B 1 inhomogeneity-corrected T 1 mapping and quantitative magnetization transfer imaging via simultaneously estimating Bloch-Siegert shift and magnetization transfer effects
Albert Jang, Paul K. Han, Chao Ma, Georges El Fakhri, Nian Wang, Alexey Samsonov, Fang Liu
Abstract
Purpose
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 (T 1F), macromolecular fraction (f ), magnetization exchange rate (k F), and local transmit field (B 1+).
Theory and Methods
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+ compensation was further investigated through experiments, both ex vivo and in vivo.
Results
Comparing BTS with existing methods, simulations showed that existing methods can significantly bias T 1F estimation when not accounting for transmit 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 B 1+ inhomogeneity.
Conclusion
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 (T 1F, f , k F) that are free from B 1+ bias.