Improved Bloch-Siegert Based B1 Mapping by Reducing Off-Resonance Shift

Qi Duan¹, Peter van Gelderen¹, Jeff H. Duyn¹

This abstract investigates fast simultaneous B₀/B₁⁺ mapping by Bloch-Siegert shift via lowering the off-resonance frequency of this pulse, as theoretical analysis indicated that the sensitivity of Bloch-Siegert based B₁⁺ mapping can be substantially improved when irradiating closer to resonance. Using optimized irradiation pulse shape and gradient crushers to minimize direct excitation effects, in vivo experiments on human brain at 7T confirmed the improved sensitivity available with this approach operating with peak B₁⁺ much larger than the frequency offset. This improved sensitivity translated into an 80% reduction in B₁⁺ estimation errors, without increasing tissue heating.