Preparation-based (Formula Presented.) mapping in the heart using Bloch–Siegert shifts

Abstract

Purpose: To develop and evaluate a robust cardiac (Formula presented.) mapping sequence at 3 T, using Bloch–Siegert shift (BSS)-based preparations. Methods: A longitudinal magnetization preparation module was designed to encode (Formula presented.). 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 (Formula presented.) error was simulated for various (Formula presented.) / (Formula presented.) times. The effective mapping range was determined in phantom experiments, and (Formula presented.) maps were compared to the conventional BSS method and subadiabatic hyperbolic-secant 8 (HS8) pulse-sensitized method. Cardiac (Formula presented.) 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. Results: Simulations indicate that the proposed preparation can be tailored to achieve high mapping sensitivity across various (Formula presented.) ranges, with maximum sensitivity at the upper (Formula presented.) range. (Formula presented.) / (Formula presented.) -induced bias did not exceed 5.2 (Formula presented.). Experimentally reproduced (Formula presented.) sensitization closely matched simulations for (Formula presented.) (mean difference 0.031 (Formula presented.) 0.022, compared to 0.018 (Formula presented.) 0.025 in the HS8-sensitized method), and showed 20-fold reduction in the standard deviation of repeated scans, compared with conventional BSS (Formula presented.) mapping, and an equivalent 2-fold reduction compared with HS8-sensitization. Robust cardiac (Formula presented.) map quality was obtained, with an average test-retest variability of 0.027 (Formula presented.) 0.043 relative to normalized (Formula presented.) magnitude, and plane intersection bias of 0.052 (Formula presented.) 0.031. 3D acquisitions showed good agreement with 2D scans (mean absolute deviation 0.055 (Formula presented.) 0.061). Conclusion: BSS-based preparations enable robust and tailorable 2D/3D cardiac (Formula presented.) mapping at 3 T in a single breath-hold.