B1 inhomogeneity‐corrected T1 mapping and quantitative magnetization transfer imaging via simultaneously estimating Bloch‐Siegert shift and magnetization transfer effects

Abstract

PurposeTo introduce a method of inducing Bloch‐Siegert shift and magnetization Transfer Simultaneously (BTS) and demonstrate its utilization for measuring binary spin‐bath model parameters free pool spin–lattice relaxation (), macromolecular fraction (), magnetization exchange rate () and local transmit field ().Theory and MethodsBloch‐Siegert shift and magnetization transfer is simultaneously induced through the application of off‐resonance irradiation in between excitation and acquisition of an RF‐spoiled gradient‐echo scheme. Applying the binary spin‐bath model, an analytical signal equation is derived and verified through Bloch simulations. Monte Carlo simulations were performed to analyze the method's performance. The estimation of the binary spin‐bath parameters with compensation was further investigated through experiments, both ex vivo and in vivo.ResultsComparing BTS with existing methods, simulations showed that existing methods can significantly bias estimation when not accounting for transmit heterogeneity and MT effects that are present. Phantom experiments further showed that the degree of this bias increases with increasing macromolecular proton fraction. Multi‐parameter fit results from an in vivo brain study generated values in agreement with previous literature. Based on these studies, we confirmed that BTS is a robust method for estimating the binary spin‐bath parameters in macromolecule‐rich environments, even in the presence of inhomogeneity.ConclusionA method of estimating Bloch‐Siegert shift and magnetization transfer effect has been developed and validated. Both simulations and experiments confirmed that BTS can estimate spin‐bath parameters (, , ) that are free from bias.