In vivo observation and biophysical interpretation of time-dependent diffusion in human white matter

Area of Science:

• Neuroimaging
• Biophysics
• Diffusion MRI

Background:

• Micrometer-level restrictions in biological tissues affect diffusion.
• Understanding diffusion time-dependence is crucial for interpreting diffusion MRI data.

Purpose of the Study:

• Investigate the effect of diffusion time on diffusion coefficients in human white matter in vivo.
• Explore diffusion time-dependence across a range of diffusion times (up to 600 ms) and length scales (up to 30 μm).

Main Methods:

• Stimulated echo diffusion tensor imaging (DTI) was performed on 5 healthy volunteers.
• Analysis focused on diffusion time-dependence in longitudinal and transverse directions relative to major fiber tracts.

Main Results:

• Observed weak time-dependence in diffusion transverse to major fiber tracts.
• Found notable time-dependence in the longitudinal diffusion direction.
• Time-dependence was observed across diffusion lengths up to approximately 30 μm.

Conclusions:

• Structural disorder, including axonal beading and fiber packing geometry, contributes to diffusion time-dependence.
• Time-dependent diffusion effects extend beyond simple Gaussian compartment models.
• These findings may lead to novel diffusion MRI markers specific to neuronal fiber geometry at the micrometer scale.