Fast B 1 + mapping using three consecutive RF pulses and balanced gradients for improved bSSFP imaging

To develop a B mapping during the transient phase of balanced steady state free precession (bSSFP) imaging which can be used for subsequent B inhomogeneity compensation. 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 (B ) field varies slowly in spatial domain and T and T relaxation effects are negligible during 2·TR, B was estimated using the two magnitude images and bSSFP data was sequentially acquired. B 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. The simulation results indicated that the proposed method was less sensitive to T relaxation and B mapping FA (α) of approximately 60° produced minimum estimation error. The B -induced intensity variation was reduced with the proposed method in the phantom experiment. For both the phantom and in vivo experiments, the estimated B map showed comparable to the conventional B map using spin-echo DAM. B 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 B mapping or shimming.