B1+-Mapping with the Transient Phase of Unbalanced Steady-State Free Precession

Purpose A novel: B1+ -mapping technique (B1-TRAP) is presented, which derives the actual flip angle from the frequency of signal oscillations, observed in the transient phase of unbalanced steady-state free precession sequences. Theory: For short repetition times (TR), the angular frequency of distinct oscillations in the transient phase of steady-state free precession sequences is proven to be approximately proportional to the actual flip angle: TR approximate to alpha. The result is not influenced by off-resonance and it can be shown that deviations are only of second order in the small parameter TR/T2. Methods: B1-TRAP makes use of this effect through a frequency analysis of the transient phase of a train of steady-state free precession signals. Results: In terms of reliability and time efficiency, a two-dimensional multislice implementation was found to be optimal. Unlike many steady-state B1+ -mapping methods, the accuracy of B1-TRAP was not impaired by imperfect slice profiles. Conclusion: Simulations, phantom, and in vivo measurements showed that B1-TRAP offers a good compromise with respect to speed, robustness, and accuracy.