A model-based framework for correcting inhomogeneity effects in magnetization transfer saturation and inhomogeneous...

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

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 ( $$ ) in the brain. MT_sat values were simulated for a range of $$ , R_1,obs , and $$ . An equation was fit to the simulated MT_sat , then a linear relationship between R_1,obs and $$ 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.

$$ was highly correlated with R_1,obs (r > 0.96), permitting the use of R_1,obs to estimate $$ for $$ correction. All $$ corrected MT_sat maps displayed a decreased correlation with $$ 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.

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