Magnetic resonance imaging method, magnetic resonance imaging apparatus, and control device of magnetic resonance imaging apparatus

• US 8,890,530 B2
• Filed: 07/21/2011
• Issued: 11/18/2014
• Est. Priority Date: 07/23/2010
• Status: Active Grant

• Current Assignee: Toshiba Medical Systems Corporation (Canon Inc.)
• Original Assignee: Toshiba Medical Systems Corporation (Canon Inc.), Kabushiki Kaisha Toshiba (Toshiba Corporation)
• Inventors: Umeda, Masaaki, Kamata, Mitsukazu
• Primary Examiner(s): ARANA, LOUIS M
• Application Number:
  US13/187,824
• Publication Number:
  US 20120019251A1
• Time in Patent Office: 1,216 Days
• Field of Search: 324/318, 324/320, 324/307, 324/309, 324/314
• US Class Current: 324/318
• CPC Class Codes: G01R 33/4836 using an RF pulse being spatially selective in more than one spatial dimension, e.g. a 2D pencil-beam excitation pulse
• CPC Class Codes: G01R 33/5617 using RF refocusing, e.g. RARE
• CPC Class Codes: G01R 33/56518 due to eddy currents, e.g. caused by switching of the gradient magnetic field

Abstract

According to one embodiment, an MRI apparatus includes a signal acquisition unit, an image generating unit, a position acquiring unit, and a correction unit. The signal acquisition unit acquires a magnetic resonance signal produced from an object by applying a gradient magnetic field and an RF pulse in an imaging space where the object is placed. The image generating unit reconstructs image data on the object based on the magnetic resonance signal. The position acquiring unit acquires an imaging region as positional information in the imaging space. The correction unit brings a distribution of the gradient magnetic field close to a target distribution by transforming a waveform of the gradient magnetic field based on the positional information and a time constant of an eddy-current magnetic field to cancel the eddy-current magnetic field.

Claims

1. A magnetic resonance imaging (MRI) apparatus comprising:

gradient magnetic field coils, a static magnetic field generator, at least one radio frequency (RF) coil, at least one RF transmitter and at least one RF receiver, said components being coupled to an MRI control system configured to;

acquire a magnetic resonance signal produced from an object by applying a gradient magnetic field and an RF pulse in an imaging space where the object is placed;

reconstruct image data on the object based on the magnetic resonance signal;

acquire an imaging region as positional information in the imaging space; and

adjust a distribution of the gradient magnetic field to approximate a target distribution by transforming a waveform of the gradient magnetic field based on the positional information and a time constant of an eddy-current magnetic field to cancel at least part of the eddy-current magnetic field, the eddy-current magnetic field being induced by an eddy current caused by application of the gradient magnetic field,

wherein the MRI control system is further configured

(a) to calculate, for each direction of application of the gradient magnetic field, an amplitude of a component of the eddy-current magnetic field in an X-axis direction of an apparatus coordinate system, an amplitude of a component of the eddy-current magnetic field in a Y-axis direction of the apparatus coordinate system, and an amplitude of a component of the eddy-current magnetic field in a Z-axis direction of the apparatus coordinate system based on a distance between the imaging region and a magnetic field center obtained from the positional information, and

(b) to adjust the distribution of the gradient magnetic field to more closely approximate the target distribution by temporally varying a value of the current supplied to the gradient magnetic field coil according to the time constant of the eddy-current magnetic field and a result of the calculations.

2. The magnetic resonance imaging apparatus according to claim 1, wherein:

wherein the MRI control system is also configured to preliminarily store the time constant of the eddy-current magnetic field as a value based on a relative positional relationship between the gradient magnetic field coils and the static magnetic field magnet and a shape and material of the gradient magnetic field coils, and to transform the waveform of the gradient magnetic field according to the time constant.

3. The magnetic resonance imaging apparatus according to claim 1, wherein:

the MRI control system is also configured to acquire a center position of the imaging region based on the positional information, and to perform the calculations based on a distance between the center position and the magnetic field center.

4. The magnetic resonance imaging apparatus according to claim 3, wherein:

the MRI control system is also configured to adjust the distribution of the gradient magnetic field closer to the target distribution by temporally varying a value of the current supplied to the gradient magnetic field coil according to the time constant of the eddy-current magnetic field.

5. The magnetic resonance imaging apparatus according to claim 3, wherein:

the MRI control system is also configured;

to acquire the magnetic resonance signal from a plurality of imaging regions corresponding to a plurality of images respectively;

to acquire the plurality of imaging regions as a plural sets of positional information; and

to correct the current supplied to the gradient magnetic field coil for each of the imaging regions.

6. The magnetic resonance imaging apparatus according to claim 1, wherein:

the MRI control system is also configured to perform the calculations so as to calculate amplitudes of components of the eddy-current magnetic fields in an X-axis direction, a Y-axis direction and a Z-axis direction in an apparatus coordinate system as average values over a width of the imaging region by using respective distances from the magnetic field center to a plurality of positions in the imaging region.

