Purpose: Inhomogeneous magnetization transfer (ihMT) effect reflects dipolar order with a dipolar relaxation time (T₁₃), specific to motion-restricted macromolecules. We aim to quantify T₁₃ using spin-lock MRI implemented with a novel rotary-echo sequence. Methods: In proposed method, we defined a relaxation rate R₃₎ₒ₋ that is specific to dipolar order and obtained as the difference of dual-frequency R₁ₑ₇₎^dual relaxation and single-frequency R₁ₑ₇₎^single relaxation. A novel rotary-echo spin-lock sequence was developed to enable dual-frequency acquisition. We derive the framework to estimate T₁₃ from R₃₎ₒ₋ under macromolecular pool fraction (MPF) map constraints. The proposed approach was validated via Bloch-McConnell-Provotorov simulation, phantom studies, and in-vivo white matter studies on a 3T scanner. Results: Simulations demonstrated that R₃₎ₒ₋ exhibits an approximately linear relationship with T₁₃. Phantom experiments showed robust ihMT contrast in R₃₎ₒ₋ and confirmed the feasibility and reliability of T₁₃ quantification via R₃₎ₒ₋. In vivo white-matter studies further supported the clinical potential of this T₁₃ mapping approach. Conclusion: We propose a novel, clinical feasible method for T₁₃ quantification based on spin-lock MRI. This method requires substantially fewer contrast-prepared images compared to the conventional T₁₃ quantification approach. This technique provides a promising pathway for robust MPF and T₁₃ quantification in a single rapid scan with reduced confounds.
Gao et al. (Fri,) studied this question.