System and method of receive sensitivity correction in MR imaging

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

a magnetic resonance imaging (MRI) system having a plurality of gradient coils positioned about a bore of a magnet, and an RF transceiver system and an RF switch controlled by a pulse module to transmit RF signals to an RF coil assembly to acquire MR images; and

a computer programmed to:

generate an original image of an object;

generate a proton density weighted image of the object;

generate a B 1 + field map of the object;

generate a transmit-corrected image based on the B 1 + field map and based on the proton density weighted image;

electronically filter the transmit-corrected image to generate a receive sensitivity map; and

revise the original image using the receive sensitivity map to yield a final image.

2. The MRI apparatus of claim 1 wherein the computer is programmed to calculate a transmit sensitivity at each voxel of the B 1 + field map based on a signal equation for a Bloch-Siegert (B-S) magnitude signal, and generate the transmit-corrected image using the transmit sensitivity.

3. The MRI apparatus of claim 2 wherein the signal equation is given by:

S

n

=

R

n

⁢

M

0

⁢

sin

⁡

(

α

)

⁢

1

-

E

1

1

-

E

1

⁢

cos

⁡

(

α

)

,

where S n is a signal from an n-th coil, R n is a receive sensitivity of the n-th coil, M 0 is a proton density, α is a flip angle, and E 1 =exp(−TR/T1), wherein TR is a repetition time and T1 is a time constant pertaining to longitudinal relaxation.

4. The MRI apparatus of claim 2 wherein the signal equation is given by S n =R n M 0 sin(α), where S n is a signal from an n-th coil, R n is a receive sensitivity of the n-th coil, M 0 is a proton density, α is a flip angle.

5. The MRI apparatus of claim 1 wherein the computer is programmed to execute a B-S sequence to generate the proton density weighted image and to generate the B 1 + field map of the object.

6. The MRI apparatus of claim 5 wherein the computer is programmed to execute the B-S sequence and generate the proton density weighted image from a single B-S image derived therefrom.

7. The MRI apparatus of claim 5 wherein the computer is programmed to:

execute the B-S sequence by being programmed to obtain a positive off-resonance B-S image and a negative off-resonance B-S image;

combine the positive and the negative off-resonance B-S images to generate the proton density weighted image; and

combine the positive and negative off-resonance B-S images to generate the B 1 + field map.

8. The MRI apparatus of claim 1 wherein the computer is programmed to:

electronically filter the transmit-corrected image by low-pass filtering the transmit-corrected image having a cutoff frequency of σ=0.4 cm −1 .

9. A method of magnetic resonance imaging (MRI) comprising:

generating a proton density weighted image of an object;

determining a transmit sensitivity map from a B 1 + field map of the object;

generating a transmit-corrected image of the object based on the transmit sensitivity map and based on the proton density weighted image;

low-pass filtering the transmit-corrected image to generate a receive sensitivity map; and

correcting an MR image using the receive sensitivity map to yield a final image.

10. The method of claim 9 comprising generating the B 1 + field map of the object and generating a proton weighted image of the object using a Bloch-Siegert (B-S) mapping sequence.

11. The method of claim 10 wherein using the B-S mapping sequence comprises obtaining a positive off-resonance B-S image and a negative off-resonance B-S image; and further comprises:

combining the positive and the negative off-resonance B-S images to generate the proton density weighted image; and

generating the B 1 + field map from the positive and the negative off-resonance B-S images.

12. The method of claim 9 wherein determining the transmit sensitivity map comprises calculating a transmit sensitivity at each voxel of the B 1 + field map based on a signal equation for a B-S magnitude signal.

13. The method of claim 9 wherein low-pass filtering the transmit-corrected image comprises electronically filtering the transmit-corrected image using a cutoff frequency of σ=0.4 cm −1 .

14. A non-transitory computer readable storage medium having stored thereon a computer program comprising instructions, which, when executed by a computer, cause the computer to:

generate an original image of an object;

obtain an MR image of an object;

obtain a proton density weighted image of the object;

obtain a B 1 + field map of the object;

generate a transmit-corrected image based on the B 1 + field map and based on the proton density weighted image;

electronically filter the transmit-corrected image to generate a receive sensitivity map; and

refine the original image using the receive sensitivity map to generate a final image having reduced image artifacts.

15. The non-transitory computer readable storage medium of claim 14 wherein the computer is programmed to control a magnetic resonance imaging (MRI) system in order to:

generate the MR image of the object; and

execute a Bloch-Siegert (B-S) sequence in order to:

generate the proton density weighted image; and

generate the B 1 + field map.

16. The non-transitory computer readable storage medium of claim 15 wherein the computer is programmed to:

execute the B-S sequence by being programmed to obtain a positive off-resonance B-S image and a negative off-resonance B-S image;

derive the proton density weighted image from the positive and the negative off-resonance B-S images; and

derive the B 1 + field map from the positive and negative off-resonance B-S images.

17. The non-transitory computer readable storage medium of claim 16 wherein the proton density weighted image is an average of the positive and negative off-resonance B-S images.

18. The non-transitory computer readable storage medium of claim 14 wherein the computer is programmed to calculate a transmit sensitivity at each voxel of the B 1 + field map based on a signal equation for a Bloch-Siegert (B-S) magnitude signal.

19. The non-transitory computer readable storage medium of claim 18 wherein the signal equation is given by S n =R n M 0 sin(α), where S n is a signal from an n-th coil, R n is a receive sensitivity of the n-th coil, M 0 is a proton density, α is a flip angle.

20. The non-transitory computer readable storage medium of claim 14 wherein the computer is programmed to electronically filter the transmit-corrected image by low-pass filtering the transmit-corrected image having a cutoff frequency of σ=0.4 cm −1 .