B1 Correction Map

Class: NodeBOneCorrection

Node Icon

Calculates a B1 correction map (i.e. a correction of the transmit \(B_1^+\) map) using the double angle method. The nominal flip angles are automatically dected from the metadata and the second angle must be twice as large as the first. The method requires gradient echo images (\(S_1\) and \(S_2\)) with long repetition time, i.e. \(TR \gg T1\) and that the echo time is the same for both images. When these conditions are fullfilled the actual flip-angle can be calculated as [1]:

\[ \begin{equation} FA = \arccos\left( \frac{S_2}{2S_1}\right). \end{equation} \]

The output is a B1 correction map meaning that the map represents a scaling of flip angles and a value of 1 indicates that the nominal FA is achieved.

Example Workflows

B1 example

Inputs

Angle 1

Gradient echo image with nominal \(FA = \alpha\).

MetaData: Requires that the field FlipAngle is present.

Type: Image4DFloat, Required, Single

Angle 2

Gradient echo image with nominal \(FA = 2\alpha\).

MetaData: Requires that the field FlipAngle is present.

Type: Image4DFloat, Required, Single

Outputs

B1 Map

B1 correction map. The values in the map are relative to the nominal flip angle. For example if the actual produced flip angle in a voxel is 8° while the nominal flip angle is 10°, the B1 correctgion value is 0.8 in this voxel.

Type: Image4DFloat

References

1. Wang, W. Mao, M. Qiu, M. B. Smith, and R. T. Constable, “Factors Influencing Flip Angle Mapping in MRI : RF Pulse and B 0 Inhomogeneities Measurement of Relative Flip Angles,” Magn. Reson. Med., vol. 56, no. 2, pp. 463–468, 2006

Keywords: B1 map, B1 correction, B1, Dubble angle method