1. Noise SNR and CNR Types of noise Thermal noise SNR - definitions
Physiological
Artifacts
Thermal noise
Averaging
Bandwidth
SNR - definitions
Noise

2. SNR: why do we care? Typical BOLD contrast: 1% change
Typical image SNR: 100:1
Typical CNR: 1!
Noise

3. SNR: why do we care?
Temporal averaging beats down noise:
Noise

4. SNR: why do we care? Doubling SNR (CNR) = 4X reduction in scan time
Noise

5. Types of noise Thermal Physiological noise Artifacts
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
Noise

6. Ways to reduce thermal noise
Refrigerate sample
Noise

7. Ways to reduce thermal noise
Refrigerate sample
Average multiple image acquisitions
Noise

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)
Noise

9. Ways to reduce thermal noise
Refrigerate sample
Average multiple image acquisitions
Decrease bandwidth
Increase field of view (while maintaining resolution)
Noise

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)
Noise

11. Ways to quantify SNR
mean(S) in sample region / stdev(noise) in background
Noise

12. Ways to quantify SNR
mean(S) in sample region / stdev(noise) in background
mean(S) in sample region / stdev(S) in sample region
Noise

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?
Noise