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

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

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

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

determining a plurality of mean flip angles for a plurality of slice locations in a pre-scan imaging volume from the B 1 field map;

determining a transmit attenuation correction value for each of the slice locations based on a prescribed flip angle and the mean flip angle determined for the respective slice location;

correcting the reference value of transmit attenuation with the transmit attenuation correction value at each of the slice locations to obtain a final value of transmit attenuation for each of the slice locations; and

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

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

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

deriving the three-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 for a slice location 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 the 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 voxel location of the pre-scan imaging volume, the pre-scan imaging volume located in same area as a prescription scan location.

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 the mask is shaped to match the size and shape of a bore of the MM system.

9. The method of claim 1 , wherein the mask is a localized mask based on area of interest.

10. 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;

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

determine a plurality of mean flip angles for a plurality of slice locations in a pre-scan imaging volume from the B 1 field map;

determine a transmit attenuation correction value for each of the slice locations based on a prescribed flip angle and the mean flip angle determined for the respective slice location;

correct the reference value of transmit attenuation with the transmit attenuation correction value at each of the slice locations to obtain a final value of transmit attenuation for each of the slice locations; and

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

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

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

13. The system of claim 10 , wherein the reference value of transmit attenuation value is set based on one or more anatomical features of an imaging subject to be scanned.

14. The system of claim 10 , wherein determining the mean flip angle for a slice location 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.

15. The system of claim 10 , wherein the B 1 field map represents a strength of a B 1 field at each voxel location of the pre-scan imaging volume, the pre-scan imaging volume located in same area as a prescription scan location.

16. The system of claim 10 , 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.

17. A method for a magnetic resonance imaging (MM) system, comprising:

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

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

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

determining a transmit attenuation correction value based on a prescribed flip angle and the mean 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.

18. The method of claim 17 , wherein the B 1 field map comprises a two dimensional (2D) B 1 field map or a three dimensional (3D) B 1 field map.

19. The method of claim 17 , wherein acquiring the B 1 field map comprises acquiring the B 1 field map during the 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.

20. The method of claim 19 , wherein the imaging plane is located in a center of a bore of the MM system or in an off-center location of the bore of the MM system based on user prescription.