Dipolar order mapping based on spin-lock magnetic resonance imaging

Purpose: Inhomogeneous magnetization transfer (ihMT) effect reflects dipolar order with a dipolar relaxation time ($T_{1D}$), specific to motion-restricted macromolecules. We aim to quantify $T_{1D}$ using spin-lock MRI technique. Methods: In the proposed method, we introduce a $T_{1D}$-specific ratio, denoted as $RATIO_{dosl}$. This ratio is derived from the distinct relaxation rate $R_{dosl}$, calculated as the difference between dual-frequency relaxation $R_{1\rho}^{dual}$ and single-frequency $R_{1\rho}^{single}$ relaxation measurements. A novel rotary-echo spin-lock sequence was developed to enable dual-frequency spin-lock acquisition. We established a framework to estimate $T_{1D}$, as well as the macromolecular pool fraction (MPF) map. The proposed approach was validated via numerical simulations, phantom studies, and demonstrated in vivo in human white matter. Results: Simulations revealed the high sensitivity of $RATIO_{dosl}$ to $T_{1D}$, and substantiated the accuracy and robustness of the proposed methods. Phantom experiments demonstrated robust ihMT contrast and confirmed the capability of $T_{1D}$ quantification via $RATIO_{dosl}$. In vivo studies supported the clinical viability of this approcah, achieving simultaneous $T_{1D}$ and MPF mapping using only three spin-lock prepared images. Across ten healthy volunteers, the mean white matter $T_{1D}$ ranged from approximately 3.70 to 4.80 ms. Conclustion: We propose a novel method for $T_{1D}$ quantification based on spin-lock MRI. By requiring only three contrast-prepared images, this technique provides a promising pathway for robust, rapid, and simultaneous $T_{1D}$ and MPF quantification with fewer confounds