At high magnetic field strengths (≥3T), the radiofrequency wavelength used in MRI is of the same order of magnitude of (or smaller than) the typical sample size, making transmit magnetic field (B 1+) inhomogeneities more prominent. Methods such as radiofrequency-shimming and transmit SENSE have been proposed to mitigate these undesirable effects. A prerequisite for such approaches is an accurate and rapid characterization of the B 1+ field in the organ of interest. In this work, a new phase-sensitive three-dimensional B 1+-mapping technique is introduced that allows the acquisition of a 64 × 64 × 8 B 1+-map in ∼20 s, yielding an accurate mapping of the relative B 1+ with a 10-fold dynamic range (0.2-2 times the nominal B 1+). Moreover, the predominant use of low flip angle excitations in the presented sequence minimizes specific absorption rate, which is an important asset for in vivo B 1+-shimming procedures at high magnetic fields. The proposed methodology was validated in phantom experiments and demonstrated good results in phantom and human B 1+-shimming using an 8-channel transmit-receive array. Magn Reson Med, 2011. © 2011 Wiley-Liss, Inc.