====== Spectral Diffusion Analysis ======

**Definition:**  
'''[[Spectral diffusion]] [[analysis]]''' is an advanced [[MRI]]-based [[computational method]] that decomposes [[diffusion-weighted]] [[signal]]s into frequency components to estimate tissue-specific microstructural properties.

This technique allows quantification of compartmentalized diffusion behaviors—such as intracellular, interstitial, and restricted diffusion—by analyzing the diffusion spectrum rather than assuming a single apparent diffusion coefficient (ADC).

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**Purpose and Utility:**
  * Estimates surrogate markers for:
    - **Interstitial fluid volume fraction** (F<sub>int</sub>)
    - **Interstitial diffusivity** (D<sub>int</sub>)
  * Differentiates between tissue compartments (cellular vs. extracellular)
  * Detects subtle alterations in microstructural water dynamics
  * Enhances diagnostic sensitivity in conditions like:
    - [[Idiopathic normal pressure hydrocephalus]] (iNPH)
    - [[White matter pathology]]
    - [[Brain tumors]]

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**Methodological Principles:**
  * Uses multi-b-value and/or multi-diffusion time datasets
  * Applies Fourier or inverse Laplace transforms to diffusion signals
  * Generates a **diffusion spectrum**, characterizing signal contributions from various mobility ranges
  * Allows non-invasive inference of tissue complexity and fluid dynamics

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**Advantages:**
  * More sensitive than conventional ADC to subtle microstructural changes
  * Enables modeling of **fluid mobility** and **volume fraction** in interstitial compartments
  * Provides physiologically interpretable parameters

**Limitations:**
  * Requires high-quality multi-shell or multi-tensor diffusion MRI
  * Computationally intensive
  * Interpretation may depend on model assumptions

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**Clinical Relevance:**
  * In iNPH, increased F<sub>int</sub> and altered D<sub>int</sub> may reflect glymphatic dysfunction and extracellular space expansion
  * Helps in evaluating response to shunt surgery or fluid clearance impairment


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