Magnetic resonance imaging apparatus and irradiation magnetic field distribution measurement method

1. A magnetic resonance imaging apparatus comprising:

an imaging unit which includes a transmission unit configured to irradiate a high-frequency magnetic field onto an examination target and a reception unit configured to receive a nuclear magnetic resonance signal from the examination target;

an arithmetic unit which processes the nuclear magnetic resonance signal acquired by the reception unit and performs an arithmetic operation including image reconstruction; and

a control unit which controls imaging by the imaging unit,

wherein the transmission unit includes a transmission coil having multiple channels, and

the control unit has

an image acquisition sequence in which an image is acquired by irradiation with one channel or a combination of two or more of the multiple channels, and

an irradiation magnetic field distribution measurement sequence in which an irradiation magnetic field distribution upon irradiation with all of the multiple channels is measured, and

the arithmetic unit includes

a first irradiation magnetic field distribution calculation unit which calculates the irradiation magnetic field distribution of all of the multiple channels using data acquired in the irradiation magnetic field distribution measurement sequence, and

a second irradiation magnetic field distribution calculation unit which calculates the irradiation magnetic field distribution of each channel of the multiple channels using multiple pieces of image data acquired in the image acquisition sequence and the irradiation magnetic field distribution of all of the multiple channels calculated by the first irradiation magnetic field distribution calculation unit, and

wherein the control unit has additionally a second image acquisition sequence in which an image of an object is acquired, and

the arithmetic unit includes a shimming unit which calculates a set of amplitude and phase of a high-frequency magnetic field to be irradiated in the second image acquisition sequence, using the irradiation magnetic field distribution calculated by the second irradiation magnetic field distribution calculation unit, for each channel of the multiple channels.

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

wherein the second irradiation magnetic field distribution calculation unit calculates the irradiation magnetic field distribution of each channel using phase of an overall image acquired in one image acquisition sequence by irradiation with all of the multiple channels, phase of a partial irradiation image acquired in another image acquisition sequence by irradiation with some channels of the multiple channels, and the irradiation magnetic field distribution calculated by the first irradiation magnetic field distribution calculation unit.

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

wherein the image acquisition sequence and the irradiation magnetic field distribution measurement sequence are the same pulse sequence.

4. The magnetic resonance imaging apparatus according to claim 1 ,

wherein the irradiation magnetic field distribution measurement sequence is one of the pulse sequences based on a double angle method (DAM), a fitting method, and an actual flip angle method (AFI).

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

wherein the irradiation magnetic field distribution measurement sequence includes application of a high-frequency magnetic field pre-pulse by the transmission unit and multiple signal acquisition sequences having different elapsed times from the application of the high-frequency magnetic field pre-pulse.

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

wherein the image acquisition sequence includes a pulse sequence in which irradiation with one channel of the multiple channels is performed, and the pulse sequence is repeated a number of times as a number of the multiple channels while changing the channels for use in irradiation, and

the second irradiation magnetic field distribution calculation unit calculates the irradiation magnetic field distribution of each channel using the image acquired for each channel and the image acquired by irradiation with all of the multiple channels.

7. The magnetic resonance imaging apparatus according to claim 1 ,

wherein the image acquisition sequence includes a pulse sequence in which irradiation with all other channels excluding one channel of the multiple channels is performed, and the pulse sequence is repeated the same number of times as the number of channels while changing a channel to be excluded, and

the second irradiation magnetic field distribution calculation unit calculates, using an image acquired with the pulse sequence excluding one channel and an overall image acquired in an image acquisition sequence by irradiation with all of the multiple channels, phase difference between phase of the image acquired with the pulse sequence excluding one channel and phase of the overall image and calculates the irradiation magnetic field distribution of each channel using the phase difference and the irradiation magnetic field distribution calculated by the first irradiation magnetic field distribution calculation unit.

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

wherein the image acquisition sequence has a sequence in which, when the multiple channels are divided into two groups and division is repeated until a number of channels after division becomes one, multiple images by irradiation using the channel groups and all or some of the channels of each division stage are acquired, and

the second irradiation magnetic field distribution calculation unit calculates the irradiation magnetic field distribution of each channel using image data of the channel groups and image data of the channels.

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

wherein the arithmetic unit includes a determination unit which determines that a difference between (i) a phase difference between the phase of the partial irradiation image of said some channels and the phase of the overall image and (ii) a phase difference between phase of a partial irradiation image of remaining channels, other than said some channels, amongst the multiple channels, and the phase of the overall image is equal to or greater than, or is smaller than a predetermined threshold value for each pixel, and recomputation of the irradiation magnetic field distribution is performed based on a determination result of the determination unit.

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

wherein, when the determination unit determines that the difference between the phase differences is smaller than the predetermined threshold value, the control unit repeats imaging by the imaging unit, and the arithmetic unit performs recomputation of irradiation magnetic field strength for pixels in which the difference between the phase differences is determined to be smaller than the predetermined threshold value.

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

wherein, when the determination unit determines that the difference between the phase differences is smaller than the predetermined threshold value, the arithmetic unit changes a combination of channels of a partial image used in computation of the irradiation magnetic field distribution and performs recomputation of irradiation magnetic field strength for pixels in which the difference between the phase differences is determined to be smaller than the predetermined threshold value.

12. The magnetic resonance imaging apparatus according to claim 1 , wherein the arithmetic unit includes an image synthesizing unit which synthesizes an image acquired from data of all of the multiple channels using multiple images each acquired by irradiation with some channels.

13. The magnetic resonance imaging apparatus according to claim 1 , wherein the control unit executes the image acquisition sequence immediately before the irradiation magnetic field measurement sequence.

14. The magnetic resonance imaging apparatus according to claim 1 , wherein the control unit executes the image acquisition sequence after TR of the irradiation magnetic field measurement sequence.

15. A method of measuring an irradiation magnetic field distribution of a transmission coil having plural channels of a magnetic resonance imaging apparatus, the method comprising:

an image acquisition step of, for multiple channels which are all or some of the plural channels of the transmission coil, performing irradiation using one channel or channels excluding at least one channel to acquire image data;

an irradiation magnetic field distribution acquisition step of acquiring an irradiation magnetic field distribution upon irradiation using all of the multiple channels;

an irradiation magnetic field distribution calculation step of calculating the irradiation magnetic field distribution of each channel of the multiple channels using image data acquired in the image acquisition step and the irradiation magnetic field distribution acquired in the irradiation magnetic field distribution acquisition step;

a second calculation step of calculating, using the irradiation magnetic field distribution calculated in the irradiation magnetic field distribution calculation step, at least one of amplitude and phase of a high-frequency magnetic field to be irradiated with each channel of the multiple channels; and

a second image acquisition step of acquiring another image, by irradiating, with each channel of the multiple channels, the high-frequency magnetic field having said at least one of amplitude and phase calculated in the second calculation step.

16. The method according to claim 15 , further comprising:

a step of performing irradiation using all of the multiple channels to acquire image data of all channels or a step of synthesizing image data of all channels using image data acquired in the image acquisition step.