===== Interstitial Fluid Volume Fraction  =====

**Definition:**  
The '''interstitial fluid volume fraction''' ('''F<sub>int</sub>''') refers to the proportion of tissue volume occupied by interstitial fluid (ISF), excluding cellular and vascular compartments.

It is expressed as a dimensionless ratio between '''0 and 1'''. Higher values suggest an increased extracellular (interstitial) space, which may be relevant in pathological states such as:
  * [[idiopathic normal pressure hydrocephalus]] (iNPH)
  * [[brain edema]]
  * [[white matter hyperintensities]]
  * [[neurodegeneration]]

**Measurement:**  
F<sub>int</sub> can be estimated using advanced MRI techniques, including:
  * [[diffusion MRI]]
  * [[spectral diffusion analysis]]
  * [[multi-compartment modeling]]

**Clinical Relevance:**  
  * In [[iNPH]], increased F<sub>int</sub> may reflect impaired glymphatic flow or extracellular matrix expansion.
  * Changes in F<sub>int</sub> can serve as a non-invasive biomarker for tracking fluid dynamics in neurodegenerative or hydrocephalic disorders.

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**Related terms:**
  * [[Interstitial Fluid (ISF)]]
  * [[Diffusivity (D<sub>int</sub>)]]
  * [[Spectral Diffusion MRI]]
  * [[Glymphatic system]]