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

PURPOSE

B mapping is important for many quantitative imaging protocols, particularly those that include whole-brain T mapping using the variable flip angle (VFA) technique. However, B mapping sequences are not typically available on many magnetic resonance imaging (MRI) scanners. The aim of this work was to demonstrate that B mapping implemented using standard scanner product pulse sequences can produce B (and VFA T ) 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 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 maps were used as a reference for comparison (Ref. DA). VFA flip angles were scaled using each B map prior to fitting T ; the nominal flip angle case was also compared.

RESULTS

The pooled-subject voxelwise correlation (ρ) for B maps (BS/AFI/EPI-DA) relative to the reference B scan (Ref. DA) were ρ = 0.92/0.95/0.98. VFA T correlations using these maps were ρ = 0.86/0.88/0.96, much better than without B correction (ρ = 0.53). The relative error for each B map (BS/AFI/EPI-DA/Nominal) had 95 percentiles of 5/4/3/13%.

CONCLUSION

Our findings show that B mapping implemented using product pulse sequences can provide excellent quality B (and VFA T ) 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 pulse sequences.

LEVEL OF EVIDENCE

1 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2017;46:1673-1682.