Definition: 'Diffusivity' (symbol: 'D') refers to the rate at which molecules (typically water in MRI) spread out over space due to random Brownian motion. It is measured in units of mm²/s.

In brain imaging, diffusivity quantifies the movement of water molecules within tissue microstructure, providing insight into:

• cellularity
• extracellular space
• tissue anisotropy
• edema and inflammation

Types of Diffusivity in MRI:

• 'Apparent Diffusion Coefficient (ADC):' A scalar value representing the average water diffusion in a voxel.
• 'Axial Diffusivity (AD):' Diffusion along the principal axis of white matter tracts.
• 'Radial Diffusivity (RD):' Diffusion perpendicular to the principal axis.
• 'Mean Diffusivity (MD):' Average of diffusivity along all directions.
• 'Interstitial Fluid Diffusivity (Dint):' Estimated using advanced techniques like spectral diffusion MRI; reflects fluid mobility in extracellular (interstitial) space.

Clinical Relevance:

• ↓ Diffusivity → may indicate cytotoxic edema (e.g., in acute stroke).
• ↑ Diffusivity → may reflect increased extracellular space (e.g., neurodegeneration, vasogenic edema, iNPH).
• Alterations in D_int may correlate with changes in interstitial fluid dynamics and glymphatic dysfunction.

Measurement Techniques:

• Diffusion-weighted imaging (DWI)
• Diffusion tensor imaging (DTI)
• Spectral diffusion analysis (for compartment-specific diffusivity like D_int)

See also:

• F_int — Interstitial fluid volume fraction
• Diffusion MRI
• Glymphatic system
• Isotropic vs. anisotropic diffusion