Preparation-based B   1 + mapping in the heart using Bloch-Siegert shifts

To develop and evaluate a robust cardiac $B_1^{+}$ mapping sequence at 3 T, using Bloch-Siegert shift (BSS)-based preparations.

A longitudinal magnetization preparation module was designed to encode $\left|B_1^{+}\right|$ . After magnetization tip-down, off-resonant Fermi pulses, placed symmetrically around two refocusing pulses, induced BSS, followed by tipping back of the magnetization. Bloch simulations were used to optimize refocusing pulse parameters and to assess the mapping sensitivity. Relaxation-induced $B_1^{+}$ error was simulated for various $T_1$ / $T_2$ times. The effective mapping range was determined in phantom experiments, and $\left|B_1^{+}\right|$ maps were compared to the conventional BSS method and subadiabatic hyperbolic-secant 8 (HS8) pulse-sensitized method. Cardiac $B_1^{+}$ maps were acquired in healthy subjects, and evaluated for repeatability and imaging plane intersection consistency. The technique was modified for three-dimensional (3D) acquisition of the whole heart in a single breath-hold, and compared to two-dimensional (2D) acquisition.

Simulations indicate that the proposed preparation can be tailored to achieve high mapping sensitivity across various $B_1^{+}$ ranges, with maximum sensitivity at the upper $B_1^{+}$ range. $T_1$ / $T_2$ -induced bias did not exceed 5.2 $\%$ . Experimentally reproduced $B_1^{+}$ sensitization closely matched simulations for $B_1^{+}\ge 0.3B_{1,\max }^{+}$ (mean difference 0.031 $\pm$ 0.022, compared to 0.018 $\pm$ 0.025 in the HS8-sensitized method), and showed 20-fold reduction in the standard deviation of repeated scans, compared with conventional BSS $B_1^{+}$ mapping, and an equivalent 2-fold reduction compared with HS8-sensitization. Robust cardiac $B_1^{+}$ map quality was obtained, with an average test-retest variability of 0.027 $\pm$ 0.043 relative to normalized $B_1^{+}$ magnitude, and plane intersection bias of 0.052 $\pm$ 0.031. 3D acquisitions showed good agreement with 2D scans (mean absolute deviation 0.055 $\pm$ 0.061).

BSS-based preparations enable robust and tailorable 2D/3D cardiac $B_1^{+}$ mapping at 3 T in a single breath-hold.