Reducing bias in dual flip angle T<sub>1</sub>‐mapping in human brain at 7T
Hampus Olsson, Mads Andersen, Jimmy Lätt, Ronnie Wirestam, Gunther Helms
Abstract
Purpose To address the systematic bias in whole‐brain dual flip angle (DFA) T 1 ‐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 1 . This can be accomplished by using only two acquisitions with different flip angles, that is, a DFA‐based approach. Although DFA‐based T 1 ‐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 0 and inhomogeneities at 7T. Experiments were performed to determine the optimal flip angle pair and appropriate RF pulse shape and duration. Obtained T 1 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 0 offsets. A pulse duration of 700 μs minimized effects from incidental magnetization transfer. Conclusion When performing DFA‐based T 1 ‐mapping one should (a) limit the higher flip angle to avoid incomplete spoiling, (b) use a RF pulse shape insensitive to B 0 inhomogeneities and (c) apply a constant RF pulse duration, balanced to minimize incidental magnetization transfer.