Double-wave-vector diffusion-weighted imaging reveals microscopic diffusion anisotropy in the living human brain

Area of Science:

• Neuroimaging
• Biophysics
• Diffusion MRI

Background:

• Diffusion tensor imaging (DTI) provides anisotropy information (e.g., fractional anisotropy) in biological tissues.
• DTI-derived anisotropy is ambiguous, potentially indicating reduced axon density or increased fiber crossing.
• Standard DTI struggles to differentiate between fiber loss and reduced coherence.

Purpose of the Study:

• To investigate the utility of double-wave-vector experiments for detecting microscopic diffusion anisotropy in white matter.
• To assess if this novel technique can provide reliable white matter integrity independent of macroscopic fiber orientation.

Main Methods:

• Utilized double-wave-vector (or double-pulsed-field-gradient) experiments with successive diffusion-weighting periods.
• Applied experiments to white matter regions in the living human brain, including macroscopically isotropic areas (fractional anisotropy = 0).
• Analyzed signal differences between parallel and orthogonal wave vector orientations.

Main Results:

• Successfully detected microscopic diffusion anisotropy in human white matter, even in regions with fractional anisotropy of 0.
• Observed signal modulation specific to the double-wave-vector experiment, not present in standard diffusion-weighting.
• Found similar modulation amplitudes across regions with varying fractional anisotropy values.

Conclusions:

• Double-wave-vector experiments can detect microscopic diffusion anisotropy in vivo.
• This technique offers a direct and reliable method to assess white matter integrity.
• It provides insights independent of the voxel's macroscopic fiber orientation distribution.