Robust 3D Bloch-Siegert based B1+ mapping using multi-echo general linear modeling

Quantitative MRI applications, such as mapping the T₁ time of tissue, puts high demands on the accuracy and precision of transmit field ( $B_1^{+}$ ) estimation. A candidate approach to satisfy these requirements exploits the difference in phase induced by the Bloch-Siegert frequency shift (BSS) of 2 acquisitions with opposite off-resonance frequency radiofrequency pulses. Interleaving these radiofrequency pulses ensures robustness to motion and scanner drifts; however, here we demonstrate that doing so also introduces a bias in the $B_1^{+}$ estimates.

It is shown here by means of simulation and experiments that the amplitude of the error depends on MR pulse sequence parameters, such as repetition time and radiofrequency 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 $B_1^{+}$ estimation that uses a multi-echo acquisition and a general linear model to estimate the correct BSS-induced phase is presented.

In line with simulations, phantom and in vivo experiments confirmed that the general linear model-based method removed the dependency on tissue properties and pulse sequence settings.

The general linear model-based method is recommended as a more accurate approach to BSS-based $B_1^{+}$ mapping.