7. The magnetic resonance imaging apparatus according to claim 6, wherein:

the MRI control system is also configured;

to acquire the magnetic resonance signal from a plurality of imaging regions corresponding to a plurality of images respectively;

to acquire the plurality of imaging regions as a plural sets of positional information; and

to correct the current supplied to the gradient magnetic field coil for each of the imaging regions.

8. The magnetic resonance imaging apparatus according to claim 1, wherein:

the MRI control system is also configured;

to acquire the magnetic resonance signal from a plurality of imaging regions corresponding to a plurality of images respectively;

to acquire the plurality of imaging regions as plural sets of positional information; and

to correct the current supplied to the gradient magnetic field coil for each of the imaging regions.

9. The magnetic resonance imaging apparatus according to claim 1, wherein:

the MRI control system is configured;

to perform, as multi-slice imaging, acquisition of a magnetic resonance signal from an imaging region by applying a gradient magnetic field and an RF pulse that selectively excite the imaging region and acquisition of a magnetic resonance signal from another imaging region by applying a gradient magnetic field and a RF pulse that selectively excite the another imaging region during a period until a repetition time for the imaging region is elapsed;

to acquire positional information on each of imaging regions selectively excited in the multi-slice imaging; and

to perform calculation of amplitudes of components of the eddy-current magnetic fields in an X-axis direction, a Y-axis direction and a Z-axis direction in an apparatus coordinate system for each of the imaging regions, and to correct current supplied to the gradient magnetic field coil for each of the imaging regions based on a result of the calculation.

10. The magnetic resonance imaging apparatus according to claim 1, wherein:

the positional information is information that prescribes a three-dimensional range of the imaging region.

11. The magnetic resonance imaging apparatus according to claim 1, wherein:

the positional information is a coordinate position of each vertex of an outer edge of the imaging region in an apparatus coordinate system.

12. The magnetic resonance imaging apparatus according to claim 1, wherein:

the positional information is a set of information including a coordinate of a center of the imaging region, a thickness of a slice, a cross-sectional direction of the slice, and longitudinal and lateral dimensions of the slice.

13. A control device for a magnetic resonance imaging (MRI) apparatus,

the magnetic resonance imaging apparatus being configured to (i) supply a current to at least one gradient magnetic field coil to generate a gradient magnetic field to an imaging space where an object is placed, and (ii) perform magnetic resonance imaging of the object by using the gradient magnetic field,

the control device including at least one MRI control processor configured to:

acquire an imaging region of the magnetic resonance imaging apparatus, as positional information in the imaging space; and

adjust a distribution of the gradient magnetic field to more closely approximate a target distribution by transforming a waveform of the gradient magnetic field based on the positional information and a time constant of an eddy-current magnetic field to at least partly cancel the eddy-current magnetic field, the eddy-current magnetic field being induced by an eddy current caused by application of the gradient magnetic field,

wherein said at least one MRI control processor is configured

(a) to calculate, for each direction of application of the gradient magnetic field, an amplitude of a component of the eddy-current magnetic field in an X-axis direction of an apparatus coordinate system, an amplitude of a component of the eddy-current magnetic field in a Y-axis direction of the apparatus coordinate system, and an amplitude of a component of the eddy-current magnetic field in a Z-axis direction of the apparatus coordinate system based on a distance between the imaging region and a magnetic field center obtained from the positional information, and

(b) to adjust the distribution of the gradient magnetic field to more closely approximate the target distribution by temporally varying a value of the current supplied to the gradient magnetic field coil according to the time constant of the eddy-current magnetic field and a result of the calculations.

14. A magnetic resonance imaging (MRI) method comprising:

acquiring an imaging region as positional information in an MR imaging space where an object is placed;

adjusting, as a correction step, a distribution of a gradient magnetic field to more closely approximate a target distribution by transforming a waveform of the gradient magnetic field based on the positional information and a time constant of an eddy-current magnetic field to at least partly cancel the eddy-current magnetic field, the eddy-current magnetic field being induced by an eddy current caused by application of the gradient magnetic field;

acquiring a magnetic resonance signal produced from the object by applying the gradient magnetic field transformed in the correction step and an RF pulse in the imaging space; and

reconstructing image data on the object based on the magnetic resonance signal,

wherein said correction step comprises

(a) calculating, for each direction of application of the gradient magnetic field, an amplitude of a component of the eddy-current magnetic field in an X-axis direction of an apparatus coordinate system, an amplitude of a component of the eddy-current magnetic field in a Y-axis direction of the apparatus coordinate system, and an amplitude of a component of the eddy-current magnetic field in a Z-axis direction of the apparatus coordinate system based on a distance between the imaging region and a magnetic field center obtained from the positional information, and

(b) adjusting the distribution of the gradient magnetic field to more closely approximate the target distribution by temporally varying a value of the current supplied to the gradient magnetic field coil according to the time constant of the eddy-current magnetic field and a result of the calculations.