Dipolar Order Mapping Based on Spin-Lock Magnetic Resonance Imaging

Inhomogeneous magnetization transfer (ihMT) is sensitive to dipolar order associated with motion-restricted macromolecules and can be characterized by the dipolar relaxation time $T_{1D}$ . In this study, we propose a spin-lock MRI framework for $T_{1D}$ quantification. Specifically, we introduce a $T_{1D}$ -sensitive metric, $\text{RATI}{O}_{\text{dosl}}$ , derived from the distinct relaxation rate $R_{\text{dosl}}$ , defined as the difference between dual-frequency and single-frequency $R_{1\rho }$ measurements. To enable dual-frequency spin-lock acquisition, we developed a dedicated rotary-echo spin-lock sequence. Based on this framework, we further estimated $T_{1D}$ and the macromolecular proton fraction (MPF) within a unified acquisition. The proposed method was evaluated using numerical simulations, phantom experiments, and in vivo imaging in the healthy human brain. Simulations demonstrated high sensitivity of $\text{RATI}{O}_{\text{dosl}}$ to $T_{1D}$ and supported the robustness of the proposed approach under the investigated conditions. Phantom experiments showed measurable ihMT contrast and supported the feasibility of $T_{1D}$ estimation using $\text{RATI}{O}_{\text{dosl}}$ . In vivo experiments demonstrated simultaneous $T_{1D}$ and MPF mapping using only three spin-lock-prepared images. Across 10 healthy volunteers, mean white matter $T_{1D}$ values ranged from approximately 3.70 to 4.80 ms. By requiring only three contrast-prepared images, the proposed technique provides a rapid framework for simultaneous $T_{1D}$ and MPF mapping and may facilitate further investigation of dipolar-order-sensitive microstructural imaging in vivo.