Dipolar Order Mapping Based on Spin-Lock Magnetic Resonance Imaging

Area of Science:

• Biomedical Imaging
• Magnetic Resonance Imaging
• Biophysics

Background:

• Inhomogeneous magnetization transfer (ihMT) is sensitive to dipolar order in motion-restricted macromolecules.
• The dipolar relaxation time (T1D) characterizes this dipolar order.
• Current methods for T1D quantification are limited.

Purpose of the Study:

• To propose and validate a novel spin-lock MRI framework for T1D quantification.
• To develop a T1D-sensitive metric (RATIO_dosl) derived from dual-frequency spin-lock measurements.
• To enable simultaneous T1D and macromolecular proton fraction (MPF) mapping.

Main Methods:

• Development of a dedicated rotary-echo spin-lock sequence for dual-frequency acquisition.
• Introduction of the RATIO_dosl metric based on the difference between dual-frequency and single-frequency R1ρ measurements.
• Evaluation using numerical simulations, phantom experiments, and in vivo human brain imaging.

Main Results:

• Simulations confirmed high sensitivity and robustness of RATIO_dosl to T1D.
• Phantom experiments demonstrated measurable ihMT contrast and feasibility of T1D estimation.
• In vivo imaging achieved simultaneous T1D and MPF mapping with only three prepared images.
• Mean white matter T1D values in healthy volunteers ranged from 3.70 to 4.80 ms.

Conclusions:

• The proposed spin-lock MRI framework enables rapid and simultaneous T1D and MPF mapping.
• This technique offers a promising tool for investigating dipolar-order-sensitive microstructural imaging in vivo.
• The method requires minimal imaging time, facilitating clinical applications.