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

Patent metadata

Field	Value
Publication number	US-11175367-B2
Application number	US-201816101199-A
Country	US
Kind code	B2
Filing date	Aug 10, 2018
Priority date	Aug 10, 2018
Publication date	Nov 16, 2021
Grant date	Nov 16, 2021

Abstract

Various methods and systems are provided for correcting transmit attenuation of an amplifier of a transmit radio frequency (RF) coil for use in a magnetic resonance imaging (MRI) system. In one example, a method includes setting a reference value of transmit attenuation for an amplifier of a transmit radio frequency (RF) coil, acquiring a two-dimensional B1 field map with the transmit attenuation set at the reference value, determining a mean flip angle from the B1 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 value.

First claim

The invention claimed is:

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.

Assignees

• Gen Electric

Inventors

• Venkatachari Anand Kumar
• Sun Ling
• Sun Wei

Classifications

• G01R33/586 Primary

  for optimal flip angle of RF pulses · CPC title

• G01R33/5659

  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) · CPC title

• A61B5/055 Primary

  involving electronic [EMR] or nuclear [NMR] magnetic resonance, e.g. magnetic resonance imaging · CPC title

• G01R33/246 Primary

  Spatial mapping of the RF magnetic field B1 · CPC title

• G01R33/3415

  comprising arrays of sub-coils {, i.e. phased-array coils with flexible receiver channels} · CPC title