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

Purpose: 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.

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 (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.

Results: 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.

Conclusion: 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.