FMRI: Biological Basis and Experiment Design Lecture 12: Signal-to-Noise Ratio Things that determine signal strength –voxel size –RF coil Things that determine.

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Presentation transcript:

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/ap html 1 light year = 5,913,000,000,000 miles?

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

SNR, definition 1 SNR regular = 317 SNR sense = 251

Spatial dependence of SNR with parallel imaging

SNR definition 2

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

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

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

SNR maps in water phantoms New category of noise: inanimate processes? Siemen’s implementation of SENSERegular EPI

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

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

Surface coils vs. volume coils:

Nova “visual coil” Nova birdcage head coil

Pulse sequence diagram: Spoiling in FLASH/GRASS/RAGE... N rep = 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.

Image: N avg = 4

Averaging beats down noise

Physiological vs. thermal noise