SNR efficiency of combined bipolar gradient echoes: Comparison of three-dimensional FLASH, MPRAGE, and multiparameter mapping with VFA-FLASH and MP2RAGE

Purpose: High-bandwidth bipolar multiecho gradient echo sequences are increasingly popular in structural brain imaging because of reduced water-fat shifts, lower susceptibility effects, and improved signal-to-noise ratio (SNR) efficiency. In this study, we investigated the performance of three three-dimensional multiecho sequences (MPRAGE, MP2RAGE, and FLASH) with scan times < 9 min and 1-mm isotropic resolution against their single-echo, low-bandwidth counterparts at 3T. We also compared the performance of multiparameter mapping (PD, T₁ , and T2*) with bipolar multiecho MP2RAGE versus the variable flip angle technique with multiecho FLASH (VFA-FLASH).

Methods: Multiecho sequences were optimized to yield equivalent contrast and improved SNR compared with their single-echo counterparts. Theoretical SNR gains were verified with measurements in a multilayered phantom. Robust image processing pipelines extracted PD, T₁ , and T2* maps from MP2RAGE or VFA-FLASH, and the corresponding SNR was measured with varying SENSE accelerations (R = 1-5) and number of echoes (N = 1-12). All sequences were tested on four healthy volunteers.

Results: Multiecho sequences achieved SNR gains of 1.3-1.6 over single-echo sequences. MP2RAGE yielded comparable T₁ -to-noise ratio to VFA-FLASH, but significantly lower SNR (<50%) in PD and T2* maps. Measured SNR gains agreed with the theoretical predictions for SENSE accelerations ≤3.

Conclusion: Multiecho sequences achieve higher SNR efficiency over conventional single-echo sequences, despite three-fold higher sampling bandwidths. VFA-FLASH surpasses MP2RAGE in its ability to map three parameters with high SNR and 1-mm isotropic resolution in a clinically relevant scan time (∼8:30 min), whereas MP2RAGE yields lower intersubject variability in T₁ . Magn Reson Med 77:2186-2202, 2017. © 2016 International Society for Magnetic Resonance in Medicine.