MAGNETIC RESONANACE IMAGING APPARATUS AND METHOD FOR MEASURING IRRADIATION MAGNETIC FIELD

• US 20130207653A1
• Filed: 10/13/2011
• Published: 08/15/2013
• Est. Priority Date: 11/05/2010
• Status: Active Grant

• Current Assignee: FUJIFILM Healthcare Corp. (Fujifilm Holdings Corporation)
• Original Assignee: Hitachi, Ltd.
• Inventors: Ito, Kosuke, Takizawa, Masahiro
• Granted Patent: US 9,400,319 B2
• Time in Patent Office: Days
• US Class Current: 324/309
• CPC Class Codes: A61B 5/055 involving electronic [EMR] or nuclear [NMR] magnetic resonance, e.g. magnetic resonance imaging
• CPC Class Codes: G01R 33/246 Spatial mapping of the RF magnetic field B1
• CPC Class Codes: G01R 33/443 Assessment of an electric or a magnetic field, e.g. spatial mapping, determination of a B0 drift or dosimetry
• CPC Class Codes: G01R 33/56 Image enhancement or correction, e.g. subtraction or averaging techniques , e.g. improvement of signal-to-noise ratio and resolution
• CPC Class Codes: G01R 33/5659 caused by a distortion of the RF magnetic field, e.g. spatial inhomogeneities of the RF magnetic field G01R33/56509, G01R33/56518, G01R33/56536 take precedence

Abstract

There is provided an MRI apparatus capable of measuring the B1 distribution of an RF transmission coil in a short time with high accuracy. In order to realize this, imaging means of the MRI apparatus includes a B1 distribution measurement sequence that includes an application of a pre-pulse by RF radiation means and a plurality of signal acquisition sequences with different elapsed time (TI) from the pre-pulse. The signal acquisition sequence uses a pulse having a small flip angle as an RF pulse and is executed before the longitudinal relaxation after the pre-pulse ends. Calculation means calculates the B1 distribution of the RF radiation means using image data with different TI acquired in the respective signal acquisition sequences.

Claims

1. A magnetic resonance imaging apparatus comprising:

an RF radiation unit that radiates a high-frequency magnetic field (B1) to cause nuclear magnetic resonance in an object;

an imaging unit that images the object using a B1 distribution measurement sequence that includes a signal acquisition sequence to acquire a nuclear magnetic resonance signal by setting elapsed time (TI) from application of a high-frequency magnetic field pre-pulse; and

a calculation unit that reconstructs an image of the object using the nuclear magnetic resonance signal,

wherein the calculation unit calculates an irradiation magnetic field distribution of the RF radiation unit using the image, and

the calculation unit calculates the irradiation magnetic field distribution using a plurality of images with the different elapsed time.

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

wherein the B1 distribution measurement sequence includes a plurality of signal acquisition sequences with the different elapsed time, and

the calculation unit reconstructs the plurality of images with the different elapsed time using nuclear magnetic resonance signals acquired in the plurality of signal acquisition sequences with the different elapsed time.

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

wherein the imaging unit acquires images by executing a plurality of B1 distribution measurement sequences in which application conditions of the high-frequency magnetic field pre-pulse are different, and

the calculation unit calculates the irradiation magnetic field distribution using images acquired in the plurality of B1 distribution measurement sequences in which the application conditions of the high-frequency magnetic field pre-pulse are different.

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

wherein among the plurality of B1 distribution measurement sequences, a first B1 distribution measurement sequence is a signal acquisition sequence executed without using the high-frequency magnetic field pre-pulse, and a second B1 distribution measurement sequence includes an application of the high-frequency magnetic field pre-pulse and at least two signal acquisition sequences subsequent to the application of the high-frequency magnetic field pre-pulse.

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

wherein the calculation unit calculates the irradiation magnetic field distribution by solving a determinant using a ratio between a signal intensity of an image, which is obtained by reconstructing a nuclear magnetic resonance signal acquired in a signal acquisition sequence of the first B1 distribution measurement sequence, and each signal intensity of a plurality of images, which are obtained by reconstructing nuclear magnetic resonance signals acquired in a plurality of signal acquisition sequences of the second B1 distribution measurement sequence.

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

wherein the imaging unit executes the B1 distribution measurement sequence for each of a plurality of cross-sections, and

the calculation unit calculates the irradiation magnetic field distribution for each of the plurality of cross-sections.

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

wherein the signal acquisition sequence is a multi-slice imaging sequence or a three-dimensional imaging sequence.

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

wherein the imaging unit includes a plurality of the B1 distribution measurement sequences in which application conditions of the high-frequency magnetic field pre-pulse are the same, and separately acquires all pieces of data required to reconstruct the images in the plurality of B1 distribution measurement sequences.

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

wherein the imaging unit executes the B1 distribution measurement sequence multiple times while changing the elapsed time, and

the calculation unit reconstructs the plurality of images with the different elapsed time using nuclear magnetic resonance signals acquired in the respective B1 distribution measurement sequences.

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

wherein the high-frequency magnetic field pre-pulse is a high-frequency magnetic field pulse having a flip angle of 90°

or more.

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

wherein, in the signal acquisition sequence, a high-frequency magnetic field pulse having a flip angle of 10°

or less is repeatedly applied to acquire the nuclear magnetic resonance signal.

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

wherein, in the imaging unit, the plurality of signal acquisition sequences are continuous, and the high-frequency magnetic field pulse is applied at a fixed repetition time.

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

wherein the signal acquisition sequence is a pulse sequence to acquire the nuclear magnetic resonance signal by scanning k-space radially.

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

wherein the RF radiation unit has a plurality of channels,

the imaging unit images the object by executing the B1 distribution measurement sequence for each of the channels, and

the calculation unit calculates the irradiation magnetic field distribution for each of the channels.

13-14. -14. (canceled)

16. (canceled)

18. A method for measuring the irradiation magnetic field distribution of an RF radiation unit in a magnetic resonance imaging apparatus including an RF radiation unit, the irradiation magnetic field measuring method comprising:

a measurement step of executing a signal acquisition sequence to acquire a nuclear magnetic resonance signal by setting elapsed time (TI) from application of a high-frequency magnetic field pre-pulse by the RF radiation unit;

an image reconstruction step of reconstructing an image of the object using the nuclear magnetic resonance signal; and

an irradiation magnetic field distribution calculation step of calculating the irradiation magnetic field distribution of the RF radiation unit using the image,

wherein, in the irradiation magnetic field distribution calculation step, the irradiation magnetic field distribution is calculated using a plurality of images with the different elapsed time.

19-20. -20. (canceled)