MAGNETIC FIELD ADJUSTMENT FOR MRI APPARATUS

• US 20110089943A1
• Filed: 05/08/2009
• Published: 04/21/2011
• Est. Priority Date: 05/09/2008
• Status: Active Grant

• Current Assignee: Hitachi, Ltd.
• Original Assignee: Hitachi, Ltd., Hitachi Medical Corporation (Hitachi, Ltd.)
• Inventors: Nakayama, Takeshi, Ando, Ryuya, Abe, Mitsushi
• Granted Patent: US 8,947,089 B2
• Time in Patent Office: Days
• US Class Current: 324/301
• CPC Class Codes: A61B 5/055 involving electronic [EMR] or nuclear [NMR] magnetic resonance, e.g. magnetic resonance imaging
• CPC Class Codes: G01R 33/38 Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field
• CPC Class Codes: G01R 33/3806 Open magnet assemblies for improved access to the sample, e.g. C-type or U-type magnets
• CPC Class Codes: G01R 33/383 using permanent magnets
• CPC Class Codes: G01R 33/3873 using ferromagnetic bodies ; Passive shimming
• CPC Class Codes: G01R 33/3875 using correction coil assemblies, e.g. active shimming
• CPC Class Codes: G01R 33/389 Field stabilisation , e.g. by field measurements and control means or indirectly by current stabilisation
• CPC Class Codes: H01F 7/02 Permanent magnets [PM]
• CPC Class Codes: H01F 7/0284 using a trimmable or adjustable magnetic circuit, e.g. for a symmetric dipole or quadrupole magnetic field

Abstract

A measured error magnetic field distribution is divided into eigen-mode components obtained by a singular decomposition and iron piece arrangements corresponding to respective modes are combined and arranged on a shim-tray. An eigen-mode to be corrected is selected in accordance with an attainable magnetic field accuracy (homogeneity) and appropriateness of arranged volume of the iron pieces. Because the adjustment can be made with the attainable magnetic field accuracy (homogeneity) being known, an erroneous adjustment can also be known, and the adjustment is automatically done during repeated adjustments. As a result, an apparatus with a high accuracy can be provided. In addition, there is an advantageous effect of being able to detect a poor magnet earlier by checking the attainable homogeneity.

Claims

1. A magnetic field adjustment method for reducing an error magnetic component in a magnetic field distribution in a region of a target magnetic field distribution in a magnetic generation device to bring the magnetic field close to the target, characterized by:

having, as adjusting means, a magnetic field adjustment mechanism in shape of a curved surface or a flat surface for arranging a current loop, a ferromagnetic material that is passively magnetized, or a permanent magnet not dependent on an external magnetic field; and

a magnetic field adjustment work process that performs a magnetic measurement on a predetermined number of nodes, calculates an error magnetic field that is a difference from the target magnetic field, obtains a current potential distribution on the magnetic field adjustment mechanism that can approximately correct the error, converts the current potentials distribution into a magnetic moment, and arranges a loop current, a permanent magnet, or a ferromagnetic material piece that correspond to the magnetic moment.

2. The magnetic field adjustment method as claimed in claim 1, characterized in that

a distribution function is selected from eigen-distribution functions that are basis obtained by singular value decomposition, and

a current potential distribution for approximately correcting the error magnetic field with the combination is expressed.

3. The magnetic field adjustment method as claimed in claim 2, characterized in that

correction magnetic field quantities of the region having a target magnetic field and a magnetic field measurement node from the approximately corrected current potential are calculated,

a residual error magnetic field from the target magnetic field after a magnetic field adjustment work process is obtained,

appropriateness of selecting an eigen-distribution function is checked, as well as,

selection within the target residual error magnetic field is performed.

4. The magnetic field adjustment method as claimed in claim 1, characterized in that

the current potential is converted into an iron volume density as a quantity proportional to a magnetic moment, and

an iron piece or a permanent magnet is arranged in accordance with the converted distribution.

5. The magnetic field adjustment method as claimed in claim 1, characterized in that selection of the eigen-distribution functions for obtaining a current potential distribution necessary for correcting an error magnetic field is made with a number (order) numbered in accordance with a magnitude order of singular values on a correlation chart (spectrum chart) of eigen-distribution intensity included in the error magnetic field.

6. The magnetic field adjustment method as claimed in claim 1, characterized in that

a display including contour lines is made on the magnetic field adjustment mechanism plates on which the iron piece is arranged to show density distribution of the current potentials, or magnitudes of the magnetic moment, volumes of iron or permanent magnets, and

the iron piece is arranged in accordance with the display.

7. The magnetic field adjustment method as claimed in claim 6, characterized in that

the contour line as well as the plates for arranging the iron piece are divided into polygons, and

a magnitude of the magnetic moment or the volume of the iron or permanent magnet are displayed for each divided region with an area integration value together with the contour line or without the contour line.

8. The magnetic field adjustment method as claimed in claim 6, characterized in that

peaks or valleys indicated with contour line are collectively integrated, and

volumes thereof are arranged at one place or dispersed at a plurality of places within the peak or the valley.

9. The magnetic field adjustment method as claimed in claim 1, characterized in that calculation and a work process from a magnetic field measurement to arrangement of the magnetic moment or volumes of iron or permanent magnet are repeatedly made.

10. The magnetic field adjustment method as claimed in claim 9, characterized in that a progress of the magnetic field adjustment is grasped by checking, in the iterative calculations and works, a magnitude of individual intensity regarding eigen-distribution function which is a base indicating a magnetic field distribution obtained by a singular value decomposition together with a magnitude of the error magnetic field.

11. The magnetic field adjustment method characterized in that a representative value of the residual error magnetic field in claim 3, such as a value obtained by dividing a difference between a maximum value and a minimum value by a target or an average magnetic field intensity is displayed together with the correlation chart in claim 5, the volume of the ferromagnetic material, the permanent magnet, or a size of the current loop arranged in the magnetic field adjustment work in claim 6, 7, or 8.

12. A quality control method, including the magnetic field adjustment method as claimed in claim 3, for an electromagnet which includes a coil for generating a magnetic field and parts generating magnetizing force, characterized in that

it can be determined whether a magnetic field adjustment can be normally executed at a start of a magnetic field adjustment work on the basis of that it is possible to arrange an intensity of the magnetic moment, the volume of the iron or the permanent magnet in the claim 6 and claim 7 in a condition of making it lower than the target error magnetic field.

13. A magnetizing-force-parts-arrangements-in-a-magnet designing method for designing of an electromagnet including a coil that generates a magnetic field and parts generating magnetizing force of a ferromagnetic material, characterized in that

a magnetic field distribution is calculated where a target magnetic field distribution is given,

a magnetic field calculation value is entered in place of the magnetic field measurement value in claim 12,

an appropriateness of the part generating magnetizing force is checked, and

when there is no appropriateness, the magnetizing force parts arrangements in the magnet is changed until the magnetic field adjustment becomes possible to obtain magnetizing force parts arrangement capable of the magnetic field adjustment.

14. An apparatus including the magnetic field adjustment method therein as claimed in any one of claims 1 to 11.

15. A device including the quality control method as claimed in claim 12 therein.

16. An apparatus including the magnetizing-force-parts-arrangements-in-a-magnet designing method as claimed in claim 13 therein.