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 degrees radiofrequency pulse and gave a flip angle of 88.6 +/- 3.42 degrees when tested in vivo in 32 volunteers. Magn Reson Med 66: 1333-1338, 2011. (C) 2011 Wiley Periodicals, Inc.