Measuring intra-axonal T₂ in white matter with direction-averaged diffusion MRI

Area of Science:

• Neuroimaging
• Biophysics
• Magnetic Resonance Imaging

Background:

• White matter integrity is crucial for neurological function.
• Accurate measurement of intra-axonal water properties is essential for understanding white matter microstructure.
• Diffusion MRI offers a non-invasive window into tissue microstructure.

Purpose of the Study:

• To establish a method for measuring the T2 relaxation time of intra-axonal water (T2a) in white matter using direction-averaged diffusion MRI.
• To quantify T2a and T2e in healthy human white matter at 3 Tesla.

Main Methods:

• Utilized direction-averaged diffusion MRI with high b-values (>4000 s/mm²) to isolate the intra-axonal water signal.
• Acquired data across multiple echo times (TE) to fit a mono-exponential decay curve for T2a estimation.
• Calculated extra-axonal relaxation time (T2e) by incorporating intra-axonal diffusivity.

Main Results:

• Estimated T2a values ranged from 50 ms to 110 ms, exhibiting significant regional variation in white matter.
• T2a demonstrated angular dependence in tracts with collinear fibers, suggesting susceptibility-induced field inhomogeneity effects.
• T2e values were consistently lower than T2a across white matter regions for plausible intra-axonal diffusivity values.

Conclusions:

• Intra-axonal water T2 relaxation time (T2a) in white matter can be reliably measured using direction-averaged diffusion MRI with large b-values and multiple TEs.
• In healthy brains, T2a exceeds T2e and shows considerable anatomical variability.
• This technique provides a valuable tool for characterizing white matter microstructure non-invasively.