JP2014004169A - Magnetic field adjustment method, magnetic field generator, magnetic resonance imaging device and magnetic field adjuster - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic field adjustment method capable of highly accurately determining positions to arrange magnetic body pieces and an amount thereof.SOLUTION: In a magnetic field adjustment method for a magnetic field generator having a superconducting coil for generating a static magnetic field and a shim tray in which magnetic body pieces for adjusting magnetic homogeneity of a static magnetic field in a predetermined space are arranged in a plurality of positions, intensity of magnetization of the magnetic body pieces assumed to be positioned in respective predetermined positions is obtained for each predetermined position (S13), and the amount of the magnetic body pieces arranged in respective predetermined positions is calculated by using the magnetization intensity obtained for each predetermined position (S3). To obtain the intensity of magnetization (S13), the intensity of a static magnetic field is calculated for each predetermined position (S11), and the intensity of magnetization is obtained for each predetermined position based on the magnetic field intensity by using database D1 of magnetization characteristics by which the intensity of magnetization relative to a magnetizing force can be determined (S13). Furthermore, the intensity of an additional magnetic field created by magnetic substances arranged to surround the magnetic field generator may also be calculated for each predetermined position and added to calculate the intensity of a static magnetic field for each predetermined position.

Description

  The present invention relates to a magnetic resonance imaging (MRI) apparatus and a magnetic field generation apparatus, and more particularly to a magnetic field adjustment method and a magnetic field adjustment apparatus for adjusting the magnetic field uniformity of a magnetic field generated by the magnetic resonance imaging (MRI) apparatus.

  The magnetic field generator is mounted on an MRI apparatus or the like. A magnetic field generator mounted in an MRI apparatus is required to achieve high-precision magnetic field uniformity (for example, several ppm or less) in an imaging region (FOV: Field Of View) in order to obtain a clear image. At the design stage of the MRI apparatus (magnetic field generator), the coil arrangement of the superconducting coils provided in the magnetic field generator is optimized in order to satisfy the required magnetic field uniformity specifications. However, in the manufacturing stage of the MRI apparatus (magnetic field generator), dimensional errors and installation errors are unavoidably caused in the superconducting coil by processing and assembly, and the magnetic field uniformity in the FOV is deteriorated from the required magnetic field uniformity. In addition, the magnetic field uniformity in the FOV is required due to the inevitable magnetic field created by the magnetic material installed around the MRI apparatus (magnetic field generator) in the manufacturing and use stages of the MRI apparatus (magnetic field generator). It deteriorates more than once. For this reason, in an MRI apparatus (magnetic field generator), an operation called shimming is performed in which the magnetic field piece for correction is installed in an appropriate position to adjust the magnetic field distribution of the FOV to improve the magnetic field uniformity. carry out. Various methods have been proposed for shimming (see, for example, Patent Documents 1 and 2). Patent Documents 1 and 2 propose a method for calculating the position and amount at which a magnetic piece should be arranged.

JP 2009-268791 A JP 2008-289703 A

  As in Patent Documents 1 and 2, even when a calculated amount of magnetic pieces are arranged at the calculated positions to be arranged, the required magnetic field uniformity may not be achieved with low accuracy. In some cases, the calculation of the position and amount at which the magnetic pieces 1 and 2 are to be arranged, the arrangement thereof, and the measurement of the magnetic field uniformity in the FOV are repeatedly performed. In the measurement of the magnetic field uniformity, the superconducting coil is excited, and in the arrangement of the magnetic piece, the superconducting coil is demagnetized. In the demagnetization of the superconducting coil, since the heater of the permanent current switch is energized, a large amount of refrigerant is consumed. For this reason, in order to reduce the cost, it is important to reduce the number of repetitions in order to reduce the number of times of excitation / demagnetization of the superconducting coil.

  That is, the problem to be solved by the present invention is to provide a magnetic field adjustment method and a magnetic field adjustment apparatus that can determine the position and amount at which a magnetic piece should be arranged with high accuracy. Moreover, it is providing the magnetic field generator and MRI apparatus adjusted with these magnetic field adjustment methods and magnetic field adjustment apparatuses.

