MAGNETIC RESONANCE IMAGING METHOD, MAGNETIC RESONANCE IMAGING APPARATUS, AND CONTROL DEVICE OF MAGNETIC RESONANCE IMAGING APPARATUS

• US 20120019251A1
• Filed: 07/21/2011
• Published: 01/26/2012
• 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
• Granted Patent: US 8,890,530 B2
• Time in Patent Office: Days
• US Class Current: 324/322
• 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 apparatus that includes “

a signal acquisition unit that 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” and

“

an image generating unit that reconstructs image data on the object based on the magnetic resonance signal”

, comprising;

a position acquiring unit that acquires an imaging region as positional information in the imaging space; and

a correction unit that 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, the eddy-current magnetic field being induced by an eddy current caused by application of the gradient magnetic field.

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

a static magnetic field magnet that produces a static magnetic field; and

a gradient magnetic field coil that produces the gradient magnetic field according to a current supplied thereto;

wherein the correction unit is 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 coil and the static magnetic field magnet” and

“

a shape and material of the gradient magnetic field coil”

, 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, further comprising:

a gradient magnetic field coil that produces the gradient magnetic field according to a current supplied thereto;

wherein the correction unit is configured to transform the waveform of the gradient magnetic field by correcting a current supplied to the gradient magnetic field coil by the signal acquisition unit.

4. The magnetic resonance imaging apparatus according to claim 3, wherein the correction unit is configured:

to perform an operational processing 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 in an apparatus coordinate system, an amplitude of a component of the eddy-current magnetic field in a Y-axis direction in the apparatus coordinate system, and an amplitude of a component of the eddy-current magnetic field in a Z-axis direction in the apparatus coordinate system based on a distance between the imaging region and a magnetic field center obtained from the positional information, and

to correct the current supplied to the gradient magnetic field coil based on a result of the operational processing.

5. The magnetic resonance imaging apparatus according to claim 4;

wherein the correction unit is configured to bring the distribution of the gradient magnetic field close to the target distribution “

by temporally varying a value of the current supplied to the gradient magnetic field coil by the signal acquisition unit”

according to the time constant of the eddy-current magnetic field.

6. The magnetic resonance imaging apparatus according to claim 4;

wherein the correction unit is configured to acquire a center position of the imaging region based on the positional information, and to perform the operational processing based on a distance between the center position and the magnetic field center.

7. The magnetic resonance imaging apparatus according to claim 6;

wherein the correction unit is configured to bring the distribution of the gradient magnetic field close to the target distribution “

by temporally varying a value of the current supplied to the gradient magnetic field coil by the signal acquisition unit”

according to the time constant of the eddy-current magnetic field.

8. The magnetic resonance imaging apparatus according to claim 6;

wherein the signal acquisition unit is configured to acquire the magnetic resonance signal from a plurality of imaging regions corresponding to a plurality of images respectively;

the position acquiring unit is configured to acquire the plurality of imaging regions as a plural sets of positional information; and

the correction unit is configured 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 4;

wherein the correction unit is configured to perform the operational processing so as to calculate the amplitudes of the components of the eddy-current magnetic field in the X-axis direction, the Y-axis direction and the Z-axis direction in the 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.

10. The magnetic resonance imaging apparatus according to claim 9;

wherein the correction unit is configured to bring the distribution of the gradient magnetic field close to the target distribution “

by temporally varying a value of the current supplied to the gradient magnetic field coil by the signal acquisition unit”

according to the time constant of the eddy-current magnetic field.

11. The magnetic resonance imaging apparatus according to claim 9;

wherein the signal acquisition unit is configured to acquire the magnetic resonance signal from a plurality of imaging regions corresponding to a plurality of images respectively;

the position acquiring unit is configured to acquire the plurality of imaging regions as a plural sets of positional information; and

the correction unit is configured to correct the current supplied to the gradient magnetic field coil for each of the imaging regions.

12. The magnetic resonance imaging apparatus according to claim 3;

wherein the signal acquisition unit is configured to acquire the magnetic resonance signal from a plurality of imaging regions corresponding to a plurality of images respectively;

the position acquiring unit is configured to acquire the plurality of imaging regions as plural sets of positional information; and

the correction unit is configured to correct the current supplied to the gradient magnetic field coil for each of the imaging regions.

13. The magnetic resonance imaging apparatus according to claim 3;

wherein the signal acquisition unit 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”

;

the position acquiring unit is configured to acquire positional information on each of imaging regions selectively excited in the multi-slice imaging; and

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

14. The magnetic resonance imaging apparatus according to claim 3;

wherein the correction unit is configured to bring the distribution of the gradient magnetic field close to the target distribution “

by temporally varying a value of the current supplied to the gradient magnetic field coil by the signal acquisition unit”

according to the time constant of the eddy-current magnetic field.

15. 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.

16. 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.

17. 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.

18. A control device for a magnetic resonance imaging apparatus, the magnetic resonance imaging apparatus supplying a current to a gradient magnetic field coil to apply a gradient magnetic field to an imaging space where an object is placed, and performing magnetic resonance imaging of the object by using the gradient magnetic field, the control device comprising:

a position acquiring unit that acquires an imaging region of the magnetic resonance imaging, as positional information in the imaging space; and

a correction unit that 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, the eddy-current magnetic field being induced by an eddy current caused by application of the gradient magnetic field.

19. A magnetic resonance imaging method comprising the steps of:

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

bringing, as a correction step, a distribution of a 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, 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.