SA2RAGE: A new sequence for fastB1+-mapping

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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 (BSymbol) 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 theBSymbolfield in the organ of interest. In this work, a new phase-sensitive three-dimensionalBSymbol-mapping technique is introduced that allows the acquisition of a 64 × 64 × 8BSymbol-map in ˜20 s, yielding an accurate mapping of the relativeBSymbolwith a 10-fold dynamic range (0.2–2 times the nominalBSymbol). Moreover, the predominant use of low flip angle excitations in the presented sequence minimizes specific absorption rate, which is an important asset for in vivoBSymbol-shimming procedures at high magnetic fields. The proposed methodology was validated in phantom experiments and demonstrated good results in phantom and humanBSymbol-shimming using an 8-channel transmit-receive array.

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