psy 8960, fall ‘06 noise1 snr and cnr types of noise –thermal –physiological –artifacts...
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Noise 1Psy 8960, Fall ‘06
Noise
• SNR and CNR• Types of noise
– Thermal– Physiological– Artifacts
• Thermal noise– Averaging– Bandwidth
• SNR - definitions
Noise 2Psy 8960, Fall ‘06
SNR: why do we care?
• Typical BOLD contrast: 1% change• Typical image SNR: 100:1• Typical CNR: 1!
Noise 3Psy 8960, Fall ‘06
SNR: why do we care?
• Temporal averaging beats down noise:
Noise 4Psy 8960, Fall ‘06
SNR: why do we care?
• Doubling SNR (CNR) = 4X reduction in scan time
Noise 5Psy 8960, Fall ‘06
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
Noise 6Psy 8960, Fall ‘06
Ways to reduce thermal noise
• Refrigerate sample
Noise 7Psy 8960, Fall ‘06
Ways to reduce thermal noise
• Refrigerate sample• Average multiple image acquisitions
Noise 8Psy 8960, Fall ‘06
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 9Psy 8960, Fall ‘06
Ways to reduce thermal noise
• Refrigerate sample• Average multiple image acquisitions• Decrease bandwidth• Increase field of view (while maintaining resolution)
Noise 10Psy 8960, Fall ‘06
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 11Psy 8960, Fall ‘06
Ways to quantify SNR
• mean(S) in sample region / stdev(noise) in background
Noise 12Psy 8960, Fall ‘06
Ways to quantify SNR
• mean(S) in sample region / stdev(noise) in background• mean(S) in sample region / stdev(S) in sample region
Noise 13Psy 8960, Fall ‘06
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?