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?

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

S

SSNR

SNRregular = 317SNRsense = 251

Spatial dependence of SNR with parallel imaging

SNR definition 2

N

SSSSNR

measNN 86.025.1 ,

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

SNR definition 2

N

SSSSNR

measNN 86.025.1 ,

= 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 = 5s dwellTime = 10s

Surface coils vs. volume coils:

Nova “visual coil”

Nova birdcage head coil

Pulse sequence diagram: Spoiling in FLASH/GRASS/RAGE ...

Nrep = 256

256 points

RF

GSS

GPE

GRO

DAC

Flip angle = 7 deg. TR = 10ms

TE ~ 5ms

Goal: obliterate any remaining transverse magnetization before next excitation.

Image: Navg = 4

Averaging beats down noise

Physiological vs. thermal noise

Physiological vs. thermal noise