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Noise, SNR, and CNR in Imaging: Importance and Reduction Techniques

Explore the significance of Noise, SNR, and CNR in imaging, types of noise like Thermal, Physiological, and Artifacts, and effective ways to reduce noise, improve SNR, and enhance image quality. Learn why SNR matters and techniques to quantify and enhance it for clearer images. Discover how to reduce thermal noise through methods like sample refrigeration, averaging, and bandwidth adjustments.

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Noise, SNR, and CNR in Imaging: Importance and Reduction Techniques

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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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