In order to solve the above problems, the present invention provides:
A superconducting coil that generates a static magnetic field;
For a magnetic field generator having a shim tray that arranges magnetic pieces for adjusting the magnetic field uniformity of the static magnetic field in a predetermined space at a plurality of predetermined locations,
Obtain the magnetization intensity of the magnetic piece that is assumed to have been arranged there for each predetermined location,
The amount of the magnetic piece disposed at each predetermined location is calculated using the magnetization intensity at each predetermined location.

  ADVANTAGE OF THE INVENTION According to this invention, the magnetic field adjustment method and magnetic field adjustment apparatus which can determine the position and quantity which should arrange | position a magnetic body piece with high precision can be provided. Moreover, the magnetic field generator and MRI apparatus adjusted by these magnetic field adjustment methods and magnetic field adjustment apparatuses can be provided.

It is sectional drawing which cut | disconnected the MRI (magnetic resonance imaging) apparatus (magnetic field generator) with which the magnetic field adjustment method which concerns on the 1st Embodiment of this invention is implemented with 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) with which the magnetic field adjustment method which concerns on the 1st Embodiment of this invention is implemented with the plane which passes along Y-axis Z-axis. It is a top view of a shim tray. It is a longitudinal cross-sectional view of a shim tray. It is a longitudinal cross-sectional view of the shim pocket peripheral portion of the shim tray, and is an enlarged view of a part of FIG. 2B. It is a graph (magnetic field strength distribution) which shows the magnetic field intensity of the static magnetic field in the position in the axis (Z) direction on the shim tray. It is the magnetization characteristic of a magnetic body piece. It is a block diagram of the magnetic field adjustment system which implements the magnetic field adjustment method (shimming method) which concerns on the 1st Embodiment of this invention. It is a flowchart of the magnetic field adjustment method (shimming method) which concerns on the 1st Embodiment of this invention. It is sectional drawing which cut | disconnected the MRI (magnetic resonance imaging) apparatus (magnetic field generator) provided with the refrigerator with which the magnetic field adjustment method concerning the 2nd Embodiment of this invention was implemented with the plane parallel to an X-axis Y-axis. . It is a flowchart of the magnetic field adjustment method (shimming method) which concerns on the 3rd 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) in which the magnetic field adjustment method according to the first embodiment of the present invention is implemented, 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 that irradiates a high-frequency signal to cause nuclear magnetic resonance to occur, a gradient magnetic field generator for providing positional information to each magnetic resonance signal emitted from the subject, and magnetic resonance emitted from the subject A receiving coil for receiving signals and a bed (not shown) on which a subject is loaded are configured.

  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 shim tray 6 provided on the surface (inner peripheral surface) of the vacuum vessel 2c on the FOV 3 side.

  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 are discretely arranged in the Z direction (six in the example of FIG. 1B). The plurality of main coils 2a generate, in the FOV 3, a static magnetic field that has a uniform magnetic field strength distribution, a uniform magnetic field direction, and a uniform uniform magnetic field. 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 static magnetic field is also generated at the position where the shim tray 6 is disposed. Is generated. The plurality of shield coils 2b can generate a magnetic field that cancels out the leakage magnetic field to reduce the magnitude of the leakage magnetic field, and also generate a static magnetic field at the position where the shim tray 6 is disposed. The plurality of shield coils 2b have a ring shape having the Z axis as a common central axis. The plurality of shield coils 2b are discretely arranged (two in the example of FIG. 1B) in the Z direction. The plurality of shield coils 2b are disposed in the vicinity of a pair of main coils 2a disposed at both ends of the plurality of shield coils 2b arranged in the Z direction. 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 in their cross-sectional center position, 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. Has been. Although details will be described later, these values are stored in a storage device and can be obtained by a computer.

  A plurality of shim trays 6 are provided on the surface (inner peripheral surface) of the vacuum vessel 2c on the FOV 3 side. The plurality of shim trays 6 are arranged such that the longitudinal direction thereof is parallel to the Z direction. The shim tray 6 is disposed 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 shimming work using the shim tray 6, so that the environmental magnetic field at the location where the MRI apparatus 1 is installed and the error magnetic field due to the installation error of the main coil 2 a and the shield coil 2 b of the superconducting coil are expressed in the FOV 3. The magnetic field uniformity in the FOV 3 is set to a predetermined value (for example, several ppm) or less.

