TIME: Tractography-Informed myelin estimation

Investigating myelin integrity within multiple sclerosis (MS) lesions and in normal-appearing white matter is crucial for understanding demyelination and remyelination processes. While most approaches assess global myelin changes or compare lesions with homologous regions in healthy controls, they do not allow direct within-tract comparisons between lesional and non-lesional tissue. We introduce the tractography-informed myelin estimate (TIME), a novel map designed to quantify tract-specific myelin loss. TIME integrates tractography with myelin-sensitive imaging, such as myelin volume fraction, to compare lesional and non-lesional segments within the same white matter tract. By modeling local deviations from the expected myelin volume fraction signal along streamlines, TIME captures tract-specific myelin damage while accounting for within-tract variability. TIME is based on a microstructure-informed tractography framework, with an extra compartment to model signal loss caused by lesions. We evaluated TIME in 159 MS patients, assessing its association with neurological disability at baseline and longitudinally over a median follow-up of two years. At baseline, higher myelin loss captured by TIME was significantly associated with worse disability (beta = 0.14, p = 0.015). Longitudinally, greater baseline disability predicted faster TIME-quantified myelin loss, which was in turn associated with a higher risk of disability worsening. In contrast, lesion-averaged myelin volume fraction showed no significant associations with either baseline disability or its progression. TIME provides a detailed, tract-specific assessment of myelin damage, providing greater sensitivity than conventional metrics, highlighting its potential as a biomarker in MS.