A model‐based framework for correcting inhomogeneity effects in magnetization transfer saturation and inhomogeneous magnetization transfer saturation maps

Abstract

In this work, we propose that ΔB + 1 -induced errors in magnetization transfer (MT) saturation (MT sat ) maps can be corrected with use of an R 1 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 R 1 (R 1,obs ) and apparent bound pool size (M B 0,app ) in the brain. MT sat values were simulated for a range of B + 1 , R 1,obs , and M B 0,app . An equation was fit to the simulated MT sat , then a linear relationship between R 1,obs and M B 0,app 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 B 0,app was highly correlated with R 1,obs (r > 0.96), permitting the use of R 1,obs to estimate M B 0,app 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. K E Y W O R D SB 1 correction, ihMT, inhomogeneous magnetization transfer, magnetization transfer, MT sat , Myelin | 2193 ROWLEY Et aL.

T1D‐weighted ihMT imaging – Part II. Investigating the long‐ and short‐T1D components correlation with myelin content. Comparison with R1 and the macromolecular proton fraction

“…Moreover, as shown in this study, the relative WM/ GM contrast is qualitatively enhanced for ihMTsat and the specificity to myelination is considerably improved. While various available approaches 49,57,58 demonstrated their utility, the ihMTsat approach presented by Munsch et al 38 and implemented here has the advantage of a minimum number of assumptions.…”

T1D‐weighted ihMT imaging – Part I. Isolation of long‐ and short‐T1D components by T1D‐filtering

“…An increasing number of ihMT studies have focused on central nervous system (CNS) applications 1,2,5–25 : more specifically—myelin imaging. Indeed, because of its highly organized multilayered stack ultrastructure, 26 myelin represents a suitable candidate for ihMT imaging.…”

Reliability of quantitative multiparameter maps is high for magnetization transfer and proton density but attenuated for R1 and R2* in healthy young adults

“…To do so, the acquired 3D‐EPI‐based B 1 + maps (Lutti et al, 2012 ) were used after correcting them for EPI‐specific distortions by means of the gradient echo‐based B 0 fieldmaps. Also the MT maps were corrected for residual local RF transmit field inhomogeneities—using a semi‐empirical approach (Rowley et al, 2021 ; Weiskopf et al, 2013 ). Imperfect RF spoiling was also corrected for in the T 1 maps using the approach described by Preibisch and Deichmann ( 2009 ), which was adapted to the FLASH acquisition parameters used here.…”