  2A shows a plan view of the shim tray 6, FIG. 2B shows a side view of the shim tray 6, and FIG. 2C shows a cross-sectional view of the periphery of the shim pocket 6a of the shim tray 6 which is an enlarged view of a part of FIG. 2B. Show. The shim tray 6 has an elongated bar shape. In the shim tray 6, a plurality of shim pockets 6a are arranged in a line at discrete intervals at equal intervals in the rod-like longitudinal direction (Z direction). Further, the bar-shaped width direction of the shim tray 6 coincides with the cylindrical circumferential direction of the inner peripheral surface of the vacuum vessel 2c (see FIG. 1A and the like). A storage container 7 is fitted in the plurality of shim pockets 6a. The shim tray 6 can be fixed to the vacuum vessel 2c (see FIG. 1A and the like). Thereby, the position of each shim tray 6 in the magnetic field generator 2 (MRI apparatus 1) can be specified, and the position (predetermined location) of each shim pocket 6a (storage container 7) can also be specified. Although details will be described later, these positions (predetermined locations) are stored in a storage device and can be acquired by a computer. The shim tray 6 and the storage container 7 can be made of a non-magnetic material such as a resin material, an aluminum material, or a copper material.

  As shown in FIG. 2C, a plurality of shim iron pieces (magnetic body pieces) 5 and a plurality of spacers 5 a are stored inside the storage container 7 in a stacked state. As the magnetic piece 5, a thin plate-like magnetic body, for example, an iron plate, preferably 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 5 a is provided so that no gap is generated in the storage container 7 so that the magnetic piece 5 does not move in the storage container 7. Thereby, the position (predetermined location) of each magnetic piece 5 in the magnetic field generator 2 (MRI apparatus 1) can be specified as the position (predetermined location) of each shim pocket 6a. By adjusting the amount of the magnetic piece 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 adjusted (changed) by adjusting (changing) the number of the magnetic pieces 5 stored in the storage container 7 and the thickness of each magnetic piece 5. Can be changed.

  FIG. 3 shows the magnetic field strength (magnetic field strength distribution) of the static magnetic field at the Z-direction position on the shim tray 6. The solid line in FIG. 3 indicates the magnetic field strength (magnetic field strength distribution) of the static magnetic field at the position in the axis (Z) direction on the shim tray 6. As shown in FIG. 1A, since the shim tray 6 is separated from the FOV 3, the magnetic field strength distribution of the static magnetic field in the axial (Z) direction on the shim tray 6 is uniform as shown by the solid line in FIG. Absent. The magnetic field strength distribution of the solid line in FIG. 3 depends on the positional relationship in the axis (Z) direction of the plurality of main coils 2a. That is, the plurality of main coils 2a are discretely arranged in the axis (Z) direction, and the magnetic field strength is relatively high at adjacent positions in the axis (Z) direction where the main coil 2a exists. The magnetic field strength is relatively low at the position in the axial (Z) direction between the two (where the main coil 2a does not exist).

  In the axial (Z) direction on the shim tray 6, a plurality of magnetic body pieces 5 (shim pockets 6a) are discretely arranged. The magnetic field strength at the Z-direction position (predetermined location) where each of the plurality of magnetic body pieces 5 (the shim pocket 6a) is arranged is indicated by the mark ● in FIG. It can be seen that the magnetic field strength differs at the Z-direction position (predetermined location) where each of the magnetic body pieces 5 (the shim pockets 6a) is disposed. In the first embodiment, details will be described later (described in step S11 in FIG. 6). However, based on the discrete arrangement of the main coil 2a and the like, the magnetic piece 5 of the static magnetic field created by the main coil 2a and the like. The magnetic field strength (marked with ● in FIG. 3) at the Z-direction position (predetermined location) where each of the shim pockets 6a is arranged is calculated.

