Fast radiofrequency flip angle calibration by Bloch–Siegert shift

In a recent work, we presented a novel method for B   1 +field mapping based on the Bloch–Siegert shift. Here, we apply this method to automated fast radiofrequency transmit gain calibration. Two off‐resonance radiofrequency pulses were added to a slice‐selective spin echo sequence. The off‐resonance pulses induce a Bloch–Siegert phase shift in the acquired signal that is proportional to the square of the radiofrequency field magnitude B 1 2 . The signal is further spatially localized by a readout gradient, and the signal‐weighted average B 1 field is calculated. This calibration from starting system transmit gain to average flip angle is used to calculate the transmit gain setting needed to produce a desired imaging sequence flip angle. A robust implementation is demonstrated with a scan time of 3 s. The Bloch–Siegert‐based calibration was used to predict the transmit gain for a 90° radiofrequency pulse and gave a flip angle of 88.6 ± 3.42° when tested in vivo in 32 volunteers. Magn Reson Med, 2011. © 2011 Wiley Periodicals, Inc.