B1

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Calculate 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]:

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

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.

Inputs

α

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

Type: Image, Required, Single

2α

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

Type: Image, Required, Single

Mask

Binary mask that defines the region in which the B1 map is calculated.

Type: Mask, Optional, Single

Outputs

B1

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: Image

Settings

Metadata

Flip Angle Source Selection

Select the Flip Angle source for the calculation. The Flip Angle can be taken from the metadata, or can be supplied as an input Numeric Array.

Values: Metadata, Numeric Array Input

Flip Angle Tag Text

Name of the Flip Angle tag in the metadata.

B1