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Human gray matter microstructure mapped using neurite exchange imaging (NEXI) on a clinical scanner
Quentin Uhl1, Tommaso Pavan1, Thorsten Feiweier2
1Department of Radiology, Lausanne University Hospital (CHUV) and University of Lausanne, Lausanne, Switzerland.
Imaging Neuroscience (Cambridge, Mass.)
|August 13, 2025
Summary
Neurite Exchange Imaging (NEXI) provides insights into brain microstructure using clinical MRI. This study reports the first NEXI estimates in the human cortex, showing good repeatability and a correlation with myelin content.
Area of Science:
- Neuroimaging
- Biophysics
- Medical Physics
Background:
- Biophysical models of diffusion-weighted imaging (DWI) offer valuable insights into human brain microstructure, crucial for understanding neurological health and disease.
- Clinical compatibility of these models is essential for widespread application in healthcare settings.
- Neurite Exchange Imaging (NEXI) is a two-compartment model that captures inter-compartment exchange within gray matter (GM), requiring multi-shell, multi-diffusion time data for parameter estimation.
Purpose of the Study:
- To report the first estimates of NEXI in the human cortex using a clinical MRI scanner.
- To develop and validate an acquisition protocol and fitting routine suitable for clinical environments.
- To compare the performance of the narrow-pulse approximation (NEXI) with the broad-pulse model (SMEX) on clinical scanners.
Main Methods:
- Implementation of NEXI and SMEX models on a clinical MRI scanner.
- Establishment of a compatible acquisition protocol and fitting routine.
- Evaluation of NEXI parameter repeatability in the human cortex.
- Correlation analysis between NEXI parameters and Myelin Water Fraction (MWF) derived from multicomponent T2 relaxation.
Main Results:
- Successful acquisition of NEXI data in the human cortex on a clinical MRI scanner.
- SMEX model provided estimates more comparable to literature values in the context of clinical scanners with longer gradient pulses.
- NEXI estimates demonstrated lower intra-subject variability than inter-subject variability, indicating good repeatability.
- A strong correlation was confirmed between exchange time (tex) estimates and MWF, though with brain-region-specific spatial correspondence.
Conclusions:
- NEXI is feasible for human cortical microstructure analysis on clinical MRI scanners.
- The SMEX model is relevant for clinical data due to longer gradient pulse durations.
- Repeatability of NEXI estimates is promising for clinical cohort studies.
- Myelin content influences cortical microstructure exchange, but other factors also contribute to tex.
