fMRI: Biological Basis and Experiment DesignLecture 14: Localization I
• Spin echo BOLD• Experiment design
1 light year = 5,913,000,000,000 miles?
BOLD fMRI is differentially sensitive to large and small vessels
Spin echo sequences refocus dephasing caused by susceptibility-induced gradients near large veins
In both cases magnitude of field perturbation depends on:- field strength- deoxyhemoglobin concentration
Dynamic averaging regime: diffusion of water molecule is large compared to field gradient
Static averaging regime: diffusion of water molecule is small compared to field gradient
Spin Echo
90 deg.180 deg.
time (ms)
M
T2*
T2
Spin echo does not form – BOLD contrast is measured
Spin echo forms – BOLDcontrast is erased
Signal contributions: gradient echo (T2*)
100m
Intravascular
Small venuole/capillary
Large venuoleField strength
Extravascular protons near large vessels
Extravascular protons near small vessels
Rel
ativ
e co
ntri
buti
on
Blood signal
Harrison, Harel et al., Cerebral Cortex 12:225 (2002)
100m
Signal contributions: spin echo (T2)
Intravascular
Small venuole/capillary
Large venuoleField strength
Extravascular protons near small vessels
Rel
ativ
e co
ntri
buti
on
Blood signal
BOLD response: 1st approximation
~0.75 mmincreased blood flow
Signal near smallveins and venuoles
~2.0mm
capillarysignal
Neural activity (column)
2.5 mm FWHM, GE
1.5mm FWHM, SE
GE BOLD, 2.5 mm PSF:
SE BOLD, 1.5 mm PSF:
Cortical “columns”: