SAR-reduced spin-echo-based Bloch–Siegert B1+ mapping: BS-SE-BURST



Fast and accurate B1+ mapping is possible using phase-based Bloch–Siegert (BS) methods. Importantly, the off-resonant pulses needed for BS B1+ mapping methods can easily be implemented in multiple MR sequences. BS-based B1+ mapping has thus been introduced for gradient echo (BS-FLASH), spin-echo (BS-SE), and Carr, Purcell, Meiboom, Gill (CPMG)-based multi-SE and turbo-SE sequences. When using SE and multi-SE/turbo-SE-based BS sequences, however, the high intrinsic specific absorption rates must be considered in clinical situations. This study introduces a fast BS B1+ mapping method based on a SE-BURST sequence (BS-SE-BURST). With SE-BURST sequences, multiple low-magnitude excitation pulses are applied prior to the refocusing pulse. Thus, multiple and different phase-encoded echoes can be acquired per excitation cycle. Compared with a SE sequence, this excitation strategy results in a similar signal-to-noise ratio (SNR) per unit time but with reduced specific absorption rate. The proposed BS-SE-BURST sequence was implemented on a conventional 3 T whole body MRI scanner and applied successfully. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.