  Purpose: To address the systematic bias in whole-brain dual flip angle (DFA)
T1-mapping at 7T by optimizing the flip angle pair and carefully selecting RF
pulse shape and duration. Theory and Methods: Spoiled gradient echoes can be
used to estimate whole-brain maps of T1. This can be accomplished by using only
two acquisitions with different flip angles, i.e., a DFA-based approach.
Although DFA-based T1-mapping is seemingly straightforward to implement, it is
sensitive to bias caused by incomplete spoiling and incidental magnetization
transfer (MT) effects. Further bias is introduced by the increased B0 and B1+
inhomogeneities at 7T. Experiments were performed to determine the optimal flip
angle pair and appropriate RF pulse shape and duration. Obtained T1 estimates
were validated using inversion recovery prepared EPI and compared to literature
values. A multi-echo readout was used to increase SNR, enabling quantification
of R2* and susceptibility, X. Results: Incomplete spoiling was observed above a
local flip angle of approximately 20 degrees. An asymmetric gauss-filtered sinc
pulse with a constant duration of 700 us showed a sufficiently flat frequency
response profile to avoid incomplete excitation in areas with high B0 offsets.
A pulse duration of 700 us minimized effects from incidental MT. Conclusion:
When performing DFA-based T1-mapping one should (i) limit the higher flip angle
to avoid incomplete spoiling, (ii) use a RF pulse shape insensitive to B0
inhomogeneities and (iii) apply a constant RF pulse duration, balanced to
minimize incidental MT.
