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fMRI: Biological Basis and Experiment Design Lecture 12: Signal-to-Noise Ratio

Explore the biological basis and experiment design of fMRI signal-to-noise ratio (SNR) including factors determining signal strength, noise contributions, spatial dependence, and optimization techniques. Learn about RF coils, voxel size, bandwidth, and noise sources affecting SNR. Discover the impact of physiological and thermal noise on imaging quality.

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fMRI: Biological Basis and Experiment Design Lecture 12: Signal-to-Noise Ratio

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  1. fMRI: Biological Basis and Experiment DesignLecture 12: Signal-to-Noise Ratio • Things that determine signal strength • voxel size • RF coil • Things that determine noise contribution • Bandwidth (thermal noise) • Brains (physiological noise) • Averaging antwrp.gsfc.nasa.gov/apod/ap041008.html 1 light year = 5,913,000,000,000 miles?

  2. Thermal noise Assumed white Fluctuations in RF path (coupling to sample, amplifiers ...) Physiological noise Probably colored Vasoreactivity Motion Respiration Cardiac ... Uncontrolled cognitive processes Broad definitions

  3. SNR, definition 1 SNRregular = 317 SNRsense = 251

  4. Spatial dependence of SNR with parallel imaging

  5. SNR definition 2

  6. Rician distribution: effect of magnitude reconstruction on noise close to zero

  7. SNR definition 2  = 48.2 = 58.6  = 56.7 = 61.0 S = 3012 S = 3226 ?????

  8. SNR maps, 2 definitions Definition 1: mean through time divided by standard deviation through time Definition 2: static signal intensity divided by background noise

  9. SNR maps in water phantoms New category of noise: inanimate processes? Regular EPI Siemen’s implementation of SENSE

  10. SNR – dependence on coil properties Body coil transmit, 8-channel coil receive Body coil transmit, body coil receive (These are just static images, not SNR maps.)

  11. SNR – dependence on bandwidth dwellTime = 5s dwellTime = 10s

  12. Surface coils vs. volume coils:

  13. Nova “visual coil” Nova birdcage head coil

  14. Pulse sequence diagram: Spoiling in FLASH/GRASS/RAGE ... Nrep = 256 Flip angle = 7 deg. TR = 10ms RF GSS Goal: obliterate any remaining transverse magnetization before next excitation. GPE TE ~ 5ms GRO 256 points DAC

  15. Image: Navg = 4

  16. Averaging beats down noise

  17. Physiological vs. thermal noise

  18. Physiological vs. thermal noise

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