FIG. 4 shows the magnetization characteristics of the magnetic material used for the magnetic piece 5. As the magnetization characteristics, the relationship between the magnetization force of the magnetic field in which the magnetic piece 5 (magnetic body) is placed and the magnetization intensity in the magnetic piece 5 (magnetic body) is shown. In the example of FIG. 4, in the region where the magnetizing force is low (the region where the relative scale (au) is approximately 2.E + 01 (2.0 × 10 ⁺⁰¹ ) or less), the magnetization strength rapidly increases as the magnetizing force increases. It has increased. In a region having a high magnetizing force (region in which the relative scale (au) exceeds approximately 2.E + 01 (2.0 × 10 ⁺⁰¹ )), the magnetization strength increases as the magnetizing force increases, and saturation magnetization (relative scale) (Au) is asymptotic to approximately 2.3). In the case of a small piece (thin plate) such as the magnetic piece 5, from the magnetic field strength B of the external magnetic field (static magnetic field formed by the main coil 2a etc.) ignoring the demagnetizing force in the magnetic piece 5, B = The magnetizing force H can be obtained by using a relational expression of μH (μ: permeability). Thereby, the magnetization intensity of the magnetic piece 5 can be acquired from the obtained magnetization force H using the database (formulated function) of the magnetization curve (relationship between magnetization intensity and magnetization force) of FIG. Then, the relational expression of B = μH (μ: permeability) and the database (formulated function) of the magnetization curve (relationship between magnetization intensity and magnetization force) in FIG. A database (formulated function) can be created. According to this, from the magnetic field strength B of the external magnetic field (static magnetic field formed by the main coil 2a and the like) in the magnetic piece 5, using a database (formulated function) of the relationship between the magnetization strength and the magnetic field strength B, The magnetization intensity of the magnetic piece 5 can be acquired. In the first embodiment, details will be described later (described in steps S12 and S13 in FIG. 6), but each of a plurality of magnetic body pieces 5 (shim pockets 6a) indicated by ● in FIG. 3 is arranged. From the magnetic field intensity B at the Z-direction position (predetermined location), using the database (formulated function) of the relationship between the magnetization intensity and the magnetic field strength B prepared in advance, each Z-direction position (predetermined location, shim) The magnetization intensity (magnetization distribution) of the magnetic piece 5 in the pocket 6a) is acquired. The magnetic field distribution after installing the magnetic piece 5 is shown by a one-dot chain line in FIG. By installing the magnetic piece 5, the influence of the discretized coil 2a is reduced, and a magnetic field distribution suitable for generating a uniform magnetic field on the FOV 3 is obtained.

  FIG. 5 is a block diagram of the magnetic field adjustment apparatus 10 that performs the magnetic field adjustment method (shimming method) according to the first embodiment of the present invention. The magnetic field adjustment device 10 includes a magnetic field measurement device 11 and a computer (computer) 12. In the magnetic field measurement apparatus 10, for example, a plurality of magnetic field measurement probes 11a are arranged along an arc of a semicircle of the spherical space of the FOV 3. The plurality of magnetic field measuring probes 11a are arranged along the outer periphery of the FOV 3, and measure the magnetic field strength at each arrangement position. Then, after rotating a minute angle around the Z axis shown in FIG. 1B, the measurement is repeated, and the entire outer periphery of the FOV 3 is measured. The measurement result (data) is transmitted to the arithmetic unit 12a of the computer 12 as FOV3 magnetic field distribution data.

  The computer 12 includes an arithmetic device 12a, a storage device 12b, a monitor (output device) 13, and a printer (output device) 14. Based on the magnetic field distribution data of the FOV 3 and the magnetic field distribution (data) on the shim tray 6 that has been acquired in advance by measurement or calculation and stored in the storage device 12b, the arithmetic unit 12a calculates the magnetic material distribution on the shim tray 6 ( Data). This magnetic material distribution (data) is displayed by the display 13 or the printer 14. In the magnetic material distribution (data), the amount of shim iron pieces (magnetic material pieces) 5 to be added to each shim pocket (predetermined location) 6a is determined. Based on the magnetic material distribution (data), the operator places shim iron pieces (magnetic material pieces) 5 of the designated quantity at the designated position (the shim pocket 6a). In calculating the magnetic material distribution (data) on the shim tray 6, not only the magnetic field distribution data of the FOV 3 but also the magnetic field distribution (data) on the shim tray 6 is used. Piece) The calculation accuracy of the quantity of 5 can be improved. The magnetic field distribution on the shim tray 6 can be applied to other magnetic field generators 2 (MRI apparatus 1) having the same coil arrangement as long as it is obtained by measurement or calculation once unless the coil arrangement is changed. As the storage device 12b, in addition to the fixed disk device built in the computer 12, an external storage medium such as a CD-ROM, a DVD, or a USB memory can be used. The storage device 12b may also serve as the storage device included in the MRI apparatus 1.

  FIG. 6 shows a flowchart of the magnetic field adjustment method (shimming method) according to the first embodiment of the present invention.

  First, in step S1, 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 magnetic field measurement apparatus 11 measures a magnetic field (intensity) distribution (third magnetic field intensity distribution) B1 on the FOV 3.

