Magnetic resonance imaging system, static magnetic field homogeneity adjusting system, magnetic field homogeneity adjusting method, and magnetic field homogeneity adjusting program

1. A magnetic resonance imaging system comprising:

a static magnetic field generating device that includes a superconducting coil and generates a static magnetic field in an imaging space where an object is placed;

a correction magnetic field generating unit that generates a correction magnetic field to reduce inhomogeneity of a static magnetic field distribution in the imaging space;

a measurement unit that measures the static magnetic field distribution in the imaging space;

an excitation power source that selectively supplies either of an electric current at a rated current value or an electric current at a predetermined (greater than zero) low current value smaller than the rated current value to the superconducting coil; and

a calculation control unit that evaluates a distribution of a correction magnetic field that should be generated by the correction magnetic field generating unit using calculation based on the static magnetic field distribution in the imaging space measured by the measurement unit,

wherein the calculation control unit supplies the electric current at the rated current value from the excitation power source to the superconducting coil, evaluates the distribution of the correction magnetic field that should be generated by the correction magnetic field generating unit using first calculation based on the static magnetic field distribution measured by the measurement unit in a state where the electric current at the rated current value is flowing in the superconducting coil, reduces the electric current value of the superconducting coil to the low current value using the excitation power source, notifies an operator to set the correction magnetic field of the correction magnetic field generating unit to the correction magnetic field distribution evaluated by the first calculation in a state where the electric current at the low current value is flowing in the superconducting coil, and repeats the above operations.

2. The magnetic resonance imaging system according to claim 1 ,

wherein, when the electric current value of the superconducting coil reaches the low current value before reaching the rated current value, the calculation control unit evaluates a distribution of the correction magnetic field that should be generated by the correction magnetic field generating unit using second calculation based on the static magnetic field distribution

measured by the measurement unit in a state where the electric current at the low current value is flowing in the superconducting coil and notifies the operator to set the correction magnetic field of the correction magnetic field generating unit to the correction magnetic field distribution evaluated by the calculation control unit using the second calculation in a state where the electric current at the low current value is flowing in the superconducting coil.

3. The magnetic resonance imaging system according to claim 1 ,

wherein, when the operator sets the correction magnetic field distribution for the correction magnetic field generating unit after notification to the operator, the calculation control unit evaluates a distribution of the correction magnetic field that should be generated by the correction magnetic field generating unit using second calculation based on the static magnetic field distribution measured by the measurement unit in a state where the electric current at the low current value is flowing in the superconducting coil and notifies the operator to set the correction magnetic field of the correction magnetic field generating unit to the correction magnetic field distribution evaluated by the calculation control unit using the second calculation in a state where the electric current at the low current value is flowing in the superconducting coil.

4. The magnetic resonance imaging system according to claim 2 ,

wherein, when the correction magnetic field distribution is evaluated by the calculation control unit using the second calculation, the calculation control unit evaluates the inhomogeneity of the static magnetic field distribution in a state where the electric current at the rated current value is flowing in the superconducting coil and evaluates the distribution of the correction magnetic field based on the inhomogeneity of the evaluated static magnetic field distribution.

5. The magnetic resonance imaging system according to claim 1 ,

wherein the correction magnetic field generating unit includes magnetic shims and one or more shim trays supporting the magnetic shims,

the shim trays are detachably arranged in a position closer to the imaging space than the static magnetic field generating device, and

the calculation control unit evaluates arrangement of the magnetic shims on the shim trays as the correction magnetic field distribution.

6. The magnetic resonance imaging system according to claim 5 ,

wherein the low current value is set to a value at which the operator can attach and detach one of the shim trays, to which the magnetic shim were attached, in a static magnetic field to be generated by the superconducting coil in which the electric current at the low current value flows.

7. The magnetic resonance imaging system according to claim 5 ,

wherein the low current value is set to a value at which an electromagnetic attraction force to be applied to one of the shim trays attached to the magnetic shims is more than 0 kg and equal to or less than 5 kg, by a static magnetic field to be generated by the superconducting coil in which the electric current at the said low current value flows.

8. The magnetic resonance imaging system according to claim 2 ,

wherein the measurement unit includes measurement elements for a rated static magnetic field that measure a static magnetic field distribution in a state where the electric current at the rated current value is flowing in the superconducting coil and measurement elements that measure a static magnetic field distribution in a state where the electric current at the low current value is flowing.

9. The magnetic resonance imaging system according to claim 2 ,

wherein an external magnetic field generating unit that generates a magnetic field to a periphery of the static magnetic field generating device is further included in order to equalize a leakage magnetic field to the periphery of the static magnetic field generating device in a state where the electrical current at the low current value is flowing in the superconducting coil, to a leakage magnetic field in a state where the electrical current at the rated current value is flowing in the superconducting coil.

