B 1 mapping for bias‐correction in quantitative T 1 imaging of the brain at 3T using standard pulse sequences

Purpose B 1 mapping is important for many quantitative imaging protocols, particularly those that include whole‐brain T 1 mapping using the variable flip angle (VFA) technique. However, B 1 mapping sequences are not typically available on many magnetic resonance imaging (MRI) scanners. The aim of this work was to demonstrate that B 1 mapping implemented using standard scanner product pulse sequences can produce B 1 (and VFA T 1 ) maps comparable in quality and acquisition time to advanced techniques. Materials and Methods Six healthy subjects were scanned at 3.0T. An interleaved multislice spin‐echo echo planar imaging double‐angle (EPI‐DA) B 1 mapping protocol, using a standard product pulse sequence, was compared to two alternative methods (actual flip angle imaging, AFI, and Bloch‐Siegert shift, BS). Single‐slice spin‐echo DA B 1 maps were used as a reference for comparison (Ref. DA). VFA flip angles were scaled using each B 1 map prior to fitting T 1 ; the nominal flip angle case was also compared. Results The pooled‐subject voxelwise correlation ( ρ ) for B 1 maps (BS/AFI/EPI‐DA) relative to the reference B 1 scan (Ref. DA) were ρ  = 0.92/0.95/0.98. VFA T 1 correlations using these maps were ρ  = 0.86/0.88/0.96, much better than without B 1 correction ( ρ  = 0.53). The relative error for each B 1 map (BS/AFI/EPI‐DA/Nominal) had 95 th percentiles of 5/4/3/13%. Conclusion Our findings show that B 1 mapping implemented using product pulse sequences can provide excellent quality B 1 (and VFA T 1 ) maps, comparable to other custom techniques. This fast whole‐brain measurement (∼2 min) can serve as an excellent alternative for researchers without access to advanced B 1 pulse sequences. Level of Evidence: 1 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2017;46:1673–1682.