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Noise

Noise. SNR and CNR Types of noise Thermal Physiological Artifacts Thermal noise Averaging Bandwidth SNR - definitions. SNR: why do we care?. Typical BOLD contrast: 1% change Typical image SNR: 100:1 Typical CNR: 1!. SNR: why do we care?. Temporal averaging beats down noise:.

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Noise

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  1. Noise • SNR and CNR • Types of noise • Thermal • 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 • 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

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