10. A static magnetic field homogeneity adjusting system comprises:

a measurement unit that includes a superconducting coil and measures a static magnetic field distribution in an imaging space of a static magnetic field generating device that generates a static magnetic field in the imaging space where an object is placed;

an excitation power source that selectively supplies either of an electric current at a rated current value or an electric current at a predetermined (greater than zero) low current value smaller than the rated current value to the superconducting coil; and

a calculation control unit that evaluates a distribution of a correction magnetic field that should be generated by a correction magnetic field generating unit disposed in a vicinity of the imaging space, by calculation based on the static magnetic field distribution in the imaging space measured by the measurement unit, in order to reduce inhomogeneity of a static magnetic field distribution in the imaging space,

wherein the calculation control unit supplies the electric current at the rated current value from the excitation power source to the superconducting coil, evaluates the distribution of the correction magnetic field that should be generated by the correction magnetic field generating unit using first calculation based on the static magnetic field distribution measured by the measurement unit in a state where the electric current at the rated current value is flowing in the superconducting coil, reduces the electric current value of the superconducting coil to the low current value using the excitation power source, notifies an operator to set a correction magnetic field of the correction magnetic field generating unit to the correction magnetic field distribution evaluated by the calculation control unit using the first calculation in a state where the electric current at the low current value is flowing in the superconducting coil, and repeats the above operations.

11. The static magnetic field homogeneity adjusting system according to claim 10 ,

wherein, when the electric current of the superconducting coil reaches the low current value before reaching the rated current value, the calculation control unit evaluates a distribution of the correction magnetic field that should be generated by the correction magnetic field generating unit using second calculation based on the static magnetic field distribution measured

by the measurement unit in a state where the electric current at the low current value is flowing in the superconducting coil and notifies the operator to set the correction magnetic field of the correction magnetic field generating unit to the correction magnetic field distribution evaluated by the calculation control unit using the second calculation in a state where the electric current at the low current value is flowing in the superconducting coil.

12. The static magnetic field homogeneity adjusting system according to claim 10 ,

wherein an external magnetic field generating unit that generates a magnetic field to a periphery of the static magnetic field generating device is further included in order to equalize a leakage magnetic field to the periphery of the static magnetic field generating device in a state where the electrical current at the low current value is flowing in the superconducting coil, to a leakage magnetic field in a state where the electrical current at the rated current value is flowing in the superconducting coil.

13. A magnetic field homogeneity adjusting method of a static magnetic field generating device repeatedly performing the following steps:

a first step in which a static magnetic field inhomogeneity in an imaging space is measured in a state where an electric current at a rated current value is flowing in a superconducting coil of the static magnetic field generating device;

a measurement step in which a static magnetic field distribution in the imaging space is measured;

a second step in which a distribution of a correction magnetic field that should be generated by a correction magnetic field generating unit disposed in a vicinity of the imaging space is evaluated by a calculation control unit using calculation based on the static magnetic field distribution measured in the measurement step; and

a third step in which the electric current value of the superconducting coil is reduced to a predetermined (greater than zero) low current value smaller than the rated current value and notifies an operator to set a correction magnetic field of the correction magnetic field generating unit to the correction magnetic field evaluated by the calculation control unit using the calculation in a state where the electric current at the low current value is flowing in the superconducting coil.

14. The magnetic field homogeneity adjusting method of the static magnetic field generating device according to claim 13 , performing the following steps:

a 1-1 step in which the static magnetic field distribution is measured in a state where the electric current at the low current value is flowing in the superconducting coil when the electric current value of the superconducting coil reaches the low current value before the first step;

a 1-2 step in which the distribution of the correction magnetic field that should be generated by the correction magnetic field generating unit is evaluated by the calculation control unit using the calculation based on the measurement result of the 1-1 step; and

a 1-3 step in which the operator is notified to set the correction magnetic field of the correction magnetic field generating unit to the correction magnetic field evaluated in the 1-2 step in a state where the electric current at the low current value is flowing in the superconducting coil.

15. The magnetic field homogeneity adjusting method of the static magnetic field generating device according to claim 13 , performing the following steps before repeating the first step again after the third step:

a fourth step in which the static magnetic field inhomogeneity is measured in a state where the electric current at the low current value is flowing in the superconducting coil when the operator sets the correction magnetic field for the correction magnetic field generating unit after the notification;

a fifth step in which the distribution of the correction magnetic field that should be generated by the correction magnetic field generating unit is evaluated by the calculation control unit using the calculation based on the measurement result of the fourth step; and

a sixth step in which the operator is notified to set the correction magnetic field of the correction magnetic field generating unit to the correction magnetic field evaluated in the fifth step in a state where the electric current at the low current value is flowing in the superconducting coil.