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?