Orientation independent quantification of macromolecular proton fraction in tissues with suppression of residual dipolar coupling

Motivation: The residual dipolar coupling (RDC) can lead to the orientation-dependent measurements in ordered tissues in MRI, potentially confounding their clinical applications. Goal(s): We demonstrate the potential confounding effect from tissue orientation in quantitative magnetization transfer can be suppressed by using a new technique Macromolecular Proton Fraction Mapping based on Spin-Lock (MPFSL). Approach: Applying MPFSL, we can adjust both the resonance frequency offset and the amplitude of spin-lock radiofrequency pulse to achieve a strong effective spin-lock field to suppress RDC, eliminating orientation-dependency of MPF measurement. Human knee specimen experiments conducted verified this finding. Results: The MPF measured using MPFSL shows insensitivity to tissue orientations. Impact: Spin-lock based quantitative magnetization transfer imaging can achieve orientation-independent quantification, thus having potential applications in characterization of highly-ordered tissues such as cartilage and myelin.