A practical method for post-acquisition reduction of bias in fast, whole-brain B1-maps.

Large consistent differences have been observed between maps of the flip angle correction factor (commonly called "B1-maps") produced with different fast methods in the human brain. We present an empirical procedure for first-order multiplicative bias correction that can be applied when more than one B1-mapping method is available. We use a B1-map measurement in a calibration phantom as a reference and the voxel-wise histogram mode between ratios of B1-maps produced from different methods to calculate determine the bias as a multiplicative correcting scale factor. Institutional implementations of four common methods of B1-mapping were assessed: Method of Slopes, FSE and EPI double angle methods (DAM), and Bloch-Siegert. In human subjects, the multiplicative bias used to correct for each of the four methods was: Method of Slopes = 1.005, FSE-DAM = 0.956, EPI-DAM = 1.080, and Bloch-Siegert = 1.128. Scaling to remove this bias between methods produces more consistent B1-maps which enable more consistent values for any computations requiring flip angle correction. In addition, we present evidence that the corrected B1 maps, using our calibration method, are also more accurate.