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

다국어 초록 (Multilingual Abstract)

<P><B>Abstract</B></P> <P><B>Purpose</B></P> <P>To develop a B<SUB>1</SUB> <SUP>+</SUP> mapping during the transient phase of balanced steady state free precession (bSSFP) imaging which can be used for subsequent B<SUB>1</SUB> <SUP>+</SUP> inhomogeneity compensation.</P> <P><B>Methods</B></P> <P>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<SUB>1</SUB> <SUP>+</SUP>) field varies slowly in spatial domain and T<SUB>1</SUB> and T<SUB>2</SUB> relaxation effects are negligible during 2·TR, B<SUB>1</SUB> <SUP>+</SUP> was estimated using the two magnitude images and bSSFP data was sequentially acquired. B<SUB>1</SUB> <SUP>+</SUP> 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.</P> <P><B>Results</B></P> <P>The simulation results indicated that the proposed method was less sensitive to T<SUB>1</SUB> relaxation and B<SUB>1</SUB> <SUP>+</SUP> mapping FA (α) of approximately 60° produced minimum estimation error. The B<SUB>1</SUB> <SUP>+</SUP>-induced intensity variation was reduced with the proposed method in the phantom experiment. For both the phantom and in vivo experiments, the estimated B<SUB>1</SUB> <SUP>+</SUP> map showed comparable to the conventional B<SUB>1</SUB> <SUP>+</SUP> map using spin-echo DAM.</P> <P><B>Conclusion</B></P> <P>B<SUB>1</SUB> <SUP>+</SUP> 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<SUB>1</SUB> <SUP>+</SUP> mapping or shimming.</P>