  In step S2, the computer 12 (arithmetic unit 12a) calculates the magnetic field homogeneity of the magnetic field (intensity) distribution B1 in the FOV 3 based on the measurement result of the magnetic field distribution B1 in the FOV 3 (for example, when calculating with peak to peak, (Highest magnetic field-lowest magnetic field) / average magnetic field is calculated). This computer 12 may or may not be a computer attached to the MRI apparatus 1 (magnetic field generator 2). The operator of the shimming work or the computer 12 determines whether or not the magnetic field uniformity of the calculation result has achieved a predetermined specification value (highly accurate magnetic field uniformity; for example, several ppm or less). If it has been achieved (step S2, Yes), the flow (chart) of this magnetic field adjustment method is stopped, and the shimming operation is terminated. If not achieved (No at Step S2), the process proceeds to Steps S3 and S11 in parallel.

  In step S3, the computer 12 obtains a magnetic moment distribution on the shim tray 6 that cancels the magnetic field inhomogeneity using singular value decomposition or spherical harmonic expansion based on the measurement result of the magnetic field distribution B1 in the FOV 3. . Specifically, the computer 12 calculates the difference target magnetic field (intensity) distribution ΔB1 between the target magnetic field (intensity) distribution B0 targeted when adjusting the magnetic field uniformity in the FOV 3 and the magnetic field distribution B1 measured in step S1. Is calculated (ΔB1 = B1−B0). The calculator 12 calculates the magnetic moment ΔM1 at the location (predetermined location) of the plurality of magnetic body pieces 5 (the shim pockets 6a) so as to create the differential target magnetic field distribution ΔB1 in the FOV 3. For the calculation of the magnetic moment ΔM1, the singular value decomposition method or the expansion method based on the spherical harmonic function described in Patent Documents 1 and 2 shown as the prior art documents can be used.

  In step S11, the computer 12 acquires the magnetomotive force arrangement (coil arrangement) of the main coil 2a and the shield coil 2b of the superconducting coil by reading from the storage device or inputting 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 position and the side length of each turn. 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. Then, the computer 12 calculates a magnetic field (intensity) distribution (a plurality of magnetic body pieces 5 (shims) formed on the shim tray 6 by the main coil 2a and the shield coil 2b of the superconducting coil based on the acquired magnetomotive force arrangement (coil arrangement). The magnetic field intensity at each location (predetermined location) of the pocket 6a) is calculated.

  In the magnetic field adjustment method (shimming method) according to the first embodiment, prior to starting this method, the computer 12 executes step S12 to determine the magnetization strength of the magnetic piece 5 and the magnetic piece 5. A database (formulated function) D1 of the relationship between the magnetic field strengths of the magnetic fields to be placed is created and stored in the storage device 12b. Specifically, in step S12, the computer 12 calculates the relational expression between the magnetic field strength B and the magnetizing force H (B = μH (μ: permeability)) and the magnetization curve of FIG. 4 (relationship between the magnetizing strength and the magnetizing force H). Is used to create a magnetization characteristic database D1 that can determine the magnetization intensity with respect to the magnetic field intensity B and store it in the storage device 12b.

  In step S13, the computer 12 uses the database D1 to calculate a plurality of magnetic field (intensity) distributions (magnetic field strengths for each location (predetermined location) of the plurality of shim pockets 6a) created on the shim tray 6 calculated in step S11. The magnetization intensity (magnetization distribution) of the magnetic piece 5 is acquired (calculated) for each location (predetermined location) of the shim pocket 6a.

  In step S4, the computer 12 calculates the magnetic moment ΔM1 at the location (predetermined location) of the plurality of magnetic pieces 5 (shim pockets 6a) calculated in step S3, and the magnetic piece 5 (shim pockets 6a) obtained in step S13. ) And the magnetic strength (magnetization distribution) of the magnetic piece 5 for each arrangement location (predetermined location), and the magnetic piece (magnetic material) for each arrangement location (predetermined location) of the magnetic piece 5 (shim pocket 6a) ) Calculate the volume (amount) of 5. Specifically, the volume of the magnetic piece (magnetic body) 5 is calculated by dividing the magnetic moment ΔM1 in step S3 by the magnetization intensity in step S13.

