1. fMRI: Biological Basis and Experiment Design Lecture 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. Broad definitions Thermal noise –Assumed white –Fluctuations in RF path (coupling to sample, amplifiers...) Physiological noise –Probably colored –Vasoreactivity –Motion –Respiration –Cardiac –... –Uncontrolled cognitive processes

3. SNR, definition 1 SNR regular = 317 SNR sense = 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 = 3226S = 3012

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? Siemen’s implementation of SENSERegular EPI

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

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

12. Surface coils vs. volume coils:

13. Nova “visual coil” Nova birdcage head coil

15. Pulse sequence diagram: Spoiling in FLASH/GRASS/RAGE... N rep = 256 256 points RF G SS G PE G RO DAC Flip angle = 7 deg. TR = 10ms TE ~ 5ms Goal: obliterate any remaining transverse magnetization before next excitation.

16. Image: N avg = 4

17. Averaging beats down noise

18. Physiological vs. thermal noise