Journal
MAGNETIC RESONANCE IN MEDICINE
Volume 70, Issue 6, Pages 1515-1523Publisher
WILEY-BLACKWELL
DOI: 10.1002/mrm.24598
Keywords
B1-mapping; actual flip angle; SSFP; transient phase; off-resonance
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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.
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