Robust 3D Bloch-Siegert based B1+ mapping using Multi-Echo General Linear Modelling

Purpose: Quantitative MRI applications, such as mapping the T1 time of tissue, puts high demands on the accuracy and precision of transmit field (B1+) estimation. A candidate approach to satisfy these requirements exploits the difference in phase induced by the Bloch-Siegert Shift (BSS) of two acquisitions with opposite off-resonance frequency RF pulses. Interleaving these RF pulses ensures robustness to motion and scanner drifts, however, here we demonstrate that doing so also introduces a bias in the B1+ estimates. Methods: It is shown here via simulation and experiments that the amplitude of the error depends on MR pulse sequence parameters, such as TR and RF spoiling increment, but more problematically, on the intrinsic properties, T1 and T2, of the investigated tissue. To solve these problems, a new approach to BSS-based B1+ estimation that uses a multi-echo acquisition and a general linear model (GLM) to estimate the correct BSS-induced phase is presented. Results: In line with simulations, phantom and in-vivo experiments confirmed that the GLM-based method removed the dependency on tissue properties and pulse sequence settings. Conclusion: The GLM-based method is recommended as a more accurate approach to BSS-based B1+ mapping.