Definition

1
SNR

• Definition
• Averaging uncorrelated noise
• Thermal vs. physiological

2
Averaging to beat down noise
If noise is uncorrelated
3
Signal-to-Noise Ratio (SNR)

• Thermal noise (?th) is uncorrelated and normally
  and uniformly distributed
• Imaging noise
• Physiological noise (?phys) has all sorts of
  correlations and is different in gray matter,
  white matter
• Brain noise

4
Caveat estimating zero-mean noise in magnitude
images
Gaussian distribution
Rician distribution
5
Small voxel low SNR
6
Big voxel dominated by phys. noise
Phys. noise masked by thermal noise
SNR lower in gray matter