====== Diffusion Signal-to-Noise Ratio (SNR) ======

**Diffusion signal-to-noise ratio (SNR)** refers to the **quantitative measure of signal strength relative to background noise** in diffusion-weighted magnetic resonance imaging (dMRI). It is a key determinant of image quality and **tractography accuracy**.

===== Definition =====
Mathematically:

  * **SNR = Mean Signal Intensity / Standard Deviation of Noise**

In diffusion imaging, the signal is affected by diffusion gradients and becomes lower than in standard (non-diffusion) MRI sequences, making **high SNR more difficult to achieve**.

===== Importance =====
  * **High SNR** → clearer visualization of diffusion anisotropy → more accurate fiber orientation modeling.
  * **Low SNR** → increased noise, leading to:
    * Spurious tracts
    * Unreliable fractional anisotropy (FA) values
    * Reduced confidence in tracking small or curved fibers (e.g., cranial nerves)

===== Factors Affecting SNR =====
  * **Field strength** (e.g. 3T vs. 7T MRI)
  * **Voxel size** (larger voxels = higher SNR, but lower spatial resolution)
  * **Number of signal averages (NSA)**
  * **Gradient strength and b-value**
  * **Acquisition time** and **patient motion**