High Field and Spin Echo: the Minnesota story
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Transcript of High Field and Spin Echo: the Minnesota story
Spin Echo, high field 1Psy 8960, Fall ‘06
High Field and Spin Echo: the Minnesota story
• Preview: fMRI decision tree
• Why image at 7T?
Spin Echo, high field 3Psy 8960, Fall ‘06
Harrison, Harel et al., Cerebral Cortex 12:225 (2002)
100m
Spin Echo, high field 5Psy 8960, Fall ‘06
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, high field 6Psy 8960, Fall ‘06
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
Spin Echo, high field 7Psy 8960, Fall ‘06
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)
Spin Echo, high field 8Psy 8960, Fall ‘06
100m
Signal contributions: spin echo (T2)
Intravascular
Small venuole/capillary
Large venuoleField strength
Rel
ativ
e co
ntri
buti
on
Blood signal
Extravascular protons near small vessels