Non-uniformity correction of human brain imaging at high field by RF field mapping of B1+ and B1-

A new method of non-uniform image correction is proposed. Image non-uniformity is originated from the spatial distribution of RF transmission and reception fields, represented as B-1(+) and B-1(-), respectively. In our method, B-1(+) mapping was performed in vivo by a phase method. In B-1(-) mapping, images with multiple TEs were acquired with a multi-echo adiabatic spin echo (MASE) sequence which enables homogeneous excitation. By T-2 fitting of these images an M-0 map (M-0(MASE)) was obtained, in which signal intensity was expressed as the product of B-1(-) and M-0(1 - e(-TR/Tt)). The ratio of this M-MASE map to the B-1(+) map showed a similar spatial pattern in different human brains. These ratios of M-0(MASE) map to B-1(+), in 24 subjects were averaged and then fitted with a spatially polynomial function to obtain a ratio map of B-1(-)/B-1(+) (alpha). Uniform image was achieved in spin echo (SE), MASE and inversion recovery turboFLASH (IRTF) images using measured B-1(+) and calculated B-1(-) by alpha B-1(+). Water fractions in gray and white matters obtained from the M-0 images corrected by this method were in good agreement with previously reported values. From these experimental results, the proposed method of non-uniformity correction is validated at 4.7 T imaging. (C) 2011 Elsevier Inc. All rights reserved.