SA2RAGE: A new sequence for fast B 1 + ‐mapping

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.