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 "B-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 B-mapping method is available. We use a B-map measurement in a calibration phantom as a reference and the voxel-wise histogram mode between ratios of B-maps produced from different methods to calculate determine the bias as a multiplicative correcting scale factor. Institutional implementations of four common methods of B-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 B-maps which enable more consistent values for any computations requiring flip angle correction. In addition, we present evidence that the corrected B maps, using our calibration method, are also more accurate.