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.

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.

• 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)

• 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