title: SNR efficiency of combined bipolar gradient echoes: Comparison of three-dimensional FLASH, MPRAGE, and multiparameter mapping with VFA-FLASH and MP2RAGE.
abstract: 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).
  
  
  
  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.
  
  
  
  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.
  
  
  
  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.
authors.affiliation: Department of Oncology, University of Alberta, Edmonton, Alberta, Canada.
authors.forename: Jean-David
authors.initials: JD
authors.lastname: Jutras
authors.affiliation: Department of Oncology, University of Alberta, Edmonton, Alberta, Canada.|Department of Medical Physics, Cross Cancer Institute, Edmonton, Alberta, Canada.
authors.forename: Keith
authors.initials: K
authors.lastname: Wachowicz
authors.affiliation: MR Clinical Science, Philips Healthcare Canada, Markham, Ontario, Canada.
authors.forename: Guillaume
authors.initials: G
authors.lastname: Gilbert
authors.affiliation: Department of Oncology, University of Alberta, Edmonton, Alberta, Canada.|Department of Medical Physics, Cross Cancer Institute, Edmonton, Alberta, Canada.
authors.forename: Nicola
authors.initials: N
authors.lastname: De Zanche
mesh_terms: D000465:Algorithms; D001921:Brain / Q000033:anatomy & histology*; D006801:Humans; D007089:Image Enhancement / Q000379:methods*; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D019047:Phantoms, Imaging; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D012815:Signal Processing, Computer-Assisted; D059629:Signal-To-Noise Ratio
keywords: 
  T2*; FLASH; MP2RAGE; MPRAGE; SNR efficiency; T1; Variable Flip Angle; bipolar echoes; high resolution brain MRI; multiparameter mapping; proton density