MAGNETIC FIELD ADJUSTMENT METHOD FOR MRI DEVICE

• US 20120268119A1
• Filed: 11/24/2010
• Published: 10/25/2012
• Est. Priority Date: 11/24/2009
• Status: Abandoned Application

• Current Assignee: Hitachi, Ltd.
• Original Assignee: Hitachi, Ltd.
• Inventors: Abe, Mitsushi, Ando, Ryuya
• Application Number:
  US13/511,786
• Publication Number:
  US 20120268119A1
• Time in Patent Office: Days
• US Class Current: 324/307
• 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

Abstract

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 grasped, an erroneous adjustment can be grasped, and the adjustment is automatically done during repeated adjustment. When the magnetic field adjustment is carried out with support by the method of the present invention according to the first and second embodiments or an apparatus including this method therein, the magnetic field adjustment can be surely completed. As a result, the apparatus with a high accuracy can be provided. In addition, there is an advantageous effect of earlier detection of a poor magnet by checking the attainable homogeneity. They are applicable to magnet devices for the horizontal magnetic field type, being an open type MRI, and vertical magnetic field type MRI.

Claims

1. A magnetic field adjustment method for an MRI device having a magnetic field generating device having an region to which a target magnetic field distribution is set, the method bringing the magnetic field distribution in the region close to the target magnetic field distribution, characterized by:

having, as adjusting means, a magnetic field adjustment mechanism capable of arranging a current loop, a ferromagnetic material that is passively magnetized such as an iron piece, or a permanent magnet not depending on an external magnetic field in a sleeve region including the region; and

a magnetic field adjustment work process that performs a magnetic field measurement at a predetermined number of points, calculates an error magnetic field that is a difference from the target magnetic field, obtains a current potential distribution in a region of the magnetic field adjustment mechanism that can approximately correct the error on a plurality of ring shape planes intersecting the magnetic field direction within the region of the magnetic field adjustment mechanism,

converts the current potential distribution into magnetic moments which are summed in a section including one or more calculation points, and arranges a loop current, or a ferromagnetic material piece that corresponds to the summed magnetic moment.

3. The magnetic field adjustment method for the MRI device 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

the current potential distribution or the magnetic moment distribution for approximately correcting the error magnetic field with the combination of the distribution functions is calculated.

4. The magnetic field adjustment method for the MRI device as claimed in claim 3, characterized in that

correction magnetic field quantities at a magnetic field measurement point in the region to which the target magnetic field is set on the basis of the approximately corrected current potential, or the magnetic moment is calculated,

a predictive value of a residual error magnetic field is obtained by subtracting the corrected magnetic field quantity from the target magnetic field is obtained,

a eigen-distribution function which makes the predictive value of the residual error magnetic field is within a target range of an allowed residual error magnetic field is selected.

5. The magnetic field adjustment method for the MRI device as claimed in claim 1, characterized in that

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

the iron piece is arranged in accordance with the converted distribution.

6. The magnetic field adjustment method for the MRI device as claimed in claim 1, characterized in that

selection of eigen-distribution functions for obtaining a current potential distribution or a magnetic moment necessary for correcting the error magnetic field is made on a correlation chart (spectrum chart) with a number (order) numbered in accordance with a magnitude order of singular values and eigen-distribution intensity included in the error magnetic field.

7. The magnetic field adjustment method for the MRI device, as claimed in claim 1, characterized in that

a density distribution display including contour lines is made on a circular sleeve included in the magnetic field adjustment mechanism regarding a quantity corresponding to the current potential, or a quantity of the magnetic moment that are calculation result, and

the iron piece is arranged in accordance with the display by an operator.

8. The magnetic field adjustment method for the MRI device as claimed in claim 7, characterized in that

the contour line as well as the plane included in the magnetic field adjustment mechanism for arranging the iron piece are divided into polygons, and

a magnitude of the magnetic moment, or the volume of the iron piece or a permanent magnet is displayed for each divided region with a surface integration value together with the contour line or without the contour line.

9. The magnetic field adjustment method for the MRI device as claimed in claim 8, characterized in that

a peak or valley part indicated with the contour line is collectively integrated, and

the volume thereof is arranged at one place or dispersed over a plurality of places within the peak or valley part.

10. The magnetic field adjustment method for the MRI device as claimed in claim 1, characterized in that

the calculation and a work process from the magnetic field measurement and the magnitude calculation of the magnetic moment to the arrangement of an iron piece volume or a permanent magnet value are repeatedly made.

11. The magnetic field adjustment method for the MRI device as claimed in claim 10, characterized in that

in the repeated calculation and work process, a magnitude of intensity of each eigen-distribution function which is a base for indicating the magnetic field distribution obtained by singular value decomposition is displayed in addition to the error magnetic filed to grasp a progress of the magnetic field adjustment.

2. A magnetic field adjustment method for an MRI device having a magnetic field generating device having an region to which a target magnetic field distribution is set, the method bringing the magnetic field distribution in the region close to the target magnetic field distribution, characterized by:

having, as adjusting means, a magnetic field adjustment mechanism capable of arranging a current loop, a ferromagnetic material that is passively magnetized such as an iron piece, or a permanent magnet not depending on an external magnetic field in a sleeve region including the region; and

a magnetic field adjustment work process that performs a magnetic field measurement at a predetermined number of points, calculates an error magnetic field that is a difference from the target magnetic field, obtains magnitudes of magnetic moments at a lot of locations for magnetic moment calculation arranged in a region of the magnetic field adjustment mechanism where the error magnetic field can be approximately corrected, makes addition in a section including one or more magnetic moment calculating points, and arranges a loop current, or a ferromagnetic material piece that corresponds to the magnitude of the magnetic moment.

15. The magnetic field adjustment method for the MRI device as claimed in claim 2, characterized in that

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

the current potential distribution or the magnetic moment distribution for approximately correcting the error magnetic field with the combination of the distribution functions is calculated.

16. The magnetic field adjustment method for the MRI device as claimed in claim 2, characterized in that

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

the iron piece is arranged in accordance with the converted distribution.

17. The magnetic field adjustment method for the MRI device as claimed in claim 2, characterized in that

selection of eigen-distribution functions for obtaining a current potential distribution or a magnetic moment necessary for correcting the error magnetic field is made on a correlation chart (spectrum chart) with a number (order) numbered in accordance with a magnitude order of singular values and eigen-distribution intensity included in the error magnetic field.

18. The magnetic field adjustment method for the MRI device, as claimed in claim 2, characterized in that

a density distribution display including contour lines is made on a circular sleeve included in the magnetic field adjustment mechanism regarding a quantity corresponding to the current potential, or a quantity of the magnetic moment that are calculation result, and

the iron piece is arranged in accordance with the display by an operator.

19. The magnetic field adjustment method for the MRI device as claimed in claim 2, characterized in that

the calculation and a work process from the magnetic field measurement and the magnitude calculation of the magnetic moment to the arrangement of an iron piece volume or a permanent magnet value are repeatedly made.

12. (canceled)

13. (canceled)

14. A designing method of magnet magnetomotive force arrangements in a designing a electromagnet including a coil for generating a magnetic field and magneto motive force of a magnetic material, characterized in that:

a target magnetic field distribution is given, a magnetic field distribution is calculated from the magnetomotive force arrangement, a magnetic field calculation value is inputted in place of the magnetic field measurement value in claim 13, acceptability in arrangement of the magnetomotive force source arranged is confirmed, the arrangement of the magnetomotive force source is changed until the magnetic field adjustment is acceptable to obtain magneto motive force source arrangement allowing a magnetic field adjustment when the acceptability is not confirmed.