Both noise-floor and tissue compartment difference in diffusivity contribute to FA dependence on b-value in diffusion...

Area of Science:

• Neuroimaging
• Biophysics
• Biomedical Engineering

Background:

• Noninvasive diffusion magnetic resonance imaging (dMRI) is crucial for studying brain microstructure.
• Fractional anisotropy (FA) derived from dMRI correlates with white matter properties but lacks specificity.
• FA's dependence on b-value is attributed to microstructure and noise, with limited understanding of their interplay.

Purpose of the Study:

• To systematically investigate the contributions of intra- and extra-neurite diffusivity differences and measurement noise to observed FA changes with b-value.
• To quantify the inter-relationship between tissue microstructure and noise effects on FA.

Main Methods:

• Utilized in-vivo and post-mortem human brain imaging.
• Employed numerical simulations and histological validation (Bielschowsky's silver impregnation).
• Analyzed neurite orientation dispersion and density imaging (NODDI) for neurite density index (NDI).

Main Results:

• Percentage difference in FA between b-values (pdFA) positively correlated with NDI.
• This association varied with Signal-to-Noise Ratio (SNR), indicating a nonlinear interaction.
• Multicompartment model simulations confirmed that differing intra-/extra-neurite diffusivities drive these findings, amplified by noise.

Conclusions:

• Both variations in intra- and extra-neurite diffusivity and noise-floor effects significantly contribute to b-value-dependent FA differences.
• Understanding these factors is crucial for accurate interpretation of dMRI-derived FA in white matter research.