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

Purpose: In this work, we propose that Delta B-1(+)-induced errors in magnetization transfer (MT) saturation (MTsat) 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,R-obs) and apparent bound pool size (M-0,app(B)) in the brain. MTsat values were simulated for a range of B-1(+), R-1,R-obs, and M-0,app(B). An equation was fit to the simulated MTsat , then a linear relationship between R-1,R-obs and M-0,app(B) was generated. These results were used to generate correction factor maps for the MTsat 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,R-obs (r > 0.96), permitting the use of R-1,R-obs to estimate M-0,app(B) for B-1(+) correction. All B-1(+) corrected MTsat maps displayed a decreased correlation with B-1(+) compared to uncorrected MTsat and MTsat 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) MTsat maps. Conclusion: The proposed correction decreases the dependence of MTsat 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.

Automated summary

In this study, a method is proposed to correct Delta B-1(+)-induced errors in MTsat maps using R-1 and B-1(+) maps and numerical simulations. Experimental data from a partial quantitative MT protocol showed that the proposed correction can reduce the dependence of MTsat on B-1(+) inhomogeneities.