Neurite Exchange Imaging cross-validation against ihMT and MPF in the human brain

Motivation: The NEXI model aims to characterize gray matter microstructure. Validation against established techniques is crucial for assessing its sensitivity, specificity, and clinical potential. Goal(s): Evaluate NEXI against ihMT and MPF, highly sensitive biomarkers of tissue myelination. Approach: Compare NEXI maps against MPF and ihMT in the human cortex. Results: ihMT ratio correlated exclusively with exchange time (tex, cell-membrane permeability proxy), while MPF correlated exclusively with cell process density (f). High ihMTR/MPF patterns matched high tex/f patterns in myelin-rich regions. Discrepancies indicate unique contributions from different cell populations and can offer insights into distinct microstructural features, enhancing our understanding of brain tissue composition in-vivo. Impact: We performed a cross-validation of the sensitivity of NEXI-derived membrane permeability with myelin content, and neurite density with macromolecular proton fraction in the human brain, thus paving the way for the translation of NEXI to studies on patient populations.