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

To 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 ( T1F$$ {T}_1^{\mathrm{F}} $$ ), macromolecular fraction ( f$$ f $$ ), magnetization exchange rate ( kF$$ {k}_{\mathrm{F}} $$ ) and local transmit field ( B1+$$ {B}_1^{+} $$ ).Bloch-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 B1+$$ {B}_1^{+} $$ compensation was further investigated through experiments, both ex vivo and in vivo.Comparing BTS with existing methods, simulations showed that existing methods can significantly bias T1$$ {T}_1 $$ estimation when not accounting for transmit B1$$ {B}_1 $$ 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 B1+$$ {B}_1^{+} $$ inhomogeneity.A 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 ( T1F$$ {T}_1^{\mathrm{F}} $$ , f$$ f $$ , kF$$ {k}_{\mathrm{F}} $$ ) that are free from B1+$$ {B}_1^{+} $$ bias.