===== Diffusivity (D) =====

**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 (D<sub>int</sub>):''' Estimated using advanced techniques like spectral diffusion MRI; reflects fluid mobility in extracellular (interstitial) space.

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**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<sub>int</sub> 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<sub>int</sub>)

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**See also:**
  * [[F<sub>int</sub> — Interstitial fluid volume fraction]]
  * [[Diffusion MRI]]
  * [[Glymphatic system]]
  * [[Isotropic vs. anisotropic diffusion]]