A qMRI approach for mapping microscopic water populations and tissue relaxivity in the in vivo human brain.

Quantitative magnetic resonance imaging (qMRI) enables non-invasive mapping of brain tissue microstructure and is widely used for monitoring various physiological and pathological brain processes. Here, we introduce a qMRI approach for enriching the microstructural characterization of the sub-voxel environment. Inspired by pioneering magnetization transfer (MT) models, this approach employs MT saturation to differentiate between various water populations within each voxel. Our in vivo results align well with theoretical predictions and are reproducible using standard qMRI protocols. We present an array of new quantitative maps, highlighting different aspects of the tissues water. Furthermore, by manipulating the effective water content and relaxation rate with MT, we approximate within the voxel the tissue relaxivity. This property reflects the dependency of R1 on the macromolecular tissue volume (MTV) and is associated with the lipid and macromolecular composition of the brain. Our approach also enables biophysically-informed modulation of the R1 contrast, resulting in a set of unique cortical profiles. Finally, we demonstrate the effectiveness of our technique in imaging the common pathology of white matter hyperintensities (WMH), revealing tissue degradation and molecular alterations.