Simultaneous 3D quantitative magnetization transfer imaging and susceptibility mapping.

Purpose: Introduce a unified acquisition and modeling strategy to simultaneously quantify magnetization transfer (MT), tissue susceptibility ( $\chi$ ) and $T_2^{*}$ .

Theory and methods: Magnetization transfer is induced through the application of off-resonance irradiation between excitation and acquisition of an RF-spoiled gradient-echo scheme, where free pool spin-lattice relaxation ( $T_1^F$ ), macromolecular proton fraction ( $f$ ) and magnetization exchange rate ( $k_F$ ) were calculated by modeling the magnitude of the MR signal using a binary spin-bath MT model with $B_1^{+}$ inhomogeneity correction via Bloch-Siegert shift. Simultaneously, a multi-echo acquisition is incorporated into this framework to measure the time evolution of both signal magnitude and phase, which was further modeled for estimating $T_2^{*}$ and tissue susceptibility. In this work, we demonstrate the feasibility of this new acquisition and modeling strategy in vivo on the brain tissue.

Results: In vivo brain experiments were conducted on five healthy subjects to validate our method. Utilizing an analytically derived signal model, we simultaneously obtained 3D $T_1^F$ , $f$ , $k_F$ , $\chi$ and $T_2^{*}$ maps of the whole brain. Our results from the brain regional analysis show good agreement with those previously reported in the literature, which used separate MT and QSM methods.

Conclusion: A unified acquisition and modeling strategy based on an analytical signal model that fully leverages both the magnitude and phase of the acquired signals was demonstrated and validated for simultaneous MT, susceptibility and $T_2^{*}$ quantification that are free from $B_1^{+}$ bias.