B1 Mapping by Bloch-Siegert Shift

Laura Sacolick¹, Florian Wiesinger¹, W. Thomas Dixon², Ileana Hancu², Mika W. Vogel¹

Here we present a novel method for B1+ field mapping based on the Bloch-Siegert shift. The Bloch-Siegert shift refers to the effect where the resonance frequency of a nucleus shifts when an off-resonance RF field is applied. This shift is proportional to the square root of the RF field magnitude B12. An off-resonance RF pulse is added to an imaging sequence following spin excitation. This pulse induces a B1 dependent phase in the acquired image. A B1 map is calculated from the square of the phase difference between two images, with the RF pulse applied at two frequencies symmetrically around the water resonance. In-vivo Bloch-Siegert B1+ maps with 25.6 seconds/ 128x128 slice were found to be quantitatively comparable to 13 minute conventional double-angle maps. The method can be integrated into a wide variety of fast imaging sequences, and is compatible with EPI, alternative readout trajectories, receive array acceleration, etc. Insensitivity to B0, chemical shift, TR, T1, and magnetization transfer is shown as well.