magnetic material) and the net magnetomotive force arrangement e1 calculated in step S60, as described in FIG. 6 and FIG. 8B. Implement the simulation method of pre-sim iron distribution. According to this, the amount of shim iron in the whole MRI apparatus 1 can be reduced. Further, the time required for the shimming work can be shortened, and the number of times of exciting / demagnetizing the superconducting coil can be reduced in the shimming work.

DESCRIPTION OF SYMBOLS 1 MRI (magnetic resonance imaging) apparatus 2 Magnetic field generator 2a Superconducting coil (main coil)
2b Superconducting coil (shielded coil)
2c vacuum chamber 2d refrigerant container 3 imaging region (FOV: F ield O f V iew, predetermined space)
4 Magnetic field adjustment device 5 Shim iron piece (magnetic piece)
5a Spacer 6 Shim tray 6a Shim pocket 7 Storage container 7a Container body 7b Container lid 7c Opening 7d Storage space 7e Partition plate 8a Magnetic shield for protecting the refrigerator motor (magnetic material)
8b Magnetic shield (magnetic material) to reduce the leakage magnetic field
8c Magnetic shield (magnetic material) to protect precision machinery
9 beds

Claims (2)

A plurality of thin plate-like magnetic pieces and a shim tray provided with a plurality of pockets for storing the magnetic pieces, the magnetic pieces are stacked and placed in the pocket, and the superconducting coil is placed in a predetermined space. In the magnetic field adjustment device for adjusting the magnetic field uniformity of the generated static magnetic field,
It has an opening for taking in and out the magnetic pieces in a direction different from the laminating direction of the magnetic pieces, and comprises a storage container for storing a plurality of the magnetic pieces and being stored in the pockets. Magnetic field adjustment device. The storage container is
The magnetic field adjustment device according to claim 1, further comprising a partition plate that divides a storage space in which the magnetic piece is stored in the stacking direction.

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