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

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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?
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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

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SNR, definition 1
SNRregular 317 SNRsense 251
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Spatial dependence of SNR with parallel imaging
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SNR definition 2
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Rician distribution effect of magnitude
reconstruction on noise close to zero
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SNR definition 2
? 48.2 ? 58.6
? 56.7 ? 61.0
S 3226
S 3012
?????
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SNR maps, 2 definitions
Definition 1 mean through time divided by
standard deviation through time
Definition 2 static signal intensity divided by
background noise
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SNR maps in water phantoms
New category of noise inanimate processes?
Siemens implementation of SENSE
Regular EPI
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SNR dependence on coil properties
Body coil transmit, body coil receive
Body coil transmit, 8-channel coil receive
(These are just static images, not SNR maps.)
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SNR dependence on bandwidth
dwellTime 5?s
dwellTime 10?s
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Surface coils vs. volume coils
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Nova visual coil
Nova birdcage head coil
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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
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Image Navg 4
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Averaging beats down noise
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Physiological vs. thermal noise
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Physiological vs. thermal noise