Dipolar Order Mapping Based on Spin‐Lock Magnetic Resonance Imaging

ABSTRACT Inhomogeneous magnetization transfer (ihMT) is sensitive to dipolar order associated with motion‐restricted macromolecules and can be characterized by the dipolar relaxation time . In this study, we propose a spin‐lock MRI framework for quantification. Specifically, we introduce a ‐sensitive metric, , derived from the distinct relaxation rate , defined as the difference between dual‐frequency and single‐frequency measurements. To enable dual‐frequency spin‐lock acquisition, we developed a dedicated rotary‐echo spin‐lock sequence. Based on this framework, we further estimated 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 to and supported the robustness of the proposed approach under the investigated conditions. Phantom experiments showed measurable ihMT contrast and supported the feasibility of estimation using . In vivo experiments demonstrated simultaneous and MPF mapping using only three spin‐lock‐prepared images. Across 10 healthy volunteers, mean white matter 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 and MPF mapping and may facilitate further investigation of dipolar‐order‐sensitive microstructural imaging in vivo.