Fast B 1 + 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.