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.