Diffusion time dependence of microstructural parameters in fixed spinal cord.

Area of Science:

• Neuroimaging
• Biophysics
• Diffusion MRI

Background:

• Accurate biophysical modeling of diffusion MRI is crucial for determining tissue microstructural properties.
• Estimating parameters from low-strength diffusion gradient data is unreliable due to optimization challenges.
• Diffusion kurtosis imaging (DKI) offers more robust parameter estimation but faces challenges in uniquely determining all parameters, particularly concerning intra-axonal and extra-axonal diffusivities.

Purpose of the Study:

• To develop a biophysical model of white matter diffusion incorporating axonal dispersion.
• To achieve stable estimation of all model parameters using DKI in fixed pig spinal cord.
• To investigate the diffusion time dependence of microstructural parameters.

Main Methods:

• Developed a novel white matter diffusion model including axonal dispersion.
• Utilized fast axisymmetric DKI with stimulated echo acquisition mode.
• Acquired data across a wide diffusion time range (two orders of magnitude) with narrow diffusion gradient pulses.

Main Results:

• Demonstrated stable estimation of all model parameters from DKI data.
• Observed significant time dependence in most estimated parameters, excluding volume fractions.
• The diffusion time dependence provided evidence, though not definitive, for higher intra-axonal diffusivity than extra-axonal diffusivity in spinal cord white matter.

Conclusions:

• The developed model and DKI approach enable robust estimation of white matter microstructural parameters.
• The time dimension in DKI measurements is essential for resolving parameter degeneracies.
• Findings align with effective medium theory predictions for time-dependent compartmental diffusivities.