Reducing bias in dual flip angle T -mapping in human brain at 7T.

PURPOSE

To address the systematic bias in whole-brain dual flip angle (DFA) T -mapping at 7T by optimizing the flip angle pair and carefully selecting radiofrequency (RF) pulse shape and duration.

THEORY AND METHODS

Spoiled gradient echoes can be used to estimate whole-brain maps of T . This can be accomplished by using only two acquisitions with different flip angles, that is, a DFA-based approach. Although DFA-based T -mapping is seemingly straightforward to implement, it is sensitive to bias caused by incomplete spoiling and incidental magnetization transfer effects. Further bias is introduced by the increased B and inhomogeneities at 7T. Experiments were performed to determine the optimal flip angle pair and appropriate RF pulse shape and duration. Obtained T estimates were validated using inversion recovery prepared echo planar imaging and compared to literature values. A multi-echo readout was used to increase signal-to-noise ratio, enabling quantification of and susceptibility, χ.

RESULTS

Incomplete spoiling was observed above a local flip angle of approximately 20°. An asymmetric gauss-filtered sinc pulse with a constant duration of 700 μs showed a sufficiently flat frequency response profile to avoid incomplete excitation in areas with high B offsets. A pulse duration of 700 μs minimized effects from incidental magnetization transfer.

CONCLUSION

When performing DFA-based T -mapping one should (a) limit the higher flip angle to avoid incomplete spoiling, (b) use a RF pulse shape insensitive to B inhomogeneities and (c) apply a constant RF pulse duration, balanced to minimize incidental magnetization transfer.