Definition: The 'interstitial fluid volume fraction' ('Fint') 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_int can be estimated using advanced MRI techniques, including:

• diffusion MRI
• spectral diffusion analysis
• multi-compartment modeling

Clinical Relevance:

• In iNPH, increased F_int may reflect impaired glymphatic flow or extracellular matrix expansion.
• Changes in F_int can serve as a non-invasive biomarker for tracking fluid dynamics in neurodegenerative or hydrocephalic disorders.

Related terms:

• Interstitial Fluid (ISF)
• Diffusivity (D_int)
• Spectral Diffusion MRI
• Glymphatic system