A model-based framework for correcting B1+ inhomogeneity effects in magnetization transfer saturation and inhomogeneous magnetization transfer saturation maps

Abstract

Purpose: In this work, we propose that Δ $B_1^{+}$ -induced errors in magnetization transfer (MT) saturation (MT_sat ) maps can be corrected with use of an R₁ and $B_1^{+}$ map and through numerical simulations of the sequence.

Theory and methods: One healthy subject was scanned at 3.0T using a partial quantitative MT protocol to estimate the relationship between observed R₁ (R_1,obs ) and apparent bound pool size ( $M_{0,app}^B$ ) in the brain. MT_sat values were simulated for a range of $B_1^{+}$ , R_1,obs , and $M_{0,app}^B$ . An equation was fit to the simulated MT_sat , then a linear relationship between R_1,obs and $M_{0,app}^B$ was generated. These results were used to generate correction factor maps for the MT_sat acquired from single-point data. The proposed correction was compared to an empirical correction factor with different MT-preparation schemes.

Results: $M_{0,app}^B$ was highly correlated with R_1,obs (r > 0.96), permitting the use of R_1,obs to estimate $M_{0,app}^B$ for $B_1^{+}$ correction. All $B_1^{+}$ corrected MT_sat maps displayed a decreased correlation with $B_1^{+}$ compared to uncorrected MT_sat and MT_sat corrected with an empirical factor in the corpus callosum. There was good agreement between the proposed approach and the empirical correction with radiofrequency saturation at 2 kHz, with larger deviations seen when using saturation pulses further off-resonance and in inhomogeneous (ih) MT_sat maps.

Conclusion: The proposed correction decreases the dependence of MT_sat on $B_1^{+}$ inhomogeneities. Furthermore, this flexible framework permits the use of different saturation protocols, making it useful for correcting $B_1^{+}$ inhomogeneities in ihMT.

Keywords: B1 correction; MTsat; Myelin; ihMT; inhomogeneous magnetization transfer; magnetization transfer.