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

Abstract

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:We show 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, we present 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. 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. It also showed greater robustness to hardware imperfections. Conclusion:The GLM-based method is recommended as a more accurate approach to BSS-based B1+ mapping.

A multi‐inversion multi‐echo spin and gradient echo echo planar imaging sequence with low image distortion for rapid quantitative parameter mapping and synthetic image contrasts

“…However, when we use the presented methods to mitigate distortion, a GRE scan may be sufficient which can reduce coil sensitivity calibration scan time to <5 s. For acquisitions that use dynamic shimming, additional B 0 maps are needed for the slice-optimized MC shimming calibration, which also adds to the overall scan time (~90 s). Recently, a Physics Calibration (PhysiCal) 31 scan consisting of a rapid multi-echo GRE Bloch-Siegert [32][33][34] acquisition has been proposed to quickly map B0, B1, and RF coil sensitivities for the whole-brain in ~10 s. These fast acquisition maps could not only be used to inform the dynamic shimming calibration, but also eliminate the need for a reference scan for coil sensitivities. Moreover, if needed, the B 0 maps can be used to distort coil sensitivities to match the EPI data to improve parallel imaging reconstruction accuracy.…”

Mitigating the impact of flip angle and orientation dependence in single compartment R2* estimates via 2‐pool modeling

“…An in‐house sequence exploiting the Bloch‐Siegert shift was used to map the effective transmitting field ($$$ {{\mathrm{B}}_1^{+}}_{\mathrm{eff}} $$$) 37 . Relevant parameters included: single echo, TE/TR = 6.77/40 ms, 14° flip angle, FOV of 256 × 256 × 192 mm 3 with 4 mm isotropic resolution, using a Fermi pulse with an off‐resonance frequency of ±2 kHz and 4‐ms duration to impart the Bloch‐Siegert phase that encodes $$$ {{\mathrm{B}}_1^{+}}_{\mathrm{eff}} $$$.…”