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

Purpose: To develop and evaluate a robust cardiac B(1)(+)mapping sequence at 3 T, using Bloch-Siegert shift (BSS)-based preparations. Methods: A longitudinal magnetization preparation module was designed toencode |B-1(+)|. After magnetization tip-down, off-resonant Fermi pulses, placed sym-metrically around two refocusing pulses, induced BSS, followed by tipping backof the magnetization. Bloch simulations were used to optimize refocusing pulseparameters and to assess the mapping sensitivity. Relaxation-induced B-1(+) errorwas simulated for various T-1/T-2 times. The effective mapping range was determined in phantom experiments, and |B-1(+)| maps were compared to the conventionalBSS 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 forthree-dimensional (3D) acquisition of the whole heart in a single breath-hold, andcompared to two-dimensional (2D) acquisition. Results: Simulations indicate that the proposed preparation can be tailored toachieve 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%. Experimentallyreproduced B-1(+) sensitization closely matched simulations for B-1(+)>= 0.3B(1,max)(+)(mean difference 0.031 +/- 0.022, compared to 0.018 +/- 0.025 in the HS8-sensitized method),and showed 20-fold reduction in the standard deviation of repeated scans, comparedwith conventional BSS B(1 )(+)mapping, and an equivalent 2-fold reduction comparedwith HS8-sensitization. Robust cardiac B-1(+) map quality was obtained, with an average test-retest variability of 0.027 +/- 0.043 relative to normalized B(1 )(+)magnitude, and plane intersection bias of 0.052 +/- 0.031. 3D acquisitions showed good agreement with 2D scans (mean absolute deviation 0.055 +/- 0.061). Conclusion: BSS-based preparations enable robust and tailorable 2D/3D cardiac B(1)(+)mapping at 3 T in a single breath-hold.