Noise SNR and CNR Types of noise Thermal
- Slides: 13
Noise • SNR and CNR • Types of noise – Thermal – Physiological – Artifacts • Thermal noise – Averaging – Bandwidth • SNR - definitions Psy 8960, Fall ‘ 06 Noise 1
SNR: why do we care? • Typical BOLD contrast: 1% change • Typical image SNR: 100: 1 • Typical CNR: 1! Psy 8960, Fall ‘ 06 Noise 2
SNR: why do we care? • Temporal averaging beats down noise: Psy 8960, Fall ‘ 06 Noise 3
SNR: why do we care? • Doubling SNR (CNR) = 4 X reduction in scan time Psy 8960, Fall ‘ 06 Noise 4
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 Psy 8960, Fall ‘ 06 Noise 5
Ways to reduce thermal noise • Refrigerate sample Psy 8960, Fall ‘ 06 Noise 6
Ways to reduce thermal noise • Refrigerate sample • Average multiple image acquisitions Psy 8960, Fall ‘ 06 Noise 7
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) Psy 8960, Fall ‘ 06 Noise 8
Ways to reduce thermal noise • • Refrigerate sample Average multiple image acquisitions Decrease bandwidth Increase field of view (while maintaining resolution) Psy 8960, Fall ‘ 06 Noise 9
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) Psy 8960, Fall ‘ 06 Noise 10
Ways to quantify SNR • mean(S) in sample region / stdev(noise) in background Psy 8960, Fall ‘ 06 Noise 11
Ways to quantify SNR • mean(S) in sample region / stdev(noise) in background • mean(S) in sample region / stdev(S) in sample region Psy 8960, Fall ‘ 06 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 • mean(S) in voxel / std(Svoxel) through time SNR ~ 15? Psy 8960, Fall ‘ 06 Noise 13
