Reducing bias in dual flip angle T₁‐mapping in human brain at 7T

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 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, i.e., 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 (MT) effects. Further bias is introduced by the increased B 0 and B 1 + 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 EPI and compared to literature values. A multi-echo readout was used to increase SNR, enabling quantification of R 2 * 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 MT. Conclusion: When performing DFA-based T 1 -mapping one should (i) limit the higher flip angle to avoid incomplete spoiling, (ii) use a RF pulse shape insensitive to B 0 inhomogeneities and (iii) apply a constant RF pulse duration, balanced to minimize incidental MT.

Imperfect spoiling in variable flip angle T1 mapping at 7T: Quantifying and minimizing impact

“…It has recently been suggested that the maximum flip angle should be minimized to mitigate spoiling-induced errors. 37 However, for the TR and target T 1 range considered here, this would compromise the precision of the T 1 estimates by as much as 50%. 38 An alternative would be to reduce both flip angles and TR, which could be achieved by adopting a single echo protocol.…”

Multiparameter mapping of relaxation (R1, R2*), proton density and magnetization transfer saturation at 3 T: A multicenter dual‐vendor reproducibility and repeatability study

“…The product sequences did not allow the user to precisely control the characteristics of the MT saturation pulses, thus a rescaling of MT maps was implemented to harmonize MT maps across manufacturers. The proposed harmonization also accounts for systematic differences in TR and measured R1 due to incidental MT by the excitation pulse (Olsson, Wirestam, Lätt, & Helms, 2020).…”

Mapping magnetization transfer saturation (MTsat) in human brain at 7T: Protocol optimization under specific absorption rate constraints

“…The full derivation is given in Supporting Information, which is available online. As in the DFA experiment, 28 a global optimization of is not possible.…”