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In vivo human neurite exchange imaging (NEXI) at 500 mT/m diffusion gradients
Kwok-Shing Chan1,2, Yixin Ma1,2, Hansol Lee1,2
1Athinoula A. Martinos Center for Biomedical Imaging, Charlestown, MA, United States.
Biorxiv : the Preprint Server for Biology
|January 7, 2025
Summary
This study demonstrates in vivo imaging of brain gray matter water exchange using advanced diffusion MRI on the Connectome 2.0 scanner. Faster exchange times were observed, highlighting the importance of diffusion times and noise reduction for accurate neuroimaging analysis.
Area of Science:
- Neuroimaging
- Biophysics
- Medical Physics
Background:
- Diffusion-water exchange imaging (dEXI) is crucial for assessing brain microstructure and membrane integrity.
- Challenges in dEXI include achieving high signal-to-noise ratios and short diffusion times for fast exchange processes.
- Accurate measurement of intra-neurite and extracellular water exchange is vital for understanding neuronal function and disease.
Purpose of the Study:
- To demonstrate the feasibility of in vivo imaging of tissue micro-geometries and water exchange in human brain gray matter.
- To utilize the state-of-the-art Connectome 2.0 scanner with an ultra-high-performance gradient system for advanced diffusion MRI.
- To apply the anisotropic Kärger model for estimating water exchange times in gray matter.
Main Methods:
- Diffusion MRI measurements were performed on 15 healthy volunteers using the Connectome 2.0 scanner.
- Multiple diffusion times (13-30 ms) and b-values up to 17.5 ms/μm² were employed.
- The anisotropic Kärger model was fitted to estimate the non-exchanging and exchanging water populations' residence times, with GPU acceleration for analysis.
Main Results:
- Estimated water exchange time in gray matter was significantly faster (median 13±8 ms) on Connectome 2.0 compared to Connectome 1.0-alike systems.
- Connectome 1.0 compatible protocols showed increased susceptibility to noise floor biases with fast exchange processes.
- Spatial variations in exchange time were observed across cortical regions, with motor, somatosensory, and visual cortices showing longer exchange times.
Conclusions:
- Advanced diffusion MRI hardware and optimized protocols are essential for accurate in vivo measurement of fast water exchange in the brain.
- Careful consideration of diffusion times and noise reduction strategies (e.g., addressing Rician noise) is critical for reliable NEXI (neurite exchange imaging) estimations.
- Understanding spatial variations in cortical water exchange may provide insights into regional functional differences and pathologies.
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