B1

The B1 (RF) fields inside the scanner are not perfectly flat (homogeneous), particularly at field strengths of 3T or above. Hence in methods where the local flip-angle must be known (e.g. Relaxometry) the B1 field must be measured or fitted for. A distinction is drawn between B1+ (the transmit coil inhomogeneity) and B1- (the receive coil inhomogeneity). Generally it is B1+ that is important to know for quantitative techniques, and throughout QUIT it is expressed as a fraction - a value of 1 means the desired flip-angle was achieved in a voxel, and values lower/higher than this imply a lower/higher flip-angle. B1- is important if you are interested in measuring the Proton Density (PD / M0).

The following commands are available:

• qi afi
• qi dream
• qi b1_papp

qi afi

Calculates a B1+ map using the Actual Flip-angle Imaging method.

Example Command Line

qi afi input_file.nii.gz

Does not read any input from stdin. The input file should contain two volumes, corresponding to TR1 and TR2.

Outputs

• AFI_B1.nii.gz - The relative flip-angle map.

Important Options

• --flip, -f

  The nominal flip-angle that should have been achieved, default 55 degrees.

• --ratio, -r

  The ratio of TR2 to TR1, default 5.

• --save, -s

  Output AFI_angle.nii.gz, the actual achieved angle in each voxel.

References

• Original AFI Paper
• Optimal parameters
• Steady-State Conditions

qi dream

Calculates a B1+ map using the DREAM method.

Example Command Line

qi dream input_file.nii.gz

Does not read any input from stdin. The input file should contain two volumes, the FID and stimulated echo (STE).

Outputs

• DREAM_B1.nii.gz - The relative flip-angle map.
• DREAM_angle.nii.gz - The actual achieved angle in each voxel.

Important Options

• --alpha, -a

  The nominal flip-angle that should have been achieved, default 55 degrees.

• --order, -O

  • f - FID is the first volume, STE is second
  • s - STE is the first volume, FID is second
  • v - VST (Virtual Stimulated Echo) is the first volume, FID is second

References

• Original DREAM Paper
• Virtual Stimulated Echo

qi b1_papp

Implements the method of measuring B1- from Papp et al, which takes the ratio of an image acquired with the body coil as receiver and one with the receive coil that was used to acquire the quantitative images.

Example Command Line

qi b1_papp input_file.nii.gz

Does not read any input from stdin. The input file should contain two volumes, corresponding to the body coil image and the receive coil respectively.

Outputs

• B1minus.nii.gz - The relative receive coil intensity. Images should be divided by this to correct for receive coil profile.

References

• Papp et al