SNR and CNR

1
Noise

• SNR and CNR
• Types of noise
• Thermal
• Physiological
• Artifacts
• Thermal noise
• Averaging
• Bandwidth
• SNR - definitions

2
SNR why do we care?

• Typical BOLD contrast 1 change
• Typical image SNR 1001
• Typical CNR 1!

3
SNR why do we care?

• Temporal averaging beats down noise

4
SNR why do we care?

• Doubling SNR (CNR) 4X reduction in scan time

5
Types of noise

• Thermal
• Random fluctuations in current in receive coil,
  amplifier
• Spatially independent
• Physiological noise
• Respiration
• Cardiac
• Vasomotor
• Uncontrolled cognitive factors
• Artifacts
• Spikes
• Fluctuations in Nyquist ghost
• Segmentation ghosts
• Motion

6
Ways to reduce thermal noise

• Refrigerate sample

7
Ways to reduce thermal noise

• Refrigerate sample
• Average multiple image acquisitions

8
Ways to reduce thermal noise

• Refrigerate sample
• Average multiple image acquisitions
• Decrease bandwidth

(This should be a picture of one noisy image and
one ugly image)
9
Ways to reduce thermal noise

• Refrigerate sample
• Average multiple image acquisitions
• Decrease bandwidth
• Increase field of view (while maintaining
  resolution)

10
Ways to reduce thermal noise

• Refrigerate sample
• Average multiple image acquisitions
• Decrease bandwidth
• Increase field of view (while maintaining
  resolution)
• Increase voxel size (increase S, rather than
  decrease N)

11
Ways to quantify SNR

• mean(S) in sample region / stdev(noise) in
  background

12
Ways to quantify SNR

• mean(S) in sample region / stdev(noise) in
  background
• mean(S) in sample region / stdev(S) in sample
  region

13
Ways to quantify SNR

• mean(S) in sample region / stdev(noise) in
  background
• mean(S) in sample region / stdev(S) in sample
  region
• mean(S) in voxel / std(Svoxel) through time

SNR 15?