Fast B₁⁺ mapping using three consecutive RF pulses and balanced gradients for improved bSSFP imaging

Purpose To develop a B₁⁺ mapping during the transient phase of balanced steady state free precession (bSSFP) imaging which can be used for subsequent B₁⁺ inhomogeneity compensation. Methods Two images with different flip angles (FA) are acquired using single-shot spiral technique during the transient phase of bSSFP with three consecutive RF pulses and balanced gradients. Under the assumptions that the transmit (B₁⁺) field varies slowly in spatial domain and T₁ and T₂ relaxation effects are negligible during 2·TR, B₁⁺ was estimated using the two magnitude images and bSSFP data was sequentially acquired. B₁⁺ estimation error due to the assumptions and other factors such as FA and off-resonance were assessed using Bloch simulation. Phantom and in vivo experiments were performed with α-2α-3α scheme. Results The simulation results indicated that the proposed method was less sensitive to T₁ relaxation and B₁⁺ mapping FA (α) of approximately 60° produced minimum estimation error. The B₁⁺-induced intensity variation was reduced with the proposed method in the phantom experiment. For both the phantom and in vivo experiments, the estimated B₁⁺ map showed comparable to the conventional B₁⁺ map using spin-echo DAM. Conclusion B₁⁺ map was estimated during the transient phase of bSSFP and subsequently compensated bSSFP images. There was no scan time increment and hence the technique can be used in a prescan manner for B₁⁺ mapping or shimming.