Methods and systems for estimating transmit attenuation for a magnetic resonance imaging scan

1. A method for a magnetic resonance imaging (MRI) system, comprising:

setting a reference value of transmit attenuation for an amplifier of a transmit radio frequency (RF) coil;

acquiring a two-dimensional B 1 field map with the transmit attenuation set at the reference value;

determining a mean flip angle from the B 1 field map by averaging values from the B 1 field map;

determining a transmit attenuation correction value based on a ratio of the mean flip angle to a prescribed flip angle;

correcting the reference value of transmit attenuation with the transmit attenuation correction value to obtain a final value of transmit attenuation; and

performing an MRI scan with the transmit attenuation set at the final value.

2. The method of claim 1 , wherein acquiring the two-dimensional B 1 field map comprises:

acquiring a two-dimensional map of magnetic resonance (MR) signal phase shift during a pre-scan; and

deriving the two-dimensional B 1 field map by converting the MR signal phase shift into corresponding B 1 field strength.

3. The method of claim 2 , wherein the map of MR signal phase shift is acquired by using Bloch-Siegert shift.

4. The method of claim 1 , wherein the reference value of transmit attenuation is set based on one or more anatomical features of an imaging subject to be scanned.

5. The method of claim 1 , wherein determining the mean flip angle from the B 1 field map comprises determining a mean B 1 field strength represented in the B 1 field map and determining the mean flip angle from the mean B 1 field strength.

6. The method of claim 1 , wherein acquiring the B 1 field map comprises acquiring the B 1 field map during a pre-scan performed prior to the MRI scan, and wherein the B 1 field map represents a strength of a B 1 field at each pixel location of an imaging plane, the imaging plane located in a center of a bore of the MRI system.

7. The method of claim 1 , further comprising applying a mask to the B 1 field map to generate a masked B 1 field map, and determining the mean flip angle from the masked B 1 field map.

8. The method of claim 1 , wherein determining the transmit attenuation correction value based on the ratio of the mean flip angle to the prescribed flip angle comprises determining the transmit attenuation correction value based on a logarithm of the ratio of the mean flip angle to the prescribed flip angle.

9. A magnetic resonance imaging (MRI) system, comprising:

a transmit radio frequency (RF) coil;

an amplifier configured to drive the transmit RF coil; and

a controller coupled to the transmit RF coil and the amplifier, the controller configured to:

set a reference value of transmit attenuation for the amplifier based on one or more anatomical features of an imaging subject to be scanned;

acquire a two-dimensional B 1 field map with the transmit attenuation set at the reference value;

determine a mean flip angle from the B 1 field map;

determine a transmit attenuation correction value based on a ratio of the mean flip angle to a prescribed flip angle;

correct the reference value of transmit attenuation with the transmit attenuation correction value to obtain a final value of transmit attenuation; and

perform an imaging scan with the transmit attenuation set at the final value.

10. The system of claim 9 , wherein the two-dimensional B 1 field map is acquired by acquiring a two-dimensional map of magnetic resonance (MR) signal phase shift during a pre-scan, and deriving the two-dimensional B 1 field map by converting the MR signal phase shift into corresponding B 1 field strength.

11. The system of claim 10 , wherein the map of MR signal phase shift is acquired by using Bloch-Siegert shift.

12. The system of claim 9 , wherein determining the mean flip angle from the B 1 field map comprises determining a mean B 1 field strength represented in the B 1 field map and determining the mean flip angle from the mean B 1 field strength.

13. The system of claim 9 , wherein the B 1 field map represents a strength of a B 1 field at each pixel location of an imaging plane, the imaging plane located in a center of a bore of the MRI system.

14. The system of claim 9 , wherein the controller is further configured to apply a mask to the B 1 field map to generate a masked B 1 field map, and determine the mean flip angle from the masked B 1 field map.

15. A non-transitory computer-readable medium comprising instructions that, when executed, cause a processor to:

set a reference value of transmit attenuation for an amplifier of a transmit radio frequency (RF) coil of a magnetic resonance imaging (MRI) device;

operate the MRI device to acquire a two-dimensional B 1 field map with the transmit attenuation set at the reference value;

determine a mean flip angle from the B 1 field map by averaging values from the B 1 field map;

determine a transmit attenuation correction value based on a ratio of the mean flip angle to a prescribed flip angle;

correct the reference value of transmit attenuation with the transmit attenuation correction value to obtain a final value of transmit attenuation; and

operate the MRI device to perform an imaging scan with the transmit attenuation set at the final value.

16. The computer-readable medium of claim 15 , wherein to acquire the two-dimensional B 1 field map, the instructions, when executed, cause the processor to operate the MRI device to:

acquire a two-dimensional map of magnetic resonance (MR) signal phase shift during a pre-scan; and

derive the two-dimensional B 1 field map by converting the MR signal phase shift into corresponding B 1 field strength.

17. The computer-readable medium of claim 16 , wherein the map of MR signal phase shift is acquired by using Bloch-Siegert shift.

18. The computer-readable medium of claim 15 , wherein the reference value of transmit attenuation is set based on one or more anatomical features of an imaging subject to be scanned.

19. The computer-readable medium of claim 15 , wherein determining the mean flip angle from the B 1 field map comprises determining a mean B 1 field strength represented in the B 1 field map and determining the mean flip angle from the mean B 1 field strength.

20. The computer-readable medium of claim 15 , wherein the B 1 field map is acquired during a pre-scan performed prior to the imaging scan, and wherein the B 1 field map represents a strength of a B 1 field at each pixel location of an imaging plane, the imaging plane located in a center of a bore of the MRI device.