  In step S5, the shimming work operator calculates the magnetic piece for each location (predetermined location) of the magnetic piece 5 (shim pocket 6a) calculated in step S4 and displayed on the display 13 or the printer 14 (output device). Based on the volume (quantity) of 5, the quantity of the magnetic piece 5 is adjusted. Specifically, the MRI apparatus 1 (magnetic field generating apparatus 2) demagnetizes the main coil 2a and the shield coil 2b, erases the static magnetic field, and then magnetic pieces 5 and spacers in the storage container 7 (shim pocket 6a). By removing and adjusting 5a, the volume (amount) of the magnetic piece 5 calculated in step S4 is realized. The storage container 7 is disposed in the corresponding shim pocket 6a of the corresponding shim tray 6, and the shim tray 6 is disposed at the corresponding position of the vacuum container 2c. Then, the process returns to step S1.

  In the first embodiment, when the volume (amount) of the magnetic piece 5 in each shim pocket 6a is calculated (step S4), step S3 is performed using the magnetization intensity of the magnetic piece 5 in each shim pocket 6a. Since the volume (quantity) capable of realizing the magnetic moment calculated in (1) is calculated, the volume can be obtained with high accuracy. After arranging the number corresponding to the calculated volume of the magnetic piece 5 in the shim pocket 6a, the magnetic field distribution on the FOV 3 is measured again in step S1, and the magnetic field uniformity is compared with the specification value in step S2. The series of steps S1 to S5 are repeated until the magnetic field uniformity becomes a value equal to or less than the specification value. In steps S11 and S13, the magnetization distribution calculated for the first time can be used for the second and subsequent repetitions. For this reason, as shown in FIG. 8 of the third embodiment, this magnetization distribution may be stored as a database D2 and used after the second repetition. According to the first embodiment, since the accuracy of the volume obtained for each repetition is improved, it is possible to reduce the number of repetitions, and to reduce the work cost related to shimming. Can do. Moreover, since the time required for shimming can be shortened, the operating rate of the MRI apparatus 1 (magnetic field generator 2) can be increased.

  In addition, in the high magnetic field MRI apparatus 1 (magnetic field generator 2), since the electromagnetic force applied to the magnetic piece 5 is strong, it is difficult to insert and remove the shim tray 6 and the magnetic piece 5 while being excited. For this reason, it is necessary to demagnetize the main coil 2a and the like which are superconducting coils every time the above is repeated. Since the consumption of liquid helium, which is a refrigerant, increases during excitation and demagnetization of the main coil 2a and the like, in the first embodiment, the number of repetitions can be reduced, so that the number of excitation and demagnetization can be reduced. Reduce consumption. The cost of the refrigerant can be reduced.

(Second Embodiment)
FIG. 7 shows a cross-sectional view of an MRI apparatus 1 (magnetic field generator 2) in which the magnetic field adjustment method according to the second embodiment of the present invention is performed, cut along a plane parallel to the X axis and the Y axis. The MRI apparatus 1 (magnetic field generator 2) is provided with a refrigerator in order to suppress the consumption of liquid helium. In the motor part of the refrigerator, a magnetic shield 8 is installed to protect the motor from a strong magnetic field generated by the main coil 2a and the like. Since the magnetic shield 8 is magnetized by the magnetic field generated by the main coil 2a or the like, the irregular magnetic field generated by the magnetic shield 8 is superimposed on the magnetic field generated by the main coil 2a or the like around the magnetic shield 8. Not only the magnetic shield 8 but also a magnetic body arranged around the MRI apparatus 1 (magnetic field generator 2) generates the irregular magnetic field. These irregular magnetic fields are also distributed in the arrangement position of the shim tray 6 (the shim pocket 6a), and the magnetization intensity of the magnetic piece 5 is changed. Thus, in the second embodiment, the irregular magnetic field is added to the magnetic field generated by the main coil 2a and the like, and the magnetization intensity of the magnetic piece 5 is calculated with higher accuracy than in the first embodiment.

  Specifically, in step S11 shown in FIG. 6, the computer 12 calculates the magnetic field (intensity) distribution generated on the shim tray 6 by the main coil 2a and the like based on the magnetomotive force arrangement (coil arrangement) of the main coil 2a and the like. In addition, the magnetic material installed around the magnetic field generator 2 such as the magnetic shield 8 calculates the magnetic field (intensity) distribution (additional magnetic field strength) of the irregular magnetic field formed on the shim tray 6. Then, the magnetic field (intensity) distribution (additional magnetic field intensity) of the irregular magnetic field is added to the magnetic field (intensity) distribution created on the shim tray 6 by the main coil 2a and the like. The added magnetic field (intensity) distribution is used in step S13 in place of the magnetic field (intensity) distribution that the main coil 2a or the like creates on the shim tray 6. Thus, by calculating the magnetic field distribution of the shim pocket 6a with a non-linear magnetic field analysis code in consideration of the irregular magnetic field, the calculation accuracy of the magnetization distribution in the shim pocket 6a can be improved, and the accuracy of shimming can be further improved. . It should be noted that the magnetic body installed around the magnetic field generator 2 such as the magnetic shield 8 also has a magnetic field strength at the position where the magnetic body (magnetic shield 8) described with reference to FIG. In consideration of the magnetization intensity of the magnetic body (magnetic shield 8) based on the magnetic field strength described in FIG. 4, the magnetic moment generated in the magnetic body (magnetic shield 8) is calculated, and the magnetic body (magnetic It is preferable to calculate the magnetic field (strength) distribution (additional magnetic field strength) of the irregular magnetic field created by the shield 8) on the shim tray 6. According to this, it is possible to further improve the accuracy of shimming.

(Third embodiment)
FIG. 8 shows a flowchart of a magnetic field adjustment method (shimming method) according to the third embodiment of the present invention. The third embodiment is different from the first embodiment in that a database D2 of magnetization intensity (magnetization distribution) of the magnetic piece 5 for each location (predetermined location) of the shim pocket 6a is created. It is the point used. The method for deriving the magnetization distribution stored as the database D2 is also different between the third embodiment and the first embodiment. The method for deriving the magnetization distribution is performed in steps S21 to S25 in the third embodiment, and in steps S11 to S13 in FIG. 6 in the first embodiment. Both of these can be performed prior to steps S1 to S5. Both of the magnetization distributions derived in the third embodiment and the first embodiment are used in step S4. In steps S1 to S5 including step S4, the third embodiment is the same as the first embodiment. Therefore, in the following description, description of steps S1 to S5 is omitted, and steps S21 to S25 will be described.

In steps S21 to S25, the magnetization distribution is indirectly measured. Since it is difficult to directly measure the magnetization, the magnetization is indirectly measured.
First, in step S21, the operator of the shimming work removes the magnetic piece 5 from all the shim pockets 6a. 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 magnetic field measuring apparatus 11 measures the magnetic field (intensity) distribution (first magnetic field intensity distribution) B11 on the FOV 3 in a state where the magnetic piece 5 is removed from all the shim pockets 6a.

  In step S22, the operator of the shimming work puts the magnetic material piece 5 capable of acquiring a volume (including known) into one shim pocket 6a whose magnetization is to be measured in the entire shim pocket 6a. 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 magnetic field measuring apparatus 11 measures a magnetic field (intensity) distribution (second magnetic field intensity distribution) B12 on the FOV 3 with the magnetic piece 5 placed in one shim pocket 6a. This measurement is repeated by changing the shim pocket 6a in which the magnetic piece 5 is inserted. For each shim pocket 6a, a magnetic field (intensity) distribution (second magnetic field intensity distribution) B12 when the magnetic material piece 5 is inserted only there is measured.

  In step S23, the computer 12 calculates a differential magnetic field strength distribution ΔB11 that is a difference between the first magnetic field strength distribution B11 and the second magnetic field strength distribution B12 for each shim pocket 6a (ΔB11 = B12−B11). Then, the calculator 12 calculates a magnetic moment ΔM11 in the shim pocket 6a so as to create a differential magnetic field strength distribution ΔB11 in the FOV 3 for each shim pocket 6a. For the calculation of the magnetic moment ΔM11, similarly to the magnetic moment ΔM11 in step S3, the singular value decomposition method or the expansion method based on the spherical harmonic function described in Patent Documents 1 and 2 shown as the prior art documents can be used. .

  In step S24, the calculator 12 calculates the magnetization (strength) for each shim pocket 6a by dividing the magnetic moment ΔM11 calculated in step S23 by the volume of the magnetic piece 5 placed in the shim pocket 6a. . Thereby, the magnetization distribution of the whole several shim pocket 6a is acquirable.

In step S25, the computer 12 stores the magnetization distribution obtained in step S24 in the storage device 12b, and creates a database D2 capable of reading the corresponding magnetization intensity for each shim pocket 6a. This database D2 provides the magnetization distribution used in step S4. The database D2 can be used for a magnetic field adjustment method (shimming method) of the MRI apparatus 1 (magnetic field generator 2) in which the main coils 2a and the like have the same magnetomotive force arrangement. That is, once the database D2 is acquired by a certain type of MRI apparatus 1 (magnetic field generator 2), this database D2 can be used for the same type of MRI apparatus 1 (magnetic field generator 2). According to the third embodiment, even when the magnetization characteristics as shown in FIG. 4 are not obtained, the magnetization distribution is obtained, and shimming can be performed with high accuracy.

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 container 2d Refrigerant container 3 Imaging area (FOV: Field Of View)
5 Shim iron pieces (magnetic piece)
5a Spacer 6 Shim tray 6a Shim pocket (predetermined location)
7 Storage container 8 Magnetic shield to protect the refrigerator motor (magnetic material installed around the magnetic field generator)
DESCRIPTION OF SYMBOLS 10 Magnetic field adjustment apparatus 11 FOV magnetic field measurement apparatus 11a Magnetic field measurement probe 12 Computer (computer)
12a arithmetic device 12b storage device 13 display (output device)
14 Printer (output device)

Claims (8)

A superconducting coil that generates a static magnetic field;
In a magnetic field adjustment method of a magnetic field generator having a shim tray in which magnetic pieces for adjusting the magnetic field uniformity of the static magnetic field in a predetermined space are arranged at a plurality of predetermined locations,
Obtain the magnetization intensity of the magnetic piece that is assumed to have been arranged there for each predetermined location,
A magnetic field adjustment method, comprising: calculating the amount of the magnetic piece disposed at each predetermined location using the magnetization intensity at each predetermined location. When determining the magnetization intensity,
Calculate the magnetic field strength of the static magnetic field for each predetermined location,
2. The magnetization intensity at each predetermined location is acquired based on the magnetic field intensity using a magnetization characteristic database capable of determining the magnetization intensity with respect to the magnetization force or the magnetic field intensity. Magnetic field adjustment method. Calculate the additional magnetic field strength at each predetermined location of the magnetic field created by the magnetic body installed around the magnetic field generator,
The magnetic field adjustment method according to claim 2, wherein the additional magnetic field strength is added to the magnetic field strength of the static magnetic field at each predetermined location. When determining the magnetization intensity,
Measure the first magnetic field strength distribution of the static magnetic field in the predetermined space with the magnetic piece removed from all the predetermined locations,
Measuring the second magnetic field strength distribution of the static magnetic field in the predetermined space with the magnetic piece inserted in each predetermined place,
Calculating a differential magnetic field strength distribution that is a difference between the first magnetic field strength distribution and the second magnetic field strength distribution for each of the predetermined locations;
Calculate the magnetic moment to generate the differential magnetic field strength distribution there for each predetermined location,
2. The magnetic field adjustment method according to claim 1, wherein the magnetic moment is divided by a volume of the magnetic piece placed in each predetermined location. When calculating the amount of the magnetic piece,
Measuring a third magnetic field strength distribution of the static magnetic field in the predetermined space;
Calculating a difference target magnetic field strength distribution which is a difference between the third magnetic field strength distribution and the target magnetic field strength distribution to be adjusted;
Generating the differential target magnetic field strength distribution, calculating a magnetic moment located at the predetermined location;
The magnetic field adjustment method according to any one of claims 1 to 4, wherein the magnetic moment is divided by the magnetization intensity at each predetermined location.   6. A magnetic field generation apparatus, wherein the magnetic field uniformity of the static magnetic field in the predetermined space is adjusted by the magnetic field adjustment method according to claim 1. A magnetic field generator according to claim 6,
A magnetic resonance imaging apparatus, wherein the predetermined space is an imaging region. In a magnetic field adjustment device of a magnetic field generation device comprising a superconducting coil that generates a static magnetic field and shim trays that arrange magnetic pieces for adjusting the magnetic field uniformity of the static magnetic field in a predetermined space at a plurality of predetermined locations.
Computer
Obtain the magnetization intensity of the magnetic piece that is assumed to have been arranged there for each predetermined location,
A magnetic field adjustment device that calculates the amount of the magnetic piece to be arranged at each predetermined location by using the magnetization intensity at each predetermined location.

Images